Steel and Manufacturing Handbook

HEAT
RESISTANT
 ALLOYS 


TABLE OF CONTENTS 1.

INTRODUCTION ...................................................................................... 1-1 What Are Heat Resistant Alloys? ....................................................... 1-2 Rolled Alloys Products Nominal Compositions ............................... 1-4 Heat Resistant Alloy Specifications................................................... 1-5

2.

EFECT OF ALLOYING ELEMENTS ...................................................... 2-1

3.

RESISTANCE TO THE ENVIRONMENT................................................. 3-1 Oxidation .............................................................................................. 3-3 Laboratory Oxidation Testing ............................................................ 3-10 Carburization........................................................................................ 3-15 Carburization Testing .......................................................................... 3-14 Vacuum carburizing ............................................................................ 3-19 Metal Dusting/Catastrophic Carburization/Carbon Rot ................... 3-20 Nitriding ................................................................................................ 3-23 Sulphidation ......................................................................................... 3-25 Halogen Gas Hot Corrosion .............................................................. 3-30 Molten Salts.......................................................................................... 3-34 Molten Metals ....................................................................................... 3-36 Magnetism ............................................................................................ 3-41

4.

STRENGTH AT TEMPERATURE ............................................................ 4-1 Hot Tensile Properties ........................................................................... 4-1 Creep and Rupture ................................................................................. 4-3 Creep-Rupture Testing ........................................................................... 4-5 10,000 hour Rupture Strength Data ...................................................... 4-7 0.0001% per hour Minimum Creep Rate Data ...................................... 4-8

5.

THERMAL FATIGUE ............................................................................... 5-1

6.

WEAR, EROSION, GALLING .................................................................. 6-1

7.

PHYSICAL METALLURGY...................................................................... 7-1 Sigma ...................................................................................................... 7-1 Grain Growth.......................................................................................... 7-5

8.

HEAT RESISTANT ALLOY GRADES ..................................................... 8-1 Iron-Chromium Alloys ............................................................................ 8-1 Fe-Cr-Ni alloys, Ni under 20%............................................................... 8-3 Fe-Ni-Cr alloys, Ni 30 to 40% ................................................................ 8-5 Ni-Cr-Fe alloys, Ni 45 to 60% ................................................................ 8-7 Ni-Cr-Fe alloys, Ni over 60%, 15 to 25%Cr .......................................... 8-8 Cast heat resistant grades ..................................................................... 8-10

9.

DESIGN .................................................................................................... 9-1 Thermal Strain......................................................................................... 9-2 Weldments............................................................................................... 9-4 Thermal Expansion................................................................................. 9-5 Thermal Expansion Coefficients ........................................................... 9-6 Section Size............................................................................................. 9-8 i

10.

SELECTING THE ALLOY ................................................................................... 10-1 Temperature ....................................................................................................... 10-1 Atmosphere ........................................................................................................ 10-2

11.

CUTTING AND FORMING .................................................................................. 11-1 Shearing.............................................................................................................. 11-1 Bending and Forming ........................................................................................ 11-1 Spinning and Deep Drawing ............................................................................. 11-4 Machining ........................................................................................................... 11-5 Forging................................................................................................................ 11-6

12.

WELDING ............................................................................................................ 12-1 Carbon Steel vs Stainless ................................................................................. 12-2 Shielding Gases ................................................................................................. 12-3 Cold Cracking versus Hot Cracking................................................................. 12-4 Distortion ............................................................................................................ 12-5 Penetration ......................................................................................................... 12-6 Fabrication Time ................................................................................................ 12-6 Welding Austenitic Alloys ................................................................................. 12-7 Alloys under 20% Nickel ................................................................................... 12-8 Alloys over 20% Nickel ...................................................................................... 12-9 Gas Metal Arc Welding ...................................................................................... 12-10 Flux Cored Welding ........................................................................................... 12-11 Shielded Metal Arc Welding .............................................................................. 12-12 Gas Tungsten Arc Welding ............................................................................... 12-14 Plasma Arc Welding .......................................................................................... 12-15 Submerged Arc Welding ................................................................................... 12-16 Resistance Welding ........................................................................................... 12-17 Weld Filler Selection .......................................................................................... 12-18 Dissimilar Metal Joints ...................................................................................... 12-19 Heat Resistant Alloy Weld Filler Metals ........................................................... 12-20 Brazing and Soldering ....................................................................................... 12-21

13.

APPLICATIONS Muffles ................................................................................................................ 13-1 Retorts ................................................................................................................ 13-5 Baskets, rod frame............................................................................................. 13-6 Radiant tubes ..................................................................................................... 13-8 Rotary Kilns & Calciners ................................................................................... 13-12 Cast Link Belts ................................................................................................... 13-15 Neutral Salt Pots ................................................................................................ 13-16 Bolts .................................................................................................................... 13-21 Springs................................................................................................................ 13-22

14.

THUMBNAIL BIOGRAPHIES OF RA ALLOYS.................................................. 14-1 CHEMICAL SYMBOLS ....................................................................................... 14-2 BIBLIOGRAPHY ................................................................................................. 14-3 HISTORY ............................................................................................................. 14-5 TRADEMARKS ................................................................................................... 14-6 GERMAN STANDARDS VS AMERICAN ........................................................... 14-7

ii

INTRODUCTION We cannot but marvel at the fact that fire is necessary for almost every operation. By fire minerals are disintegrated, and copper produced, in fire is iron born and by fire it is subdued, by fire gold is purified. Pliny the Elder, Natural History, Book XXXVI, 200 Rolled Alloys has specialized in supplying wrought heat and corrosion resistant alloys over a half century now. We have an experienced sales force, laboratory personnel and a more detailed inventory of heat and specialty corrosion resistant alloys than any other supplier. Our technical expertise, to which this paper is an introduction, includes documented field experience and laboratory studies back to 1952. Our current laboratory data, both oxidation testing and metallography, has been generated under the direction of Jason D. Wilson. During these years, Rolled Alloys worked with the industrial furnace builders, and with those fabricators who also specialize in heat resistant alloy fabrication, to create the present market for RA330®, RA333®, RA 253 MA®, RA 353 MA®, and RA 602 CA® alloys. We have modified the chemistry and mill processing of RA330 on three separate occasions to maximize its effectiveness in heat treat applications. Beginning in the 1970’s, Rolled Alloys initiated and drafted eleven separate ASTM specifications for our own alloys, RA330® and RA333®. We, together with our suppliers, generated the data to obtain ASME Code coverage of RA330 to 1650°F (900°C). Although RA330 and RA333 are both sold to published ASTM, UNS or AMS chemistries, our internal purchasing specifications are designed for more rigorous quality control levels than required by these industry-wide specifications. We currently stock a dozen different grades of heat resisting alloys, aerospace grades used in gas turbine engines, titanium alloys, specialty welding fillers and weld overlay wires, and alloys designed for corrosion applications. Several of these are proprietary to Rolled Alloys, and we have significant market share in others.

Bulletin 401 June 5, 2006

©Rolled Alloys 2006

Note: This document is available in PDF format, and in the Technical Resource Center of our web site, www.rolledalloys.com Contact Rolled Alloys Technology & Marketing Services, Temperance, Michigan U.S.A. tel +1-734-847-0561, FAX +1-734-847-3915.

1-1

What are heat resistant alloys? Heat Resistant alloys, from our perspective, are those solid solution strengthened alloys (intended) for use at temperatures over 1400°F (760°C) and limited in the extreme to 2400°F (1316°C), which is near the melting point of these materials. These materials cannot be strengthened by heat treatment, as they are used over very broad temperature ranges, and above the temperatures where age hardening mechanisms or martensitic transformation are effective. Heat resistant alloys are strengthened by solid solution, carbides, and/or nitrides. There are two fundamental types of heat resistant alloys, the “ferritic” and the “austenitic”. Nearly all heat resistant alloys of interest to us are austenitic, as they are stronger and more ductile. Austenitic alloys of interest to us cover the range from about 8 to 80% nickel. The austenitic alloys all have much greater creep-rupture strength than do the ferritics. At room temperature the austenitics are more ductile and generally easier to fabricate. They are non-magnetic as supplied, although after certain high temperature service conditions some may become magnetic. It is these austenitic alloys that are of primary interest to us. 304 stainless, often called “18-8 stainless”, is the austenitic stainless steel produced and used in the greatest quantity world-wide. As you might suppose, the chemistry is about 18% chromium, 8% nickel, with the balance mostly iron. A low carbon version, 304L, is used for corrosion resistant applications, including the stainless cookware in your kitchen. With a little higher carbon, 304H is used as a high temperature alloy having good strength and usable oxidation resistance to about 1500°F (816°C). The most commonly used true heat resistant alloy in North America is RA330®, an iron base alloy of 19% chromium, 35% nickel and 1.2% silicon. The highest nickel heat resistant alloy, again in North America, is RA600, being 15.5%Cr (chromium), 76%Ni (nickel) and about 8%Fe (iron). In other parts of the world different alloys predominate, such as 310 (20%Ni 25%Cr) in much of Asia, and 314 (20%Ni 25%Cr 2%Si) in continental Europe. The ferritic alloys, such as RA446, are simply iron with anywhere from 11% to about 26% chromium added. They have a body-centered cubic crystal structure, the same as does iron. These ferritic grades also have a little manganese, silicon, carbon and nitrogen, mostly included for their benefits in hot working the alloy at the steel mill. Ferritic grades have low creep rupture strength, are often brittle, and may be difficult to weld. They are magnetic. In spite of poor strength, ferritic heat resistant alloys may be used for their good resistance, at red heat, to sulfur bearing atmospheres. The ferritic, or body-centered-cubic, structure is also more resistant to intergranular attack by low melting point metals, in particular molten copper and its alloys. The “straight chromium” grades also divide into two more classes of stainless steel, the martensitic and the precipitation hardening. However, neither of these is of any use above 1200°F (~650°C). An addition of carbon permits the chromium-iron grades to be hardened by heat treatment. These constitute the martensitic stainlesses, including types 410, and the 440’s A, B and C. Martensitic stainlesses can be heat treated to maintain high strength through about 900°F (482°C), with a maximum use temperature of 1200°F (649°C). 1-2

Like the ferritic stainlesses, all straight chromium grades embrittle severely after being held in the 700 to 1000°F (370 to 540°C) temperature range, the so-called “885°F” (475°C) embrittlement. There is a class of very low carbon martensitic stainlesses which obtain their high strength by an age-hardening, or precipitation hardening, process. An addition of copper, molybdenum or titanium is responsible for the precipitation hardening mechanism. The most commonly available of these is 17-4PH®, or RA17-4, which uses a copper addition. A point of occasional confusion to note—the ferritic grades have some resistance to hot SO2 or H2S gas, but they are not resistant to aqueous corrosion by sulphuric acid. Whereas RA446 might be chosen to resist a sulfidizing atmosphere at 1500°F, it has almost no resistance to liquid sulfuric acid. On the other hand, the nickel alloy RA20 (20-Cb-3® stainless) is often chosen to resist sulfuric acid. But, because of its high nickel, RA20 will not withstand hot gas corrosion by sulfur compounds at red heat. When enough nickel is added to the iron—chromium mix, the alloy becomes austenitic. That is, the atoms form a different arrangement, known as face-centered-cubic. This structure by its nature is more ductile than the ferritic, or body-centered-cubic atomic arrangement. An example is RA310, which contains 25% chromium, just like RA446, but also has 20% nickel. With the addition of this 20% nickel to a 25%Cr-iron base, the alloy becomes austenitic, with roughly ten times the strength of RA446, and much greater ductility. The one or two letter symbol is normally used for various chemical elements, rather than writing out the full name. To make this book easier to read, here are some of the most common elements used in various heat and corrosion resistant alloys. A more complete list is on page 140:

Al B C Cb Ce Co Cr Cu Fe Mn

aluminum boron carbon columbium (a.k.a. niobium, Nb) cerium cobalt chromium copper iron manganese

Mo N Ni O Si Ti V W Y Zr

1-3

molybdenum nitrogen nickel oxygen silicon titanium vanadium tungsten yttrium zirconium

Rolled Alloys Products, Nominal Composition Cr RA333® RA330® RA330HC RA 253 MA® RA 353 MA® RA 602 CA® RA800H/AT RA309 RA310 RA600 RA601 RA446

Ni

Si

Heat Resistant 25 45 1.0 19 35 1.25 19 35 1.25 21 11 1.7 25 35 1.2 25 63 -21 31 0.4 23 13 0.8 25 20 0.5 15.5 76 0.2 22.5 61.5 0.2 25 -0.5

Mo Co

W

Al

Ti

Other

3 ------------

3 ------------

3 ------------

-----2.2 0.4 --0.2 1.4 --

------0.6 --0.2 ---

0.05C 18Fe 0.05C 43Fe 0.40C 43Fe 0.17N 0.08C 0.04Ce 65Fe 0.16N 0.05C 0.05Ce 36Fe 0.1Y 0.08Zr 9.5Fe 0.07C 45Fe 0.05C 62Fe 0.05C 52Fe 0.08C 8Fe 0.05C 14Fe 0.08 0.15N 73Fe

-2.1 ------

--------

--------

--------

--0.2 -----

0.05C 1.7Mn 70Fe 0.02C 1.6Mn 69Fe 0.01C 70Fe 0.5Cb 0.04C 70Fe 0.14C 87Fe 0.06C 87Fe 3.3Cu 0.3Cb 0.05C 75Fe

Stainless 304H 316L RA321 RA347 RA410 RA410S RA17-4

18.3 16.4 17.3 17 12 12 15.5

8.2 10.2 9.3 9.5 --4.7

0.5 0.5 0.7 0.7 0.3 0.3 0.3

Corrosion Resistant AL-6XN® RA20

20.5 20

24 33

0.4 0.4

6.3 - 2.2 - -

---

---

---

RA2205 LDX 2101

22.1 21.5

5.6 1.5

0.45 0.7

3.1 - 0.3 - -

---

---

---

0.22N 0.02C 48Fe 3.3Cu 0.5Cb 0.02C 40Fe 0.16N 0.02C 67Fe 5Mn 0.22N 0.3Cu 0.03C 70Fe

0.3 0.1 0.1 -0.07 ----

9 9 3 5.8 9.4 4 ---

1.7 --20 10.6 13 37.7 50

0.6 -----14 15

-0.4 0.5 0.4 1.5 1.4 ---

-0.4 0.9 2.2 3.2 3 ---

0.08 19Fe 3.6 Cb 0.05C 5Fe 5Cb 19Fe 0.02Zr 0.001B 0.08C 3Fe 0.06C 2Fe 0.10C 2Fe 0.10C 2Fe

--

--

--

--

6.3 89.4 4V 0.16O 0.15Fe

Aerospace RA X 22 47 RA625 21.5 61 RA718 19 52 C-263 20 51 ® René 41 19.3 52.7 TM WASPALOY 19 57 RA188 22 22.5 L-605 20 10.5 Ti 6Al-4V

Titanium ---

1-4

Heat Resistant Alloy Specifications alloy

UNS Product Form (W.Nr.) RA333 N06333 Plate, sheet, strip (2.4608) Bar Smless pipe, tube Welded pipe Welded tube RA330 N08330 Plate, sheet, strip (1.4886) Bars & shapes Billets & bars Smless pipe, tube Welded pipe Welded tube Fusion weld pipe RA 253 MA S30815 Plate, sheet, strip (1.4893) Bars and shapes Pipe Welded tube RA 353 MA S35315 Plate, sheet, strip (1.4854) Bars and shapes Pipe RA 602 CA N06025 Plate, sheet, strip (2.4633) Rod, bar, wire RA800H/AT N08811 Plate, sheet, strip (N08810) Rod and bar Smlss pipe &tube RA309 S30908 Plate, sheet, strip (1.4833, Bars and shapes 1.4933) Pipe RA310 S31008 Plate, sheet, strip (1.4845) Bars and shapes Pipe RA446 S44600 Plate, sheet, strip (1.4763) RA600 N06600 Plate, sheet, strip (2.4816) Rod, bar, wire Smlss pipe & tube RA601 N06601 Plate,sheet, strip (2.4851) Rod, bar, wire Bar, forgings,rings Smlss pipe & tube

ASME

ASTM AMS W.Nr./EN

-B 718 5593 2.4608 -B 719 5717 -B 722 -B 723 -B 726 SB-536 B 536 5592 1.4886 SB-511 B 511 5716 -B 512 SB-535 B 535 SB-710 B 710 -B 739 -B 546 SA-240 A 240 - 1.4893 SA-479 A 479 1.4835 SA-312 A 312 SA-249 A 249 ASME Code Case 2033-1 -A 240 - 1.4854 ---A 312 -B 168 2.4633 -B 166 SB-409 B 409 - -SB-408 B 408 - (1.4876) SB-407 B 407 SA-240 A 240 - 1.4833 SA-479 A 479 1.4833 SA-312 A 312 SA-240 A 240 5521 1.4845 SA-479 A 479 5651 1.4845 SA-312 A 312 -A 176 - 1.4763 SB-168 SB-166 SB-167 SB-168 SB-166 -SB-167

1-5

B 168 B 166 B 167 B 168 B 166 -B 167

-2.4816 5665 5870 2.4851 5715

EFFECT OF ALLOYING ELEMENTS Starting with a base of iron, the most important alloying elements in heat resisting alloys are: chromium (Cr) for oxidation resistance and nickel (Ni) for strength and ductility. Other elements are added to improve these properties, but heat resistant alloys are primarily alloys of iron, chromium and nickel, and a few are mainly nickel and chromium. Silicon is one of the most effective elements in contributing carburization resistance. CHROMIUM (symbol Cr) Chromium is the one element present in all heat resisting alloys. Oxidation resistance comes mostly from the chromium content (the same is true of aqueous corrosion resistance). Chromium adds to high temperature strength, and to carburization resistance. Metallurgically speaking, chromium tends to make the atomic structure “ferritic”, that is, with a body centered cubic (BCC) crystal structure. High chromium also contributes to sigma formation. RA446, which is essentially 25% chromium, 75% iron, is a ferritic alloy. The tendency to form ferrite, and to form sigma, is counteracted by nickel. NICKEL (symbol Ni) Nickel is present, anywhere from 8% up to 80%, in all of the “austenitic” heat resistant alloys. When added to a mix of iron and chromium, nickel increases ductility, high temperature strength, and resistance to both carburization and nitriding. Nickel decreases the solubility of both carbon and nitrogen in austenite. High nickel is bad for sulphidation resistance. Again speaking metallurgically, nickel tends to make the atomic structure “austenitic”, that is, with a face centered cubic (FCC) crystal structure. For example, the alloy RA446 with 25%Cr 75%Fe (iron) is ferritic, and rather rather weak. If one were to substitute 20% nickel for some of that iron one the result would be a 25% chromium, 20% nickel, 55% iron austenitic alloy. This is RA310, which, because it is austenitic, is much stronger and more ductile than RA446. Nickel counteracts, but doesn’t necessarily stop, the tendency for an alloy to form sigma. Here, both RA446 and RA310 may form sigma when exposed to intermediate temperatures. IRON (symbol Fe) Heat resistant alloys may contain anywhere from 8 to 75% iron. In some proportions iron is a strengthening element, but it is easily oxidized and carburized unless protected by other elements. Metallurgically speaking, iron is a ferritizing element. Iron itself has a ferritic, or body centered cubic (BCC) crystal structure. Iron base alloys require a certain amount of nickel to be added before they become austenitic.

2-1

EFFECT OF ALLOYING ELEMENTS, continued THE NEXT GROUP of alloying elements present in all heat resisting alloys is silicon, carbon, nitrogen, sulphur and phosphorus. All may be considered impurities arising from the steel making process. They may be either tolerated at some level as undesirable impurities, or controlled for their effects on metal properties. Silicon, for example, affects the fluidity of the molten metal, which is an important variable in the steelmaking process. Carbon is controlled within certain limits in heat resisting alloys as a strengthening element, normally above 0.04%. In corrosion resistant grades carbon is considered an undesirable element, and is kept as low as practical, under 0.03%. Nitrogen may be controlled like carbon as a strengthening element in both the heat and the corrosion resistant grades. When not used deliberately, there may be about 0.05% or so N in austenitic stainless and nickel alloys. Sulphur is generally undesirable, but some sulphur is used to improve machinability. Phosphorus is quite harmful to weldability in nickel alloys. SILICON (symbol Si) Silicon improves both carburization and oxidation resistance, as well as resistance to absorbing nitrogen at high temperature. At high enough levels, silicon improves resistance to alkali metal hot corrosion. Silicon can decrease weldability in some, not all, alloys. In the U.S., Rolled Alloys has long been the only company to produce wrought heat resistant alloys containing silicon. RA330® has 1.2%Si, RA333® about 1% silicon. All the cast heat resistant alloys have silicon, in part because it increases fluidity of the molten metal. In Europe silicon is used to improve a number of heat resistant alloys, such as the Outokumpu grades, RA 253 MA® and RA 353 MA®, and the German alloys 314 (1.4841) and 1.4828. The metallurgical effects of silicon are that it tends to make the alloy ferritic, or to form sigma. In RA330, the 35% nickel content is more than enough to prevent any embrittling sigma to form from the Si. Silicon decreases the solubility of carbon in the metal (technically it increases the chemical “activity” of carbon in the alloy). A silica (silicon oxide) layer, just under the chromium oxide scale on the alloy, is what helps the alloy resist carburization. CARBON (symbol C) Carbon, even a few hundredths of a per cent, is a strengthening element. As the carbon level increases, the alloy becomes stronger, but it also becomes less ductile. Most wrought heat resisting alloys contain around 0.05 to 0.10% carbon, with RA 602 CA near 0.2%, and RA330HC at 0.4% C. The cast heat resisting alloys usually have from 0.35% up to 0.75% carbon. While strong, the cast alloys are not very ductile. Corrosion resistant grades, by contrast, have less than 0.03% carbon, and sometimes much less.

2-2

CARBON (symbol C), continued Carbon is an “austenitizing element”, and tends to retard or prevent formation of ferrite and sigma. Carbon may actually be dissolved in the alloy, or, more commonly, it is present as small, hard particles called carbides. These are chemical compounds of carbon with chromium, molybdenum, tungsten, titanium, zirconium or columbium (niobium). NITROGEN (symbol N) A small amount of nitrogen serves to strengthen austenitic heat resisting alloys. Too much nitrogen can embrittle them. Nitrogen is used to strengthen Outokumpu’s heat resistant grades 153 MA®, 253 MA® and 353 MA®, likewise Haynes® HR-120®. Nitrogen is also an “austenitizing” element. It tends to retard or prevent ferrite and sigma formation. A small amount of nitrogen is specified in RA446. This causes a little austenite to form (in a predominately ferritic structure) while it is being hot worked. This in turn helps keep the ferrite grain size from getting too large. Nitrogen at 0.23% is used in the corrosion resistant alloy AL-6XN® to prevent sigma formation. It also raises the tensile and yield strengths of AL-6XN, and increases its resistance to chloride pitting corrosion. SULPHUR (symbol S) Sulphur is normally regarded as an impurity, and is commonly below 0.010% in most nickel alloys. It has the benefit of improving machinability, so for 304 and 316 bar it is kept up around 0.02%. Free machining stainless steels, such as Type 303, may have much higher sulphur, about 0.3%. To improve hot workability, and therefore maximize yields, the steel mill normally refines the metal to a very low sulphur content. This is fairly easy to do with current melting processes such as the AOD (argon-oxygen decarburization) or ESR (electro-slag remelt) furnaces. AL-6XN stainless, for example, is refined to extremely low sulphur, 0.001% being not uncommon. Sulphur is also detrimental to weldability. Along with simply removing the sulphur in the refining process, the harmful effects of S on hot working and welding may be reduced to a degree by the addition of some manganese. PHOSPHORUS (symbol P) Phosphorus is harmful to weldability. Phosphorus cannot be removed during the refining process. To produce alloys with low phosphorus, one must start with low phosphorus raw materials. Because phosphorus is so harmful to nickel alloy weldability, the nickel weld fillers themselves are normally specified to have no more than 0.015% phosphorus, and even lower P would be preferred.

2-3

EFFECT OF ALLOYING ELEMENTS, continued OTHER METALLIC ALLOYING ELEMENTS include cobalt, manganese, tungsten, molybdenum, titanium, aluminum, columbium (also called niobium), zirconium, the rare earth elements such as cerium, lanthanum and yttrium, and boron. Copper and vanadium are used in some corrosion resistant alloys but not in the heat resistant grades. Some of these elements are added for strength, others like aluminum and the rare earth elements are largely for oxidation resistance. COBALT (symbol Co) Cobalt at the 3% level in RA333 improves strength slightly and enhances oxidation resistance at extreme temperatures. Larger amounts are required for a significant strengthening, such as the 15%Co in cast Supertherm® or 12.5% in the LBGT combustor alloy 617. Cobalt base alloys L605 and 188 are very strong, but oxidation resistant only to 1800 or 2000°F (1000 or 1100°C). Cobalt is an austenitizing element, like nickel. High cost and variations in availability of cobalt tend to limit the use of cobalt alloys to gas turbine engine applications. MANGANESE (symbol Mn) Manganese is used in steelmaking to improve hot workability. It is mildly detrimental to oxidation resistance, so is limited to 2% maximum in most heat resistant alloys, and restricted further, to 0.80% max, in RA 253 MA. Manganese improves weldability, and is added to many austenitic weld fillers. RA330-04 achieves its hot cracking resistance from about 5% manganese added to the 35%Ni 19%Cr base. Manganese is usually considered an austenitizing element. It increases solubility for nitrogen and has for decades been used in the Nitronic® series of stainless steels from AK Steel (formerly Armco), both as a partial substitute for nickel and to permit a substantial nitrogen addition. Mn decreases solubility for carbon, i.e., decreases the chemical activity of carbon in austenitic alloys. TUNGSTEN (symbol W) Tungsten is a large, heavy atom used as a strengthening addition, 3% in RA333, 5% in the cast alloy Supertherm and 14% in Haynes alloy 230. Tungsten is a carbide forming element, that is, it reacts with the carbon in the alloy to form a hard particle, which may incorporate other carbide forming elements such as chromium. Tungsten also promotes formation of sigma, and of ferrite. Tungsten metal, with thoria or rare earth oxide additions, is used for the electrode in gas tungsten arc welding. It is tungsten’s very high melting point, 6170°F (3410°C), which is required for this application. Tungsten oxidizes in air readily above 950°F, but is prevented from doing so by the argon or helium weld shielding gas. High levels of tungsten in heat resistant alloys may promote catastrophic oxidation. 2-4

MOLYBDENUM (symbol Mo) Molybdenum is another large, heavy atom used to increase high temperature creep-rupture strength. 3% Mo is used in RA333. This is about as much Mo as can be tolerated in a heat resistant alloy without serious oxidation problems in heat treat furnace applications. Alloys X, 625 and 617 all contain 9% molybdenum, which is very good for strength but not so good for oxidation at extreme temperatures (2100°F/1150°C) or under stagnant conditions. Molybdenum promotes sigma formation, unless counterbalanced by austenitizing elements such as nickel, cobalt, etc., and is a ferritizer. Molybdenum is also a carbide forming element. Molybdenum helps weldability in austenitic alloys, both stainless and nickel base. Commercially pure molybdenum metal is used for vacuum furnace fixturing, because of its very high melting point, 4730°F (2610°C), and high temperature strength. However, molybdenum metal has no oxidation resistance above 800°F (427°C) and would literally disappear in a cloud of white smoke if exposed to air at red heat. TITANIUM (symbol Ti) Titanium is added in small amounts, about 0.3—0.7%, for strength in austenitic alloys. Around 0.1—0.2%Ti is used, as part of steel mill melting practice, in deoxidation of nickel alloys. Ti is a strong carbide former, and it is the titanium carbides that strengthen RA800AT. Titanium also promotes sigma and ferrite, but it is normally used in such small amounts as to be inconsequential in this respect. In aqueous corrosion alloys titanium is referred to as a “stabilizing” element. Age hardening alloys used in aerospace, such as A-286, X750, C-263, the various Nimonic® alloys, Renè 41®, WASPALOYTM and 718, depend upon some larger amount of titanium, to 3% or so, for their age hardening properties. The titanium in these grades may form a thin oxide layer on the surface which inhibits flow of braze metal. For the most part, the age hardening grades must be nickel plated before brazing. Titanium metal itself, although it has a very high melting point (3040°F/1671°C), is not really a heat resistant metal. Titanium alloys are used up to about 600°F (316°C) in aerospace applications. ALUMINUM (symbol Al) Aluminum is added at the 1 to 5% level for oxidation resistance. RA 602 CA® has 2.2% Al, alloy 601 contains 1.4% Al, and there is 4.5% aluminum in Haynes alloy 214. Aluminum increases the activity of carbon, similar to silicon, which increases resistance to carburization. Al is used in the age hardening (precipitation hardening) alloys, in which it forms the ductile intermetallic phase gamma prime, Ni3Al. At around 0.1 to 0.4%, aluminum is added to most nickel alloys as a deoxidizing agent, in the last stages of AOD refining. Aluminum promotes sigma formation. It is also a ferritizing element. Aluminum may be added to a Cr-Fe stainless grade, e.g. type 405, so that it forms no austenite when welded, hence does not harden. 2-5

COLUMBIUM (symbol Cb) . . . also called NIOBIUM (symbol Nb) Columbium is added at the 0.4 to 0.8% level for strength in several heat resisting alloys, and to prevent corrosion after welding in 347 stainless and nickel corrosion resistant alloys 20Cb3® (RA20), G-3 and G-30. This low amount of Cb is harmful to weldability, while higher amounts are beneficial. About 2 to 2.7%Cb is used in various high nickel weld fillers (82, 182), while the 3.6% Cb in 625 is good for both strength and weldability. Columbium is very harmful to oxidation resistance, practically speaking around 1800°F/980°C and higher. For this reason we limit the amount of residual Cb that may be present in RA330, and in RA333. Columbium is a strong carbide former, a ferritizing element and promotes sigma formation. At the 5% level, it is the age hardening element in alloy 718. ZIRCONIUM (symbol Zr) Zirconium is a strong carbide former. It is added in very small amounts, less than 0.1%, to increase strength in RA 602 CA®, and in alloy 214. THE RARE EARTH ELEMENTS cerium, lanthanum and yttrium are used singly or in combination to increase oxidation resistance in austenitic alloys both wrought and cast, and in the newer ferritic heat resistant alloys. The technology has been known, but little used, since about 1940 in Germany. CERIUM (symbol Ce) Cerium is the major rare earth element responsible for the excellent oxidation resistance of RA 253 MA. The cerium is added as an alloy of several rare earths, called mischmetal. For chemistry control purposes, the mill analyzes only for Ce. Mischmetal is encountered in everyday life as the “flint” in a cigarette lighter. It oxidizes (burns) very readily. In RA 253 MA and RA 353 MA the Ce helps chromium form a thinner, tighter and more protective oxide scale. Residual cerium oxides in the metal may contribute to creep-rupture strength. LANTHANUM (symbol La) Used at the 0.02 to 0.05% range to for oxidation resistance in Haynes alloys 230, S, 556 and 188. YTTRIUM (symbol Y) Used at the 0.005 to 0.1% level for oxidation resistance in 214 and RA 602 CA. A larger amount, 0.5%, of yttria (Y2O3 ) is used as an oxide dispersion strengthening element in the oxide dispersion strengthened (ODS) ferritic alloys such as PM 2000l® and Kanthal APM®. As yttria, it also increases oxidation resistance.

2-6

BORON (symbol B) Boron increases creep-rupture strength, and is used at rather low concentrations, 0.002% is typical. Boron is somewhat harmful to weldability of nickel alloy plate. For this reason boron may be restricted in nickel alloy weld fillers. Boron is an interstitial element and tends to concentrate at the grain boundaries. High boron may reduce the formation of continuous carbide network in austenitic alloys. Boron is an essential ingredient in many high temperature braze alloys, specifically the Nickel-SiliconBoron braze alloys developed by Dr. Robert Peaslee of Wall Colmonoy®.

2-7

RESISTANCE TO THE ENVIRONMENT It is difficult to run high temperature corrosion tests in the laboratory and obtain results that can be used to predict metal behavior in service. Even two laboratories running the same type of test may not come up with numerical results that agree with one another, although the alloy rankings should be similar. Based on extensive experience and lab work, we have confidence that our oxidation data may be used to compare relative performance of one alloy with another, at that test temperature. There are certain limitations, such as catastrophic oxidation, that may not be revealed by our testing. Good performance of a new alloy in this test only indicates that the alloy MAY perform well in service. We do regard poor results from the lab test as a very strong indication that the product will be unsatisfactory in service at that temperature. As oxidation rates vary with thermal cycling, among other variables, the data are not directly useful for predicting metal wastage/corrosion rates of high temperature equipment in service. It is even more difficult to obtain data useful as an engineering tool to predict life of equipment in sulphidation, carburization, liquid metal environments and other types of high temperature corrosion. We must emphasize that laboratory data are a necessary first step. That laboratory test itself must then be validated by documented service experience. Only after validation by service experience may this test be considered a useful engineering tool. We would make a distinction between fundamental studies of the nature of oxidation, and engineering data. In the following pages we will present the results of laboratory testing, controlled service exposure, and service experience reported by others. Read all such high temperature laboratory data, including these, with a critical eye. Results may be very sensitive to exactly how the test was run, as well as to the investigator’s unstated assumptions. Oxidation For our purposes, this means the high temperature chemical reaction of a metal with the oxygen in the air. Simply put, most metals can burn when they get hot enough. Some, like magnesium (once used in flash bulbs) and titanium do burn in the conventional sense and may cause a serious industrial fire. Even iron burns. Of course, lighting a match to a nail does absolutely nothing. But if one takes very fine iron wire—specifically, 0000 steel wool—it may indeed be ignited by a match. There is no actual flame, but a red hot “coal” develops and enough sparks fly to endanger clothing. There are two basic ways in which a metal may be resistant to oxidation. First, it may be inert and simply not react chemically with oxygen in the air. Two examples come to mind, the precious metals gold (Au) and platinum (Pt). Because of its high melting point, 3217°F (1769°C), coupled with oxidation resistance, platinum is actually used for some laboratory ware and other items that must withstand extreme temperature. The second way a metal may resist oxidation, and the one of interest to us, is that the metal or alloy may form an adherent oxide film, which protects it from further oxidation. The element most often used to form such a protective oxide layer, or scale, is chromium. It forms the oxide Cr2O3, also known as chromia. 3-1

Oxidation, continued Although chromium itself oxidizes even more readily than iron, the oxide it forms is very thin, and adheres tightly to the metal. This oxide layer forms very quickly at high temperature, but once formed it protects the metal against further oxidation. The chromia scale also protects the alloy against carburization and sulfidation, to a degree. The protection is by no means perfect. The scale contains defects through which oxygen and other elements may pass, to continue to react with the alloy. Scale also cracks from both thermal and mechanical strains, and small pieces spall off each time the metal is cooled down. For a high temperature alloy to have useful oxidation resistance, the scale must be able to “heal” these defects, by more chromium diffusing to the surface to form a new protective film. Other elements are added to the alloy to improve the protective nature of this oxide film or scale. One of the most effective is silicon. Silicon oxidizes to SiO2, or silica. If enough silicon is present, the silica forms a sub-scale underneath the chromium oxide scale. This silica subscale is how silicon provides resistance to carburization, in alloys such as RA330. At the 1.2% Si level in RA330, silicon also contributes to oxidation resistance. In RA85H, which is no longer produced, the silicon was much higher, 3.5%. At this high level silicon appeared to offer resistance to molten alkali salt corrosion. The effectiveness of the chromium oxide scale may be improved by very small additions of rare earth elements, such as cerium. Cerium promotes a thinner scale, which is more protective against oxidation because it cracks and spalls off less than would a thicker scale. It is the 0.04% cerium in RA 253 MA that is largely responsible for the excellent oxidation resistance of this rather lean 21Cr, 11Ni alloy. Aluminum is also used to improve oxidation resistance. In order to actually develop an Al2O3, or alumina, scale a rather high amount of aluminum is required. At 2.2% aluminum, RA 602 CA alloy will form an alumina subscale. This contributes to the oxidation resistance of RA 602 CA. RA 602 CA does not oxidize internally. The 1.4% Al typical in alloy 601 is not enough to form an alumina scale, but it is enough to enhance oxidation resistance of 601. Because of aluminum at this somewhat lower level, 601 oxidizes internally. This is not a problem in plate gauges, though perhaps it may be a consideration in thin sheet. The 4.5% aluminum in Haynes alloy 214 is enough to form an actual alumina scale. and 214 is extremely oxidation resistant above 1800°F/982°C. The Protective Film While chromium is given credit for promoting oxidation resistance and is without question the most effective element in this respect, it is actually the most easily oxidized. This may sound like double talk, but it really isn’t. When pure chromium or a chromium-bearing alloy is exposed to oxygen, even at room temperature, it is oxidized and a layer of chromium oxide (and oxides of other elements as well) is formed. Even the chromium plate on automobiles, or the cutlery on our dinner tables, has a microscopically thin and transparent film of chromium oxide present.

3-2

The Protective Film, continued When formed at high temperatures, the oxide coating becomes green, black, blue or yellow, depending upon its thickness and which of the numerous chromium oxide compounds is formed. This in turn depends upon the temperature and availability of oxygen to combine with chromium. The oxide layer is dense, is inclined to be tightly adhering, and effectively seals out the air or oxygen from the metal underneath. So long as the oxide layer is intact, the metal is protected and further oxidation proceeds very slowly. Several things may tend to destroy our protective layer: Expansion and contraction, as the result of heating and cooling, will “pop” the oxide layer, because the base metal and the oxide expand and contract at different rates. The more rapid the rate of expanding and contracting, or the more quickly the metal is heated and cooled, the more hazard there is of the protective coating flaking off. Certain combinations of chromium, iron, nickel, silicon and other oxides are more tightly adhering than others at different temperatures. With some alloys it is possible to reach a temperature where the scale or oxide is no longer tightly adhering and will be loose, thereby offering little or no protection. Thus, an excessive temperature for the specific alloy can destroy the protection normally offered by the oxide layer. Some examples are 321, which is acceptable at 1600°F (870°C) but scales unacceptably at 1800°F (980°C), and 309, which isn’t very useful above 1900°F (1040°C).

Composite radiant tube, RA333 for 4 feet (1.2metre) on the firing end, middle portion RA330 and exhaust end fabricated of RA309. Used at a nominal furnace operating temperature 1750°F (955°C) for annealing malleable iron castings. It is to be expected that the tube metal temperature would be perhaps 100— 150°F (55—85°C) higher. A jam-up in the furnace broke the tube. Note the crater-like appearance of local oxidation or “warts” the RA309.

3-3

The Protective Film, continued When an alloy is used at a temperature exceeding its capabilities the scale may break down locally, a condition sometimes called “warts”, or “nodules”. We have observed this on 309 (above) and 310, occasionally on RA 253 MA, RA330 and 600 alloy. On one occasion we saw warts on an RA333 brazing muffle. Upon investigation we found that the RA333 had been heated in service to the incipient melting temperature, perhaps somewhere above 2370°F. The grains were sliding apart so as to leave voids at the triple points, resulting in apparent porosity of the 11gage (3mm) muffle wall. Mechanical deformation and creep, such as the stretch of a bar under load, may also destroy the protection. While the metal is ductile and yields in creep, the oxide coating is fragile and brittle and will spall off. In service, a given item may appear to have insufficient oxidation resistance, whereas that particular property would have been more than adequate had the strength been sufficient to avoid excessive creep. Laboratory data which do not duplicate cyclic conditions or stresses imposed in actual service can be misleading as a measurement of an alloy’s oxidation resistance. Of great concern are environments that promote the destruction of the protective layer by some chemical reaction. For example, we know of one case where minute amounts of potassium nitrate/nitrite austempering salts were present on fixturing used in a carburizing atmosphere. The salts attacked the protective oxide coating, so that a normally carburization resistant alloy carburized very quickly and uniformly. In years past, we knew of a few cases where parts being heat-treated were first coated with sal ammoniac (ammonium chloride). The presence of this chloride salt resulted in a chemical attack upon the protective oxide coating of the furnace fixtures. Alloys normally selected for the strength, oxidation resistance and thermal shock resistance requirements were not suitable. Another form of chemical destruction that may be encountered is corrosion from welding fluxes. Fluoride-bearing fluxes from coated welding electrodes must be carefully and thoroughly removed. Otherwise they continue to function as a flux, damaging not only oxidation resistance, but also carburization resistance. Green rot might be considered one form of destruction of the protective oxide coating. To the best of our knowledge, green rot tends to be more prevalent in alloys containing about 65% or more nickel. Green rot is the result of the alloy being alternately exposed to oxidizing and reducing conditions. When the alloy is exposed to the oxidizing environment, a protective oxide coating is formed, as we previously discussed. When the alloy is exposed to highly reducing conditions, the nickel and other less stable oxides may be reduced to pure metal, which disappears as a powder; but the chromium oxide, being more stable, is not reduced. Upon exposure of the alloy to an oxidizing environment once more, the oxygen is free to penetrate to the metal and form another layer of oxide, since there are now voids in the coating where some of the oxides previously existed. 3-4

The Protective Film, continued With continuous exposure to the two conditions, a mass is eventually formed consisting only of porous chromium oxide, with or without other oxides that may have been sufficiently stable to resist reducing. This actually has little strength and no ductility. It has the characteristic greenish-black color of chromium oxide and, upon fracture, has the appearance of rotten wood. Hence the name, green rot. Catastrophic Oxidation Catastrophic oxidation is, as its name implies, oxidation that proceeds so rapidly that complete failure of the material occurs in an extremely short time. Certain elements, such as molybdenum, columbium (niobium), vanadium, and tungsten, form oxides that are volatile at relatively low temperatures. If these oxides are formed and retained in the scale, they act as fluxes and destroy the protective film1, 2, 3.

This 316 stainless (S31600) bar was originally 3/4” (19mm) dia. It operated at 1800°F (982°C) as an electrical heating element. Sections that were covered with ceramic insulation suffered catastrophic oxidation from the 2%Mo in 316, and reduced in section to less than 1/4” (6mm) diameter. The typical chemistry of 316 is 16.4Cr 10.2Ni 2.1Mo. Generally 1500°F (816°C) is considered the maximum long-time use temperature of 316 even in a freeflowing atmosphere

The effect of molybdenum is important enough that we would like to quote directly from the late Howard S. Avery’s classic work on heat resistant alloys, Cast Heat-Resistant Alloys for High-Temperature Weldments: “Where in fact the addition of molybdenum has conferred better hot strength, the chief problem may be surface stability, especially in the 1800— 2300°F (980—1260°C) range. This is most serious under those conditions that cause catastrophic oxidation which stems from the volatile nature of molybdenum oxide (MoO3). This oxide is likely to form in stagnant atmospheres, with a threshold for trouble around 1400—1500°F (760—816°C).”

3-5

Catastrophic oxidation, continued Catastrophic oxidation may be a serious problem under certain operating conditions. That is, a stagnant atmosphere, or solid deposits under which the atmosphere is of course stagnant, and extreme temperatures. Alloy X (N06002, W.Nr. 2.4665), containing 47% nickel, 22% chromium and 9% molybdenum, may completely disappear from catastrophic oxidation when heated for some months at 2200°F (1200°F). At lower temperatures in free-flowing atmospheres alloy X is highly oxidation resistant. It has, after all, served for decades as the primary alloy used in gas turbine flight engine combustors. However, alloy X may not well tolerate stagnant conditions or temperature extremes.

Alloy X (N06002) bar exposed to a metal dusting environment. To keep the temperature near 1100°F (~600°C), where metal dusting is most likely to occur, the rod was packed in fibrous insulation. Because the atmosphere under the insulation was stagnant, in our opinion the attack shown here is more likely representative of catastrophic oxidation, rather than metal dusting.

Metal Thickness Thin things burn faster than thick. Thin sections have a lesser total amount of chromium available to reform the protective scale. Most of our data and experience is with plate gauges, roughly 3/16—1/2” (~5—13mm). The normal concern is that the plate not lose enough thickness that it is no longer structurally sound. One should be cautious about applying this experience to thin sheet. For example, a metal loss of about 0.020” (1/2mm) per side may not seriously impede the operation of a plate item 1/2” (12.7mm) thick. But that same loss on 16 gage (1.6mm) sheet would quite destroy its usefulness.

3-6

Metal Thickness, continued

Isothermal oxidation at 2100F (1149C) for 97 hours, versus thickness of RA330. Test coupon thickness: sheet 0.005, 0.060, 0.120, plate 1/4, and 3/8”.

Grain Size As the protective oxide layer flakes away or is otherwise damaged, diffusion of chromium to the surface continually reforms, or “heals”, the scale. The diffusion rate of chromium is orders of magnitude greater along grain boundaries than it is through the grain itself. Fine grain size improves the ability of the scale to re-form and to heal damage4. We observed the effect of grain size on oxidation of S30400 stainless flat bar after long time service.

Type 304 stainless “belly band”, cross section 11.5 x 38 mm. This band was used to reinforce corrugated RA309 inner covers used for batch annealing carbon steel coils.

3-7

Grain Size, continued In service this band was exposed to products of combustion of natural gas with excess air at about 1600°F (870°C) for perhaps five years. The flat bar for this band was produced by shearing strips from 1/2” (12.7mm) plate. The shearing operation heavily cold worked the edges.

Metal loss due to oxidation was 0.6mm per side over most of the band. There was a narrow zone of deep attack parallel to and about 2,5-3mm from each sheared edge.

The bulk of the metal had grain size ASTM 7. The heavily cold worked sheared surfaces recrystallized in service to grains as fine as ASTM 8. A short distance back from the edge, coincident with the heavily oxidized zone, the grain size was as coarse as ASTM 4. This is the zone which was cold worked in the critical range for grain growth. The relation between metal wastage and grain size of this 18-8 stainless is shown at left.

The effect of grain size on hot salt corrosion is similar. An Alloy Casting Institute study5 of the grain size effect on intergranular corrosion rates of low carbon cast Ni-Cr-Fe alloys in molten chloride salts is in agreement. The ACI exposed samples for 50 hours at 1600°F (871C) in a neutral salt bath containing 55% BaCl2, 25%KCl & 20% NaCl. The reduction in attack of HW (12%Cr 60%Ni) and HT (15%Cr 35%Ni) with decreasing grain size is shown at left.

3-8

Laboratory Oxidation Testing In order to evaluate new and competitive alloys we perform considerable laboratory oxidation testing at Rolled Alloys, at temperatures up to 2250°F (1232°C)6, 7 . We measure weight gain, that is, the total amount of oxygen (and nitrogen) that has reacted with the test specimens. Specimens are usually of plate gages, and the tests are cyclic. Samples are heated in porcelain crucibles, 4 to 6 in a tray, for about 160 hours (one week) at temperature. The tray is then removed from the furnace, lids are quickly placed on the crucibles to contain spalling oxide, and the assembly allowed to air cool to room temperature. The entire crucible, containing specimen and scale, is then weighed every cycle. Results are reported as weight gain, in milligram/centimeter2 . The numerical results are valid only for the specific conditions of the test. Which means they are not useful for predicting metal wastage of components in actual service. However they are of value when one compares the data from new alloys, with those of existing grades. For example, we have a great deal of experience with the good performance of RA333 and RA330. Likewise, 309 is about the only one of our heat resistant alloys that occasionally gives disappointing performance, generally around 1900°F (1040°C) or above. If an alloy performs well on test, we feel that means it MAY perform well in service. A simple coupon test does not simulate all the things that can happen in service. So, it is possible for an alloy to look very good in the laboratory and not at all so good in production equipment. While we like to think our tests provide useful guidance, there are a number of conditions, common in high temperature equipment use, which are not well simulated by laboratory oxidation testing: 1. Thermal Cycling. This is more or less addressed by cycling the specimen to room temperature weekly. More rapid cycling means more scale spalls off, increasing oxidation rates, more so for some alloys than others. For example, in static 1000 hour oxidation testing 310 is somewhat superior to RA330. When thermal cycling is added, RA330 better retains its protective oxide. 2. Creep Strain. This is not at all addressed in the lab. Creep strain, as well as thermal cycling, increases the amount of scale which spalls off the coupon. 3. Stagnant Atmospheres. There is little or no flow of atmosphere in certain areas of electrically heated equipment, and underneath insulation or solid deposits. Alloys with high molybdenum contents are subject to catastrophic oxidation under these conditions, though they may perform rather well in our open air test. 4. Atmospheres other than dry air. The H2O content of the atmosphere affects oxidation rates. High water content increases the metal wastage of low nickel alloys faster than it does the higher nickel grades

3-9

Laboratory Oxidation Testing, continued

Oxidation test after 2880 hours at 2200°F (1200°C)

And, finally, the laboratory test does not properly simulate time. 3000 hours seems a reasonable length of time to run a test in our laboratory, but that is still only about 4 months. If one expects the equipment to last 1, 2 or 10 years, it would be hard to make a case that a 4 month test adequately represents service conditions. The specimen continually changes chemistry throughout the test (it loses chromium, silicon and aluminum by scaling). Thin samples, simply from having less total chromium, may show greater oxidation rates than thick specimens. In our considered view, significant extrapolations of oxidation, or other high temperature corrosion data, are not valid. The declining availability of experienced engineers in the U.S.A. has generated pressure to extrapolate such data, valid or not. Data shown on the following bar graphs is all for 3000 hour (~18 weeks) exposure, in order to compare all alloys for about the same exposure time. All but the RA309 tests were run for 3000 hours, that one being extrapolated from a 1600 hour run. As this is weight gain data, high numbers mean heavy oxidation, small numbers a relatively light degree of oxidation. One would expect to use these numbers, along with service experience, as a guide to an alloy’s usefulness. We emphasize that, unlike what is assumed about aqueous corrosion data, oxidation data ought in our opinion be viewed qualitatively.

3-10

Laboratory Oxidation Testing, continued Numbers under 20 may give assurance that the alloy, in plate form, should not lose structural integrity due to metal loss. One might want a little actual service background when considering alloys with weight gains in the 100-300 mg/cm2 range. One example is 304 stainless, which gains 64 mg/cm2 at 1600°F (871°C), and in the neighborhood of 300 mg/cm2 at 1800°F (982°C). By experience, we know that 304 1/4” plate will simply disappear in 2-3 months when used in air around 1700-1800°F (930—980°C). We would look at the higher alloys, for more elevated temperatures, somewhat differently. Note 600 alloy which shows a 153 mg/cm2 weight gain at 2100°F (1149°C). Nevertheless, alloy 600 plate is a useful material for retorts and muffles operating in the 2100-2200°F (1150—1200°C) temperature range. Likewise RA 353 MA is used quite successfully at such temperatures. RA 602 CA is clearly the best by far in our test series. The good resistance to scaling of RA 602 CA in test has also been borne out by service experience in rotary calciners, CVD retorts, and at least one AOD charge chute.. One should also bear in mind that these data still represent simple laboratory oxidation testing, which does not take into account many of the ways by which the protective oxide scale may be damaged. The alloys were cycled to room temperature once a week. More rapid thermal cycling would not only increase oxidation rates but might also change the relative performance of some alloys. In static 1000 hour oxidation testing, for example, 310 is somewhat superior to RA330. When thermal cycling is added, RA330 better retains its protective oxide. Another point to remember is that alloys high in molybdenum and columbium may be sensitive to catastrophic oxidation, particularly under stagnant atmospheres. References, Oxidation 1. Stanislaw Mrowec, Teodor Werber, Gas Corrosion of Metals, 1975, Warsaw, Poland 2. Leslie and Fontana, Transactions ASTM, Vol 41, pages 1213-1247, 1949, ASTM Philadelphia, Pennsylvania 3. H.S. Avery, Cast Heat-Resistant Alloys for High-Temperature Weldments, WRC Bulletin 143, August 1969, Welding Research Council, New York, New York 4. J.C. Kelly and J.D. Wilson, Oxidation Rates of Some Heat Resistant Alloys, Heat Resistant Materials nd II,Conference Proceedings of the 2 International Conference on Heat-Resistant Materials 11-14 September, 1995, Gatlinburg, Tennessee, U.S.A., ASM International 5. J.H. Jackson, Cast Alloys for Salt-Bath Heat Treating, Alloy Casting Bulletin No. 16, November 1952, Alloy Casting Institute 6. Gene Rundell and James McConnell, Oxidation Resistance of Eight Heat-Resistant Alloys at 870, 980 1095 and 1150C, Oxidation of Metals, Vol. 36, Nos. 3/4, 1991 7. J.C. Kelly and J.D. Wilson, Oxidation Rates of Some Heat Resistant Alloys, Heatnd Resistant Materials II, Conference Proceedings of the 2 International Conference on Heat-Resistant Materials 11-14 September, 1995 Gatlinburg, Tennessee

3-11

Exposed for 3000 Hours Cycled Every 160 Hours

250

1600F 200

Weight Gain (mg/cm2)

200

1800F 2000F

154*

2100F

150 113

100 64 54

53

50

32 20

19 5

1

0 RA304

RA309

RA310 Alloys 3-12

11* 1

RA 253 MA

4

RA330

Exposed For 3000 Hours Cycled Every 160 Hours

350

1800F

333 324

2000F

295

300

Weight Gain (mg/cm

2)

2100F 2200F

250 232

2250F 200

150

156

153

145

143

138

100 83 61

58

50

50

36

32 21 4

11

49

34 18

4

21 12

11

18

0 RA800H

RA625

RA600

RA X

RA 353 MA

Alloys

3-13

RA333

RA601

RA 602 CA

CARBURIZATION The temper of Iron for Files It must be made of the best Steel, and excellently tempered, that it may polish, and fit other iron as it should be: Take Ox hoofs, and put them into an Oven to dry, that they may be powdered fine: mingle well one part of this with as much common Salt, beaten Glas, and Chimney-soot, and beat them together, and lay them up for your use in a wooden Vessel hanging in the Smoak; for the Salt will melt with any moisture of the place or Air. The powder being prepared, make your iron like to a file: then cut it checquerwise, and crossways, with a sharp edged tool: having made the Iron tender and soft, as I said, then make an Iron chest to lay up your files in, and put them into it, strewing on the powders by course, that they may be covered all over: then put on the cover, and lute well the chinks with clay and straw, that the smoak of the powder may not breath out; and then lay a heap of burning coals all over it, that it may be red-hot about an hour: when you think the powder to be burnt and consumed, take the chest out from the coals with Iron pinchers, and plunge the files into very cold water, and so they will become extremely hard. This is the usual temper for files; for we fear not if the files should be wrested by cold waters. But I shall teach you to temper them excellently G. B. Della Porta, 1589, Sources for the History of the Science of Steel 1532—1786, Ed. Cyril Stanley Smith

Carburizing is one of the most commonly performed steel heat treatments. For perhaps three thousand years it was performed by packing the low carbon wrought iron parts in charcoal, then raising the temperature of the pack to red heat for several hours. The entire pack, charcoal and all, was then dumped into water to quench it. The surface became very hard, while the interior or “core” of the part retained the toughness of low carbon steel. Pack hardening is uncommon today, except in a few custom made sporting arms. Now, low carbon steel parts are heated in a prepared furnace atmosphere that provides the carbon that diffuses into the surface layers of the steel. Temperatures are usually around 1750°F (950°C). This atmosphere has traditionally been “endothermic”, using a catalyst to partially burn natural gas. Typical composition1 of an endothermic gas (Class 302) is 39.8% nitrogen, 20.7% carbon monoxide, 38.7% hydrogen and 0.8% methane, with a dew point –5°F (–20°C). This carrier gas is subsequently enriched by a small, controlled addition of a hydrocarbon gas, such as propane, or an easily vaporized liquid, which is the source of carbon. 100% nitrogen, from bulk tanks, may also be used as a carrier gas, with propylene or other hydrocarbon injected to provide the necessary carbon. In vacuum, or low pressure, carburizing acetylene, C2H2 or cyclohexane, C6H12, is used as the source of carbon. The end result is that low carbon steel parts acquire a high carbon steel surface. When the steel is quenched it combines the hardness and wear resistance of this high carbon steel “case” with the toughness of the low carbon steel interior (core). The alloy bar frame baskets2, radiant tubes and other fixturing in the furnace also pick up carbon through many, many heat treat cycles. These fixtures are made of carburization resistant alloys. Even though the atmosphere is reducing to iron, it is still oxidizing to chromium, silicon, and aluminum. An oxide scale, some mixture of Cr2O3, SiO2 and Al2O3, forms on or just beneath the surface. This oxide layer is what provides most of the alloy’s resistance to carburization.

3-14

Carburization, continued Carburization embrittles high temperature alloys, so that they can neither be straightened nor weld repaired. The degree of embrittlement depends upon the amount of carbon absorbed3, and upon the microstructure. Generally speaking, once an alloy has absorbed about 1% carbon it will no longer have measurable room temperature ductility. We once examined a sample of 310 sheet which contained 4% carbon, and could readily be broken by hand. With carburized alloy, enough ductility may remain while at red heat for the metal to perform its task. This, so long as it is not excessively strained at high temperature, or impacted at room temperature.

Alloy 601 used in a powdered iron sintering muffle. Grain growth is from the operating temperature. Brittle fracture at room temperature comes from the large amount of carbon, 2.34%, absorbed during service. The nitrogen-hydrogen atmosphere is not supposed to be carburizing. Nevertheless, carbon enters the atmosphere from the organic compounds used as binders in the “green” powder compact.

Carburization resistance in an alloy is conferred almost entirely by the protective oxide scale4, along with the nickel content. The oxide scale is primarily chromia, with silicon being a very potent assist5. Nickel lowers the solubility of carbon in the alloy, so that a very high nickel grade simply will not carburize to the same level as will a lower nickel material. In vacuum carburizing there is too little oxygen present to form chromium or silicon oxides for protection. Such carburization resistance as an alloy has may come largely from its aluminum content, which will form alumina in this process. Among the alloys, RA330 usually does the best job for the money. RA333, RA600, RA 353 MA, RA601 and RA 602 CA are all more carburization resistant but also more expensive. 800H does not well tolerate the effects of carburization, in part because it lacks silicon but also, and more importantly, because it is invariably coarse grained.

3-15

Carburization, continued RA 253 MA has worked as furnace fixturing because it is strong, but RA 253 MA is not resistant to carburization. Even RA309 has somewhat better carburization resistance than does RA 253 MA. The common stainlesses 304 and 316L do not possess adequate resistance to carburization for use as fixturing in commercial carburizing heat treat furnaces. The ferritic grade 446 is quite poor in carburization resistance. When nickel heat resisting alloys become carburized, it happens that many also become magnetic. A pocket magnet, then, becomes a handy tool to judge whether or not alloy fixturing has enough ductility remaining to be weld repaired or straightened. Not due to carburization, but a purely mechanical problem that may occur in a carburizing atmosphere is of some concern. Soot may deposit from the atmosphere and “coke” in any crevices, such as cracks in weld joints or surface defects on castings. The growth of this soot deposit acts like tree roots growing in rock. It literally pries open lack of fusion in the weld or opens small pin-holes in castings into large cavities. In the case of wrought alloys, which are free of surface defects, we emphasize the need to have designs and weldments that do not provide crevices in which carbon deposition may occur. This is one reason why full penetration welds of the return bend to straight leg are essential for maximum life in radiant tubes. On low fire soot may deposited in the root crevice (as well as in surface defects of cast return bends). On high fire this soot burns out, locally overheating and weakening the metal. Carburization testing Laboratory carburization testing must be carried out in some approximation of the industrial atmosphere of interest. The test temperature should be similar to that anticipated in service. In addition it would be a good idea to include thermal cycles about like the expected service conditions6. Finally, duration of the test is important. Carburization resistance depends upon the chromia scale, the silica subscale, and the alumina scale in some alloys. For this reason the test atmosphere should, in our opinion, contain an oxygen partial pressure comparable to the expected service atmosphere, in order to form a similar protective scale7. One may also wish to consider nitrogen, as nitrogen the atmosphere reacts with alloying elements such as chromium, and may affect carburization. There have been laboratory carburization tests run in an atmosphere of hydrogen—2% methane, with no control of oxygen partial pressure. In this environment the alloy will not develop much of a protective scale. Such an atmosphere is one way to achieve the objective of actually carburizing most alloys. Very small amounts of oxygen can form enough alumina or titania scale, for example, to inhibit braze flow in many vacuum furnaces. Alloy 800H contains enough titanium to turn light gray in aome vacuum heat treat furnaces. 3-16

Carburization testing, continued In order to braze even stainless steel (with no Al or Ti) in hydrogen it is normally considered that the dew point should be –60°F (–51°C) or lower8. This is necessary to dissociate the oxides of most alloying elements. Alumina and titania will not be dissociated by this atmosphere. One might expect that grades such as N06601, N06025 and N0811 would form aluminum and titanium oxide films in a nominal hydrogen—methane atmosphere. Such films may affect carburization. Recent work by George Lai11, in H2-CH4, ranks several alloys in the same order as does service experience. Oxygen partial pressure was not indicated. Dr. Lai’s ranking of wrought alloys, from best to worst is: Haynes® 214, RA 602 CA©, Incoloy® 803, 800H, and 310 stainless. The ranking is the same, whether based on weight change or on measured depth of carburization. Alloy ranking is approximately in accordance with their aluminum + titanium contents. Al and Ti are is the elements most likely to form a scale in this test. We might infer that these methane-hydrogen test results could have some relevance to performance of alloy fixturing in a vacuum carburizing furnace. In vacuum carburizing very small amounts of oxygen are present from the furnace leak rate, if nothing else. Carbon Content, weight %, Before and After Testing Alloy

Original

Final

Increase

%Al

214 RA 602 CA 803 800H 310

0.042 0.19 0.084 0.082 0.08

0.50 0.36 0.99 1.23—0.95 2.73

0.008 0.17 0.91 0.86—1.14a 2.65

4.5 2.2 0.3 0.4 0.05

a

range of five specimens

When the atmosphere does simulate that of industrial interest, carburization testing may require long exposure. There is some period of time during which significant carbon absorption does not take place. Experience related to us from one furnace company indicated that the test had to be run for at least 1000 hours, before their results correlated with service experience. Their test data, shared with us, is below. The tests were conducted in an electrically heated industrial carburizing furnace. The higher temperature results, 1900°F (1038°C) are from a composite electric heating element made of the five alloys shown, and the 1750°F (954°C) results are from plate samples exposed to the actual furnace operating temperature. In both cases the total exposure hours were distributed as follows: 20% of the time in endothermic gas enriched with natural gas to carbon potential 1.0-1.2%C relative to iron, 70% of the hours in nitrogen, and 10% of the time reflected air burnout cycles at 100°F (56°C) reduced temperature. Various depth of cuts were machined in the samples and the carbon contents analyzed. Results here are reported at 0.045” (1.14mm) depth on the element and 0.20” (0.508mm) depth on the plate sample. 3-17

Carburization testing, continued 1900°F (1038°C) 2260 hour exposure

1750°F (954°C) 4300 hour exposure

alloy

%carbon

%carbon

RA333 RA330 617 601 600 310

1.53 3.03 2.86 2.98 1.56 --

0.344 0.443 1.6 1.096 -3.92

Vacuum Carburizing Vacuum carburizing presents a different environment. Behavior of alloys in conventional atmosphere carburizing do not necessarily predict how they will perform in low pressure carburizing. Still, there is always a small amount of oxygen in a vacuum carburizing furnace. Some may be introduced through traces of acetone in the acetylene used as a carburizing gas. The leak rate in the furnace will always permit some oxygen to be present. While not enough to form a stable chromia or silica scale, the 2.2% aluminum alloy RA 602 CA will form an alumina film on the surface. It is this oxide that is responsible for the alloy’s resistance to carburization in this environment.

RA 602 CA alloy baskets designed for low pressure carburizing, This captive shop processes transmission sprockets and gears at 1650°F (900°C) in Abar Ipsen furnaces with integral oil quench. The carburizing gas is acetylene. Baskets are stacked three high, and will see two cycles per day on average.

3-18

METAL DUSTING A somewhat aggravating problem in carburizing atmospheres is “metal dusting”, a.k.a. “catastrophic carburization”, or “carbon rot”. This occurs at lower temperatures, typically 800—1200°F (430—650°C) in heat treating furnaces. Such temperatures exist in a carburizing furnace (nominal 1750°F/950°C) where alloy tube hangers, atmosphere sampling tubes or electrical leads pass through furnace walls, and in some areas of Ipsen® furnace chains. The exact mechanism may be disputed, but the effect is that the metal disappears. A bar may look just like a beaver had chewed away on it. In other cases, the metal literally appears worm-eaten on the surface. In the petrochemical industry, a small amount of sulphur (40—50 ppm H2S) is sometimes added to the process gas stream to “poison” the high temperature chemical reaction that is metal dusting. Alloys vary greatly in susceptibility to metal dusting. RA333, by four decades’ experience and several years testing in the heat treat industry, is the best known choice. The high silicon grade RA85H (no longer produced) was also good, though not quite so resistant as is RA333. RA330 is average, while 800H is markedly inferior. Both 600 alloy and its matching weld filler, 82 (ERNiCr-3) are the least resistant. Neither 310 nor 601 will solve metal dusting problems. RA 602 CA is being tested in several metal dusting environments. To date it appears superior to RA330, but we do not yet know how it compares with RA333. One direct alloy comparison, below, shows two RA333 GMAW beads with minor smoothing, while the 3/16” (4.8mm) RA310 plate between them suffered nearly complete loss of section9.

Sample from a rotary retort used to carburize small parts at an operating temperature of 1750°F (940°C). Spiral flights of RA310 welded to the inside transport work pieces through the retort. As the retort was externally fired, the 3/8” (7.9mm) 600 alloy shell was above the temperature range for metal dusting. Metal dusting was a serious problem with flights at the entry end of the retort. Here the cold work pieces chilled the RA310 flights down into the metal dusting temperature range.

3-19

Metal Dusting, continued

Furnace chain severely attacked by metal dusting. This is an application where RA333 has given the ® best service life in original equipment here in the U.S.A. Nicrofer 6025HT (RA 602 CA) is being used in Europe.

RA330 carburizing furnace anchor bolt, 3/4” (19 mm) diameter. Failure by metal dusting. RA330 gave better life than 600 alloy in this application, but is still not satisfactory. The longest lasting tube hangers to date are those made of 5/8” (16mm) dia RA333 rod. At furnace temperatures the mechanical strength of 5/8” RA333 is comparable to that of 3/4” dia. RA330, while the metal dusting resistance of RA333 is greatly superior. RA 602 CA, with somewhat higher strength and good metal dusting resistance, may replace RA333 in this application.

3-20

Metal Dusting, continued The following test results are from a direct comparison of alloys for 25,594 hours (3 years) at temperature, in the metal dusting zone of a Surface Combustion carburizing furnace. 1” (25.4mm) Sch 40 oxygen probes of various alloys with different surface treatments were inserted through the furnace roof. The atmosphere is endothermic enriched with 0.7—0.8% methane to a 1.20% carbon potential, operating temperature 1700°F (927°C). Metal dusting occurs in the region where temperatures are roughly 1100°F (600°C), as the pipe passes through the refractory. Alloy

Condition

Results

RA333®

As received Preoxidized

Dark, no pits at 27,594 hours Some pits at 16,183 hours

RA85H®

As received Preoxidized

Black, no pits at 8122 hours Black, no pits at 7549 hours

RA330®

As received

Pitted, test stopped at 19,472 hours

214®

As received Preoxidized

Many pits, test stopped at 19,472 hours Many pits, test stopped at 19,472 hours

HR-120M

As received

Pitted—removed from test at 11,264 hours

HR-160®

As received

Pitting started at 24,422 hours

Preoxidizing treatments provided no benefits or were counter productive. Although surface treatment by aluminum diffusion coat is usually considered to provide resistance to metal dusting, aluminum as an alloy addition, even the 4.5% nominal aluminum content of alloy 214, did not appear effective.

This a most direct performance comparison of RA333 with RA330 in carburizing service. The flights in this rotary carburizing retort were RA330, but they were GMAW welded using RA333 wire. Metal dusting is normally most severe at the entry end, where the flights are cooled by incoming work pieces. Because of the high carbon potential in this furnace, the first RA330 flight is completely corroded through. The RA333 weld bead is essentially unaffected, and remains even though the RA330 plate on each side has been destroyed. One might consider using RA333 plate for the flights, or at least the first set of them, in this particular application. The shell and balance of flights might continue as RA330.

3-21

Metal Dusting, continued

Here RA330 plate is compared with alloy 82 (ERNiCr-3) weld filler, operating 1700°F (927°C) in a high carbon potential atmosphere. The weld bead in this corrugated retort has been selectively attacked. To prevent this, GMAW welding with RA333 wire would be appropriate.

3-22

NITRIDING Nitrogen reduces alloy ductility in a manner similar to carbon. A great deal of attention is given to carbon-pickup in alloys at high temperature, but the nitrogen content is rarely analyzed. An increase in nitrogen content may normally be expected to occur during high temperature service in air. RA446 plate coupon exposed 3000 hours in air at 2100°F (1150°C). The initial nitrogen level was 0.089%. After exposure nitrogen reached 1.15%. The needles at 60° angles in this photomicrograph are chromium nitrides. The surface, at top, shows some internal oxidation. An absence of nitrides near the surface is probably due to chromium depletion from oxidation. Mill certification, Jessop Steel Co. Heat 26445: 0.18C 0.70Mn 0.47Si 0.26Ni 24.84Cr 0.03Mo 0.03Cu 0.089N

Nitrogen has been associated with blistering and severe reduction of creep-rupture strength in carburized HL (30Cr 20Ni) steam-methane reformer tubes10. Carburization decreased nitrogen solubility in Ni-Cr-Fe alloys by removing chromium from the matrix. Because of the reduced solubility, nitrogen then diffused ahead of the advancing carburized front. This locally concentrated the nitrogen, and contents as high as 0.462% were measured. It was postulated that this could result in high nitrogen gas pressure, which the authors associated with microvoids and cracks. A lamellar phase near the grain boundaries was apparently a nitride phase. Commercial nitriding, e.g. the Floe process (U.S. Patent 2,437,249), usually is done with RA600 alloy fixturing. The first stage of the Floe process is done at 925-975°F (495525°C), the second at 1025-1050°F (550-565°C). Ferritic nitrocarburizing, done around 1100°F (600°C), may be carried out in an RA600 retort, with containers of RA330, 304 stainless, or a mixture of both. At this temperature grades such as RA309, RA310 or RA 253 MA are not suggested, due to embrittlement from sigma formation. A higher temperature process, carbo-nitriding is carried out in an atmosphere containing both carbon and nitrogen. Temperatures are high enough to austenitize the steel workpieces. That is, higher than for nitriding but lower than carburizing, roughly 1300— 1650°F (705—900°C), with shorter cycle times than for carburizing. The life of alloy fixturing in a carbo-nitriding application cannot be expected to equal that in a straight carburizing environment, probably for two reasons. First, because the embrittling effect of carbon and nitrogen combined is more drastic. Second, because the cycles are much shorter.

3-23 Nitriding, continued The hours or years of exposure are not the important things affecting an alloy’s (quenching fixture) life. Rather it is the total number of cycles that determines life. Thermal fatigue cracking gradually develops and grows with each cycle. A part in a carbonitriding environment will receive many more cycles in a given length of time than if it were in a carburizing application, and its life will be shortened accordingly. References, Carburization, Metal Dusting and Nitriding 1. Furnace Atmospheres, Metals Handbook® Ninth Edition, Volume 4 Heat Treating, ASM, Metals Park, Ohio 1981 2. G. R. Rundell, Evaluation of Heat Resistant Alloys in Composite Fixtures, Corrosion 86 Paper Number 377, National Association of Corrosion Engineers, Houston, Texas 1986 3. D. E. Wenschof and J. A. Harris, The Influence of Carburization on the Mechanical Properties of Wrought Nickel Alloys, Corrosion/77 Paper No. 9, National Association of Corrosion Engineers, Houston, Texas 1977 4. R. H. Kane, Carburization of Cast Heat-Resisting Alloys in Synthetic Petrochemical Environments, Corrosion/83 Paper Number 266, National Association of Corrosion Engineers, Houston, Texas 1983 5. D. B. Roach, Carburization of Cast Heat-Resistant Alloys, Corrosion/76 Paper No. 7, National Association of Corrosion Engineers, Houston, Texas 1976 6. D. J. Hall, M. K. Hossain, and J. J. Jones, Factors affecting carburization behavior of cast austenitic steels, Materials Performance, January 1985, Houston Texas 7. R. H. Kane, Effects of Silicon Content and Oxidation Potential on the Carburization of Centrifugally Cast HK-40, Corrosion/80 Paper Number 168, National Association of Corrosion Engineers, Houston, Texas 1980 8. Brazing of Heat-Resistant Alloys, Low-Alloy Steels, and Tool Steels, ASM Handbook® Volume 6, Welding, Brazing and Soldering, ASM International, Metals Park, Ohio 1993 9. James Kelly, Metal Dusting in the Heat Treating Industry, Stainless Steel World 99 Comference, The Hague, Netherlands 1999 10. J. R. Schley and F. W. Bennett, Destructive Accumulation of Nitrogen in 30 Cr 20Ni Cast Furnace Tubes in Hydrocarbon Cracking Service at 1100C, Corrosion, September, 1967 National Association of Corrosion Engineers, Houston, Texas 11.

George Lai, Proposed Standard Carburization Test Method, Corrosion 2003 Paper No. 3473, National Association of Corrosion Engineers, Houston, Texas 2003

3-24 SULFIDATION Environments containing sulfur may rapidly attack high nickel alloys. The problem is more severe under reducing, or low oxygen, environments. The higher the nickel the more sensitive the alloy is to sulfidation attack. If sulfur is a problem, we do not suggest using any alloy with more than 20% nickel. RA310, with 25% chromium and 20% nickel, is useful in many sulfur bearing environments. RA309, at 13% nickel, may be preferred for some applications. Under the most severe conditions an alloy completely free of nickel, such as RA446 may be required, in spite of other disadvantages it has. When the environment is oxidizing the alloy is more likely to form a protective chromium oxide scale, rather than a chromium sulfide. Under reducing environments the alloy forms chromium sulfide, which is non-protective. An oxidizing environment is one in which sulfur is present as sulfur dioxide (SO2), and there is some excess oxygen (O2), or even carbon dioxide (CO2) and/or water vapor (H2O). In reducing environments sulfur is in the form of hydrogen sulfide (H2S), there may be hydrogen (H2), carbon monoxide (CO), methane (CH4) or other sources of carbon, and rather little CO2 or H2O. Sometimes the distinction isn’t obvious. For example, there may be solid deposits on metal in an oxidizing environment. Underneath those deposits, in contact with the metal the actual amount of oxygen available to form a scale is miniscule. It has been stated that oxygen partial pressures may be about 10-8 under calcium sulfate deposits on fluidized bed components. If the deposit contains sulfur, then the metal may be heavily attacked under the deposit, regardless of oxygen is in the atmosphere above it. An example of under deposit attack is shown below. This 1/4” (6.35mm) RA 253 MA plate sample came from a kiln processing ferrous sulfate monohydrate to red iron oxide pigment. The atmosphere was air, plus the SO2 and SO3 driven off in the process, operating temperature 1840°F (1004°C). After about a year the RA 253 MA kiln shell had developed holes roughly 3/4” (20mm) across, some rather long. Previously used RA310 had failed by more uniform thinning, and lasted 2 to 2 1/2 years.

From Rolled Alloys Report Number 94-72

3-25

Sulfidation, continued

Types of Scale Developed on Type 310 Stainless Steel as a Function of Oxygen and Sulfur Partial Pressures in the Gas Environment at Temperatures of 750, 875, and 1000C. Conversion factor: 1 atm = 0.101356 MPa. ANL 1 Neg. No. 306-79-625 These diagrams illustrate whether oxides or sulfides are formed at equilibrium.

“For a given sulfur partial pressure, there exists a threshold value for oxygen partial pressure beyond which a continuous protective oxide scale is developed on the specimens. This threshold 3 oxygen partial pressure, represented by the kinetic boundaries in the figure, is ~10 times the oxygen partial pressure for he Cr oxide/Cr sulfide equilibrium.” In our opinion, Dr. K. Natesan of Argonne has done, and continues to do, the best high temperature corrosion work of our time.

The meaning of these diagrams is simply that if not enough oxygen is present, or the sulfur is too high, the alloy will form a chromium sulfide rather than an oxide. While the oxide may be protective, the sulfide offers little resistance to further attack.

3-26 Sulfidation, continued Nickel reacts chemically with sulfur very readily. Unlike metal oxides, which at least are solid, metal sulfides, or metal-metal sulfide eutectics, are often molten at operating temperature. If sufficient molten metal sulfide forms underneath the chromium oxide scale, it may literally wash that scale away. Useful corrosion testing for sulfidation resistance requires very long time exposure. In general, the corrosion rate in sulfidation may be more or less parabolic for some period of time. Eventually, corrosion enters a “break-away” mode2, where corrosion rates accelerate dramatically. The most comprehensive study of heat resistant alloy sulfidation was carried out in the late 1970’s through early 1980’s under the direction of the Metals Properties Council. Some of their results, from the 1987 Final Report, are included here. We present Table 8 from that report, time to breakaway corrosion. This is, in our view, the significant measure of sulfidation resistance. ESTIMATED TIME TO BREAKAWAY CORROSION FOR VARIOUS ALLOYS Based on Metallographic Measurements and Gravimetric Analysis in 1000 psig Tests Estimated Time, 1000 hour Alloy 671 657 HL-40A Co-Cr-W No. 1 T63WCB 6B HK-40A 30/50WB RA333 Crutemp 25 310 310 (Al) 446 309 188 556 617B Alloy X 21.9

32X N-155 800H 800H (Al)

>10

1650°F— Wt % 0.5 vol% H2S Cr Met. Grav.

1650°F— 1.0 vol% H2S Met. Grav.

1800°F— 0.5 vol% H2S Met. Grav.

1800°F— 1.0 vol% H2S Met. Grav.

50.2 48.0 30.9 30.0 28.2 28.1 28.0 27.9 26.2 25.4 25.0 25.0 24.0 23.0 22.0 22.0 22.0

>10 >10 3 >9 2 >10 2 ->8 >10 10 10 -10 >10 >10 >10 >10 >10 1 -5 >10 1 >10 >4 3 10 ->8

>10 >10 >10 >10 >10 >10 1 -4 >10 10 1 4 >10 ->8

>10 >10 >10 >10 >10 >10 7 >10 2 >8 ->10 2 -10 >10 >10 >10 8 >10 5 >10 2 6 ->10 c -1 -4

1 >10 ->5

->10 -10

-9 ->10

>10

21.6 20.9 20.6 20.6

>10 >10 >10 >10 >10 >10 >10 ->10 >10 10 >10 3 5 >10 10 >10

10 >10 >10

>10 >10 >10 >10 >10 >10 >10 ->10 >10 3 >10 >10 2 >10 >10 >10 >10

4 >10 10 >10

10

5 >10 8 >10

>10 >10 2 >9 4 >10 1 -7 3 10 >10 -10 7 >10

1

1

1 >10 6 >5

3-27 Sulfidation, continued Alloys Tested Alloy Name

UNS Cr Ni 671 R20500 50.20 47.80 A IN-657 R20501 48.00 50.00 HL-40 J94614 30,90 19.40 Co-Cr-W No. 1 R30001 30.0 -® Thermalloy 63WC -28.20 36.00 ® Stellite 6B R30106 28.10 2.80 A HK-40 J94204 28.00 20.00 ® Wiscalloy 30/50W -27.90 48.70 ® RA333 N06333 26.20 45.00 ® Crutemp 25 -25.40 24.80 310 S31008 25.00 20.20 310 (Al) S31008 25.00 20.20 446 S44600 24.00 0.40 309 S309008 23.00 14.70 ® Haynes 188 R30188 22.00 22.90 556

Co ---55.50 15.00 57.10 --2.50 -----37.96

W ---12.0 5.0 4.8 -3.6 2.7 -----14.5

Alloy Compositions Mo Si C -0.39 0.06 ---0.01 1.40 0.47 --2.50 -1.40 0.52 1.20 0.50 1.00 0.50 2.0 0.40 -1.00 0.51 3.80 1.40 0.05 0.40 0.60 0.07 -0.68 0.06 -0.68 0.06 -0.38 0.10 -0.11 0.11 0.60 0.40 0.09

Fe 1.10 -47.10 -13.78 1.90 47.10 17.42 15.50 47.20 52.20 52.20 74.60 62.50 1.20

A

R30556

22.00

20.00

20.00

2.5

3.00

0.40

0.10

29.18

A

N06617 N06002

22.00 21.90

54.00 44.60

12.50 2.50

-0.5

8.00 9.10

-0.44

0.07 0.09

-19.50

21.60

32.00

--

3.1

--

1.10

0.10

40.72

617 Alloy X ®

Sanicro 32X ®

--

N-155 , Multimet®

R30155

20.90

19.80

19.50

2.8

3.00

0.60

0.10

29.00

800H

N08810

20.35

30.00

--

--

--

0.23

0.08

46.13

800H (Al)

N08810

20.35

30.00

--

--

--

0.23

0.08

46.13

A

Other 0.24Ti 1.5Cb 0.60Mn -0.10Mn 1.40Mn 2.00Mn 0.87Mn 1.50Mn 1.50Mn 1.71Mn 1.71Mn 0.45Mn 0.54Mn 0.06Mn 0.22Al 0.07La 1.50Mn 1.1Cb+Ta 0.30Al 0.02La 1.00Al 0.69Mn 0.21Al 0.65Mn 0.40Al 0.33Ti 1.30Mn 1.1Cb+Ta 0.94Mn 0.37Al 0.38Ti 0.72Cu 0.94Mn 0.37Al 0.38Ti 0.72Cu

Nominal chemistry only

(Al) means the sample was aluminum diffusion coated. This test data gives an indication of how much sulfur heat resistant alloys might tolerate at what temperature in a mildly reducing atmosphere. What it says about alloys is that high chromium, at least 25%, is necessary for any degree of sulfidation resistance. It does not indicate how materials might perform under deposits. In the pilot plant exposure part of this test series, alloy 671 sulfidized badly beneath calcium sulfate deposits. RA333, chosen for an oil sands pilot project on the basis of early good results, sulfidized beneath carbon deposits. The reaction vessels were aluminum diffusion coated 310, burned out after every 1000 hour run.

3-28 Sulfidation, continued The test atmosphere supplied to the reactor for CGA (coal gasification atmosphere) was borderline oxidizing-reducing, from Table A-1 shown below:

Gas CO2 CO H2 CH4 HN3 H2 S H2 O

Inlet Gas Atmosphere for Initial Tests vol% 12 18 24 5 1 1.0, 0.5, or 0.1 Balance

Many sulfidation failures occur under highly reducing conditions. That is, where a source of carbon, such as methane (CH4) is present along with the hydrogen sulfide Even 1/2% of H2S can be quite destructive. One example is in carbon black manufacture. Low-grade oil is heated with very little oxygen to break it down into soot—which is carbon— or lamp black. The oil used as feed-stock normally contains up to 3 percent sulfur. High nickel alloys are quite unsuited for high temperature service in the sulfidizing environments of carbon black plants. Specifically, RA330, RA333, alloys X, 800H, 600, 601, 617 and some of the high cobalt alloys may fail by sulfidation. Melting points of some metal-metal sulfide eutectics are3 : 1175°F (635°C) for Ni-Ni3S2, 1611°F (877°C) for Co-Co4S3 and 1810°F (988°C) for Fe-FeS. The Fe0-FeS eutectic melts at 940C4 . CrS-Cr2S3 doesn’t melt until 2462°F (1350°C)5. References 1. K. Natesan, CORROSION AND MECHANICAL BEHAVIOR OF MATERIALS FOR COAL GASIFICATION APPLICATIONS, ANL-80-5, Argonne National Laboratory, Argonne, Illinois U.S.A. 1980 2. Maurice A. H. Howes, High-Temperature Corrosion in Coal Gasification Systems, Final Report (1 October 1972-31 December 1985) as subcontractor to The Materials Properties Council, Inc., New York, New York. 3. Binary Alloy Phase Diagrams, Thaddeus B. Massalski, Editor, 1986, American Society for Metals, Metals Park, Ohio 4. Stanislaw Mrowec, Teodor Werber, Gas Corrosion of Metals, translation published by the Foreign Scientific Publications Department of the National Center for Scientific, Technical and Economic Information, Warsaw, Poland 1978 5. Handbook of Chemistry and Physics, 65th Edition, CRC Press Inc., Boca Raton, FL 1984— 1985

3-29 HALOGEN GAS HOT CORROSION Unlike oxides, metal halides are volatile. When halogens are present in high temperature environments any oxide scale present becomes porous and non-protective. In order to form a protective scale it is generally considered that the metal chloride vapor pressure must be below 10-4 atmosphere. For CrCl3 and NiCl2 that would be about 600°C, just slightly lower for CoCl2. For FeCl3 the limit is much lower, about 160°C, and lower yet for MoCl5, 50°C, and AlCl3, about 75°C. As a practical matter, the high nickel alloys 600 (UNS N06600) and 400 (N04400) are most commonly chosen for hot halogen gas resistance. Good discussions of this subject are given in the old INCO® Corrosion Engineering Bulletins CEB-3 for HCl and Cl2, and CEB-5 for HF and F2. Data for 100% Cl2, and for HCl follow. Corrosion in dry Chlorine Gas Metal

Approximate temp, °F, at which given corrosion rate, mils/year, is exceeded in short time tests in dry Cl2 Corrosion in Dry Chlorine, 100%

Nickel alloy 600 alloy 400 316 304 Platinum CopperA SteelB Gold Silver A B

30

60

120

600

1200

950 950 750 600 550 900 350 250 250 100

1000 1000 850 650 600 950 450 350 300 150

1100 1050 900 750 650 1000 500 400 350 250

1200 1200 1000 850 750 1050 500 450 400 450

1250 1250 1000 900 850 1050 550 450 400 500

copper metal ignites in hot Cl2 at about 600°F carbon steel ignites in hot Cl2 at about 450-500°F

Suggested upper temp limit for continuous service, °F

1000 1000 800 650 600 500 400 400 ---

3-30 Corrosion in Dry Hydrogen Chloride, 100% Metal

Approximate temperature, °F, at which given corrosion rate, mils/year is exceeded in short time tests in dry HCl Corrosion rate, mil/year, in dry HCl

Nickel alloy 600 alloy 400 316 304 Platinum Copper Steel Gold Silver

30

60

120

600

1200

850 800 450 700 650 2300 200 500 1800 450

950 900 500 700 750 -300 600 -550

1050 1000 650 900 850 -400 750 -650

1250 1250 900 1100 1100 -600 1050 -850

1300 1350 1050 1200 1200 -700 1150 ---

Suggested upper temp limit for continuous service, °F

950 900 450 800 750 2200 200 500 1600 450

Both of these tables were abstracted from INCO Bulletin CEB-3. The data were obtained from short-time laboratory tests and offer only a rough guide to maximum practical temperature limit of materials. The original data from which INCO developed their table was published in 1947, M.H. Brown, W.B. DeLong and J.R. Auld, “Corrosion by Chlorine and by Hydrogen Chloride at High Temperatures”, Ind. & Eng. Chemistry, Vol 39, No. 7 pp 839-844 At lower halogen concentrations alloys forming a chromia layer can tolerate higher temperatures. Data in Bender and Schütze, Paper 00239 Corrosion 2000, show that alloy 600 can form a protective oxide at 800°C in 0.1%Cl2, 100 hour test. At 2%Cl2, same temperature, the alloy does not develop a protective scale. Grain size has an effect, 600 with finer grains, 75µm (ASTM 4.5), being superior to 600 with coarser grain size, 125µm (ASTM 3). Fine grain size increases diffusion rate of chromium to the surface.

3-31 Longer time tests show lower corrosion rates. The following industrial data were obtained from 30 day test exposures. 100% Chlorine Gas, type 304/321 stainless, 30 day test Temp mils/yr mm/yr F C 572 300 6 0.15 617 325 7 0.18 662 350 9 0.23 707 375 15.5 0.39 752 400 33 0.84 797 425 115 2.9 100% Chlorine Gas, alloy 600, 30 day test Temp mils/yr mm/yr F C 977 525 8 0.2 1022 550 12 0.3 1067 575 15 0.38 1112 600 24 0.61 1157 625 47 1.2 Corrosion of nickel alloys by hot 100% F2 gas is given in Table 14, CEB-5. Most of that data is reproduced below.

Corrosion by dry fluorine gas °F

80

400

700

1000

°C

27 204 370 Corrosion Rate, mils per year

538

2.4 0.5 -0.2 1.0 0.9 -0 1.7 0.6 0 2.7 1.1

29.8 11.3 21.3 7.2 24.5 16.1 44.5 13.8 ----3451

Temperature Material 400

200 nickel

304 304L 347 600

Exposure time, hours 5 24 24* 120 5 24 24* 120 5 24 120 5 5

0.5 0.5 0.7 0.1 3.3 0.5 0.3 0.1 6.1 7.5 25.4 4.0 0.6

1.9 1.7 2.4 1.2 1.7 1.2 0.5 0.4 1565 6018 -4248 78.0

All tests were made in flowing fluorine gas, except * which were conducted in bombs at initial pressure of 250 psi.

3-32

The original source of this fluorine data was: R.B. Jackson, General Chemical Division, Allied Chemical Company, “Corrosion of Metals and Alloys by Fluorine,” Contract AF 04 (611)-3389 Corrosion Tests in Hydrogen Fluoride Gas Temperature: 930 to 1110F (500 to 600C). Test duration 36 hours. From Table 17, INCO CEB-5 Material

Corrosion Rate mils/year mm/yr

Comments

Hastelloy® alloy C Inconel® alloy 600 Hastelloy alloy B Nickel 200 Nickel 201 Monel® alloy 400 Monel alloy K-500 70-30 Copper-Nickel

0.3 0.7 2 9 14 13 16 16

iridescent tarnish film “ black film “ “ adherent dark film “ “

0.008 0.02 0.05 0.2 0.36 0.33 0.41 0.41

Hastelloy is a registered trademark of Haynes International Inconel and Monel are registered trademarks of Special Metals, Inc.

In atmospheres containing a significant partial pressure of oxygen these laboratory data in pure halogens or halide gases have limited utility as the basis for alloy selection. The heat resistant alloy X (UNS N06002) has outperformed alloy 600 in oxidizing gases containing HCl. Alloy 59 (N06059) has performed satisfactorily in an oxidizing atmosphere with HF, where alloy 617 weld filler was inferior to alloy 600. Examination of the 600 alloy part, removed from service after many years life, showed some corrosion from sulphur and phosphorous as well. At this writing, August 2002, it is not clear to us whether it is the better oxidation resistance of the higher chromium alloys, or some molybdenum effect, that is responsible. If the customer intends to perform tests in his environment, we would suggest including a heat resistant alloy such as the 3%Mo alloy RA333® (N06333), or possibly RA 602 CA®. RA333 has shown good resistance to hot corrosion by the fluoride flux used in aluminum salt bath brazing environments.

3-33

MOLTEN SALT CORROSION When Ni-20Cr alloys and stainless steels are oxidized while submerged in molten salt (NaCl or NaCl-KCl), readily oxidizable alloy components, such as chromium, and in some cases iron, migrate to the surface to form non-adherent, granular and thus nonprotective oxides. This loss of alloy constituents causes a counter current flow of vacancies which condense into an interconnecting pore network filled with salt. Since the salt penetrates into the structure, the loss of alloying metals does not require intermetallic diffusion over long distances, but instead corrosion products are largely removed by solution of alloying metals as ions in the pre-salt network . . .chromium diffuses down grain boundaries of the grain network to be deposited at grain boundarypore intersections as chromium ions. The concentration gradient of chromium ions in the salt phase forces chromium diffusion out to the bulk salt where the higher effective oxygen pressure forms Cr2O3 and some chromate ion. A. U. Seybolt, Oxidation of Ni-20Cr Alloy and Stainless Steels in the Presence of Chlorides, Oxidation of Metals, Vol. 2, No. 2, 1970

Hot chloride salts, and particularly salt fumes mixed with air, are very corrosive to heat resistant alloys. In general the higher nickel alloys, such as 600, are preferred, although we have seen tolerable results from the 1.7% silicon grade, RA 253 MA. Corrosion in Molten Chloride Heat Treat Salts, 1100-2200°F (600-1200°C) Depth of Intergranular Attack Grade

Nickel, weight % RA85H 15 RA 253 MA 11 RA600 76 RA309 13 RA330 35

Silicon, weight % 3.5 1.7 0.2 0.8 1.2

mm 0.11 0.18 0.19 0.32 0.35

inch 0.0044 0.0069 0.0075 0.0125 0.0138

Plate samples were exposed in a commercial heat treat salt line. They saw 210 to 252 cycles in preheat salts 700°C (1290°F) and 815°C (1500°F), high heat salt 1200°C (2200°F), quench in 600°C (1100°F) nitrate/nitrite salt, air cool. Preheat and high heat salts were mixtures of potassium, sodium and barium chlorides. The alkali metals in the salt turn the protective chromium oxide scale into an alkali chromate, which is non-protective and water soluble. As fast as the scale is removed, more chromium diffusing to the surface reforms the scale. Eventually most of the chromium may be removed from the alloy, leaving primarily iron and nickel. A more detailed account of hot salt corrosion mechanisms is given under Neutral Salt Pots, 13-16.

3-34

Molten Salt Corrosion, continued Fluoride salts are more aggressive than are chloride salts. Molten fluorides are used to flux metals and alloys for brazing operations. Along with fluxing the oxide film on the workpiece, fluorides also attack the chromium oxide film on heat resistant alloy fixturing. A service trial of various alloy fixtures used in aluminum salt bath brazing at 1125°F (607°C) gave the following results: Alloy RA333 600 Nickel 200 C-276 601

Total Life, days 197 (end of test—no failure) 112 51 40 14

Other work has shown the 25% chromium ferritic grade, RA446, to be unsuitable for aluminum salt bath brazing operations. Vanadium Pentoxide Equipment fired with residual fuel oil suffers corrosion wherever the fuel ash deposits on hot metal. Heavy oils such as No. 6 or “Bunker C” may contain both sulphur and vanadium. When this oil is burned, the vanadium forms vanadium pentoxide, V2O5. This vanadium pentoxide, along with sodium sulfate, makes a molten compound which is aggressively corrosive. It will eat away most heat resistant alloys in less than a year. A high level of sulphur in the oil might be 2 or 3%, while only 0.05% (or, 500 parts per million) of vanadium is “high” enough to be destructive. Venezuelan oil is particularly high in vanadium and is often used in the Northeastern U.S.A. The alloys with good resistance to fuel ash corrosion are usually cast compositions that are both weak and brittle. 50Cr-50Ni cast IN-657 (UNS R20501) is the best, while HE (28Cr 9.5Ni) is said to be reasonable. IN-657 is expensive and readily embrittled, and HE is particularly weak and brittle. Available wrought alloys are not at all as resistant to fuel ash corrosion but are used for their much better ductility. RA333, RA625, RA330, RA 253 MA and RA310 have all been used or are on trial. Frankly, we have no good comparative field data for these wrought alloys. Mostly based on rumor, we might suggest RA333 or RA 253 MA as worth trying, but they definitely will not be as good as 50%Cr-50%Ni cast.

3-35

MOLTEN METALS From time to time one or another heat resistant alloy is used in contact with a low melting point metal in its molten state. Depending upon which metals are involved, the temperature and the state of stress, that molten metal may dissolve, or may crack, the heat resistant alloy. General Nickel—with respect to nickel-chromium-iron or nickel-chromium alloys, the higher the nickel content, the more rapidly the solid metal dissolves in the molten. As a ROUGH rule of thumb, where contact with low melting metals is concerned the lower nickel alloys, or even the ferritic stainlesses, are preferred. High nickel alloys, such as RA600 (76%Ni) tend to be attacked more severely. Molybdenum—in resisting corrosion by molten zinc alloys, a molybdenum addition appears to benefit austenitic stainless or nickel alloys. One example is 316L, which at 2% Mo seems to work better than does 304, in contact with molten zinc for galvanizing or die casting operations. AL-6XN® alloy, 6.3% Mo, has performed better than 316L at 1000°F (538°C) in 26% aluminum, 7% lead, 67% zinc. Alloy C-276, 15% Mo, has been used in continuous zinc galvanizing at about 850°F (454°C). Dissimilar Metals—must not be used in contact with molten metals. A phenomenum known as mass transfer1 may dissolve the higher nickel alloy preferentially. One example known to us was a lead pot fabricated of heavy RA330 alloy plate. It had been welded with the 72% nickel 19% chromium 2.7% columbium (niobium) weld filler 82, ERNiCr-3. It failed when the weld bead separated from the base metal. Analysis of the weld bead showed that it was now a lead alloy, with about 5% columbium (niobium) and traces of chromium and nickel. Embrittlement—liquid metal embrittlement may occur just below the melting point of the low melting metal. The same molten metal may either dissolve or crack2 the heat resisting alloy, depending upon the stress level, and how much molten metal is present. Aluminum—molten aluminum dissolves any Fe, Fe-Cr, Ni-Cr-Fe or Ni-Cr alloy, likewise for the cobalt alloys. Nevertheless, bar of alloys such as RA446 (25% Cr, balance Fe) has been used for stoppers in bottom pour aluminum ladles. Life is erratic, depending upon how long it takes the aluminum to reduce and/or wash away the hot rolling scale from the bar. RA330 (35%Ni 19%Cr 1.2%Si balance Fe) 11 gage/3mm wall cooling tubes have been used in an aluminum melting furnace, right above the metal. Wherever molten aluminum splashes on the RA330 it goes right through it like hot water through snow. Titanium tubing has been used to siphon molten aluminum. That it has been successful at all is due entirely to the tenacious oxide film on the titanium.

3-36

Molten Metals, continued Antimony—we have no definite experience. There are indications that lead baths which have been contaminated by antimony, from using scrap lead, are corrosive to Ni-Cr-Fe alloys. Bismuth—To satisfy OSHA, one American file manufacturer switched from molten lead to bismuth in its 1450°F (788°C) austenitizing baths. The lead, now bismuth, pots are fabricated of RA330 plate welded with RA330-04 (35%Ni 19%Cr %Si 5%Mn 0.25%C) weld filler. These pots have two loops of 2” Sch 40 RA330 pipe welded to the bottom. An induction coil fits through the loops and heats the bismuth. When too much heat is applied, the loops are attacked. When maintained at 1450°F (788°C) no problems have been reported to us. Cadmium—We have no experience with the effects of Cd on austenitic alloys. However cadmium is said to embrittle steel at temperatures as low as 450°F (232°C), which is about 160°F (90°C) below its melting point3. Calcium—molten calcium can crack RA330, and presumably higher nickel alloys as well. The example here is from an RA330 retorts used to process ferrites, for the electronics industry. This is done at high temperature under a hydrogen atmosphere. Calcium carbonate has been used as part of the mix. The hydrogen reduces it to calcium metal. The metallic calcium vapors haven’t been a problem. Down toward the retort base it is cooler, and molten calcium condenses on the retort wall. The retort cracks at this location. RA330 fracture surface, calcium LME. About 6X

This fracture surface is very similar in appearance to Figure 12, page 60, Volume 10, 8th Edition, Metals Handbook (ASM). That figure illustrates 2024-T4 aluminum cracked by mercury. Copper—molten copper and copper base alloys penetrate the grain boundaries of any austenitic iron, nickel-chromium-iron or nickel-chromium alloy. Even carbon steel, austenitized by immersion in molten copper, can have the austenite grain boundaries neatly outlined by copper metal. Launders for handling molten copper are successfully made of the high chromium ferritic alloy RA446 (25%Cr, balance Fe). Siphons for handling molten copper have been 446 seamless tubing. Skimmers for removing slag from ladles of molten brass or copper are mild steel, 430 stainless (16.5%Cr, balance Fe) and, more likely, RA446. All of the austenitic alloys will fail rapidly in contact with molten copper or copper alloys. 3-37

Molten Metals, Copper, continued The old Belgian alloy UMCo-50, 50%Co 28%Cr 22%Fe, was said to function well in contact with molten copper. We have no experience to confirm this. Haynes® International have made this alloy under their own trade name HS 150. Molten copper attack is a problem in muffles used for copper brazing steel. Eventually some copper braze spills onto the bottom of the muffle. With an exothermic brazing atmosphere the Ni-Cr-Fe alloy (usually RA330) muffle develops a scale which may be protective enough to prevent small amounts of copper from actually wetting the muffle floor. With enough copper the scale may be penetrated and the muffle attacked. We have seen some 15 pounds of copper, with the appearance of cast bars, removed from the corrugated bottom of an 11 gage/3mm wall RA333 muffle. One manufacturer reported longer life when RA85H was used for muffle bottoms, rather than RA330 or RA601. This 15% Ni 3.5% Si grade is no longer available. One might consider the 11% nickel—1.7% silicon alloy RA 253 MA as a replacement. However, bear in mind that all austenitic alloys will eventually fail from molten copper attack. A dry hydrogen or hydrogen-nitrogen brazing atmosphere does not permit the muffle to develop any protective oxide film. Even small amounts of spilled copper will completely penetrate the nickel alloy floor along the grain boundaries. Hydrogen then escapes through the hole and burns like a torch, locally overheating, and sometimes melting, the surrounding area. One practical solution is a sheet of ferritic stainless such as 409 or 430 on the muffle floor, to keep molten copper from contacting the austenitic alloy muffle. This RA 253 MA® tray, with RA330® expanded metal, was used to process mining tool bits at 2000°F in air. Excess brass binder melted out and cracked the tray. Analysis of the yellow metal ranged from 20 to 40% Zn, balance copper. One would expect some amount of zinc to be removed by oxidation in air at this temperature. Life is measured in days. There is no austenitic alloy that will withstand this service. An applied oxide coating, or lining the tray with ferritic stainless are suggested approaches.

3-38 Molten Metals, Copper, continued

Microstructure of cracked area above. Note copper alloy penetration into the RA 253 MA base metal. Grey phase at left is oxide.

RA330 sintering muffle cracked by molten bronze

After five RA330 muffles used for sintering powdered iron had failed, we were asked to look at one of them. Cracks as much as a foot long had developed in the sides. This muffle was intended only for powdered iron sintering. The presence of about an ounce of bronze coming out of this crack indicated that some bronze bearings had inadvertently been sintered in the same muffle. When a few bearings or bearing powder accumulated in the muffle, it melted when it was used for iron parts. Iron is sintered above 2000°F (1150°C), bearing bronze melts perhaps 1850°F (1010°C). Lead—molten lead heat treating baths, or lead pans, are fabricated of mild steel, RA309, RA310, RA 253 MA and sometimes RA330. The lead itself isn’t terribly corrosive to these alloys, although the lower nickel grades may be preferable. Alloy 600 is another matter-this high nickel alloy is dissolved by molten lead.

3-39 Molten Metals, Lead, continued With other alloys it is the lead oxide on the surface that attacks the metal sides severely at the lead-air interface. The molten lead is usually covered with so-called charcoal, more likely sulfur bearing coke of some sort, to reduce lead fumes and oxidation. The lead still oxidizes. Sulfidation and carburization also occur at the lead-air interface, caused by this protective covering. The most direct approach to this local corrosion is to make the metal wall twice as thick at the lead-air interface. Pure lead should be used. Antimony, brought in when scrap lead is used, increases attack from the molten metal itself. Lithium—a vessel fabricated in the 1970’s of RA333 for the US Navy liquid metal embrittled & cracked from residual stress in the formed head, when operated 1650°F (900°C) with molten lithium. Had we been asked, we would have suggested first annealing the head to remove forming stresses. Corrosion of RA333 occurs primarily by selective leaching of the nickel. Alloy X behaves in a similar manner. Based on laboratory tests, TZM molybdenum and pure iron (to 1000°C) are said to have good resistance to molten lithium corrosion. Ferritic stainlesses are said to be subject to chromium leaching. However E-Brite was found more resistant to molten lithium than either nickel or cobalt base alloys. These are laboratory test results, not necessarily confirmed in service. Magnesium—used in reduction of TiCl4 is normally contained in mild steel pots, or steel pots lined with 430 stainless. Melting at 1202°F (650°C), magnesium tends to leach the nickel out of Ni-Cr-Fe alloys. Because carbon steel scales on the outside (fireside) of the melting pot, a few experimental clad pots have been tried. These have been either RA 253 MA or RA330 explosively clad to mild steel. The nickel-chromium-iron alloy outside provides high temperature strength and oxidation resistance while the carbon steel inside is more compatible with the molten magnesium. Rare Earths—the same manufacturer of ferrites who had problems with molten calcium cracking RA330 has also had both cast and fabricated Ni-Cr-Fe alloy grids crack. Deposits on the cast grid analyzed 34% samarium, 10% praseodymium and 1% neodymium. Apparently rare earth compounds used in the manufacture of ferrites were reduced to metallic form by the hydrogen atmosphere. They dripped on the cooler grid at the bottom end of the retort. A lower nickel alloy would probably have been more satisfactory for these grids. Selenium—In the 1970’s, RA330 was used as 1” (25mm) diameter fabricated tubular heating elements in five 9’s purity selenium and arsenic selenide at 500 and 600°F (260 and 316°C), respectively. No degradation of product purity was reported, nevertheless we urge anyone planning to use RA330 for such an application to run their own test program.

3-40 Silver—silver braze alloys have long been known to crack or dissolve austenitic alloys. Cold worked 300 series stainless steels can not be silver brazed without danger of cracking. One reason is that silver braze, in contrast to copper braze metal, melts well below the annealing or even stress relieving temperature of the austenitic alloy to be brazed. In hydrogen atmosphere braze retorts, molten silver braze alloy dripping on the bottom of an RA330 retort will penetrate this austenitic alloy at the grain boundaries and cause hydrogen leaks. Solder (lead-tin)—no molten metal attack reported. Temperatures are low, and the chloride fluxes used are more of a corrosive problem than is the solder itself. RA333 alloy has been used in tin can soldering applications, again more to withstand the ammonium chloride flux than the Pb-Sn alloy. Tin—both RA446 3/16” (4.8mm) plate and RA 253 MA sheet have been used for side shields in the tin float process of plate glass manufacture. Tin at 600°C (1112°F) under hydrogen atmosphere is reported to have dissolved, then re-deposited, 304 stainless steel, and to have pock-marked carbon steel in the same bath, in the process of decontaminating soil. Zinc—Zinc and zinc alloys are used for both electroplating and hot-dip galvanizing of steel. Zinc, which melts at 787°F (419C°), may liquid metal embrittle steel at temperatures as low as about 750°F (400°C)3. This may occur with both Zn plated bolts and with galvanized structural steel. Molten zinc can either dissolve or liquid metal embrittle austenitic alloys, depending upon specific conditions. Zinc is the most commonly used low melting metal which may affect steel or nickel alloys. For this reason, data and applications experience are more broadly available for Zn than for other low melting metals. Molten zinc and zinc-aluminum alloys are used for galvanizing and die casting. Our observations have been that commercially pure iron, 316L stainless, RA85H, 309, AL-6XN and alloy C-276 have all been used in molten zinc/zinc alloy with some degree of success. RA330 is no good at all in molten zinc, and it seems reasonable to assume that the other nickel-chromium-iron alloys such as 800H or 600 are as bad or worse. Zinc die casting pots have been heated by gas fired immersion tubes fabricated of RA309. The tubes are usually plasma sprayed with zirconia to enhance life, but this coating is subject to damage by mechanical abuse. The 309 weld bead is attacked to a greater degree than the base metal. One failed 309 tube, which had leaked full of zinc die casting alloy, was heated rapidly with an oxy-acetylene torch to melt out the zinc. The high thermal stress coupled with zinc wetting the 309 metal inside cracked the tube. The fracture surface was typical of liquid metal embrittlement, i.e., it looked like RA330 cracked by molten calcium or 2024-T4 aluminum cracked by mercury.

3-41 Molten Metals, zinc, continued When 1” (25mm) round bars of both RA330 and 316 stainless were both used in the same zinc die cast alloy scrap recovery project the 35% nickel alloy was severely eaten away and chromium was selectively leached out. The 316 bars merely developed a galvanized coating with no appreciable metal loss. Temperature was about 1000°F (540°C) We observed that one steel company involved in continuous hot-dip galvanizing of sheet made the 850°F (454°C) zinc pot and sink arms of low carbon, low manganese, low silicon nearly pure iron. At the zinc-atmosphere interface the pot was sheathed with 316 stainless steel. The iron sink roll was weld overlaid with 316 stainless, as were the journals. Sleeve bearings, to ride on these 316 overlaid journals, were fabricated of C-276 (UNS No. N10276) sheet. The chute through which the steel sheet passes into the zinc had a tip of C-276 where it entered the molten zinc bath. We have found definite success with AL-6XN alloy for small sink rolls and bearings for galvanizing wire. Initially the company used 316, then Rolled Alloys convinced them to try RA85H, which was an improvement. On test, AL-6XN looked even better. This was confirmed in service, and for the past 3 or 4 years they have been using AL-6XN. They tried 316 for the trunnion sleeve, over AL-6XN trunnions, and the 316 did not last long. Now AL-6XN is used for both the trunnion and the sleeve bearing, as well as for the sink roll itself. Rolled Alloys laboratory immersion testing in molten zinc ranked these alloys similar to how they behaved in service: 250 hour test in molten zinc, 850°F (454°C) alloy AL-6XN 556TM 1008 RA309 RA85H® RA446 316

original thickness average metal loss inch (mm) inch (mm) 0.120 (3.05) 0.0056 (0.142) 0.110 (2.79) 0.0034 (0.086) 0.1328 (3.37) 0.0104 (0.264) 0.118 (3.0) 0.017 (0.432) 0.1164 (2.96) 0.0226 (0.574) 0.2008 (5.10) 0.0234 (0.594) 0.1188 (3.02) 0.044 (1.12)

metal loss, ratio to AL-6XN® 1.00 0.6 1.9 3.0 4.0 4.2 7.9

References 1. David H. Gurinsky, The Behavior of Materials in Aggressive Liquid Metals, pages 5-20, Nuclear Metallurgy, A Symposium on Behavior of Materials in Reactor Environment, February 20, 1956, Institute of Metals Division, American Institute of Mining and Metallurgical Engineers, New York, New York, U.S.A. 2. J.E. Cantwell and R.E. Bryant, How to Avoid Alloy Failures in: 1. Piping by liquid metal attack, 2. Flare tips by severe cracking, pages 114-117, Hydrocarbon Processing, May, 1973 3. Semih Genculu, Liquid metal embrttlement—Part I, Materials Performance p65 January 1993, National Association of Corrosion Engineers, Houston, Texas

3-42 MAGNETISM Austenitic heat resistant alloys are non-magnetic as produced. After high temperature service they sometimes become rather strongly magnetic. Usually this indicates that for one reason or other the metal is no longer fit for service, nor is it capable of being weld repaired. Service conditions that cause this magnetism are often carburization, internal attack by molten salts or selective attack by some molten metal. There are three metallic elements which, in their pure state, are magnetic. Iron, of course, is magnetic and has a ferritic (body centered cubic) structure. Pure nickel is also magnetic, even though it has an austenitic (face centered cubic) structure. Likewise ironnickel alloys, in all combinations, are magnetic, even though they may be austenitic. Cobalt is the third magnetic element, with a hexagonal close packed structure at temperatures below 783°F (417°C), and a face centered cubic structure at higher temperatures. It is easy to confuse ferritic and austenitic with magnetic and non-magnetic. Ferritic stainlesses are magnetic, the small amount of ferrite in an austenitic stainless weld bead (E308, E309, etc.) is also magnetic, and austenitic stainless and nickel alloys are usually non-magnetic. But, some austenitic alloys can also be magnetic. The 66% nickel 31% copper alloy 400 is usually non-magnetic, but depending upon the exact chemistry, a particular heat may be magnetic, at least on a cold day. And, of course, commercially pure nickel is an austenitic metal, and it is also a magnetic metal. Older Canadian coins are a high nickel-copper alloy, and are magnetic (the newer are not, to save on nickel), and many European coins are magnetic. Alloys of iron and nickel are magnetic, the magnetic properties sharply increasing at about 30% nickel and being highest in the range 50 to 80% nickel1. It is the addition of chromium that makes alloys based on iron and nickel (or cobalt) become non-magnetic. Even RA600, with 76% nickel and 8% iron, is non-magnetic, because of its 15.5% chromium. Consider going the other way, and removing chromium. If 10.5% of that chromium were removed and replaced by nickel (and the %ages recalculated), the new 85.3%Ni 8%Fe 5%Cr alloy would become a magnetic, austenitic alloy. Carburization makes nickel heat resisting alloys become magnetic because the chromium reacts chemically with carbon to form chromium carbides. Although the chromium is still present in the alloy, it is effectively removed from the solid solution matrix of nickel, iron and chromium. Lower nickel grades such as RA310 or RA 253 MA do not so readily become magnetic when carburized. This is approximately illustrated by the following Fe-Ni-Cr ternary diagram of magnetism vs alloy content2. Chromium is physically removed from the alloy by the normal corrosion mode in neutral salt pots. RA330 depleted to 12-15% Cr is common, and we have observed metal which used to be RA330 but which had become a 1%Cr-Fe-Ni alloy.

3-43 Magnetism, continued The iron oxide component of scale is also magnetic, so it is possible that a slight degree of magnetism felt on a used fixture is simply the scale, rather than carburization. Their will also be a slight chromium-depleted zone underneath the, largely chromium oxide, scale. There are a couple other times when austenitic stainless steels may be magnetic. The specified chemistry range of 309S stainless (UNS S30908) is broad enough that a small amount of ferrite may be present in the hot rolled annealed metal, just enough to feel with a magnet. This can be very upsetting to customers who think they have the wrong material, because austenitic heat resistant alloys are supposed to be non-magnetic. Also, when a leaner stainless such as 304 is cold worked it becomes magnetic because a small amount of the austenite actually transforms to martensite (a hard, magnetic phase). This is evident on sheared edges, and especially so in deep drawn sheet components. Cold working has little or no effect on the magnetism of higher alloys such as RA330 or RA333. RA310 is normally non-magnetic, right down to liquid nitrogen temperatures, and has been used for structural elements around the superconducting magnets in MRI equipment for hospitals.

References 1. R. H. Krikke, J. Hoving and K. Smit, Monitoring the Carburization of Furnace Tubes in Ethylene Plants, Paper No. 10, Corrosion 76, National Association of Corrosion Engineers, Houston, Texas 1976 2. W.P. Rees, B.D. Burns, and A.J. Cook, Constitution of Iron-Nickel-Chromium Alloys at 650° to 800°C, July 1949 JISI 3. Semih Genculu, Liquid metal embrttlement—Part I, Materials Performance p65 January 1993, National Association of Corrosion Engineers, Houston, Texas

3-44

3-45

STRENGTH AT TEMPERATURE The strength of a metal at high temperature is measured differently than at room temperature. For room temperature applications—steel guitar strings, automobile frames, claw hammers, etc.—the designer needs to know the tensile strength, yield strength or hardness. At cherry red heat, though, the only important mechanical property is creep or rupture strength. Above about 1000-1200°F (540-650°C), tensile or yield strength can NOT be used as a basis for design. This is important. Tensile Strength Tensile strength, or ultimate strength, is the stress required to pull a specimen until it breaks apart in two pieces. It is calculated by dividing the breaking load, in pounds (Newtons), by the specimen cross sectional area, in square inches (mm2), to give pounds/inch2 (Newtons/millimeter2) . The strength of wire, both steel and nickel alloy, is usually reported only by its tensile strength in psi (N/mm2, or MPa). The tensile test is carried out by mounting a specimen in a machine which pulls on it with a slowly increasing load until it breaks. The load in pounds (Newtons) is measured and recorded throughout the test. Elastic Modulus In the early stages of the tensile test the specimen is stretching elastically, like a rubber band. Were the test to be stopped, when the load was removed the specimen would go back to its original length. This is the “elastic” portion of the tensile test, where a plot of stress versus strain would be a straight line. The slope of that line, stress divided by strain, is the Elastic Modulus, also called Young’s Modulus, with the symbol E. This is the measure of the stiffness of the metal, or how “springy” it is. At ordinary temperatures, for example, the modulus of steel is about 30,000,000 psi (207 GPa), while that of 6061-T6 aluminum is only about 10,000,000 psi (69 GPa). One may say that steel is three times stiffer than aluminum. This means that, in the elastic range (room temperature, stress less than the yield strength) for a given stress aluminum will stretch or bend three times as much as will steel. At room temperature the modulus of RA333 is 29,200,000 psi (201 GPa). The modulus decreases at higher temperatures. By about 1000°F (538°C) the material is no longer elastic.

4-1

Yield Strength At some point during the tensile test, usually well before the specimen breaks, it takes a permanent stretch. This is called the “Yield Strength” (or Proof Strength). For austenitic alloys it is usually recorded on the mill test report as either the 0.1% Offset Yield Strength, or, more commonly in the U.S., the 0.2% Offset Yield Strength.

Ductility Before the specimen breaks it has stretched out a great deal, and has necked down in the area where it breaks. The amount it had stretched when it broke is the “% Elongation”, and the amount it necked down is the “% Reduction of Area”. Both are measures of ductility. For example, at room temperature an RA333 tensile specimen might have 48% Elongation and 62% Reduction of Area. The Tensile Strength could be 107,000 psi (738 MPa, or N/mm2) and the 0.2% Offset Yield Strength 47,000 psi (324 MPa) When designing a machine part, obviously the design stress has to be below the tensile strength of the metal, or the thing would break in two. But the machine would also be useless if its parts bent, or yielded, so the designer must keep the stress somewhere below the yield strength of the metal. For heat resistant alloys, yield and tensile strength may be used for design up to about 1000°F (5380°C). Above this temperature, the life of the part will be limited by the metal’s creep-rupture properties, and not by its tensile properties.

4-2

Creep-Rupture Why creep and rupture strength? Metals behave much differently at high temperatures than they do near room temperature. If a metal bar is loaded to just below its yield strength at room temperature, that load can be left there practically forever. Nothing will happen, unless it corrodes away or stress-corrosion cracks. Now let us say that this metal bar is loaded, again keeping the stress below the yield strength—while it is glowing cherry red, 1500°F (816°C). A very small amount of deformation will occur at first (first stage creep). Then that metal bar will begin to stretch, but very, very slowly. It will keep on stretching for hours, weeks, maybe years, until it finally breaks in two. All this, when it wasn’t even loaded up to the yield strength (as measured by a short-time tensile test). Creep The rate, or speed, at which the metal is stretching, in % per hour, is called its “creep rate”. Creep rate is expressed as per cent deformation per hour. For some period of time the creep rate is more or less constant. This is the “minimum creep rate”, or “secondary creep rate”. The minimum creep rate (mcr) is used as one basis for design at high temperature. That is, at high temperature one must assume that the metal is going to creep, or deform, to some degree. This is true even for light loads. Theoretically, the designer might settle on an acceptable amount of creep deformation over the projected life of the equipment. He would then pick his design strength based on the speed of deformation, that is, creep rate, acceptable in his application. In practice, in the furnace industry one design criterion is the stress required for a minimum creep rate of 1% in 10,000 hours, or 0.0001% per hour. Design stress may be set at some fraction of this number. The ASME uses for one of its criteria 100% of the extrapolated stress for 1% in 100,000 hour mcr, or 0.00001%/hr. The other measure of creep, and the one used in Europe, is “total creep”. That is, the stress required for the specimen to actually stretch a total of, say, 1%. Minimum creep rate data and total creep rate data are not interchangeable. People tend to have rather strong feelings about one or the other creep measurement, so whenever possible we provide both minimum creep and total creep data. Rupture “Rupture Stress”, or “Creep-Rupture Strength”, is reported as both a stress, and a number of hours. It is the stress required to completely break a specimen within a given amount of time. In the furnace industry another common criterion for setting design stresses is to use some fraction of the stress that would result in rupture at 10,000 hours. ASME uses whichever is lower, 67% of the extrapolated 100,000 hour rupture stress, or 100% of the extrapolated 1% in 100,000 hour minimum creep rate.

4-3

The picture below shows a broken creep-rupture specimen of RA330, tested at 2000°F (1093°C).

About full scale Creep strength is more important than rupture strength. For example, at 1800°F (982°C) the alloys RA330, RA309 and RA310 all have comparable 10,000 hour rupture strength, about 560--660 psi (3.9--4.6 N/mm2). However in service an RA330 muffle or retort can retain its shape for years, whereas one of RA309 or RA310 would collapse. The reason is, these stainless heat resisting grades have only 40—55% of the creep strength of RA330 at 1800°F (982°C). Normally we expect the strongest alloy to do the best job. This does depend on how that strength is achieved. For example, RA333, RA85H, RA 253 MA and RA 353 MA are strengthened by various alloy additions, with a medium-fine grain size. As a result, they all have good to excellent thermal fatigue resistance in quench applications. The least expensive way to obtain high creep-rupture strength is by giving the alloy a high temperature solution anneal. An aim of ASTM 5 or coarser grain size gives much better creep and rupture strength than does a finer grain size. However, coarse grained materials lose thermal fatigue resistance as they gain creep strength. In our experience, material with grain size coarser than ASTM 4 will be unsatisfactory in liquid quench applications. A quenching fixture, for example, made of 800H would resist creep deformation but quickly break up in pieces from thermal fatigue. The supposedly “weaker” RA330, with its finer grain, can give very good life in quenching service.

4-4

Creep-Rupture Testing Above 1800°F (982°C) oxidation affects the results of a creep-rupture test. As the creep voids oxidize the material undergoes an apparent strengthening. This can be seen by comparing the 2000°F (1093°C) results obtained using 0.252” (6.4mm) diameter test specimens with those from 0.505” (12.83mm) diameter specimens. For an alloy such as RA330 the results from the thinner specimen are so influenced by oxidation as to be unrealistically high. RA333 is considerably less affected at this temperature. By 2200°F (1204°C) even the largest available (0.505”/12.83mm dia.) test specimens in RA333 are probably affected. This makes it difficult to compare very high temperature creep rupture data from different sources, as the test specimen diameters are rarely recorded. For both RA330 and RA333 all currently published creep-rupture data was obtained using the larger diameter specimens, at Joliet Metallurgical Laboratories, Joliet, Illinois, U.S.A..

4-5

Cantilever Beam Creep Test

RA85H RA601

RA330

RA310 RA309 For design purposes, creep and rupture data are usually plotted on log-log charts. A visual illustration of relative creep strengths is obtained by simply clamping alloy strips at one end and measuring how much they sag or droop from their own weight. These five alloys were held at 1600°F (871°C) for 500 hours. The maximum stress in each beam, caused by its own weight, was calculated to be 1890 psi (13 N/mm2). RA309 sagged 6 inches (152 mm) in the first six hours, and continued to bend in the opposite direction once the free end touched the furnace floor. RA310 sagged 6 inches (152mm) in about 48 hours. RA85H and RA601 sagged very little in 500 hours, with RA330 showing slightly more deformation.

4-6

Average 10,000 Hour Rupture Strength, psi TEMPERATURE °F ALLOY

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

2100

2200

22,000

12,500

--

2,000 +

--

--

--

--

--

--

--

--

--

--

RA446

--

--

3,500

2,700

--

1,100

--

450

--

230

--

--

--

--

304L

--

25,000

15,600

9,700

6,000

3,700

2,300

1,400

--

--

--

--

--

--

304, 304H

--

36,000

22,200

13,800

8,500

5,300

3,250

--

--

--

--

--

--

--

316L

--

39,000

23,500

14,200

8,500

5,100

3,050

--

--

--

--

--

--

--

321

--

--

23,500

12,900

7,200

4,000

2,280

--

--

--

--

--

--

--

321H

--

--

24,800

15,200

9,200

5,600

3,400

--

--

--

--

--

--

--

347,347H

--

48,000

27,500

15,600

9,000

5,100

2,900

--

--

--

--

--

--

--

RA 253 MA

--

--

22,000

14,000

8,500

5,200

3,750

2,500

1,650

1,150

860

680

--

--

RA309

--

--

--

17,000

8,000

4,800

2,700

1,600

1,000

560

--

--

--

--

RA310

--

--

--

14,400

7,400

4,500

2,800

1,500

940

660

--

--

--

--

RA330

--

29,000

17,000

11,000

7,200

4,300

2,700

1,700

1,050

630

400

(280)

--

--

RA800AT

--

--

--

17,500

11,000

7,300

5,200

3,500

1,900

1,200

--

--

--

--

RA 353 MA

--

--

19,300

12,200

7,800

5.400

3,600

2,600

1,860

1,300

930

680

(450)

(320)

--

--

25,000

16,500

12,000

9,200

5,700

3,100

1,800

1,050

630

360

--

140

RA 602 CA

--

--

--

31,200

--

11,300

--

3,200

2180

1490

990

670

440

--

RA600

--

--

21,500

13,500

9000

6200

3700

2350

1650

1150

--

--

--

--

RA601

--

42,000

30,000

22,000

13,500

7000

3600

1850

1200

820

--

(330)

(200)

--

RA625

--

--

--

42,500

22,500

12,000

--

--

--

--

--

--

--

--

RA718

--

128,000

98,000

70,000

--

--

--

--

--

--

--

--

--

--

®

COR-TEN B

®

®

®

RA333

®

COR-TEN B is a registered trademark of US Steel Corporation RA600 and RA601 data from EN 10095

+ One Heat Tested ( ) Extrapolated data

4-7

Average Stress, psi, for 0.0001% Per Hour Minimum Creep Rate TEMPERATURE °F ALLOY

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

2100

2200

COR-TEN B

20,80 0

11,100

--

1,700 +

--

--

--

--

--

--

--

--

--

--

RA446

16,00 0

6,000

3,000

1,500

680

260

130

--

--

--

--

--

--

--

304L

--

--

7,700

4,950

3,200

2,050

1,300

--

--

--

--

--

--

--

304, 304H

--

25,500

16,500

10,800

7,000

4,600

2,950

--

--

--

--

--

--

--

316L

--

23,500

14,000

8,300

4,900

2,900

1,750

--

--

--

--

--

--

--

321

--

--

20,000

8,800

3,850

1,700

750

--

--

--

--

--

--

--

321H

--

--

20,300

12,000

7,100

4,200

2,500

--

--

--

--

--

--

--

347, 347H

--

53,000

27,500

14,800

7,800

4,100

2,150

--

--

--

--

--

--

--

RA 253 MA

--

--

18,000

11,600

7,700

5,000

3,350

2,300

1,500

890

490

(250)

--

--

RA309

--

--

--

16,000

8,800

3,400

2,400

1,400

600

220

--

--

--

--

RA310

--

--

--

14,900

5,900

3,300

2,100

1,100

570

280

--

--

--

--

RA330

--

21,000

10,500

7,600

5,300

3,600

2,700

2,100

1,000

500

--

--

--

--

RA800AT

--

--

--

17,000

9,100

6,000

--

3,600

1,500

1,050

--

--

--

--

RA333

--

--

22,000

9,800

7,700

6,400

4,200

2,700

1,650

880

--

--

--

--

RA601

--

41,000

27,000

18,000

7,200

4,100

2,700

2,000

--

760

--

430

--

--

RA718

--

--

100,000

74,000

43,000+

--

--

--

--

--

--

--

--

--

®

®

®

®

®

* COR-TEN B A Registered trademark of US Steel Corporation + One Heat Tested ( ) Extrapolated

4-8

THERMAL FATIGUE Metal parts exposed to fluctuating temperatures for long periods eventually deteriorate. Experimental work and theoretical analysis indicate the cause to be plastic flow induced by expansion and contraction during heating and cooling. The effect can be minimized by proper design, selection of alloys that combine high hot strength with low thermal expansion coefficients, and by favorable operating conditions. H. S. Avery, The Mechanism of Thermal Fatigue, Metal Progress August 1959 Thermal fatigue is the cracking which happens after a metal is repeatedly heated and cooled rapidly. Heat resistant alloys all have high coefficients of thermal expansion. Most will expand at a rate of about 2/10 inch per foot (17mm per meter) when heated from room temperature to 1800°F (982°C). Heat resistant alloys also have low thermal conductivity, perhaps one fourth that of carbon steels. Uneven heating and cooling, not only with respect to different parts of the same fixture, but from surface to center of the metal itself, is the rule for heat resistant alloy service. Rapid cooling is usually thought of as oil or water quenching. Even a nitrogen gas quench is effectively a rapid cool if carried out from 2000°F (1100°C). This is common in vacuum heat treating of tool steels and some stainless grades. Individual round bars crack because the surface of the metal heats, or cools, before the center does. Since metal expands when heated, and then contracts the same amount when cooled again, this alternately strains the center and the outside surface. After some number of these strain cycles the metal cracks. In carburizing service, and in salt bath heat treating, cracks start at the surface and grow deeply. In neutral hardening operations the bar may begin to crack internally, and give no external sign that anything is wrong until it suddenly breaks. In fixtures or bar baskets, one area individually quenches faster than another. The bottom members of deep bar frame baskets cool and contract before the middle and top do. In a rigidly welded angle frame design the long bottom side pieces may crack while the shorter ends and the top frame remain sound. The most important items to consider regarding equipment which will be thermally cycled are: 1.) Design—basically flexible or loose. This may include corrugations, serpentine rather than straight flat bars and loose, pinned joints rather than rigidly welded. 2.) Light sections. Thinner metal heats and cools more uniformly than thick. 3.) Grain size & alloy choice. Material for thermal cycling service should have a grain size ASTM 4 or finer, if possible. No alloy will compensate for inadequate design where cracking from thermal cycling is concerned.

5-1

Thermal Fatigue, continued Some alloys are better than others, of course. Both strength and ductility are important. RA333 has been our best alloy in resisting thermal fatigue, because it is both strong and ductile. The strength, in turn, can permit additional life improvement. That is, if the designer makes use of RA333’s strength to use thinner plate and smaller diameter bars. Thinner sections heat and cool more uniformly, so the thermal strains are lower. The use of the lightest possible metal sections cannot be overemphasized. We had one customer who cut his life in half simply by going from 1/2” (12.7mm) dia. RA330 in his bar frame basket, up to 5/8” (15.9mm) dia. bars. He thought he was making the basket stronger, which he was where load carrying ability was concerned. But the thermal strains from quenching the larger bar are significantly greater. It is thermal stresses that cause more distortion and cracking in heat resistant alloy equipment than do the mechanical loads imposed on the part. Ductility alone is not enough. RA600 is ductile, but RA333 survives repeated quenching better because of its strength. And for that matter, the tensile ductility of RA333 at 1600°F (871°C) has been measured at 75% elongation.

5-2

WEAR Wear resistance is often related to hardness at room temperature. But even at room temperature, heat resistant alloys are rarely harder than Rockwell B100 (Brinell 240). These austenitic heat resistant alloys do not possess wear resistance in the conventional sense. There is some limited information available for erosion, and for galling resistance. Erosion Erosion resistance appears somewhat related to oxidation resistance. AvestaPolarit provided the following information for their “MA” grades: Coupons of three different MA grades were exposed in the cyclone of the Nässjö plant in Sweden, 4200 hours fired with wood waste and 1800 hrs with Polish coal. Normal temperatures 1580-1635F (860-890C), with peak bed temperatures of 1920F (1050C) Grade RA 153 MA® RA 253 MA RA 353 MA

Maximum Thickness Reduction inch mm 0.071 0.024 0.008

1.8 0.6 0.2

Until the development of the 25Cr 35Ni grade RA 353 MA, the 21Cr 11Ni alloy RA 253 MA had been considered one of the most erosion resistant materials for fluidized bed cyclone construction. During 1999, tube shields of RA 353 MA were installed in-bed in a number of coal fired fluidized bed boilers, in particularly erosive areas. Final results are still pending. Galling Austenitic stainless and nickel alloys are known to be susceptible to galling at room temperature. The situation does not improve at elevated temperatures. The alloy Nitronic 60® (S21800, nominal 17Cr 8.5Ni 8Mn 4Si .13N) ) is one of the few austenitic alloys that resist galling at room temperature. We are aware of no published data regarding its galling resistance at elevated temperature. However we do know that axles of Nitronic 60 greatly outperform those of RA330 when used with cast heat resistant alloy wheels roughly 1600°F (870°C). Normally the cobalt alloys, e.g., L605, 188, 556, X-40 (25.5Cr 54Co 10.5Ni 7.5W 0.50C), are considered to have the best galling resistance at high temperature. These are relatively soft, solid solution or carbide strengthened grades--NOT the hardfacing Stellite® alloys. The cobalt alloys in question form a relatively soft, lubricious oxide that prevents galling. A combination of a cobalt base against nickel or iron base alloy is a good, practical approach to minimizing galling problems.

6-1

Galling, continued Due to the cost of cobalt alloys, their anti-galling properties are rarely used, or even known, outside of the gas turbine industry. In the 1960’s General Electric’s J79 engine, used to power military aircraft, used cast cobalt alloy X-40 linkage in the afterburner system. The X40 parts were regarded as “self lubricating”. In other industries it may be that a weld overlay of, for example, L605 (Haynes 25) on one of the nickel or stainless parts would function to prevent galling against the other side of the couple. For severe high temperature galling problems, where cost permits, consider weld overlaying one side with a high cobalt alloy such as L-605. The weld wire specification is AMS 5796, covered electrodes covered by AMS 5797. L-605 is also known as Haynes 25, UNS R30605, European specification W.Nr.2.4964. Note that this is not a hard-facing wire, such as Stellite® 6, but rather a material that forms a soft, lubricious oxide at high temperatures. Boron nitride spray is used for high temperature lubrication.

6-2

PHYSICAL METALLURGY An important property of alloys utilized for heat resistant service is the ability of the metal to retain its desirable characteristics throughout the range of probable operating temperatures. Some high alloy ferrous metals are subject to embrittlement at certain elevated temperatures as a result of the formation of a constituent called the “sigma phase.” If appreciable amounts of the extremely hard, brittle sigma phase can be formed, an alloy steel may lose ductility to such an extent that its usefulness may be seriously impaired. Although the existence of this phase has been observed for a number of years, the possibility of the occurrence of phases other than the well-known alpha and gamma may sometimes be overlooked in the consideration of alloys suitable for high temperature applications. Francis B. Foley, The Sigma Phase, Alloy Casting Bulletin Number 5, July 1945, Alloy Casting Institute, New York, New York, U.S.A. Sigma Phase All of our nickel-bearing stainless and nickel base alloys have an austenitic structure, and are ductile and non-magnetic when they are placed in service. Ideally, a heat resistant alloy should retain these qualities throughout its service life. Some materials change after a few hundred or thousand hours in service, and become brittle instead of tough and ductile1. This usually happens with high chromium, low nickel grades such as 309 and 310. The most common problem is that the alloy forms a hard, brittle nonmagnetic phase, called sigma. The overall chemical composition of the alloy remains the same. Sigma forms in the 1100-1600°F (600-870°C) temperature range. It happens more quickly, and embrittles more severely, when the alloy has been cold worked. Sigma may not seriously harm the alloy while it is operating at high temperature. But enough sigma can completely embrittle the alloy when it reaches room temperature. Chromium, silicon, molybdenum, columbium, aluminum and titanium promote sigma. Nickel, carbon and nitrogen retard its formation. The ASTM specifications for 310S (N31008) permit 1.5% silicon maximum, and AMS 5521 1.00% silicon. All RA310 plate, sheet and bar is made to restricted silicon, 0.75% maximum, to reduce sigma in RA310. One example of a failure due to sigma involved a long, heavy wall 310S muffle. It operated about 1200°F (650°C) with a vacuum inside, which tended to collapse it. After a few years the user inserted jacks and tried to jack up the roof which had fallen in. But, instead of straightening, the 310 roof cracked badly. And these cracks grew further when they tried to weld repair them.

7-1

Sigma phase, continued The solution would be not to use 310S or 309S at this low temperature. Indeed, below about 1400°F (760°C) these two grades have limited usefulness. Even 304H, which will form a certain amount of sigma, will not embrittle as badly as 310S. Although RA330 might normally be regarded as overkill for a 1200°F (650°C) application, RA330 does not form sigma or embrittle at any temperature range. Faced with an existing, brittle 310S muffle the only thing to do is to anneal it by heating 1900°F (1038°C) or higher. This will re-dissolve the sigma and restore ductility so that the metal can be straightened and weld repaired. Of course, after it goes back into service, sigma will again begin to form. We mentioned that silicon promotes sigma, but that RA330 does not embrittle from sigma. This is because RA330 has sufficient nickel, along with moderate chromium, that even silicon as high as 2% would be unlikely to result in sigma. There are no recorded instances, either in service or laboratory test, where RA330 has embrittled from sigma. The embrittlement due to sigma varies from alloy to alloy, can take a long time to occur and is less harmful at elevated temperature than at room temperature. The following is taken from work done for the ASME on superheater tube materials2. Charpy V-notch energy, foot-pounds (J) Test Temp

Condition

Alloy 304

321

347

316

310

800

68°F unexposed 100 (136) (20°C) 18 mo 1200°F 100 (136) 36 mo 1200°F 50 (68)

100 (136) 100 (136) 75 (102)

100(136) 50 (68) 35 (47)

-65 (88) 40 (54)

100 (136) 25 (34) 10 (14)

100 (136) 50 (68) 55 (75)

68°F 18 mo 1350°F 85 (115) (20°C) 36 mo 1350°F 75 (102)

100 (136) 95 (129)

90 (122) 45 (61)

70 (95) 30 (41)

10 5

60 30

68°F 4 mo 1500°F (20°C) 6 mo 1500°F 18 mo 1500°F 30 mo 1500°F 34 mo 1500°F 36 mo 1500°F

-100 (136) 70 (95) ----

-100 (136) 100 (136) 100 (136) ---

65 (88) ------

-65 (88) 35 (47) -25 (34) --

20 (27) ------

-100 (136) -----

1200°F unexposed 100 (136) (649°C) 18 mo 1200°F 100 (136) 36 mo 1200°F 100 (136)

100 (136) 100 (136) 100 (136)

100 (136) 85 (115) 85 (115)

100 (136) 100 (136) 100 (136)

100 (136) 85 (115) 60 (81)

100 (136) 75 (102) 80 (108)

1350°F unexposed -(732°C) 18 mo 1350°F 100 (136) 36 mo 1350°F 100 (136)

100 (136) 100 (136) 100 (136)

100 (136) 100 (136) 100 (136)

100 (136) 95 (129) 85 (115)

-35 (47) 40 (54)

-85 (115) 70 (95)

7-2

(14) (7)

(81) (41)

Sigma Phase, continued 1500°F unexposed (816°C) 4 mo 1500°F 6 mo 1500°F 12 mo 1500°F 18 mo 1500°F 30 mo 1500°F 34 mo 1500°F

100 -100 100 100 ---

100 (136) -100 (136) 100 (136) 100 (136) 100 (136) --

100 (136) 100 (136) ------

100 (136) -100 (136) 100 (136) 100 (136) -40 (54)

100 (136) 30 (41) ------

100 (136) -100 (136) -----

Chemical Compositon of Tube Materials Tested Above alloy UNS Cr Ni Mo Cb Ti C Fe A 304 S30400 18.48 10.93 ---0.07 bal 321B S32100 17.79 12.23 --0.45 0.05 bal 347 S34700 17.93 10.90 -0.56 -0.06 bal C 316 S31600 16.77 13.20 1.96 --0.06 bal 310 S31008 24.56 21.42 ---0.07 bal D 800 N08800 20.69 34.66 ---0.05 bal A current production 304 averages 9% nickel B 321 currently melted to typical 9.3% nickel C average nickel content of current production 316L is about 10.2% D titanium and aluminum not reported, specification is 0.15--0.60% each The Metal Properties Council3 performed studies on 310 and other materials after various elevated temperature exposures. The following are some test results: Test Temp ºF 80 80 80 80 1200 1500 1800

Condition AR PE 1200 PE 1500 PE 1800 PE 1200 PE 1500 PE 1800

Ultimate 0.2% Offset Tensile, Yield, psi psi 92,600 38,100 87,200 33,500 86,200 32,600 84,500 27,600 52,100 23,300 26,600 14,400 10,800 7,000 72.1

Elong %

RA %

46.5 44.6 34.9 40.4 39.8 54.4 64.6

63.2 60.9 35.7 47.0 57.8 48.6 --

Charpy energy ft-lb 88.3 76.8 24.7 87.2 ----

Lateral Expansion, inches 0.070 0.065 0.019 0.060 ---

310 Heat No. 24659, 1 inch thick plate, from Jessop Steel Co., Washington, Pennsylvania AR –as received, mill annealed PE – pre-exposed 1000 hours at temperature, ºF All data is average of three tests Some reduction of Charpy V-notch energy is shown after exposure at 1500F. However the exposure time, 1000 hours, was too short for much sigma formation to occur.

7-3

Sigma Phase, continued Both RA 353 MA® and RA 253 MA show a reduction in toughness after intermediate exposure. In this case chromium nitride precipitation is in part responsible4. Exposed 5000 hours at ºF 1292 1472 1652 200 hours at ºF 1742 1832 1922 2012

Charpy V-notch Impact, foot-pounds RA 353 MA RA 253 MA 310S 8.9 7.4 5.9

6.6 3.7 34

4.4 3.7 16

7.4 25 72 204

-----

-----

RA330® shows retains high tensile ductility and Charpy V-notch energy after 1000 hour exposure to 1400ºF5: Test Temp ºF 75 75 1400 1400

Condition

Ultimate Tensile,

0.2% Offset Elong Yield, psi %

RA %

Charpy V-notch impact energy, foot-pounds

AR PE 1400 AR PE 1400

85,000 88,500 35,000 --

34,900 32,600 18,800 --

70 60.5 59 --

240 (test machine limit) 96 167 130

47.5 40.5 65 --

AR=as received, annealed PE = pre-exposed at 1400°F This 1989 Rolled Alloys study, by Gene R. Rundell, indicates that 309 may retain better impact strength than does 310, at least after exposure to 1600°F (871°C). Room Temperature Charpy V-notch testing, three tests per alloy, material exposed 1600F (871°C). Exposure time, hours

RA309 ft-lb J

RA310 ft-lb J

RA 253 MA ft-lb J

500

54

73

8.8

11.9

13.2

17.9

1200

43.7

58.4

8.5

11.4

11.3

15.1

7-4

Grain Growth Most Rolled Alloys heat resisting alloys are produced to a medium-fine grain size, usually somewhere in the range ASTM 3-8 (125-10µm) for the smaller bar, sheet and light plate sizes. 800H/AT is a definite exception, this grade being annealed 2150°F (1177°C) minimum to deliberately coarsen grain size. With the exception of 800H/AT, the grain size of a metal sample can be an aid to estimating what temperature the metal may been subject to in service. Grain size of RA330 versus time and temperature. The following is old data, laboratory annealing of 0.04% carbon alloy, arc furnace melted (no AOD remelt), hot rolled hand mill sheet, box annealed. Response to grain growth may vary from heat to heat, and may be influenced by prior mill processing: Temperature F C 1900 1038 1950 1066 2000 1093 2050 1121 2100 1149 2150 1177 2200 1204

5 5 5 4 4 4 3 3

ASTM Grain Size Number (µm) Time at temperature, minutes 10 15 30 60 120 (63.5) 4 (90) 4 (90) 3 (127) 3 (127) 2 (180) (63.5) 4 (90) 4 (90) 3 (127) 3 (127) 2 (180) (90) 4 (90) 4 (90) 3 (127) 2 (180) 2 (180) (90) 3 (127) 3 (127) 3 (127) 2 (180) 1 (254) (90) 3 (127) 2 (180) 2 (180) 2 (180) 1 (254) (127) 3 (127) 3 (127) 2 (180) 1 (254) 1 (254) (127) 3 (127) 2 (180) 1 (254) 1 (254) 00 (508)

Light plate coupons were exposed6 in a vortex finder at an Eastern U.S.A. chemical company for 1862 hours at 1850°F (1010°C). Because of the long time exposure, as compared with the maximum 2 hours of the above table, RA309, RA310 and RA330 all experienced significant grain growth, while RA 253 MA and RA333 showed no measurable effect. Although the initial grain size was not recorded, these particular materials most likely were produced with ASTM 4-7 (88-31µm) initial grain size. Alloy RA333 RA 253 MA RA330 RA310 RA309

Sample thickness inch mm 0.259 6.58 0.242 6.15 0.243 6.17 0.190 4.83 0.192 4.88

7-5

Final Grain Size ASTM µm 5 62 4 88 00 508 4-00 88-508 00 and 508 and coarser coarser

Grain Growth, continued A test was run to compare the resistance to grain growth of RA 602 CA and several other alloys, including a version of 601 specifically intended to resist grain growth. 1000 hours exposure to 2050°F (1121°C) had no measurable effect on RA 602 CA. Grain Growth Exposure to 2050°F (1121°C), 1000 hours RA 602 CA® 601 GC® RA601 RA333® RA600 RA 353 MA® RA330® 8

7

6

5

4

3

2

1

0

00

References 1. Symposium on the Nature, Occurrence and Effects of Sigma Phase, Special Technical Publication No. 110, ASTM, Philadelphia, Pennsylvania, U.S.A. June, 1950 2. George E. Lien, editior, Behavior of Superheater Alloys in High Temperature, High Pressure Steam, The American Society of Mechanical Engineers, New York, New York, U.S.A. 1968 3. Gene R. Rundell, Rolled Alloys Investigation 27-84, August, 1984 Temperance, Michigan, U.S.A. ®

4. Rolled Alloys Bulletin 1353, RA 353 MA alloy 5. Private communications of January 10 and June 22, 1972, Crucible Inc., Materials Research Center. 6. Gene R. Rundell, Rolled Alloys Investigation 27-84, August, 1984 Temperance, Michigan, U.S.A.

7-6

HEAT RESISTANT ALLOY GRADES Now that we have reviewed the influence of the various alloying elements, and some of the environmental and mechanical requirements to be met in service, it is time to take a look at some of the available alloys on the market today. Iron-Chromium Alloys These range from simple ferritic or martensitic grades such as 409, RA446 and RA410, through enhanced oxidation resistant grades from AK Steel (formerly Armco) & Allegheny Ludlum to advanced oxide dispersion strengthened (ODS) alloys from Kanthal® and Special Metals®. The iron-chromium alloys have low coefficients of thermal expansion, comparable to or slightly lower than that of carbon steel. These alloys have low ductility, and those with higher chromium contents, such as RA446, might even be called brittle. Because of their low strength at temperature (excepting the ODS versions), their use is limited to non-stressed parts. All ferritic or martensitic alloys with 12% or more chromium embrittle very severely when held in the 800-1000F (430-540°C) temperature range. This embrittlement is well known in the petrochemical field. It is called “885°F” (475°C) embrittlement, that being the temperature of most severe embrittlement. The metal can lose ductility to the point that it will crack in several pieces just from clamping it in a vise. 409, formerly called MF-1 by Allegheny Ludlum, is the lowest chromium alloy that qualifies as stainless. It has the advantage of being low enough in Cr to avoid 885°F/475°C embrittlement for some time, although it is reported to embrittle after some 50,000 hours service. 409 is processed in the mill to be a minimum cost grade. Automotive catalytic converter shells are made of 409, as are stainless exhaust systems. Being very low carbon and titanium stabilized, 409 is formable and weldable. A matching composition flux cored wire is available, as is a columbium (niobium) stabilized solid wire. 409 plate is sometimes welded with alloy 82 wire for better weld bead toughness. 409 has usable oxidation resistance up to about 1200°F (650°C). 410 is a martensitic grade, having enough carbon, about 0.14% C, that it can be hardened by heat treatment. It also hardens when welded, and requires both pre-heat and immediate post weld anneal to keep the weldment from cracking. 410S is a lower carbon, more weldable version of 410. 430 is the most broadly available ferritic stainless, used for both corrosion resistance and as a heat resistant grade. Commercial kitchens and bake ovens use quantities of 430. Very cheap, magnetic, “silverware” is 430. 430 sheet has been used to line the bottom half of RA330 brazing muffles, to protect the austenitic alloy muffle bottom from braze attack.

8-1

Heat Resistant Grades, continued 439 is about a percent higher in chromium, and titanium stabilized. It is not broadly available from distributors. AK Steel’s (formerly Armco) 18 SRTM uses both silicon and a critical ratio of titanium to aluminum to achieve oxidation resistance well in excess of what would be expected from its chromium level. Currently, 18SR sheet is available in full coil lots only. Allegheny Ludlum’s ALFA-IVTM uses aluminum and rare earths to achieve extremely good oxidation resistance with 20%Cr. This grade is made only in very light gage strip for automotive catalyst support systems. Oxide dispersion strengthened (ODS) grades achieve extreme temperature oxidation resistance in the same manner as ALFA-IV, that is, by about 5% aluminum with rare earths. These alloys are produced by mechanically incorporating the rare earth oxide, Y2O3, into the Fe-Cr-Al matrix. As a result, the ODS alloys have very high creep rupture strength, quite unlike conventionally produced ferritic grades. The ODS ferritic grades available in the US are Inconel® MA956 and Kanthal® APM. Saqndvik has in recent years begun extruding Kanthal APM into finished radiant tubes for industrial furnace use. At Rolled Alloys we have used Kanthal APM to 2100°F (1150°C) in our oxidation test tray (currently it is RA 602 CA). The disadvantages of the ODS materials at this time include cost in the neighborhood of $50/lb ($110/kg), limited availability and fabrication. Melting from arc welding destroys the oxide dispersion, leaving the weldment with only the (very low) strength of a conventional ferritic stainless. There has been some degree of success with laser seam welding 1/4” (6.35mm) MA956 plate. RA446, at 25% chromium, has the oxidation resistance needed for 2000°F (1100°C) service. This permits it to be used around molten copper or brass. The largest single use may be as electrodes for heating neutral salt baths. This high chromium, along with no nickel at all, gives RA446 the best resistance to sulphidation—usually—of the heat resistant alloys. No longer available in sheet gages (under 3/16 inch/4.8mm), RA446 is very weak at red heat, having at best less than 10% the creep strength of an austenitic nickel alloy. RA446 has a very high ductile-to-brittle impact transition temperature, at least 250°F (120°C). This means that at room temperature RA446 plate may crack when hit in a mechanical press break. In spite of its mechanical properties, RA446 is used for applications where nothing else will handle the corrosive environment.

8-2

Heat Resistant Grades, continued Nominal Chemistry, Ferritic and Martensitic Alloys alloy 409 410 430 439 18 SR ALFA IV Kanthal APM MA956 RA446

UNS S40900 S41000 S43000 S43035 ---S67956 S44600

EN -1.4006 --------

Cr 11 12 16.5 17.2 17.3 20 22 19.4 25

Si 0.4 0.3 0.5 0.5 0.6 0.4 0.3 0.05 0.5

Al ----1.7 5 5.8 4.5 --

Ti 0.4 --0.5 0.25 --0.4 --

C 0.015 0.14 0.08 0.015 0.015 0.02 0.05 0.02 0.05

Other -----0.03 Ce+La rare earths 0.5 Y2O3 --

Iron-Chromium-Nickel Alloys, Nickel 20% and under These range from the high volume 304 and 321 up to a true heat resistant alloy, RA310. All of these grades can embrittle from sigma formation to some degree, only RA85H and 314 (W.Nr. 1.4841) have sufficient carburization resistance for heat treat service. Oxidation resistance and strength include some of the best available (RA 253 MA). This is also the group from which alloys with useful sulphidation resistance are chosen. 304 The basic “18-8” stainless is AISI type 304. Flat rolled products are usually either 304L, dual certified with 304, or 304H, likewise dual certified. Bar may actually be just plain 304, with 304L and 304H bar also available. Although the “L” grade is principally used for appearance or for aqueous corrosion resistance, the “H” version of this steel may be used to about 1500°F (815°C). 304 is limited to this temperature by oxidation resistance. Because 304 is austenitic, it retains strength at temperature. It has a fairly high coefficient of expansion. We would prefer some other material for an item that was to be heated and cooled rapidly. But for constant temperature or slow heating and cooling, at temperatures not above 1500°F/815°C, 304 can be considered and is used quite extensively. 316L Not really a heat resistant alloy but used as such anyway, particularly for fans. With 0.03% carbon maximum the design stresses at 1500°F (816°C) might be about 40% lower than for 304H. Nevertheless the 316L is chosen to better resist aqueous corrosion for fans which must operate part of the time at high temperature, and also near room temperature. 316H, even though it would be stronger at high temperatures, is rarely used because it is not broadly available.

8-3

Fe-Cr-Ni alloys, Nickel 20% and under, continued 321 This is a modification of the basic 18-8 grade with the addition of titanium to stabilize it against carbide precipitation in high temperature service or from the heat of welding. ASTM specifications require 321 to be annealed 1900°F (1038°C) minimum, which is too high for the grade to develop titanium carbides, i.e., be properly stabilized. For maximum resistance to carbide precipitation in service, hence resistance to polythionic acid stress corrosion cracking, it is suggested that welded fabrications of 321 be heat treated for 4 hours at 1600°F (871°C). 321 resists oxidation in high temperature service to about a 100°F (56°C) higher temperature than does 304, and is used up to 1600°F (871°C). RA 253 MA® achieves excellent strength and oxidation resistance through rare earths, a heavy calcium deoxidation, nitrogen and silicon. It was the first commercial NiCrFe alloy to use this technology, previously restricted to electrical resistance alloys (and the cobalt alloy 188). RA 253 MA is strong, with two to three times the creep strength of RA309. RA 253 MA is not particularly carburization resistant (RA309 is slightly better) nor has it performed well in REDUCING sulphidizing conditions (H2S). In oxidizing atmospheres RA253 MA has very good resistance to SO2 (sulphur dioxide), tolerating some 12% SO2 for extended periods at 1800F (982C). The maximum suggested continuous use temperature for RA 253 MA is 2000°F (1100°C). RA309 (really 309S, 0.08% max carbon) is one of the most widely used heat resistant alloys. Low cost, useful cyclic oxidation resistance to around 1850-1900°F (1010-1040°C) and fairly good sulphidation resistance characterize this grade. RA309 tolerates carburization well enough to be the grade of choice in carbon saggers. Fabrication is simple, and 309 weld fillers are often used for dissimilar metal welds. RA310 is one of the three alloys which should be considered where sulphidation is concerned, the other two being RA446 and RA309. The ferritic RA446 and the lower nickel RA309 might be preferred for very strongly reducing environments with sulphur present. In a more complex mix of chemicals RA310 is generally superior to RA309 in hot corrosion and is considered one of the standard materials of construction for coal gasifier and coal fired fluid bed combustor internals. RA310 has very good oxidation resistance, better than RA330 at constant temperature but not so good as RA330 when the temperature cycles. RA310 maintains useable oxidation resistance beyond 2100°F (1150°C). HR3C, also known as 310HCbN, is a nitrogen-columbium strengthened version of 310 with improved hot corrosion resistance up to perhaps 1600°F (870°C). Thousands of feet of steam boiler tubing are on test at TVA. The columbium (niobium) addition helps hot corrosion resistance at moderate temperature but is harmful to oxidation resistance around 1800°F (982°C) and upwards. Intermediate temperature embrittlement can be a problem.

8-4

Fe-Cr-Ni alloys, Nickel 20% and under, continued 314 (W. Nr. 1.4841) is essentially 310 with 2% silicon. Rolled Alloys supplanted 314 with RA330 a generation ago in the U.S.A. 314 is widely used in Europe, and is often casually referred to as “310” there. For the most part, it is the German mills that make this grade. Silicon increases the already good oxidation resistance of 310 and adds both carburization and nitriding resistance.. However, coupled with the high chromium, silicon also increases the rate, and amount, of sigma formation. 314 may have lower creep strength than 310. Nominal Chemistry, Fe-Cr-Ni Alloys, Nickel 20% and under alloy 304 321 RA 253 MA RA309 RA85H RA310 HR3C 314

UNS S30400 S32100 S30815 S30908 S30615 S31008 -S31400

EN 1.4301 1.4541 1.4835 --1.4845 ---

Cr 18.3 17.3 21 23 18.5 25 25 25

Ni 9 9.3 11 13 14.5 20 20 20

Si 0.5 0.7 1.7 0.8 3.5 0.5 0.5 2.0

C 0.05 0.01 0.08 0.05 0.20 0.05 0.06 0.10

N --0.17 ---0.25 --

Other 70Fe 0.2Ti 70Fe 0.04Ce 65Fe 62Fe 1Al 61Fe 52Fe 0.4Cb 52Fe 51Fe

Iron-Nickel-Chromium alloys, Nickel 30-40% This nickel range covers some of the most successful heat resistant grades, such as RA330 and 800H, and one of the more recent, RA 353 MA®. RA800ATTM, a.k.a. 800HT®, N08811, is a very strong alloy broadly used in the petrochemical and refining industries. In these industries 800AT is used as their basic structural material, much as RA330 is the basic heat treat alloy. 800AT gets its strength by a combination of: 1.) High temperature grain coarsening anneal, 2100°F (1149°C) minimum, commonly resulting in grain size ASTM 1-3. 2.) Carbon 0.06-0.10% 3.) Combined aluminum + titanium 0.851.20%. For the money, it is hard to beat the strength of 800AT. The alloy does have some drawbacks. First, the very coarse grains which are necessary for high creep-rupture strength are detrimental to thermal fatigue/ thermal shock resistance. This, coupled with mediocre oxidation resistance, largely keeps 800AT out of heat treat service. The second disadvantage is that the high combined aluminum + titanium content causes 800AT to form a very small amount of the age hardening constituent gamma prime at around 1100°F (600°C) or so. This has been suggested as the cause of cracking problems, and may be why the 800AT chemistry has been less well accepted in Europe. The cracking problem may be avoided by heat treating the welded fabrication 1625°F (885°C) for 1 1/2 hours, for thicknesses up to 1inch (25mm). Add one hour per inch (25mm) of thickness greater than 1” (25mm)1. There are three versions of “800” alloy. The original “Incoloy®”, 0.10% max (no minimum) carbon, was announced in July 1951. The origin of this grade goes back to an economic situation quite unlike today, and to the Korean War nickel shortage. 8-5

Iron-Nickel-Chromium alloys, Nickel 30-40%, continued Prior to this time the majority of Calrod® units for electric ranges or other electrical heaters were made using alloy 600, “Inconel®” tubing for the sheath. Inco were said to apply a commodity price for this application which made it cost just a little more than type 304 stainless. Due to the nickel shortage, the War Production Board issued a decree prohibiting the use of an alloy containing more than about 55% nickel plus chromium for most heat resisting applications. Inco faced loss of all of the Calrod business unless they could offer some metal containing less than 75% nickel 15% chromium. Incoloy was born for the Calrod industry at that time. Since then a lower nickel, lower cost, version, Incoloy 840 (W.Nr. 1.4847) 20%Cr 20%Ni, has been developed for this application. Calrod producers list the limiting temperature for Incoloy in this application as 1600°F (670°C), 1200°F (650C) for 304/316/321 stainless. By the time of the Korean War the Rolled Products Division of Michigan Steel Casting Company had begun to establish a reputation in the heat treat industry. MISCO promoted their 35%Ni 15%Cr alloy, Misco Metal, directly to the heat treater. Misco Metal was the predecessor to RA330. Prior to Incoloy the only heat resistant alloy Inco promoted was the 76%Ni alloy 600. Eventually Incoloy became Incoloy 800, and was available in two grades. Incoloy 800 Grade 1 was fine grained, annealed around 1800°F (980°C), and Grade 2 was solution annealed for greater creep-rupture strength. By applying a minimum carbon of 0.05%, Grade 2 became Incoloy 800H in the early 1970’s. During the 1980’s the ASME design stresses for 800H were challenged. In response the carbon was increased slightly to 0.06-0.10%, and the Al + Ti controlled, and raised from 0.7% typical to about 1% typical. Calrod is a GE registered trademark.

RA330® is truly the workhorse alloy of the heat treating industry. The majority of all wrought alloy fixturing in use today is RA330, with some AISI 330 and a smaller amount of RA333®, and alloys 600, 601 and RA 602 CA. 35% nickel has been found by experience to be the optimum level for carburization resistance and strength in the Fe-Ni-Cr alloy system. The 1.25% silicon addition enhances both carburization and oxidation resistance. RA330 has much better resistance to deformation (creep strength) than RA309 or RA310. At intermediate temperatures RA330 never embrittles from sigma like RA309 or RA310. A combination of fairly high melting point, 2450°F (1343°C) and good oxidation resistance permits RA330 to be used at extreme temperatures. It is not uncommon for RA330 retorts to operate as high as 2250°F (1230°C) metal temperature. With comparable melting point, better oxidation resistance and higher strength, RA 353 MA now has the advantage at very high temperatures. Our highest temperature well documented experience with RA330 was a palladium brazing muffle, 11 gage (3mm) operating 2300 to 2370°F (1260 to 1300°C), outlasting muffles of alloys 600 and 601. 8-6

.RA330HC uses 0.4% carbon to provide high shear strength for use as pins in cast link belts, usually with cast HT links. RA 353 MA® may be regarded as an improved RA330 for use at 1830°F (1000°C) and higher, where it has twice the strength. Because of its oxidation resistance, weldability by GMAW, and 100°F (56°C) higher melting point, compared to 601, RA 353 MA is finding extensive use in retorts, kilns, brazing muffles, radiant tubes, utility coal burners and boiler tube shields. Oxidizing hot corrosion resistance is good, as are its hot erosion capabilities in cyclone applications. Welding is by either RA 353 MA DC lime type covered electrodes, or matching RA 353 MA GTAW and GMAW bare wire. Nominal Chemistry, Fe-Ni-Cr alloys, Nickel 30-40% alloy RA800H/AT RA330 RA330HC RA 353 MA

UNS N08811 N08330 -S35315

EN -1.4886 -1.4854

Cr 21 19 19 25

Ni 31 35 35 35

Si 0.4 1.2 1.2 1.2

C 0.06 0.05 0.40 0.05

Other 0.4Al 0.6Ti 45Fe 43Fe 43Fe 0.16N 0.05Ce 36Fe

Nickel-Chromium-Iron alloys, Nickel 45-60% These alloys include RA333 and other superalloys developed for gas turbine use, as well as advanced grades for thermal processing. Costs are higher, and usage tends to be in niche markets. RA X, developed in the early 1950’s by Haynes® as Hastelloy® alloy X, has been for years the standard alloy for gas turbine engine combustors. It is now slowly being replaced by alloys 188 and 230 in flight engines, and by 230 and 617 in land based gas turbines. Rather few people use X in heat treat service. It has excellent oxidation resistance in freeflowing atmospheres to rather high temperature, 2100°F (1150°C). Nevertheless, at more extreme temperatures or under stagnant atmospheres the 9% molybdenum content may render this alloy susceptible to catastrophic oxidation. For example, RA333 maintains oxidation resistance to 2200°F (1200°C), while alloy X plate at that temperature may completely disappear. This was a problem for us when some RA333 Mo reduction muffles were welded with alloy X covered electrodes. In service the weld beads disappeared, causing the whole muffle to fail. RA333® has long been one of the best performing wrought alloys for industrial heating applications, through 2200°F (1200°C). RA333 is strengthened with 3% each of cobalt, tungsten and molybdenum, and has a 1% silicon addition to enhance carburization resistance. RA333 is particularly good in resisting erosion from flame impingement, as in radiant tubes. RA333 permits a thinner tube, thus better heat transfer and energy efficiency, without danger of burning a hole through the tube. Direct service comparison with 601 in the same furnace has confirmed the superiority of RA333. 8-7

Nickel-Chromium-Iron alloys, Nickel 45-60%, continued RA333, continued Rotary retorts of 3/16” (4.8 mm) RA333 plate, used to harden steel shot, have been giving 10 year life since the 1960’s. RA333 kilns 35 foot (1070 mm) long have been used to calcine zeolites for a decade now. RA333 is highly resistant to metal dusting, as shown by both years of experience and longterm comparative testing. 617 alloy is strong, and has been used for land based gas turbine combustors, and in nitric acid catalyst support grids. It is carburization resistant at high temperatures, and oxidation resistant to reasonably high temperature. Because of its 9% molybdenum, it may be susceptible to catastrophic oxidation under stagnant conditions. 230 is a strong alloy, with good retention of ductility and excellent oxidation resistance. It has also been used in nitric acid catalyst support grids, and for parts of high temperature vacuum retorts. Nominal Chemistry, Ni-Cr-Fe alloys, Nickel 45-60% alloy RA333 RA X 617 230

UNS N06333 N06002 N06617 N06230

W/Nr 2.4608 2.4665 2.4663 --

Cr 25 22 22 22

Ni 45 47 54 60

Si 1 0.3 0.03 0.4

C 0.05 0.08 0.08 0.10

Other 3Co 3Mo 3W 18Fe 9Mo 1.7Co 0.6W 19Fe 12.5Co 9Mo 1Al 0.4Ti 1Fe 14W 1.5Mo 0.3Al 0.02La

Nickel over 60%, 15 to 25% Chromium This group includes RA601, the ThyssenKrupp VDM alloy Nicrofer 6025HT (a.k.a. RA 602 CA, and RA600, often simply called by Inco’s tradename “Inconel®” RA601 is a strong, carburization resistant and very oxidation resistant alloy, developed by James Hosier. It was introduced in the 1960’s. It is used for retorts and muffles, bar product being relatively uncommon. Although 601 is very oxidation resistant, it is commonly welded with alloy 82 (ERNiCr-3), which is not. As a consequence 601 fabrications may require frequent rewelding, as the old 82 (columbium/niobium bearing) weld disappears. Using RA 602 CA weld fillers is suggested to address this problem. RA 602 CA® is the strongest and most oxidation resistant wrought alloy for service above 1900°F (1040°C). RA 602 CA resists grain growth in high temperature service. It has been used for kilns operating as high as 2100°F (1150°C), as well as powdered iron sintering muffles, CVD retorts and vacuum heat treat trays up to 2260°F (1240°C). In its highest temperature application, an AOD chute, it has outperformed both RA 353 MA and Haynes 230. 602CA has been used in Germany for steel mill annealing furnace rolls

8-8

Nickel over 60%, 15 to 25% Chromium, continued RA600 has moderate hot strength, good ductility and resistance to oxidation, and very good carburization resistance. Compared to RA330, 600 is nearly as oxidation resistant but somewhat lower in creep strength. RA600 has poor resistance to sulfidation, even in oxidizing atmospheres (sulfur present as SO2). This comes from its relatively low chromium, and high nickel, contents. RA600 has good resistance to corrosion by neutral heat treat salts and salt fumes. It is appropriate for automated salt pot fixturing. Economics generally favor RA309 or RA330 for the salt pot itself. RA600 has good resistance to dry chlorine and dry hydrogen chloride gas at temperatures up to 900-1000°F (480-540°C). As an aqueous corrosion alloy, RA600 is resistant to hot, concentrated caustic (sodium or potassium hydroxide) solutions. A bit of history. “Inconel” was originally sold for corrosion applications, not high temperature. In 1938 a salesman named Paul Goetcheus, working from the Chicago office of Steel Sales, sold the first Inconel sheet to Buick Motor Division, for carburizing boxes. Previously, Buick had used cast boxes weighing 200 pounds, to heat treat 50 pounds of work. Mr. Goetcheus moved on to head up the Rolled Products Division of Michigan Steel Casting Company, in 1944. There he worked to promote the use of a wrought (“rolled”, in their terminology) 35Ni 15Cr alloy. In 1953 Mr. Goetcheus became the first president of Rolled Alloys, and the 35-15 alloy, Misco Metal, was renamed RA330. Nominal Chemistry, Nickel over 60%, 15 to 25% Chromium alloy RA601 602CA RA600

UNS W/Nr Cr N06601 2.4851 22.5 N06025 2.4633 25 N06600 2.4816 15.5

Ni 61.5 63 76

Si 0.2 -0.2

C 0.05 0.2 0.08

Other 1.4Al 14Fe 2Al 0.1Y 0.08Zr 9.5Fe 0.2Ti 8Fe

This concludes our general discussion of the various wrought materials that might be selected for a given application and those that we have selected to cover the range of temperatures, atmospheres, stresses and cyclic conditions. We think one of our alloys will perform to best advantage in almost every application. Therefore, we are fulfilling our slogan, “ALL THE BEST HEAT RESISTING ALLOYS, READY WHEN YOU NEED THEM”.

8-9

Cast Heat Resistant Alloys2 Heat resistant alloy castings are available in chemistries similar, although never identical, to those of the wrought alloys. In addition there are a number of chemistries that are only available as castings. Selection of cast versus wrought will depend, among other things, upon experience, economics and delivery time. Two aspects which influence whether one’s experience with either is good or bad, are: 1.) Design. Appropriate design may influence life more than the simple choice of wrought versus cast. A good design in either metal form may outlast a poor one in the other. This effect of design may or may not get factored into the user’s evaluation of his own experience. 2.) Quality. In fabrications this usually means the welds. With castings it is internal shrinkage, porosity and residual casting stresses. Cast grids, for example, may last only a few months, or for many years, depending on the foundry source. Advantages of Cast Alloy 1.

Initial Cost. Since cast parts avoid all the forging, rolling, cutting and welding of a fabrication, the price per pound of fixture may be lower.

2. Creep Strength. Similar compositions are inherently stronger at high temperature in the cast form than in wrought. This is because of the microstructure, and because cast heat resistant alloys are usually much higher carbon than the wrought “equivalent”. 3.

Shapes. Certain shapes can be cast that are not commonly available hot rolled, or that cannot be fabricated economically from available wrought product forms.

4.

Compositions. Some alloys are available only as castings, because they lack sufficient ductility to be worked into wrought forms. This is particularly true of the very high chromium alloys. Disadvantages of Cast Alloy

1.

Delivery. When equipment is down, fabrications can often be delivered in a couple of days to get back on stream. This is rarely true of castings.

2.

Weight. Cast parts are almost invariably thicker and heavier than the equivalent fabrication. This increases the non-productive weight that goes through each heat treat cycle. With radiant tubes and muffles thicker cast walls increase fuel costs for the same volume of work heat treated.

3.

Embrittlement. Many cast alloys quickly become very brittle in service. They are unable to withstand rough handling when cold, and weld repair is extremely difficult.

8-10

Cast Heat Resistant Alloys, continued 4.

Soundness. Castings invariably have some degree of porosity, internal shrinkage cavities, internal oxides and cold shuts. When these defects open to the surface they are subject to attack by carbon deposits or molten salts.

5.

Pattern cost. A pattern must be made for each different part design. This is all right for production runs but quite uneconomical for one’s and two’s at a time. Advantages of Wrought Alloy

1.

Section Size. Wrought alloys are available right down to nearly foil thickness. Thinner sections often permit weight reduction of 50% or more. With lighter sections handling the fixture is easier, and much less unproductive metal goes through each furnace cycle.

2.

Thermal Fatigue. Thinner sections that reduce thermal stresses, and the inherently greater ductility of wrought metal, promote better resistance to thermal cycling and shock.

3.

Surface Finish. The smooth surface of wrought alloy helps avoid focal points for accelerated corrosion by molten salts or carbon deposits.

4.

Soundness. Wrought materials are normally free of the internal and external defects such as shrink, porosity, etc., found in castings.

5.

Availability. Wrought heat resisting alloys are available from stock in numerous product forms. Fabrications are quickly procured to maximize production up-time. Disadvantages of Wrought Alloy

1.

Creep strength. Few wrought alloys match the high strength of heat resistant castings. Where creep-rupture is truly important, this must be considered in product design.

2.

Composition. Alloys such as 50Cr 50Ni, 28Cr 10Ni or 35Cr 46Ni, all with excellent hot corrosion and/or carburization resistance, are available only as castings.

8-11

Cast Heat Resistant Alloys, continued

The effect of cast alloy surface and internal defects versus wrought alloy soundness on service performance is illustrated by our old case history, RA330-108. The application was a grid for suspending loads in a gantry furnace at a commercial heat treat shop. The work was neutral hardening from temperatures up to 1850°F (1010°C), quenched in either molten salt, oil or brine. When the cast HT (35%Ni 17%Cr) grid was practically new and had been exposed to only a few cycles, a particular job required increased working area for the grid. RA330 alloy plate was formed and welded to the outside of the existing cast grid. As can be seen in the photo, the cast alloy portion suffered surface attack from soot and the quenching salt, and failed from thermal fatigue. Note the cracks in the center of the cast ribs which occur along the plane of weakness of the dendritic structure. The wrought alloy RA330 exhibited very little surface attack and no fractures.

8-12 Cast Heat Resistant Alloys, continued Nominal Chemistry, Cast Heat Resistant Alloys3,4,5 alloy HC HD HE HH-2 Thermax® 40B HI HK HL HT HU HP Supertherm® HOM-3 22H® MO-RE® 40MA IN-657 HX

UNS W/Nr J92605 -J93005 -J93403 1.4339 J93633 1.4837 --J94003 -J94204 1.4840 J94614 -J94605 -J95405 1.4865 J95705 1.4857 -----2.4879 --R20501 2.4813 N06006 - -

Cr 28 29 28 25 25 28 25 30 17 18 26 26 26 28 35 50 17

Ni 2 5 9 13 13 16 20 20 35 38 35 35 46 48 46 47.5 66

Si 0.8 1.5 1.5 1 1 1 1.4 1.4 1.7 1.7 1.3 1.5 1 1 1 0.4 2

C 0.3 0.4 0.3 0.3 0.4 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.45 0.5 0.45 0.06 0.5

W ----0.5 ------5 3 5 ----

Co -----------15 3 -----

Other 67Fe 63Fe 61Fe 60Fe 59Fe 0.3Ti 54Fe 54Fe 47Fe 44Fe 40Fe 36Fe 13Fe 16Fe 3Mo 16Fe 14Fe 1.3Cb 1.5Cb 0.5Fe 13Fe

Where metal dusting is a problem, one large captive shop has standardized on RA333 as their wrought alloy, and on Supertherm for cast fixturing, both grades being found extremely resistant to metal dusting (carbon rot). We have been told that nickel-aluminide alloy castings for heat treat service are quite strong. However, Midwestern experience has been that they contribute to heavy sooting in carburizing furnaces. References

1. 1998 ASME Boiler & Pressure Vessel Code, Section VIII, Division 1, paragraph UNF-56 (page 205), ASME, New York, New York. 2. Selecting the Alloy, Bulletin 113, Rolled Alloys, Temperance, Michigan 48182 U.S.A. 3. High Alloy Data Sheets, Heat Series, Steel Founders’ Society of America, 1973 4. Metals & Alloys in the Unified Numbering System, ASTM DS-56G, 8th Edition, ISBN 0-7680-04071 1999 Society of Automotive Engineers, Inc., Warrendale, Pennsylvania, U.S.A. 5. Stahlschlüssel, 18th Edition, 1998, Verlag Stahlschlüssel Wegst GmbH, D-71672 Marbach, Germany.

8-13 DESIGN Stresses (compressive, tensile, or shear) due to unequal temperature distribution and non-uniform temperature gradients, cause more failures in high—temperature equipment than all other influences combined amounting . . . to about 90 per cent of the total number of cases. And it is destructive chiefly because the engineer does not include in his design proper allowance for or provision against temperature inequalities or because the operator imposes temperature differentials which cause localized dimensional changes with accompanying stresses greater than the elastic strength of the alloy at the given temperature. F. A. Fahrenwald. Some Principals Underlying the Successful Use of Metals at High Temperatures, Proceedings of ASTM, 1924 V. 24 Nothing has changed in eighty years. High temperature equipment design has certain unique features not commonly found, nor at least emphasized, in mechanical engineering texts. The first and most important is that metals expand in volume with heat. This simple statement is so obvious, yet often dismissed or given but slight consideration in design. If thermal expansion is somehow restrained, the resulting stresses will equal the yield strength of the metal at temperature. One must design to permit free expansion (and contraction) or the metal will bend, buckle or crack.

A corollary to this is that most heat resistant alloys have rather poor thermal conductivity, less than 1/4 that of carbon steel and only 1/30 that of copper. Thermal gradients, hence thermal strains, are the rule and not the exception in high temperature equipment. Next, one should be aware of the significance, and the limitations, of creep-rupture data. These data are obtained under very closely controlled laboratory conditions of constant temperature and stress. Even so, there is considerable scatter, 15 to 20%, in rupture data, and possibly more in creep. When using published average creep-rupture data for design one must include a safety factor, and be clearly aware of the range over which temperature will be controlled in service. It can be surprising how rapidly mechanical strength drops off with temperature. For example, an increase in service temperature from 1700°F (927°C) to 1800°F (982°C) could drop the life of an RA330 component from 10 years down to only 15 months, under the same load. In practice, the furnace industry often required for a minimum creep rate of ASME Boiler & Pressure Vessel Code extrapolated 0.00001%/hour minimum rupture stress, whichever is lower.

designs to an allowable stress of one half the stress 0.0001% per hour, at the service temperature. The is more conservative, designing to either 100% of the creep rate, or 67% of the extrapolated 100,000 hour

9-1 Design, continued Rotating components, such as kilns, are often designed to much higher stresses than are static components. Kiln failures may be due to hot corrosion, more often to flite design, but rarely, if ever, to fatigue from the rotation. An item of some minor confusion is elastic modulus. Although modulus data are published at elevated temperatures, the numbers are obtained by a means involving the speed of sound through the material. In practice, above about 1000°F (540°C) stress is no longer proportional to strain. In other words, at red heat these alloys are simply not elastic, and the modulus data has no real meaning. One cannot calculate a simple beam deflection at 1650°F (900°C) using anyone’s published modulus data. At such temperatures strain is proportional to both time and stress, and not simply to stress alone. Thermal Strain This point is such an important consideration for high temperature equipment design that it must be examined in some detail. A large portion of the many field failures reported to us happen because the designer or user did not appreciate the significance of thermal expansion. This expansion must be accommodated not only by design but by installation practice as well.

Heat resistant alloys expand a great deal when heated. This expansion is roughly 3/16” to 1/4” for each foot of length (16 mm per meter), when heated from room temperature to 1800°F (982°C). If the metal is not free to expand, it will stretch, bend or warp permanently with each thermal cycle. Eventually, this repeated strain will fatigue the metal and the equipment will break. It is important to recognize just how large the total expansion can be, in typical heat treat service. A 48” (1220 mm) long RA330 heat treat basket, for example, oil quenched from 1550°F (843°C) will contract 0.692 (17.6 mm)—more than 11/16”—in overall length. Since the bottom of the basket enters the quench while the top frame is still red hot, the bottom members contract before the top does. A flexible bar frame design may tolerate this. But, a mechanically strong and rigid welded angle frame design may be inclined to crack or distort. This is because this “strong” design cannot accommodate the relative thermal contraction of the bottom versus the top of the frame.

9-2 Thermal strain, continued As temperature goes up the metal not only expands but diminishes rapidly in strength. The short-term yield strength of RA330, for example, averages about 37,200 psi (256 MPa) at room temperature, but only 40% of that figure, or 15,400 psi (106 MPa) at 1600F (871C). The short-term modulus, for whatever that is worth, drops from 28.5x106 psi (196 GPa) to 19.5x106 psi (134 GPa). The combination of differential thermal expansion/contraction and reduction in strength at heat is why quenched grids or large bar frame baskets tend to bow like a rocking chair, convex to the quench. In general any piece of metal which is hotter on one side will, when cooled, become concave on what was the hot side. As well as being the cause of distortion in service, this principle may be used to straighten metal parts1,2. The equation for calculating thermal stress in the elastic region is: S = αETK 1—ν α = coefficient of thermal expansion

E = elastic modulus

T = temperature difference ν = Poisson’s ratio

K = restraint coefficient

The formula may be found in S.Timoshenko, Theory of Elasticity, McGraw-Hill, New York, NY 1934 Apply this formula to RA330. Assume a plate 1000F on one side and 800F (538 to 427°C) on the other. α = 9.3 x 10-6 inch/inch°F E = 23.8 x 106 psi T = 200°F K = 1 ν = 0.297 The calculated stress = 62,970 psi. Average 0.2% offset yield strength of RA330 at 1000°F (538°C) is 25,000 psi. So, one may assume that a temperature differential of only 200°F (110°C) in this temperature range would cause permanent plastic deformation. The restraint coefficient in real structures will be some number less than one. Nevertheless, one rough, but good, rule of thumb is that a 200°F (110°C) temperature differential will yield most austenitic heat resistant alloys.

9-3 Weldments Weldments can fail from repeated thermal cycles. All welds, butt or fillet, must be completely fused. In thermal or mechanical cycling, the unwelded areas behave as large cracks or notches. Repeated thermal strains cause the “crack” to grow outward through the weld bead, a small step each cycle. Since this crack cannot be seen from the outside, there is no warning sign that the part is about to break.

This fully welded joint can The unfused void in this fillet resist both thermal and weld acts as a stress riser and mechanical fatigue. may cause premature failure. Incompletely penetrated weld joints will not tolerate thermal strains and are the most common cause of weldment failure in high temperature service. A couple of examples: 1. Bar frame heat treating baskets. Incompletely fused welds crack a little more each time the basket is quenched. The weld may break in service, or when the basket is straightened. This happens even though the remaining weld metal is still ductile. 2. Furnace fans. Each time the fan starts up, it goes through one fatigue cycle. This is because centrifugal force, gas loading and the temperature differential between blade and hub all stress the blades. Eventually, just starting and stopping the fan will cause low cycle fatigue failure of incompletely penetrated welds. The blades may also flutter or vibrate

9-4 Weldments, continued during operation, which causes more fatigue crack growth. All welds of fan blades to the hub must be fully penetrated. A higher strength weld filler such as RA333 may be helpful in resisting mechanical loads. But no weld filler will compensate for inadequate weld joint design. Incidentally, it is more difficult to achieve weld penetration by the arc in nickel alloys than in stainless. A joint design that makes a good fan in 316L stainless (W.Nr. 1.4404) may well not allow adequate weld penetration in RA330. The result can be that the nickel alloy fan fails even though a stainless fan of same design performed well. More root gap may be required to achieve full penetration in a nickel alloy. Thermal Expansion A simple way to calculate the thermal expansion of a fixture is to use the chart below. Pick the alloy, read down the column to the operating temperature and read the number, which is how much (in inches) each foot of metal will expand. (Multiply by 83.33 to get how many millimeters each meter of metal will expand) Remember that thermal expansion occurs in all three dimensions. It is really a volume expansion, not just an expansion in one direction. So

while the fixture is increasing in length, it is also increasing in width and height. A hole, incidentally, will expand at the same rate as the piece of solid metal that would just fill that hole. Example: An RA330 D-muffle 36 inches wide and 20 feet long operates at 1800°F. How far will the free end expand? Looking down the RA330 column we find a total expansion of 0.208 inches/foot at 1800°F (982°C). Multiply this figure by the length of the muffle, 0.208 in/ft X 20 ft = 4.16 inches total expansion. How wide will it be in the hottest zone? 36 inches + 0.208 in/ft X 3 ft = 36.624 inches.

9-5

Section Size Thin, rather than thick, sections reduce the thermal gradients inherent in heat resistant alloys used under conditions of rapid thermal cycling. Bear in mind that these alloys combine high thermal expansion coefficients with low thermal conductivity. In quenching service, the effects of repeated thermal shock can be as important as mechanical loading. The lightest possible section size should be used, to permit more uniform heating and cooling. We have seen baskets used for neutral hardening (which see many, many quench cycles) last twice as long when made of 1/2” (12.7 mm) diameter RA330 bar, as when they were constructed of 5/8” (15.9 mm) dia. bar. A dramatic example of the effect bar diameter has on quench cracking is shown below.

About 4X

About 4X

1/2” (12.7 mm) dia. RA330, basket top frame

5/8” (15.9 mm) dia. RA330 from same heat treat basket

A 1/2” (12.7 mm) diameter bar, which shows essentially no cracking, was used for the basket’s top frame. The basket vertical members were 5/8” (15.9 mm) diameter. One of these is shown in cross-section, on the right. This 5/8” (15.9 mm) dia. bar has cracks extending in depth to one half its radius. Even though this heavier bar should be mechanically stronger, it is clearly weaker in resisting thermal shock. References 1. John P. Stewart, Flame Straightening Technology for Welders, 9773 LaSalle Boulevard, LaSalle, Quebec Canada H8R 2N9, 1981 2. John P. Stewart, Distortion Control, 9773 LaSalle Boulevard, LaSalle, Quebec Canada H8R 2N9, 1989 9-8

SELECTING THE ALLOY Technical data illustrating the properties of heat resistant alloys are very helpful guides in selecting an alloy suitable for a given application. However the behavior of alloys during long exposure to the many environments and temperatures that may be encountered cannot be completely documented nor described by laboratory tests. Experience obtained from many actual installations is most helpful. One must develop the judgment needed to determine which of the many factors involved are the most important. A few points to consider. Temperature is often the first—and sometimes the only—data point given when we are asked for suggestions regarding alloy selection. One cannot successfully chose an alloy based on temperature alone. Nevertheless one simple first guide to alloy selection is knowing the maximum temperature at which a given alloy may have useful long term engineering properties. Picking oxidation in air, or strength, as a limiting factor one might rate alloys as follows, in plate form. Thin sheet will have a lower limiting temperature due to proportionally greater losses to oxidation. Carbon steel, such as ASTM A 387 Grade 22 (2¼ Cr, 1 Mo). Typically considered 950°F (510°C), above which 304H is stronger. 409 ferritic stainless (UNS S40900, Werkstoff Nr. and EN 1.4512) 1200°F (650°C), limited by oxidation. Subject to embrittlement after several years’ service above about 600°F (316°C). Formable, weldable. 410S low carbon martensitic stainless (UNS S41008, W.Nr. 1.4000) 1200°F (650°C), limited by oxidation. Subject to “885°F” embritlement after long service above about 600°F (326°C). 410 martensitic stainless (UNS S41000, W. Nr. 1.4024) 1200°F (649°C), limited by oxidation. Subject to embrittlement after several years’ service above about 600°F (316°C). Can be hardened by heat treatment, difficult to weld. 304/304H & 316 stainless (S30400/S30409, W.Nr. 1.4301 & S31600, 1.4401) 1500°F (816°C). If product contamination by scale particles is a consideration, consider a 1200°F (649°C) limitation, and move up to RA309 for 1500°F (816°C) service. 321 (S32100, W.Nr. & EN 1.4541) stainless has about a 100°F (55°C) advantage over 304, and is used to 1600°F (1202°C). In Europe 316Ti (W.Nr. & EN 1.4571) is used to 1650°F (899°C), whether because of technical advantage over 321 or difference in philosophy we do not know at this time. RA309 (S30908, W.Nr. & EN 1.4833) is useful to about 1850-1900°F (1010-1038°C) above which our customers seem dissatisfied with its oxidation performance. RA800H/AT (UNS N08811) is a little more oxidation resistant, still we’d suggest keeping it below 2000°F (1093°C) 10-1

Selecting The Alloy, continued RA 253 MA® (UNS S30815, W.Nr. 1.4893, EN 1.4835) has superior oxidation resistance to a fairly definite upper limit of 2000°F (1093°C). Above this temperature the oxidation resistance may be adequate but no longer exceptional. RA310 (S31008, W.Nr. & EN 1.4845) is reasonably oxidation resistant to about 2150°F (1177°C), although the strength is quite low. RA330® (N08330) combines useful oxidation resistance and fairly high melting point so that it will tolerate more extreme temperature abuse than any other fabricable austenitic grade with which we are familiar. RA330 muffles are regularly used at 2100-2150°F (1149-1177°C). In one exceptional case an 11 gage (3mm) wall RA330 muffle provided six months service brazing with 65% palladium 35% cobalt filler at 2370°F (1300°C). RA 353 MA® (S35315, EN 1.4854 ) has a melting point similar to that of RA330, with better oxidation resistance in laboratory tests. Field experience at this time is with muffles and calciners. Based on its chemistry and test results we would expect it to tolerate extreme temperature at least as well as does RA330. RA333® (N06333, W.Nr. 2.4608) in open air use is limited more by its incipient melting point than by oxidation. Temperatures to 2200°F (1204°C) may be considered, though stagnant conditions might not be desirable. We have no experience with this grade at 2300°F (1260°C). RA600 (N06600, W.Nr. 2.4816) excellent carburization resistance. Oxidation resistance does not drop off rapidly with temperature. RA600 is used at the same high temperatures as RA330, although somewhat more creep deformation may occur in service. RA601 (N06601, W.Nr. 2.4851) has deformed more than RA333 in 2150F (1177C) applications and should have a somewhat lower maximum temperature use. RA X (N06002, W.Nr. 2.4665) is designed for gas turbine combustors where the hot gases continually sweep over the metal surface. Due to its 9% molybdenum content this grade may be subject to catastrophic oxidation under stagnant conditions, or in open air above roughly 2150°F (1177°C). Know the atmosphere which the alloy must resist—is it air, inert gas, reducing, etc.? Or vacuum? Vacuum—obviously, metal loss from oxidation doesn’t exist so rather lean alloys may be used to extreme temperature if mechanical properties suit. Occasionally chromium is a concern, as it can evaporate from the alloy fixture, then deposit on cooler areas of the furnace. Sometimes parts will diffusion bond themselves to the fixture at very high temperature. One cure for this is to paint braze stop-off on the parts or fixture. Nicrobraze® Orange Stop-Off, from Wall Colmonoy Corp. is used for this purpose. Alloys commonly used as fixturing for vacuum heat treating tool or stainless steel include RA330, RA600 and RA601. 10-2

Selecting The Alloy—atmosphere, continued Air—those alloys useful in just plain hot air are also suited for oxidizing products of combustion of natural gas and even coal. Generally oxidation and strength are the only issues. “Oxidation” usually refers to metal wastage, but concern about product contamination from scale is an occasional issue. For example, glass forming operations take place around 1100-1400°F (600-760°C) where 304 stainless might suit as a structural element. But because scale from this stainless gets onto the glass, RA330 is used for its much higher oxidation resistance. With respect to oxidizing products of combustion of coal, a major end use of RA 253 MA is for coal nozzles in powdered coal fired utility boilers. Oxidizing products of combustion from heavy fuel oils may be corrosive due mostly to small amounts of vanadium in the oil, particularly in oil from Venezuela. The vanadium pentoxide formed is very corrosive at red heat. The only alloy said to resist V2O5 hot corrosion is the cast 50Cr-48Ni alloy (“50-50”), UNS R20501. Short of that, one might consider RA333, although it is definitely not as resistant as the 50%Cr alloy. While oxidizing products of combustion of coal can readily be handled by alloys such as RA 253 MA, or higher nickel grades if one prefers, reducing products of coal combustion are another matter. In reducing atmospheres, which do occur in certain areas of the current generation of low-NOx burners, sulphidation from both coal and oil fuels can be a serious matter. We would not suggest use of any alloy with higher nickel than RA310. That is, limit nickel content of the alloy to about 20%, to minimize sulphidation attack. In all other atmospheres there may be some potential for carburization or hot corrosion. If the atmosphere really is hydrogen, argon or nitrogen then no reaction with the alloy should occur. But sometimes the atmosphere as it exists in the furnace is unintentionally different than the pure gas pumped into the furnace. A classic case is a coil annealing cover for carbon steel. The atmosphere is nominally nitrogen-hydrogen, which would be quite neutral. But residual palm oil from cold rolling steel sheet vaporizes and makes the atmosphere carburizing enough to deposit soot inside the cover. This “inert” atmosphere will also mildly carburize the cover itself, usually RA309 or RA330. When steel rod coils are annealed the carbon potential of the atmosphere is controlled to 0.4%C, to be neutral to the AISI 4140 or 1045 steel rod being annealed. This atmosphere is actually carburizing to Ni-Cr-Fe heat resistant alloy, and tends to embrittle RA 253 MA. A less common situation is sulphidation of alloy fixturing used to anneal copper cathodes, electrolytically refined copper. The cathodes have residual copper sulphate from the electrolyte used in this process. This is the source of sulphur, which will attack nickel alloy furnace fans, in particular, used for annealing cathodes in a reducing or neutral atmosphere.

10-3

Selecting The Alloy—atmosphere, continued Intentionally carburizing atmospheres are commonly used in heat treatment of steel parts. Resistance to embrittlement from carbon absorbtion is largely conferred by the total chromium , nickel and silicon content of the heat resistant alloy. Wrought alloys commonly used to resist carburization include RA330, RA333, RA600 and RA601. Of the lower nickel grades only RA85H, at 3 1/2% silicon, has useful carburization resistance in heat treat furnaces. RA800H/AT is too coarse grained, and too low in silicon, for practical use in such applications. RA X has good carburization resistance and is occasionally used. In high chlorine, or fluorine, atmospheres high nickel alloys are preferred. RA600 is the usual choice. When some amount of oxygen is also present, with only moderate halogen levels, RA601 may be useful. Under oxidizing conditions chromium, molybdenum and tungsten form highly volatile oxychlorides.

10-4

CUTTING AND FORMING Mild steel and heat resisting alloys do handle differently, and it is well to know the product you are working with to get the most out of it. There are some rules to know, but for the most part designing and fabricating alloys is using common sense based on the properties of the alloys and what you expect them to accomplish. Shearing In the first place, the yield strengths of heat resisting alloys in the annealed condition are a little higher, and their tensile strengths a lot higher, than mild steel. Shear capacity, for example, has to be about 50% greater. On our shear rated 3/8” (9.5 mm) mild steel, we regularly shear 1/4” (6.35 mm) heat resisting alloy. The hydraulic shear, rated 3/4” (19 mm) mild steel, will handle 1/2” (12.7 mm) alloy plate. Good shearing practice cuts about 20% of the metal and fractures the remaining 80%. Heavier thickness plate is best cut by abrasive wheels, which produce a smooth, close tolerance surface. Bending and Forming Austenitic heat resisting alloys should almost always be bent cold. Heating without adequate temperature control is dangerous because of the narrow working range and the possibility of over or under heating. In the 1200 to 1600°F (650-870°C), or red heat, range, both 18-8 stainless and the nickel heat resisting grades will tear or rupture in forming. It will take more power to form these alloys than it takes to form mild steel, but because of good ductility, the alloys will take a lot of deformation without rupture. Extremely severe forming may require annealing between operations. The ferritic grade RA446 does not form well at room temperature. Plates 1/4” (6.35 mm) and thicker should be preheated 250-400°F (120-205°C) for any bending or forming. Failure to preheat may result in some plates cracking apart, while others may be formed successfully. These alloys always harden on deformation and cannot be worked beyond a limit without rupture. Our stock materials have all been scientifically annealed. A given size will have limits on hardness, elongation, and reduction of area. A typical plate might have a Rockwell B hardness of 84, and elongation of 35% and a reduction of area of 60%. Every lot of RA material is checked for these properties, and the mill certifications of the material delivered to you are kept on file for ten years at Rolled Alloys. Records on your order are kept by RA for six years. After work hardening, but before rupturing, the material can be restored to its original mechanical properties by annealing. The process varies with the alloy, the mass, and the hardness. A piece of 3/16” (4.8 mm) plate, for example, that had been formed into a 4” (100 mm) tube might have had its hardness raised from the original Rockwell B 84 up to RB 96. It could be returned to RB 84 by heating to 1950°F (1065°C) and holding at this temperature for five minutes, then cooling quickly with an air blast. There probably would be little reason for annealing this shape, unless it was to be formed again, and it were required to be soft to permit further cold work. 11-1

Bending and Forming, continued These solid solution strengthened materials, therefore, can be hardened only by cold working, and softened by annealing. Occasionally tooling for aerospace requires to be stress relieved after rough machining, which, in the case of RA330, may be accomplished by heating for one hour per inch (25 mm) of thickness to 1800°F (982°C), and furnace cooling until black, then air cooling. The austenitic alloys will take a bend of 180° with a minimum inside radius equal to twice the thickness of the material. They will sometimes accept a bend flat on themselves, but they are not guaranteed to do so. The fabricator must perform bends with small radii and his own risk and be prepared to weld cracks that may develop. For extreme bends or the harder alloys it is better to bend across the grain, rather than having the grain parallel to the bending axis. The work hardened surface of a sheared or punched edge limits the amount of forming possible before cracking. As a minimum precaution, the shear burr or drag should be ground off. If severe forming is anticipated, the work hardened metal must be removed from the edge to be formed. The photo on page 80 shows the effect of edge condition on formability of 1/2” (12.7 mm) RA333 plate. With an as-sheared edge, the plate cracked after only a 40° bend. Removing the shear burr permitted a 90° bend with some cracks. With a ground or bandsawn edge, material from the same plate was bent 180°, nearly flat on itself, with no cracking. The first sign of overstretching is an orange peel appearance. This in itself is seldom detrimental, but the fracture soon to follow with further forming is incurable except by welding. It is far better to avoid a design that makes use of minimum radii. A generous radius is better for keeping the metal solid in service as well as during forming, because it gives the structure freedom to expand and contract, minimizing the thermal stresses created by heating and cooling. A thermal expansion of some 3/16” per lineal foot (16 mm per meter) is going to occur between room temperature and the average service temperature. The resulting stresses are great, and the metal is going to move; so it should be pointed in the right direction. After RA333, by a slight margin RA 353 MA, RA 602 CA, and RA 253 MA are the strongest metals of the group in most temperature ranges; likewise they are slightly tougher to work. RA309 and RA310 are a little weaker. RA600 is somewhat softer and weaker. Its high nickel makes it “gummier” than alloys with more iron. The ferritic RA446 is less ductile and requires preheating before bending.

11-2

About 0.7 Scale RA333 1/2 inch (12.7 mm) plate, formed with different edge preparations. Left - Sheared edge ground. Bent 180° flat on itself, no cracks. Middle - Shear burr removed. Bend 90° before cracking. Right- As-sheared, burr up. Cracked at 40° bend angle.

11-3

SPINNING AND DEEP DRAWING Spinning and deep drawing can be accomplished by taking into consideration the physical properties, work hardening, and annealing. RA330 spins rather well, roughly comparable to 305 stainless. None of the heat resistant alloys will deep draw as well as 304 stainless. Dies for drawing the heat resistant alloys ought not be proofed with 304, as results will be different. Illustrated below is an 11 gage RA 602 CA spun half for a radiant tube return bend.

11-4

MACHINING The alloys described here work harden rapidly during machining and require more power to cut than do the plain carbon steels. The metal is “gummy”, with chips that are stringy and tough. Machine tools should be rigid and used to no more than 75% of their rated capacity. Both workpiece and tool should be held rigidly; tool overhang should be minimized. Rigidity is particularly important when machining titanium, as titanium has a much lower modulus of elasticity than either steel or nickel alloys. Slender work pieces of titanium may deflect under tool pressures causing chatter, tool rubbing and tolerance problems.

Feed rate should be high enough to ensure that the tool cutting edge is getting under the previous cut, thus avoiding word-hardened zones. Slow speeds are generally required with heavy cuts. Sulfur-chlorinated petroleum oil lubricants are suggested for all alloys but titanium. Such lubricants may be thinned with paraffin oil for finish cuts at higher speeds. The tool should not ride on the work piece as this will work harden the material and result in early tool dulling or breakage. Use an air jet directed on the tool when dry cutting, to significantly increase tool life. Lubricants or cutting fluids for titanium should be carefully selected. Do not use fluids containing chlorine or other halogens (fluorine, bromine or iodine), in order to avoid risk of corrosion problems.

Make sure that tools are always sharp. Change to sharpened tools at regular intervals rather than out of necessity. Titanium chips in particular tend to gall and weld to the tool cutting edges, speeding up tool wear and failure. Remember— cutting edges, particularly throw-away inserts, are expendable. Don’t trade dollars in machine time for pennies in tool cost.

Material

Speed Surface ft/min

AISI B112 René 41 25 (L-605) 188 N-155 TM WASPALOY 718 625 RA X ® RA333 A-286 601 RA800H/AT Ti 6Al-4V sol’n annealed aged

The following speeds are for single point turning operations using high speed steel tools. This information is provided as a guide to relative machinability, higher speeds are used with carbide tooling.

Speed as a % of B1112

165 12 15 15 20 20 20-40 20 20 20-25 30 25-36 25-35

100 7 9 9 12 12 12-24 12 12 12-15 18 15-21 15-21

30-40 15-45

18-30 8-27

Material ®

RA330 ® RA 353 MA ® RA 253 MA RA2205 RA20 ® AL-6XN RA309 RA310 304 321 RA446 ® 17-4PH sol’n treated aged H1025 303

Speed Surface ft/min

Speed as a % of B1112

30-45 40-60 45-60 50-65 65 65 70 70 75 75 75

18-27 25-35 28-35 30-40 40 40 42 42 45 45 45

75 60 100

45 36 60

RA330 and RA333 are registered trademarks of Rolled Alloys

René 41 is a registered trademark of Teledyne Industries, Inc.

253 MA and 353 MA are registered trademarks of Outokumpu

WASPALOY is a trademark of United Technologies Corp. 17-4PH is a registered trademark of AK Steel Corp.

11-5

FORGING Hot forging should be used only if cold pressing cannot do the job. We know the materials are forgeable, because they all came from large cast ingots; but the working ranges are narrow, and close control of temperature, time, heating atmosphere and reduction are all important. Heat resistant alloys must be heated throughout the section thickness. Typically, forging should begin when the metal is around 2100-2200°F (1150-1200°C), and finish before the metal cools below 1700°F (930°C). The exact temperature ranges vary from alloy to alloy. Forging either too hot or, more likely, too cold may cause cracking. Never, never attempt to bend or form any austenitic alloy in the 1100-1600°F (590-870°C) temperature range. Whether 304 stainless or nickel alloy 600, all will tear when formed at these temperatures. Unlike carbon steel, heating locally with a torch to make bending easier just doesn’t work. It is too difficult to heat nickel alloys uniformly hot enough throughout the section.

1/2” (12.7 mm) diameter RA330

scale: 1 7/8 X

Torch heated to bend. Although the operator thought it was hot enough, the brown temper color in the crack is typical of about 1200°F (650°C) Torch heated to bend. The operator thought it was hot enough, but the brown temper color in the crack is typical of about 1200°F (650°C). This is right about the temperature where the austenitic alloys tear rather than bend.

11-6

WELDING Welding heat resistant alloys is covered in general in our Bulletin No. 115, and in more detail in No.’s 201 & 207 for RA330, 202 for RA 253 MA, 209 for RA 353 MA, with Bulletin 120 covering RA333 welding products. For our corrosion resistant alloys see Bulletins 203 for AL6XN®, 1071 for RA2205, XXXX for LDX 2101, and Bulletin 205 for 20Cb-3 stainless. Heat resistant alloys are readily welded but they do require more time, and a DIFFERENT approach than stainless, or carbon steel. A few important rules: 1. Make reinforced, stringer beads. Do not weave. Shallow fillet welds or broad, flat weld beads tend to crack down the center as they solidify. Cover or fill in craters, to prevent them from cracking.

2. Keep heat input low. Do not ever preheat, except to dry moisture off of the metal. Keep the temperature of the metal between weld passes low, below 212°F (100°C). 3. For RA330 specifically, use RA330-04 or RA330-80-15 weld fillers. Do not use AWS E330 weld wire, as it will be crack sensitive. Absolutely do not try to weld RA330 with stainless rods such as E308, E309, or E310 as they will crack. E312 electrodes, in particular, are often sold under various tradenames for general shop repair welding and dissimilar metal welds. However, E312 weldments are not suited for high temperature service. They embrittle severely with exposure above 600°F (1100°C). At red heat E312 welds are very weak, as well as brittle.

12-1

4. Make full penetration weld joints. Incompletely welded areas will open up as cracks during normal heat treat thermal cycles. Incompletely penetrated weld joints are the most common cause of weld failures in service. Weld joints in fans, in particular, must be fully penetrated.

Let us back up a bit, and first describe some of the differences between welding carbon steels, and welding either stainless or nickel alloys. Then, we will cover the important differences between stainless (under 20% nickel) and the higher nickel alloys. CARBON STEEL VERSUS STAINLESS Some important differences between welding the carbon or low alloy structural steels and the austenitic stainless and nickel alloys include: A. Surface Preparation B. Shielding Gases C. Cold cracking vs Hot Cracking D. Distortion E. Penetration F. Fabrication Time. A. Surface Preparation When fabricating carbon steels it is common practice to weld right over scale (a so-called “mill finish” is a layer of blue-black oxide, or scale, on the metal surface), red rust and even paint. The weld fillers normally contain sufficient deoxidizing agents, such as manganese and silicon, to reduce these surface iron oxides back to metallic iron. The resultant Mn-Si slag floats to the weld surface. Iron oxide, or “scale”, melts at a lower temperature, 2500°F (1371°C)1, than does the steel itself. One can see this in a steel mill when a large ingot is removed from the soaking pit for forging—the molten scale literally drips off of the white hot steel. Stainless steel, by contrast must be clean and free of any black scale from hot rolling, forging or annealing operations. Of course, stainless normally comes from the mill with a white or bright finish. A few users of heat resistant alloys, though, do prefer “black plate”, that is, plate with the mill hot rolling scale intact. This scale is thought to provide additional environmental protection at red heat.

12-2 WELDING, Surface Preparation, continued Stainless steel melts at a lower temperature than does its chromium oxide scale, and the stainless weld filler chemistry is not capable of reducing this scale back to metallic chromium. As a result, with gas shielded processes it is difficult to get the weld bead to even “wet”, or stick to, a scaled piece of stainless. With SMAW a weld of sorts can be made, as the coating fluxes away most of the scale. The need to clean or grind down to bright metal is more likely to cause trouble when stainless is being joined to carbon steel. That is because in this dissimilar metal joint it is necessary to grind that carbon steel to bright metal, on both sides of the joint, free of all rust,

mill scale, grease and paint. Incidentally, the appropriate weld fillers for this particular joint, to minimize the hard martensitic layer on the steel side, are alloy 182 covered electrode, ENiCrFe-3, or alloy 82 bare wire, ERNiCr-3. Alloy 62 bare wire, ERNiCrFe-5, and weld metal A, ENiCrFe-2 covered electrodes are also used. E309 electrodes are commonly used but may leave a hard layer on the steel side, which may crack. Both stainless and high nickel alloys which are designed for corrosion resistance are produced to very low carbon contents, less than 0.03% and sometimes less than 0.01% carbon. Any higher carbon will reduce the metal’s corrosion resistance. For this reason it is necessary to clean these alloys thoroughly of all trades of grease and oil before welding. Also the very high nickel alloys, such as 400 alloy (Monel®, UNS N04400), or commercially pure nickel 200/201, are sensitive to weld cracking from the sulphur in oil. Metallic zinc paint is a common way to protect structural steel from corrosion. Even a small amount of that zinc paint overspray on stainless will cause the stainless to crack badly when welded. Consider completing all stainless welding before painting the structural steel in the area. B. Shielding Gases For gas metal arc welding (a.k.a. MIG) carbon steel the shielding gases are usually 95% argon 5% oxygen, 75% argon 25% CO2 (carbon dioxide) or 100% CO2. These are suitable with carbon or low alloy steel welding wire but far, far too oxidizing for use with stainless or nickel alloys. It is not unknown to hear the complaint “. . . clouds of red smoke are coming off when I weld your 310. . . heavy spatter. . .” and then learn that the shielding gas used was 75%Ar 25%CO2. A fine gas for carbon steel but not for stainless. One exception to this high CO2 prohibition is when using flux cored wire, either stainless or nickel alloy. Some of these cored wires are specifically formulated to run best with 75%Ar 25%CO2.

12-3 B. Shielding gases, continued Stainless and nickel alloys are often GMAW spray-arc welded with 100% argon. Weldability is greatly improved by adding from 10 to 20% helium. Helium provides a hotter arc. This helps burn away the stable chromium oxide film which does impair weldability of stainless and chromium-nickel alloys. A very small amount of CO2, about 1%, will stabilize the arc (prevent arc wander). When 75% argon 25% helium is used for GMAW a true arc transfer cannot be obtained. Rather, the arc transfer somewhat resembles the globular transfer mode. There are shops where this is preferred. Short-circuiting arc welding generally requires the 75%Ar 25%He mix, but a 90%He 7 1/2%Ar 2 1/2%CO2 “tri-mix” is commonly used.

C. Cold Cracking versus Hot Cracking Carbon steel weldments may harden, and crack, as they cool from welding. High hardness, and the resulting cracking, are more likely when the steel contains over 0.25% carbon. Alloying elements which increase hardenability, such as manganese, chromium, molybdenum, etc. can make steels of lower carbon content also harden. Hydrogen pickup from moisture in the air causes underbead cracking in steels that harden as they cool from welding. To prevent such cracking the steel is usually preheated before welding, to retard the cooling rate of the weld and avoid martensite formation. Postweld heat treatment, or stress relief, is also applied to some steels, or for certain applications. Austenitic stainless and nickel alloys do NOT harden no matter how fast they cool from welding. So, it is not necessary to preheat stainless, nor to post weld heat treat it. As a matter of fact preheating stainless, beyond what may be necessary to dry it, can be positively harmful. Stress relief 1100-1200°F (600-650°C) as applied to carbon steel is ineffective with stainless or nickel alloys, and may damage the corrosion resistance of some grades. Stainless steel welds generally do not crack unless contaminated, possibly by zinc or copper, more rarely by aluminum. High nickel alloys are susceptible to cracking in restrained joints, or heavy sections. This is a hot tearing, not a cold crack. That is, the weld bead tears rather than stretching, as the bead contracts upon solidifying. This hot tearing/hot cracking has nothing to do with hardness. The faster a nickel alloy weld freezes solid, the less time it spends in the temperature range where it can tear. For this reason preheating, which slows down the cooling rate, is actually harmful, as it permits more opportunity for hot tearing to occur.

2,3

12-4

D. Distortion

Stainless steel has poor thermal conductivity, only about one fourth that of A 36 structural steel. This means the welding heat tends to remain concentrated, rather than spread out. Stainless also expands with heat about half again as much as does carbon steel. The combination of these two factors means that stainless or nickel alloy fabrications distort significantly more than similar designs in carbon steel. Among other things, tack welds need to be more closely spaced in stainless/nickel alloy welds. Welds should be sequenced about the neutral axis of the fabrication to balance welding stresses, hence minimize distortion. Back step welding is also helpful.

Tack welds should be sequenced.

If the tacks are simply done in order from one end, the plate edges close up and the gap disappears.

Back step welding helps reduce distortion.

12-5 D. Distortion, continued

E. Penetration The arc will not penetrate a stainless nearly as deeply as it will carbon steel. Penetration is even less in high nickel alloys. Increasing welding current will not solve the problem! Stainless, and especially nickel alloy, joints must be more open, single or double beveled, with a root gap, so that the weld metal may be placed in the joint. Lack of weld penetration is the single most important reason why austenitic alloy weldments fail in high temperature service.

F. Fabrication Time Cleanliness, distortion control measures, maintaining low interpass temperatures and even machining add up to more time spent fabricating stainless than carbon steel. A shop experienced with stainless may require one and a half times as long to complete the same fabrication in stainless, as in carbon steel. A good carbon steel shop encountering stainless or nickel alloys for the first time can easily spend twice as long, maybe even three times as long, to do the stainless fabrication, as it would the same job in carbon steel.

12-6 WELDING AUSTENITIC ALLOYS

The fundamental problem to be overcome in welding austenitic nickel bearing alloys is the tendency of the weld to hot tear upon solidification. This matter is readily handled in alloys of up to about 15% or so nickel. In these stainless grades the weld metal composition is adjusted, usually by slightly higher chromium and reduced nickel, to form a small amount of ferrite upon solidification. The amount of ferrite in the weld may be measured magnetically, and is reported as a Ferrite Number, FN. This ferrite acts to nullify the effects of the elements responsible for hot cracking in the Ni-Cr-Fe austenitics. These elements are chiefly phosphorus, sulphur, silicon and boron. In higher nickel grades, about 20% nickel and over, it is metallurgically not possible to form any measurable amount of ferrite. Therefor other means of minimizing hot cracking must be used. Foremost among these is to use high purity raw materials in the manufacture of weld fillers. Simply beginning with low phosphorus alloying elements, and reducing the amounts of harmful sulfur and silicon in the weld metal improves its ability to make a sound weld. Phosphorus, in particular, must be kept below 0.015% in the weld wire itself. Certain alloy additions such as manganese, columbium (niobium), molybdenum and carbon serve in one way or another to reduce the austenitic propensity for weld hot cracking. Manganese ranges from about 2% in AWS E310-15 covered electrodes to 5% in RA330-04 wire & electrodes and 8% in alloy 182 (ENiCrFe-3) covered electrodes. Columbium at the 0.5% level, as in 347 stainless, is harmful whereas 2 to 4% columbium is quite beneficial in many nickel base weld fillers. Molybdenum isn’t necessarily a common addition specifically for weldability but it does enhance the properties of RA333-70-16 covered electrodes. High molybdenum is responsible for the popularity of the various “C type” electrodes (15Cr 15Mo balance Ni) in repair welding. 2% Mo contributes to 316 as being the most weldable of the stainless steels. Carbon is slightly elevated in 310 weld fillers, to about 1/10%. The one welding electrode specifically using very high carbon to promote sound welds is the heat resistant grade RA330-80-15 (UNS W88338). Maintaining a weld deposit chemistry of some 0.85% carbon permits this electrode to make sound welds in either wrought or cast 35% Ni high silicon heat resistant alloys. The distinction between the lower nickel stainless grades, which depend upon ferrite to ensure weldability, and the high nickel alloys, which require high purity weld fillers, is an important one to remember. Most ferrite containing (stainless) weld fillers are useless with nickel alloy base metal, as dilution of the weld bead with nickel from the base metal eliminates this ferrite. Likewise a high purity nickel alloy weld filler, such as ER320LR, may be not quite so crack resistant when contaminated by phosphorus from 316L, cast alloy 20 (CN-7M), or carbon steel base metal. With respect to welding there are some distinctions between those alloys intended for use above 1000°F (540°C), and those meant for aqueous corrosion service.

12-7 Welding austenitic alloys, continued

One difference is in carbon content. Corrosion resistant grades are generally limited to 0.03% carbon maximum, and typically much lower. They may also have small additions of columbium or titanium. Restriction of carbon, or tying it up with a stabilizing element (Cb or Ti) is necessary to prevent heat affected zone (HAZ) intergranular corrosion and stress corrosion cracking (SCC) due to carbide precipitation. Heat resistant alloys by contrast typically require 0.04 - 0.010% carbon for good hot strength. RA 602 CA is even higher, at the 0.2% level, while RA330HC belt pin stock and the cast heat resistant alloys have a nominal 0.4% carbon. In the absence of a wet corrosive environment a little intergranular carbide precipitation is not particularly harmful to a heat resistant alloy. In both classes of material, incompletely penetrated welds and open crevices must be avoided in fabrication design. Serious aqueous corrosion can begin in crevices. In high temperature carburizing service crevices are where carbon (soot) can deposit, grow, and pry the joint apart like tree roots in rock. For both classes of alloy, weldability alone is not the entire issue. The weld filler must also have the mechanical and environmental resistance required for its intended service. Usually this point is addressed in fabricating corrosion resistant alloys. It is sometimes overlooked in heat resistant alloy fabrication and even less often considered in repair of high temperature alloy fixturing. ALLOYS UNDER 20% NICKEL Most austenitic grades containing less than 20% nickel are joined with weld fillers that utilize perhaps 4-12 FN (Ferrite Number) to ensure weldability. Heat resistant alloys with 20% or less nickel include 304H, 321, RA 253 MA, RA309, RA85H, RA310, and the cast heat resistant alloys HH and HK. All save RA310 and the cast alloys depend upon some level of ferrite in the weld bead to prevent solidification defects. The cast grades are usually welded with high carbon, fully austenitic electrodes of similar or higher nickel. RA310 stands in an odd position between the stainless and the nickel alloys, having neither ferrite nor any particular alloy addition for weldability. Not surprisingly, 310 welds have a reputation for fissuring. In the past it was possible for 310S (UNS N031008) base metal to contain as much as 1.50% silicon in the ASTM/ASME specifications. Heats on the high side of silicon and phosphorus were definitely a problem to weld (Rolled Alloys traditionally limited silicon in RA310 to 0.75% max). With the advent of 310H (UNS S31009) ASTM limited silicon to 0.75% maximum. To avoid melting two chemistries of 310, in practice all 310 varieties now melted in North America have less than 0.75% Si. Phosphorus in the weld wire may still be an issue with some lots of ER310 welding wire. The current AWS limit for ER310 wire is 0.03%P max. This is far too high. For 310 welding wire to be of practical use the phosphorus must be kept under 0.015%P max.

12-8 ALLOYS OVER 20% NICKEL

Heat resistant alloys in this category include, but are not limited to, RA800H/AT, RA330, RA 353 MA, 803, alloy X (UNS N06002), RA333, 617, Haynes alloys HR-120, 230 and 214, 601, RA 602 CA, 600, and Nimonic 75. The cobalt alloys N155, 556, 188, L605, and HR-160 may be treated in similar fashion with appropriate weld fillers. Many nickel alloys are joined with matching composition weld fillers, modified only by restrictions on phosphorus, sulphur, ilicon and boron. Titanium may be added for deoxidation. Other nickel weld fillers contain manganese, high carbon, columbium or molybdenum to improve resistance to fissuring and hot cracking. Such chemistry modifications are rarely as effective as is the use of ferrite in the lower nickel stainless weld fillers. Welding technique and attention to cleanliness, then, become increasingly important to ensure the soundness of fully austenitic welds. Techniques include reinforced, convex stringer beads and low interpass temperature. Cleanliness includes NOT using oxygen additions to the GMAW shielding gases for nickel alloys. It is worth repeating here that high nickel alloys can not reliably be welded using stainless steel weld fillers. Stainless steel weld metal (308, 309, etc.) depends upon a small amount of deposited ferrite to ensure a sound weld. But when a stainless rod is deposited on a high nickel base metal, the additional nickel melted into the weld bead makes it fully austenitic, with no ferrite at all. Without ferrite, the stainless weld bead may crack. WELDING PROCESSES Five different arc welding processes are generally used with heat resisting alloys. The most common, in North America, is Gas Metal Arc Welding (GMAW), formerly known as MIG (Metal Inert Gas), using spooled bare wire filler. Next in popularity is Shielded Metal Arc Welding (SMAW), or just plain “stick” welding, with covered electrodes. The least volume of work is done by Gas Tungsten Arc Welding (GTAW), formerly called TIG (Tungsten Inert Gas) and originally trade named Heliarc. Two other methods are Plasma Arc Welding (PAW) and Submerged Arc Welding (SAW). In addition resistance welding, particularly cross wire resistance welding, is often used in heat resistant alloy fabrication. There are two basic types of welding machines, Constant Current, and Constant Potential. A constant current machine is used for GTAW (TIG) and SMAW (stick) welding. Practically speaking it won’t work for GMAW (MIG) welding. The dial on a Constant Current machine reads in amperes, and the current is regulated by this dial. Constant Potential (voltage) machines are used for GMAW (MIG) welding. They don’t work well with covered electrodes (SMAW). The dial regulates voltage, and is marked with numbers in the 20-40 range.

12-9 Gas Metal Arc Welding

In this process, the weld filler metal is bare wire. The most common size is 0.045” (1.14 mm), though 0.035” (0.89 mm) and 0.0625” (1.59 mm) are also stocked, typically on 25-30 pound (11-14 kg) spools. Wire is fed continuously through a hollow cable to the welding gun, where it makes electrical contact. The arc between weld wire and workpiece melts the metal. Molten weld filler transfers as either a spray of fine drops, or as larger globs. The metal is protected from oxidation by a continuous flow of inert shielding gas, usually argon, through the weld torch and around the wire. Current is always Electrode Positive (DCRP, direct current reverse polarity). The GMAW process is fast and well suited to high volume work. It can be automated, as for welding long tubes. Welding with relatively high current, about 190-220 amperes for 0.045” (1.14 mm) wire, and argon shielding is used for the spray-arc transfer mode. In this mode, molten weld metal crosses the arc to the work as a fine spray. At lower current, roughly 100 amperes for 0.035” (0.89 mm) wire, with 75% argon 25% helium shielding, the molten weld metal transfers as large, individual drops. This is known as short-arc, or short-circuiting arc, welding, characterized by a noisy arc and low heat input. Choice of shielding gas is important. First, do not use oxygen additions to the gas when welding nickel alloys and NEVER use 75% argon 25% carbon dioxide for GMAW welding either stainless or nickel alloys. Oxygen above 2% starts burning out major alloying elements. CO2 above 5% adds carbon to the low carbon stainless grades. Although very small amounts of CO2 may be used in argon, at above 15% CO2 in argon the arc transfer mode is no longer spray, but rather a hot globular transfer with a great deal of spatter. For spray-arc welding the most common gas is 100% argon. To improve bead contour and reduce arc wander, respectively, 10 to 20% helium and a very small amount of CO2 may be added to the argon. RA 602 CA requires an addition of nitrogen to the shielding gas, to prevent hot cracking. One suggested mixture is 90%Ar 5%He 5%N2. The patented gas developed in Germany specifically for GMAW RA 602 CA is. A mix of 75%Ar 25%He is also used, although the transfer mode will then not quite be a true spray-arc. For short-circuiting arc transfer 75% Ar 25% He is used, as is the commonly available 90%He 7 1/2% Ar 2 1/2% CO2. Because the welding wire must be pushed through a cable, ranging from 10 to 15 foot (3 to 4 1/2m) long, there may be feeding problems. The result can be a tangle of wire known, appropriately, as a “bird’s nest”. This shuts down the operation until the welder clears it. The care with which the filler metal is wound on the spool affects how smoothly the wire feeds. While the manufacturer is often blamed for feeding problems, more often than not proper attention to machine set up will ensure freedom from “bird’s nests”.

12-10 WELDING, gas metal arc, continued

Smooth feeding depends on the cast and helix of the spooled wire. Both AWS A5.9 for stainless, and A5.14 for nickel alloy wire require cast and helix of wire on 12 inch (300mm) spools to be4 “such that a specimen long enough to produce a single loop, when cut from the spool and laid unrestrained on a flat surface, will do the following: 1. Form a circle not less than 15 in. (380mm) in diameter and not more than 50 in. (1.3m) in diameter 2. Rise above the flat surface no more than 1 in. (25mm) at any location” Our RA 253 MA wire, for example, typically has 36 to 42 inch (915 to 1070mm) cast and 1/2 inch (12.7mm) helix. The following discussion is based on information from Ron Stahura, AvestaPolarit Welding Products, Inc. Many heat resistant alloy weld wires are much higher in strength than stainless wire (e.g., ER308 or ER316L), and therefore require more care to feed smoothly. When tangling, or bird’s nest, occurs the first thing we suggest is to examine machine set-up. Does this problem occur on more than one machine? How long is the cable—the longer the cable, the more tension in the feed rolls. Are the feed rolls, inlet guide and outlet guide all clean? Incidentally, V groove rolls are used with solid stainless/nickel alloy wire, U groove for copper or aluminum and serrated rolls for flux cored wire. Use minimal pressure on the feed rolls—more is not better. A rule of thumb is to hold the wire between the fingers as it enters the feed rolls. If you can hold it back, there is not enough pressure. Adjust the pressure until you just can not hold the wire, then give it another half turn beyond that. For 0.045 inch (1.14mm) wire, use a 1/16 inch (1.6mm) conduit, instead of a 0.045”/1.14mm conduit. The oversize conduit won’t hurt, and will give more room for the wire to flex. A heavy duty contact tip is preferred instead of a standard contact tip. When spray-arc welding the tip runs hot, and the wire may swell into the tip and jam it. The heavy duty tip simply has more copper, and can handle more heat. Flux Cored Arc Welding FCAW is similar to GMAW except that the wire used is tubular, with flux and metal alloy powders inside. Because this wire contains its own flux, gas shielding may be 75% Argon 25% CO2, even with nickel alloys! The advantage of flux cored wire is that welding is easier than when solid wire is used, and the arc is “softer”. As a result there is greater overall productivity with flux cored wire. Flux cored wire is sensitive to moisture pick-up, and should be left in its sealed plastic bag until ready to use.

12-11 Flux Cored Arc Welding, continued

Shielded Metal Arc Welding Covered welding electrodes consist of an alloy core wire and a flux coating. The core

wire is usually, but not always, about the same composition as the base metal. Often, however, various alloy additions are made in the coating itself, so that the weld bead chemistry will not be the same as the chemistry of the core wire itself. In the case of RA330-80-15 or -16, and RA330-04-15 covered electrodes, a 35%Ni 15%Cr AWS E330 core wire is used. The additional carbon, manganese and chromium required in the weld deposit are added to the flux coating. During welding, these additions melt in and adjust the chemistry of the weld bead to the specified composition. RA333-70-16 electrodes do use RA333 core wire. The electrode coating does four basic jobs: 1. 2. 3. 4.

Provides a gas that shields the metal crossing the arc from oxidation Produces a molten slag which further protects the molten weld bead from oxidation, affects out-of-position weldability, and controls the bead shape Adds more alloying elements, such as manganese, carbon or chromium Promotes electrical conductivity across the arc and helps to stabilize the arc, important when alternating current (AC) is used

12-12 Shielded Metal Arc Welding, continued

There are three types of coatings used on Rolled Alloys electrodes. designated by “-15”, “-16”, or “-17” after the alloy number.

Coating type is

DC lime-type coatings are designated -15. RA330-04-15 and RA330-80-15 both have DC (Direct Current) lime coatings. This means that these electrodes can ONLY be used with direct current. Normally the current is reverse polarity (DCRP, or Electrode Positive). That is, the electrode is positive and the workpiece is the negative electrical pole of the circuit, electrons are emitted from the work and go toward the electrode. If the welder attempts to use a DC electrode with an AC (alternating current) setting on the welding machine, the electrode simply won’t run. He will not be able to keep the arc going. This is very basic knowledge, but every couple of years someone complains that RA330-0415 “won’t run”. Well, it will indeed run on DC current, but not on AC. That is, not unless that AC current is turned up so high that the whole electrode glows red and the coating spalls off. The AC/DC titania coated electrodes are designated -16. RA333-70-16 and RA330-80-16 both have AC/DC coatings. These electrodes may be used with alternating current (AC). They have compounds of potassium and titanium in the coating which stabilize the arc. This means it will not extinguish itself as the current reverses direction (and goes to zero) 60 times a second on normal 60 cycle current (50 cycle in Europe). AC/DC electrodes may also be used with direct current, DC. In fact, they run better when using DC. Weld repair with RA333-70-16 covered electrodes is best accomplished using direct current, reverse polarity (DCRP). The more recent coating designation is -17, which also operates on alternating current, as well as on direct current. RA 253 MA-17 is currently the only electrode we stock with this coating. The slag from the electrode coating is extremely corrosive at elevated temperatures. After welding, all traces of this slag must be removed, prior to using the fabrication at elevated temperature. Otherwise the slag destroys the protective chromium oxide scale on the metal. Under oxidizing conditions this simply results in excessive loss of metal to oxidation. In a carburizing atmosphere small traces of slag will cause local carburization to proceed rapidly. In any reducing atmosphere the fluoride flux will scavenge enough sulphur from the atmosphere5, even a very low-sulphur atmosphere, to cause sulphidation attack of the base metal.

12-13 Gas Tungsten Arc Welding

In GTAW, the arc is struck between the workpiece and a tungsten electrode, which remains unmelted. The argon shielding gas, which protects both the hot tungsten electrode and the molten weld puddle, is brought in through a nozzle or gas cup which is around the electrode. This process used to be called TIG (Tungsten Inert Gas), and was originally patented as Heliarc ®, a name still used occasionally. For both stainless and nickel alloys the current used is DCSP, direct current straight polarity. The work is electrically positive and the tungsten electrode is the negative electrical pole. The electrode is usually thoriated tungsten, that is, tungsten metal with 1 or 2% thorium oxide added to improve the emissivity of electrons. Rare earth oxides are also used. For aluminum welding the electrode is pure tungsten, used with AC (alternating current). Shielding gas must be pure argon or helium. Argon is used for manual welding. A helium addition may be used for automated welding, where a hotter arc is preferred. No oxygen or carbon dioxide can be tolerated or the tungsten electrode would literally burn up. For some corrosion alloys, such as AL-6XN® or RA2205, up to 4% nitrogen is added. This may cause some erosion of the tungsten electrode but improves weld bead properties in these particular alloys. In the case of RA 602 CA, it is necessary to add 2 to 2 ½% nitrogen to the argon shielding. This is to resist hot cracking. The arc between the tungsten electrode and the work is what melts the workpiece. The weld filler metal is fed by hand into the molten puddle. GTAW weld wire for heat & corrosion resistant alloys is sold as 36” (914 mm) straight lengths of bare wire, in 10 pound (4 1/2 kg) tubes. The welder has the most control when using gas tungsten arc, and this process makes the best quality weld, but it is relatively slow. It may be automated for volume production. In automatic GTAW the wire is fed into the joint from a spool of wire, just like GMAW wire. For faster welding speed helium is added to the argon shielding gas. GTAW is often used to make the root pass in pipes or whenever the joint can only be made from one side. The rest of the weld may be built up with either GMAW or SMAW, both of which are faster. Remember--the core wire of RA330-04-15 covered electrodes is AWS ER330, and not RA330-04 chemistry. Welders sometimes knock the coating off an electrode and use the core wire as GTAW filler. Do not do this with RA330-04-15 or the RA330-80 electrodes. This AWS ER330 will make a crack-sensitive weld, without the benefit of the alloying elements which were in the coating.

12-14 Gas Tungsten Arc Welding, continued

Atmospheric contamination, as from strong winds or too long an arc length, is a potential cause of porosity. Look at work to tip distance, shielding gas flow rates, cup size and consider the use of a gas lens. When using a 2—4% nitrogen addition for welding the corrosion alloys, the shielding gas will be just that much more sensitive to atmospheric contamination. Minimize the arc length, no more than 1/4 to 3/8 inch (6-9.5mm). The longer the arc length, the greater the opportunity to entrain air into the shielding gas. Gas cup size depends upon what diameter tungsten electrode is being used. A 3/32” (2.4mm) electrode should use anywhere from a No. 6 to No. 8 cup (9.5-12.7mm cup dia), No. 7 (11mm) being about right. An 1/8 inch (3.2mm) electrode requires a No. 8 (12.7mm) cup. Consider using a gas lens, a wire screen which serves to reduce turbulence of the shielding gas flow. It is this turbulence which causes air to get mixed in with the argon shielding gas. Gas Metal Arc (MIG)

Gas Tungsten Arc (TIG)

Plasma Arc Welding The plasma arc torch is roughly analogous to a GTAW torch. It generates intense heat in a very narrow zone, and has been used to weld RA330 without added filler (with GTAW this would be extremely difficult). PAW is an excellent welding process for heat resisting alloys.

12-15 Submerged Arc Welding

Submerged arc uses a spool of weld wire, much like GMAW. Instead of shielding gas, a hopper feeds granulated flux into the arc to shield the arc and molten weld puddle. While it is possible to use 0.045” (1.14 mm) dia. Wire, larger sizes such as 1/16 or 3/32” (1.6 or 2.4 mm) are generally preferred. For nickel alloys such as RA330 a strongly basic flux must be used, such as Avesta Flux 805 or Böhler-Thyssen’s RECORD NiCrW. Absolutely do not use acid fluxes or any flux meant for stainless steel. Heat input must be as low as possible. For this reason 1/8” (3.2 mm) wire is not suggested for submerged arc welding the nickel heat resistant alloys.

SAW is a process naturally inclined to high heat input, but this heat must be kept to a minimum to avoid centerbead cracking in fully austenitic alloys.

Resistance Welding6 Spot and seam welding parameters for heat resistant alloys will differ from those used with stainlesses such as 304L or 316L, and markedly from those used for carbon steel. Heat resistance alloys may have twice the yield strength of stainless and considerably higher electrical resistivity. Electrode force, welding current and time, and electrode tip contours may all need to be modified accordingly.

12-16 Resistance Welding, continued

A restricted-dome electrode is suggested for spot welding. Average dome radius may be 3 inch (76 mm) for material up to 11 gage (3mm). For a larger nugget size in material 16 to 11 gage (1.6 to 3mm) a 5 to 8 inch (127 to 203mm) radius dome is sometimes preferred.

In seam welding heat time should be adjusted to ensure that the wheel maintains pressure until the weld nugget has solidified, to avoid porosity and cracking. Likewise cool time should be sufficient that welded areas are not remelted. The metal must be clean and free of all grease, or a sound weld cannot be made. References 1. Thaddeus B. Massalski, Editor-in-Chief, Binary Alloy Phase Diagrams, Volume 1, ISBN 0-87170-262American Society for Metals, Metals Park, Ohio, U.S.A., 1986 2. Avesta handbook for the welding of stainless steel, Inf. 8901, Avesta Welding AB, S-74401 Avesta, Sweden 1989 3. Berthold Lundqvist, SANDVIK Welding Handbook, Sandvik publication 0,34 E, Sandvik AB, Sandviken, Sweden June, 1977 4. Specification for Nickel and Nickel-Alloy Bare Welding Electrodes and Rods, ANSI/AWS A5.14/A5.14M-97, ISBN 0-87171-543-0, American Welding Society, Miami, Florida, U.S.A. 5.

G. R. Pease, Corrosion of Nickel-Chromium-Iron Alloys by Welding Slags, Welding Journal Research Supplement, September, 1956

6.

Resistance Welding Manual, 4 Edition, ISBN 0-09624382-0-0, Resistance Welder Manufacturers’ Association, 1900 Arch Street, Philadelphia, Pennsylvania 19103 U.S.A. 1989

th

The best general reference we know for welding this class of materials is: R. J. Castro & J.J. de Cadenet, Welding Metallurgy of Stainless and Heat-resisting Steels, ISBN 0 521 20431 3, Cambridge University Press, 1975. First published, in French, as: Métallurgie du soudage des aciers inoxydables et 127esistant à chaud, by Dunod, Paris, 1968.

12-17 Suggested Weld Fillers

Base Metal bare wire

Preferred covered electrodes

Alternates

RA330®

RA330-04 --

RA330-04-15 RA330-80-15

RA333® , RA82 RA333-70-16

RA333

RA333

RA333-70-16

ERNiCrWMo-1

RA 602 CATM

S 6025 6225 Al (SG-, EL-NiCr25FeAlY)

RA601

RA333 601

RA333-70-16 6225 Al

RA600

82

182

RA 353 MA®

RA 353 MA

RA 353 MA

RA 253 MA®

RA 253 MA

RA 253 MA-17

RA333, RA333-70-16

RA800H/AT

RA333 556

RA333-70-16 --

ERNiCrCoMo-1 RA330-04, ENiCrFe-2

RA309

ER309

E309-16

RA330-04*

RA310

ER310

E310-15

RA330-04*

RA446

ER309 ER310

E309-16 E310-15

E312-16

HK, HT, HU

RA330-80-15 DC lime is the preferred 35% nickel rod for cast heat resistant alloys. Alternates RA333-70-16, RA330-04-15

ERNiCrCoMo-1 (lacks oxidation resistance) -RA330-04 --

General: Do choose the weld filler for its performance under the expected service conditions, as well as for weldability issues. Do not use—any stainless weld filler on nickel alloys (e.g., RA330, RA333, RA600, RA601, RA 353 MA, RA800H/AT). Dilution by nickel will eliminate ferrite, and the welds will crack. It is better not to use alloy X (ERNiCrMo-2, ENiCrMo-2) weld fillers on RA333 base metal. The X weld bead may be subject to catastrophic oxidation at the higher service temperatures where RA333 is commonly used. *Where sulphidation is an issue, do not use high nickel fillers such as RA330-04 12-18

Dissimilar Metal Joints, Weld Filler Guidelines Considerations in selecting a filler metal for a dissimilar metal weld joint include the expected service conditions at the joint, relative thermal expansion coefficients of the three metals involved, and freedom from weld metal hot cracking. The final selection should be approved by the end user and weld procedures qualified by the fabricator. Base Metals

Carbon Steel

Stainless (304,316)

RA 253 MA RA 602 CA

RA330 182 RA800H/AT RA333

RA330-04

RA333

617A RA333B

RA330-80-15 RA330-04

RA333

RA330-04 RA333

RA333 RA330-04

617A RA333B

RA333-70-16

RA 353 MA 182 RA 353 MA RA 353 MA RA 353 MA

617A

RA 353 MA RA330-80-15

RA 602 CA 82 182

S 6025 6225 Al

617

RA 253 MA E309-16 RA 253 MA RA 253 MA RA333

617 RA333B

RA333-70-16

RA600

82 182

82 182

RA333 RA333-70-16

82 182

RA333-70-16 RA330-80-15

RA601

82 182

82 182

RA333 RA333-70-16

S 6025 6225 Al

RA333-70-16 RA330-80-15

RA309

E309-16 E309-16 182 ER309

E309-16 RA 253 MA

82C 182C

RA330-80-15

RA310

E309-16 E309-16 182 E310-15

RA 253 MA --

82C 182C

RA330-80-15 RA333-70-16

RA446

E309-16 E309-16 E310-15 E310-15

E309-16 ER309

82C 182C

RA333-70-16 --

182 RA333

82 182

617 RA333*

Cast Alloys HK, HT, HP

Note: The carbon steel joint must be ground to bright metal. A “mill finish” is not acceptable. All rust, blue-black hot rolling scale and paint must be removed before welding with any stainless or nickel alloy weld wires. These alloys lack the deoxidation characteristics of carbon steel weld wires. A 617 (ERNiCrCoMo-1) lacks the oxidation resistance of RA 602 CA B The weldability of RA333 weld filler used on RA 602 CA has not yet been determined C These high nickel fillers are not suggested for sulfur bearing environments.

12-19

12-20

BRAZING and SOLDERING Heat resistant alloys are normally assembled by welding. Brazing is used on occasion to attach cooling coils or thermocouples. The age hardening aerospace grades, by contrast, are commonly joined with nickel-silicon-boron braze fillers. SOLDERING Copper cooling coils may be lead-tin soldered to heat resistant alloys. Somewhat better strength may be obtained by using a tin base solder. These may be alloyed with about 2% of either silver or antimony. The acid chloride flux is corrosive and should be washed off after soldering. BRAZING One of the issues in brazing Ni-Cr or Ni-Cr-Fe alloys is the furnace atmosphere. This atmosphere must prevent formation of any oxide film which would prevent the braze alloy from flowing. To be effective with stainless, the incoming hydrogen atmosphere should have a dew point1 –80°F ( —60°C) or lower. Aluminum Brazing From the standpoint of the heat resistant alloy supplier, the major issue in aluminum brazing is the flux. Temperatures are low enough that aluminum braze muffles are commonly made of 304 or 316L stainless. If too much flux is applied to the aluminum work pieces, that flux may spill onto the muffle. Anything that will flux aluminum oxide will quickly eat holes through stainless. Technically speaking, a high nickel alloy, such as 600, has somewhat better resistance to the fluoride flux. But the nickel alloy is unlikely to last long enough to be worth the higher cost. Keep the flux off of the alloy fixturing. Silver Brazing Often used to join carbon and low alloy steels. Austenitic alloys are prone to crack when silver brazed, from liquid metal embrittlement. Residual stresses are responsible for cracking during furnace brazing, as the braze temperature is not high enough to reduce these stresses. Stressed austenitic alloy, whether stainless or high nickel, in the presence of molten silver braze alloy will crack. In torch brazing the source of stress is the thermal stress caused by the local heating (which is normal practice when brazing steel). To furnace silver braze an austenitic alloy, consider stress relief annealing the assembly prior to brazing.

12-21

Silver Brazing, continued The lower melting silver braze alloys may require the use of flux when atmosphere brazing stainless. Dry hydrogen may not be sufficiently reducing to chromium oxide at brazing temperatures below 1800°F (980°C). Vacuum brazing requires fillers containing neither cadmium nor zinc, which would vaporize. Successful torch silver brazing of austenitic stainless depends upon technique. The metal should be heated uniformly in the area to be brazed. One approach that has been described to us is to play the torch back and forth about 6 inches (150mm) on each side of the area to be brazed. This minimizes thermal stress where the molten silver braze will contact the austenitic alloy. If there is no stress on the stainless, it will not crack. Silver braze cracking is not an issue with ferritic stainless steel. It is the austenitic structure that is sensitive to intergranular cracking by molten braze alloy. Copper Brazing Copper brazing is a common means of joining carbon steel assemblies. It is not usually chosen to join either heat or corrosion resistant alloys. The process temperature for copper brazing is usually 2050°F (1120°C). Pure copper itself melts at 1981°F (1083°C). These temperatures will quickly anneal out residual stresses from the stainless or nickel alloy parts. Even though copper, given time enough, will penetrate austenitic alloys intergranularly it is unlikely to either crack or seriously attack the metal during the brazing cycle. Nickel Brazing Nickel-base braze alloys are used to join age-hardening nickel base alloys for aerospace applications. Addition of as much as 10% silicon, in some alloys including up to 3.5% boron, greatly lowers the melting point. With AMS 4777 the brazing range is 1850—2150°F (1010— 1180°C). During the braze operation boron from the filler diffuses into the base metal, raising the remelt temperature of the braze alloy. Boron can react with nitrogen, in the alloy as well as the atmosphere, preventing braze flow. One indication of nitrogen as the brazing problem is an Iridescent bluish-gray color to the base metal. Alloys containing more than about 0.03% nitrogen2 can be difficult to braze, without special treatment. Such treatment might include about 0.001 inch (0.025mm) of nickel electroplate. This may be coupled with rapid heating and short process time to prevent diffusion of nitrogen through the nickel plate.

12-22

Nickel Brazing, continued Alloys containing aluminum and titanium require first to be electroplated with nickel. Otherwise, oxides of Al and Ti will form, even in the best of atmospheres, that prevent braze flow. For vacuum brazing 0.001” (0.025mm) is sufficient. Hydrogen brazing requires a slightly thicker plate, 0.001 to 0.0015” (0.025 to 0.038mm). The plating does need to be an electroplate and not electroless nickel. Electroless nickel contains phosphorus, enough so to depress the melting point to 1610°F (877°C)3. The braze temperature should not exceed the solution annealing temperature for the alloy in question. Effects on Furnace Equipment Excess silver or copper braze alloys dripping onto the nickel alloy muffle or fixturing will attack that alloy intergranularly (see Copper, 3-37—39). Nickel base braze alloys simply lower the alloy melting point, enough spilled braze can melt a hole through the muffle. An oxide coating on the alloy helps minimize this effect, but of course will not be present in vacuum brazing.. Likewise spilled flux is corrosive to the fixturing. Further Information Detailed insight into all manner of brazing issues is available from the Monthly Column “Brazing Q&A”, by R. L. Peaslee, in Welding Journal, American Welding Society, Maimi, Florida U.S.A. This column began in 1989 and continues as of this writing, 2002. This is far and above the best source for thorough, in-depth discussion of brazing problems. Brazing Q&A is authored by Dr. Robert L. Peaslee, of Wall Colmonoy. Dr. Peaslee invented the nickel base brazing alloys about 50 years ago. Reference 1. The absolute best source for thorough, in-depth discussion of brazing matters is: Brazing Footprints, Case Studies in High-Temperature Brazing, Robert L. Peaslee. ©2003 Wall Colmonoy Corporation, Madison Heights, Michigan [email protected] . Dr. Peaslee invented the nickel base brazing alloys about 50 years ago. There will not be another man so knowledgeable in this field during our lifetimes.

2. Brazing of Stainless Steels, in ASM Handbook Volume 6, Welding, Brazing and Soldering, ASM International, 1993 3. Brazing Q&A, September 2001 Welding Journal 4. Brazing Q&A, , January 1991 Welding Journal 12-23

APPLICATIONS Muffles Alloy Selection depends upon temperature and atmosphere. For brazing muffles the atmosphere is either reducing (cracked ammonia, H2, or N2) or slightly oxidizing to the heat resistant alloy used (endothermic). In powdered iron sintering products from binding agents tend to carburize the muffle alloy. RA 353 MA—the best choice for copper brazing and for annealing stainless 1950-2100°F (1070-1150°C). Experience to date has been that muffles of 3/16—1/4” (5—6mm) RA 353 MA plate typically outlast those of 601 by a factor of two. This is for muffles fabricated in the same shop, same design, used in the same service. One 601 muffle regularly replaced every 6 months now last 2—3 years made of RA 353 MA. A Singapore company with 3 or 4 furnaces, sintering powdered iron 2100—2150F had been getting 8—10 month life with 601 muffles. Then they tried their first 1/4” wall RA 353 MA muffle. Over a four-year period the Americans shipped them only one new muffle, probably for use as a spare. RA 602 CA—For longer life in brazing muffles operating over 2150°F (1180°C). Greater strength, oxidation and carburization resistance than does 601. Because of its high strength and oxidation resistance, 11 gage (3mm) wall is appropriate with RA 602 CA brazing muffles. RA333—at one time the traditional first choice for copper brazing muffles. RA333 is strong enough to be used in 11 gage (3 mm) wall, for maximum heat transfer. Some 3/16” (4.8 mm) RA333 has been used. RA601—oxidation resistant, not quite as strong as either RA 602 CA or RA333 above 2000°F (1100°C). To minimize leaks in weld seams consider welding with RA 602 CA or RA333 or weld fillers, rather than the commonly used 82. RA600—used for very high temperature iron sintering in strongly reducing or carburizing atmospheres. Technically performs well, but RA330 has proven more cost-effective here. RA600 is not suited for sulfur bearing environments. RA 253 MA—used for hydrogen and/or nitrogen atmospheres, not suitable for carburizing environments. Large muffles used in the production of iron powder itself are of RA 253 MA. May also suit for the bottoms of brazing muffles, being somewhat more tolerant of spilled copper than are the higher nickel heat resistant alloys. RA330—the most widespread choice for muffles up into the 2100°F (1150°C) and over range. RA330 muffles are most commonly 3/16 to 1/4” (4.8 to 6.35 mm) thick. Used for applications from copper brazing to powdered iron sintering.

13-1

RA330 “D” muffle for copper brazing, lined with 430 for protection against copper spills.

RA309—bright annealing, neutral hardening or sintering bronze powder, under 1800°F (980°C). Not for a carburizing atmosphere. Can be used for carbon fiber production when sulfur may be a problem. Muffle of 3/16” (4.8 mm) RA309 plate. Gas fired 12001600°F (650-870°C), endothermic atmosphere, bright annealing copper, brass and steel. Typical life 4-5 years. This is an old muffle design. Today we would suggest RA 253 MA for greater strength, perhaps in 11gage (3mm) for better heat transfer.

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Muffles, continued Muffle Installation and Operation Muffles are normally fixed at one end, but they absolutely must be free to expand in the other direction. Some shops weld U-bolts on the flange of the free end and run a chain over a pulley to some dead weights, or apply tension by some other means. A rule of thumb is to pull on the muffle with about half of its own weight, to aid the lengthening caused by thermal expansion. Replace the thermocouple at least quarterly. A type K thermocouple can drift 25°F (15°C) in 4-6 weeks. Type S, platinum, are preferred. For electrically heated muffles, silicon carbide elements top and bottom are the least problem. The key is to leave them on—it is cheaper to pay the electricity 7 days/week than to replace a muffle that has been periodically shut down to “save” money. Ribbon or rod elements may sag and create a hot spot. Spilled copper braze alloy, or powder from bronze sintering operations can attack the muffle bottom. Endothermic atmospheres, reducing to iron but actually oxidizing to chromium leave a scale on the muffle inside which provides some protection against this. Good, dry hydrogen or hydrogen-nitrogen mixtures leave the muffle inside bright. This clean surface is quite susceptible to attack by a small amount of copper alloy. Copper goes right through the grain boundaries. Once a hole forms, the leaking atmosphere burns. This overheats the surrounding area and often burns away any obvious traces of the cause. Silicon Carbide Silicon carbide, or more accurately silicon-silicon carbide composite, refractory is used for hearth plates, as well as for entire muffle sections. One form of this is “reaction bonded silicon carbide”. It is made by infiltrating compacts, made of a mixture of SiC and carbon, with liquid metallic silicon. Bonding agents such as borates may be added. The formed compact is then sintered 3630°F (2000°C) or higher. Silicon metal reacts with the carbon to form SiC, and the reaction product bonds the compact together. If the compact is sintered in air, it will have a surface layer of silica, SiO2 on it. Silicon itself, more properly called a “semi-metal”, has a melting point of 2577°F (1414°C). This refractory has long been known to react with nickel alloys, most commonly with the 75% nickel alloy 600. This is primarily because silicon and nickel form an eutectic which melts at 1767°F (964°C). Silicon carbide hearth plates can form a eutectic with nickel alloys, melting a hole right through the muffle. More common with 600 alloy muffles, it also happens with RA330. With RA330, our experience has been that attack is erratic, one hearth plate may attack the metal and the next not. 13-3

Silicon Carbide, continued The only known way to prevent this attack is to keep the silicon carbide from contacting the muffle. One approach is to coat either the silicon carbide hearth plate or the muffle itself with alumina castable. The castable might be plastered on, or it might be thinned and applied like heavy paint. One refractory supplier suggests that the alumina should be applied to the silicon carbide, not to the metal muffle. One may also separate the Si-SiC plate from the muffle with alumina or zirconia inert refractory cloth or board.

Holes in the bottom of an RA330 muffle sintering powdered iron. In this case only one hearth plate attacked the muffle. These holes are from the Si-SiC hearth plate. They begin on the outside surface, and are not attack from any contaminate inside the muffle (dark streaks in this photo are water marks).

13-4

Retorts A retorts is simply a large cylinder, closed at one end. The other end rests on a base, which may incorporate a sand seal, or may be water cooled for an organic seal ring. They are used for annealing, carburizing, brazing and coating operations. The wide range of materials used includes RA309, RA310, RA 253 MA, RA330, RA601, X, 230, RA333, and RA 602 CA.

30” (762mm) diameter coating retort for the aluminide coating of gas turbine blades. RA 602 CA replaced N06230 in this application. Operating temperature 1975°F (1080°C). At this temperature RA 602 CA has greater rupture strength and superior oxidation resistance.

Both of these retorts use a water-cooled base, so that the atmosphere may be retained using an organic seal ring. Most of the retort operates at very high temperature. Roughly the bottom 1—1-1/2 foot (300— 450mm) experiences a sharp thermal gradient between the process temperature and the 212F (100C) maximum temperature of the water cooled seal. The invariable result is permanent distortion from this thermal strain, which manifests itself as a circumferential bulge. The early beginnings of this may be seen on the bottom retort. What in this photo is just a different colored band, near the bottom of the retort, is the beginning of such a bulge..

An RA 353 MA® retort is used for diffused aluminide coating of turbine blades. Process temperature is 1680 to 1960°F (915 to 1070°C), depending upon the blade to be coated. As the retort is externally fired, the retort wall may be expected to operate about 100°F (50-60°C) above process temperature.

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Retorts, continued It is a good idea to choose a weld filler with properties comparable to the retort base metal. This 601 retort was used around 2200°F (1200°C). Although 601 is very oxidation resistant, the weld filler used here is not. The grooves in the retort are weld beads of filler metal 82 (ERNiCr-3). A ductile weld filler, with very good carburization resistance, but inadequate oxidation resistance above 1800°F (982°C). Use of RA 602 CA weld wire, rather than 82, could eliminate this condition.

Bar Frame Baskets Baskets having a frame of nickel alloy bars 3/8—5/8” dia, with woven wire mesh liner, are used to carry work into the heat treat furnace. Most wrought alloy used for conventional atmosphere heat treat is RA330. Because this service includes repeated liquid quenching from red heat, a fine grain size is preferred to resist thermal fatigue damage is as important as carburization. The most important alloy attribute for this service is a fine grain size, ASTM 5 or finer. Fully penetrated weldments are necessary. Usually this is achieved by pressure welding (cross-wire resistance welding) the frame members. In vacuum heat treating of tool and stainless steels, temperatures are high, often above 1900°F (1040°C). Strength is important to minimize fixture weight. Alloys such as RA 353 MA and RA 602 CA are among the strongest at such temperatures. Alloy selection for vacuum hardening and carburizing is still in a state of flux, and may include RA330, 600, 601, HR-120, 230 and RA 602 CA. Initial grain size is of somewhat less consequence as temperatures are high enough that most alloys will grain coarsen in service anyway. In vacuum carburizing the oxygen potential is too low to form protective chromium or silicon oxides. Most carburization resistant alloys, e.g. RA330, 600 and 601, carburize badly in low pressure carburizing service. Aluminum is the only common alloying element capable of forming an oxide scale under these conditions. 13-6

Bar frame baskets, continued

RA 602 CA baskets for hardening high speed steel drill bits. Bits are made of M2 and M-50 tool steels, vacuum treated at 2050-2150°F. A typical cycle is 20-45 minutes, followed by a 2 bar nitrogen quench, double tempered 1050°F. The maximum permissable load in this vacuum furnace is 600 pounds, including both weight of drill bits and alloy fixturing. Baskets of 3/8” dia RA 602 CA gave a 25 lb. (11kg) weight reduction over the previously used 1/2” 600 alloy. This permitted a 15% increase in payload. This photo shows the loaded baskets hanging from a scale.

Tests were run exposing plate coupons of various alloys in a low pressure carburizing furnace between 1650°F and 1900°F (900—1040°C) for 300 hours of boost time. Results in this particular furnace favor RA 602 CA. Alloy

Case Depth inch mm

RA 602 CA RA333 RA601 RA 353 MA RA330 RA600*

0.016 0.035 0.043 0.056 0.068 0.112

0.40 0.90 1.1 1.4 1.7 2.8

Nominal Chemistry Ni 63 35 61.5 35 35 76

Cr 25 25 22.5 25 19 15.5

Si -1 0.2 1.2 1.2 0.2

Al 2.2 -1.4 --0.2

*The deep case on RA600 should not be interpreted to mean that this alloy is more embrittled than the others. 600 alloy does tend to form a deep case of moderate carbon content, and retain a greater amount of ductility than do lower nickel grades.

In this test series RA 602 CA had less than half the case depth of the other alloys. Tests run in other furnaces, with different partial pressure of oxygen, may give different results. 13-7

Radiant Tubes Fabricated radiant tubes of 11 gauge (3mm) alloy offer several advantages over heavy wall cast tubes. Foremost, the thinner wall can mean 8-12% energy savings because of better heat transfer. Likewise furnace cycle time is shortened. Lighter tubes can often be changed in less time, maximizing furnace up-time. Reliable performance requires full penetration welds, both the seam weld and the weld of return bend to straight leg. Weld fillers should be chosen that match the strength and oxidation resistance of the tube metal. Appropriate bare welding wires include RA330-04, RA333 and RA 602 CA, all available as both GTAW and GMAW. For GTAW only, 601 (ERNiCrFe-11) has excellent oxidation resistance. Covered electrodes include RA330-04-15, RA330-80-15, RA333-70-16, and RA 602 CA. We suggest avoiding filler metal 82 (ERNiCr-3). Welds made using filler metal 82 (ERNiCr-3) are low in strength, not well suited for radiant tube service. Alloy 82 has oxidation resistance inferior to any commonly used base metal, excepting 309, above 1800°F (980°C). Tube Design & Installation Tube life is commonly limited by collapse of the firing leg, and sagging or cracking at intermediate supports. Firing legs may collapse because they are restrained from expanding by the cooler, stronger exhaust leg. This may be addressed either by a bellows on one or the other leg, or by permitting the exhaust leg to move through packing, rather than welding it securely. Intermediate supports essentially block heat flow out of the tube, and cause a local hot spot. Being weakened here, the tube deforms over the support. The local thermal gradient from the hot spot may also be a problem. Some heat treaters are considering moving away from U-tubes in their first zone to singleended tubes, in double the number. This is in order to double their heat input to the first zone. U-tubes One of the most common tube designs, U-tubes consist of two straight legs, not necessarily of the same diameter or wall thickness, welded to a semi-circular return bend. This weld of straight leg to return bend must be full penetration. Breaks at this weld due to lack of weld penetration are a common failure mode for both wrought and cast tubes. Many, if not most, U-tubes are supported by a “horn”, or chunk of pipe, welded to the return bend. 13-8

Radiant tubes, continued When the furnace has top and bottom U-tubes, with top supported by a horn and bottom just resting on a refractory ledge, failures occur first in the bottom tubes. This may be caused by the hot spot where the bottom tubes are supported. Return bends may collapse. Operating conditions, such as a long flame, may be responsible. Nevertheless, the tube producer must deal with such conditions. Common approaches are to make the bend one gauge thicker, or of a stronger alloy. For an 11 gauge (3mm) RA330 tube it might be appropriate to make the return bend either of 11 gauge RA333, or of 10 gauge (3.4mm) RA330. For 309 radiant tubes a convenient strength (and oxidation) upgrade for the return bends is RA 253 MA. This alloy has comparable thermal expansion coefficient, and is readily welded to 309. In the case of 601 radiant tubes we would suggest upgrading to RA 602 CA® for return bends or firing legs, as-needed. RA 602 CA has considerably greater creep-rupture strength, and about twice the oxidation resistance of 601. RA 602 CA weld fillers, including covered electrodes, are compatible with 601 base metal. Early promotion of RA 253 MA included U-tubes such as these below. This furnace is typically used for annealing steel at 1800°F (980°C) in a nitrogen atmosphere enriched with propylene. The tubes were fabricated with a 6 inch OD x 11ga. wall ( 152mm x 3mm) firing leg and a 5 inch OD x 11ga wall (127mm x 3mm) exhaust leg.

This is a 17-year old photo of the RA 253 MA tubes after 10 months in service. After several years the tubes carburized, and were replaced with RA330. In 2004 this same furnace was still in operation, but with RA330 tubes. Over the years our experience has been that RA 253 MA is inclined to carburize in heat-treat service.

With good maintenance and appropriate alloy, fabricated tubes have given 6 to 10 years service in carburizing furnaces. The RA333® U-tube on the next page was in a General Motors plant in Northern Ohio. At another location, vertical RA333 tubes have given 10 years life in a pit furnace deep case carburizing, high carbon potential.

13-9

Radiant tubes, continued

Fabricated 11 gage RA333 radiant tubes have given 8 to 10 years life in carburizing furnaces. This particular tube failed from local overheating after some 8-1/2 years’ service.

Most distortion in radiant tubes occurs in the firing leg. As this is the hotter leg, the metal is weaker. Being hotter, the firing leg also expands more than the cooler (and stronger) return leg. When both legs are firmly affixed to the furnace wall, this differential expansion contributes to firing leg collapse. This effect can be minimized by use of a bellows, permitting the legs to expand at different rates.

These 309 U-tubes are used in an aluminum mill. Firing legs are 3/16” wall, exhaust legs 11 gauge (0.120”). Here 321 stainless bellows are used on the exhaust legs to reduce distortion from differential expansion of the two legs. Similar bellows are used on both cast and fabricated tubes. The carbon steel box section filled with fibrous refractory is referred to as the bung.

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Radiant tubes, continued Metal dusting of the firing leg is a problem in some carburizing furnaces. This RA330 Utube metal dusted in the firing leg. One solution is to make just this portion of the firing leg of a metal-dusting resistant alloy. The traditional choice has been RA333.

The metal dusting/carbon rot shown above is being addressed by fabricated RA330 U-tubes with 2 feet (610mm) of RA333 on the firing leg, at top. The 250 pound (113kg) fabrications replace 500 pound (226kg) 6-5/8” dia 1/4” wall (168mm dia 6.35mm wall) cast tubes.

In Trident® tubes thermal expansion of the two firing legs is restrained by the central exhaust leg, which runs cooler. This collapses the firing legs. A bellows on the exhaust leg can reduce such deformation and increase useful life. Likewise permitting the exhaust leg to expand through packing, rather than welding it solidly to the bung, can increase life.

Cast Tridents are said to crack here >>>

Trident radiant tube formed out of 11ga RA 602 CA sheet. A set of these fabricated tubes replaced HT cast tubes in 1650°F (900°C) carburizing service. The lighter RA 602 CA tubes are preferred over castings, and may be changed in less time than a casting.

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Radiant tubes, continued Burner Matters One burner manufacturer, during a discussion of radiant tubes, brought to our attention the tube life advantage of recuperative burners. As an example, for a 1750°F (954°C) furnace operating temperature, heat transfer through tube wall commonly averages 35-50 Btu/inch2•°F. However the heat profile is such that heat transfer peaks at 130 Btu/in2•°F in large W-tubes, with an average of 50 Btu/in2 •°F. This is one reason that radiant tubes burn out on the firing leg. The burner manufacturer’s view was that with a recuperative burner they even out the heat transfer, reducing peak temperature & extending tube life. Tube life matters greatly to mills with large strip anneal lines, producing some $300,000 of profit/day. Downtime is costly! A Midwestern captive shop had serious failures in return bends due to inadequate combustion air, which caused a long flame to overheat the return bends. Some failures were by cracking in the base metal of the return bends, cracks running in the direction of the bend. Here the problem was that the combustion air blower filter clogged, which meant the flame was rich in gas, giving the long flame that burnt out the return bends (conversely, with too much air the flame would be concentrated right at the burner, & tend to burn out the tube in this location). ROTARY KILNS & CALCINERS Alloy selection covers the range of available high temperature grades—RA309, RA 253 MA, RA330, RA601, RA 602 CA, RA 353 MA and RA333. The design and attachment of flites inside is very important. Best success has been to weld only in the cooler zones. The flight will operate cooler than the shell in an externally fired retort or calciner. This means they will not only expand less, but they will be stronger than the shell alloy. For these reasons the flights must be free to move or they will indeed crack the hotter, weaker shell from effects of differential thermal expansion. When such cracking occurs, it may be mistaken for fatigue or rupture failure. High temperature design of static members is often based on creep or rupture strength. A common approach is to set design stress at one half of the 10,000 hour rupture strength. Or, one half of the stress for 1% in 10,000 hours minimum creep rate. The concern is with the part sagging, or breaking. Rotating equipment, so long as it is kept turning while hot, alternately sags one way, then the other. Bowing caused by a heavy load is not a common problem. For this reason, rotating equipment is usually designed to a operating stress than is a static part of comparable alloy and operating temperature. 13-12

Rotary Kilns & Calciners, continued One might prefer to design on the basis of fatigue, as one would at room temperature for any rotating member. However useful high temperature fatigue data is rarely available. As a result, experienced rotating kiln designers use some proprietary mix of hot tensile and creeprupture data. This matter is discussed by ThyssenKrupp VDM in their VDM REPORT No. 25. Just to illustrate, the 1,000,000 cycle fatigue strength of alloy 602 CA at 0.1Hz (6 rpm) is about 4350 psi (30 N/mm2) at 2012°F (1100°C), while the 10,000 hour rupture strength at this temperature is only 640 psi (4.4 N/mm2). Hot tensile strength at this temperature is 11,400 psi (79 N/mm2) These particular numbers are not directly useful in design. For one thing, kilns rotate many more cycles over their lifetime, and more commonly at fewer revolutions per minute. Many shell cracks are assoicated with flite attachment. There have been circumferential cracks associated with thermal strains from burner set-up. We see cracks in, and adjacent to, circumferential welds when the alloy used embrittles from sigma at the temperature of that weld location. We have yet to see a shell crack that in our opinion was from mechanical loading alone. The design and attachment of flites inside is important. Best success has been to weld only in the cooler zones. The flites will operate cooler than the shell in an externally fired unit. Not only will the flites expand less, but they will be stronger than the shell alloy. For these reasons, the flights must be free to move or they will crack the hotter, weaker shell from effects of differential thermal expansion. When such cracking occurs, it may be mistaken for fatigue or rupture failure. RA333 rotary calciner about 35 ft long, 5 ft diameter (10.7 m long, 1.5m dia). It processes kaolin clay to zeolite catalyst for refineries Initially RA330, increased duty cycle and higher temperatures necessitated a stronger alloy. Flites were originally welded solidly to the shell. This caused transverse cracks in RA330 units. The maintenance engineer solved the shell cracking by a redesign. The flites now consist of long bars held in cages at either end of the kiln. They are no longer welded to the shell in the hot zone.

13-13

Rotary Kilns & Calciners, continued Attention to flite attachment is important in all rotary kilns.

®

This kiln shell was fabricated of 3/8” (9.5mm) RA330 plate. It was used for soil remediation, by incinerating hydrocarbons at about 1200°F (600°C). After two years numerous branching cracks, approximately 2” (50mm) long, developed in the shell. The flites had been welded solidly to the kiln shell, all along their entire 2-3 foot (600900mm) length. Because these flites are cooled by the product inside of the eternally fired kiln, they do not thermally expand as much as does the adjacent shell. Being cooler, the flites are also stronger than the shell metal. The resulting thermal strain is what cracks the shell. A repair to this condition might be to cut slots in the existing flites, about every 8 inches (200mm) or so to reduce the thermal strains. Then air-arc out the cracks and reweld them.

13-14

Cast Link Furnace Belts Flexible belts made of cast links pinned together are used to carry heavy loads through continuous furnaces. Links are cast of alloys ranging from HT to Supertherm. Pins must be of an alloy with sufficient high temperature shear strength to avoid “crankshafting”, as well as resistance to carburization and some degree of toughness. The two basic types of cast link belts are pin-bearing, and interlocking. With a pin-bearing link the belt pin is heavily loaded in shear. If the pin is not strong enough it “crankshafts”, that is, deforms until it looks like an automobile engine’s crankshaft3.

crankshafted pin, Inadequate anneal during manufacture Belt pins for this service have largely been RA330HC for several decades, usually with cast HT links. Where strength but not carburization resistance is needed, alloys X and HR-120® pins are also used. Alloy 120 pins occasionally break after long service at around 16001700°F (870-930°C) or so. X grade may not be as consistent, due to a wider range of grain size as produced. For service 1800°F (980°C) or above, RA 602 CA is more appropriate. Interlocking links are designed to take up the shear loading on the casting itself. As pin stresses are low, both RA330 round bar and RA330 hexagonal bar have been used to pin interlocking link belts.

These interlocking links are from a belt misalignment failure

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Cast Link Furnace Belts, continued For more than 30 years the majority of belt pin failures which we have been called on to examine have been caused by belt misalignment, which fatigues the pin, or occasionally by carbon and/or salt deposits which freeze the belt solid. The following paragraphs are based largely on discussions with Omega Castings, Inc., Battle Creek, Michigan. Contamination by sodium or potassium salts from parts washing operations can cause formation of bulky oxide that shortens the life of a belt by reducing freedom of movement between pin and link. Presence of sodium contamination is indicated by a bright greenishyellow showing, once the black scale has been scraped away. We understand that another indication is, in a carburizing furnace, the presence of black “cobwebs” hanging off the radiant tubes, coupled with brickwork which is white, and not black as might be expected . When parts are washed in ionized detergents containing sodium, or in sodium hydroxide, borate or phosphate, those parts must be rinsed in water and then dried before running through the furnace. Otherwise sodium chromate will form at high temperatures. This causes a reaction which will oxidize, and continue to oxidize, the alloy components, until the volume of oxide simply freezes up the belt. For maximum belt life the furnace must be designed to minimize stress on the belt. Suggestions include the use of return rolls, rather than skid tiles, and hearth rolls sized to move at the same surface speed as the belt. Hearth rolls must be level and parallel (perpendicular to the belt travel direction). Neutral Salt Pots1 The most common industrial use of molten salts is to heat treat steel. Metallic salt pots used to contain neutral heat treating salts may last anywhere from 2 days to 18 months, depending upon maintenance and operating procedures. Alloy selection does matter somewhat. But alloy choice is outweighed in importance perhaps 50 to 1 by how the pot is maintained. The following points are important for good life in a metallic pot for neutral salt heat treating: 1.) Ensure that there is no salt whatsoever in the combustion chamber of a gas-fired pot or about the elements of an electrically heated pot. This is crucial. 2.) Rectify and desludge neutral chloride salts at least daily. 3.) Idle the pot with salt still molten, rather than shutting down completely and letting the salt freeze solid. 13-16

Salt Pots, continued 4.) Do not put oily work or any foreign matter (no floor sweepings!) into the pot. 5.) In both pots and fixtures, all welded joints must be full penetration welds. Only after these five points have been addressed should alloy selection be reviewed. Let us examine the reasons behind some of these points. Mixtures of potassium, sodium and barium chlorides are widely used as heating media that neither oxidize nor decarburize carbon, engineering alloy and tool steels. Regarded as “neutral” salts, they are actually quite oxidizing to the chromium in the Ni-Cr-Fe alloys used for pots and fixtures. While the chloride salts themselves are indeed neutral, the inevitable oxygen content of the bath is quite destructive. Oxygen is present because the surface of the bath is open to the air, and because air is always brought into the bath with the workpieces. The destructive part is that as fast as the alloy forms a protective chromium oxide scale, the alkali chlorides strip or flux that scale, forming potassium, sodium, and/or barium chromates. As fast as chromium from the alloy diffuses to the surface to re-form the oxide scale, the scale is dissolved. The chromium diffuses along grain boundaries orders of magnitude faster than through the grains themselves, and diffusion voids, or pores develop in the grain boundaries2. Eventually the molten salt physically penetrates the grain boundaries, and permeates the entire thickness of the salt pot wall, until the pot begins to leak through to the outside. This is somewhat more likely to occur in coarse grained regions, such as the fusion line of the weld or in the weld bead itself. Eventual failure in or near a weld does not necessarily mean that weld was defective. It is the combination of alkali chloride salts and oxygen that attacks the pot. If a new pot is put into a furnace contaminated with leaked salt from the previous pot, that salt will volatilize when heated. Those alkali chloride fumes will attack the chromium oxide scale on the outside of the pot, and the hot air or products of combustion provide more than enough oxygen to scale right through the pot. This occasionally happens in as little as three days. And that is why it is most important to clean out all the spilled salt from the previous pot, when installing a new one.

13-17

This photomicrograph, from Rolled Alloys Investigation No. 99-66, shows corrosion attack and corrosion assisted cracking in the fusion line between RA330 plate, top, and the RA33004-15 weld bead, bottom. In this salt pot the attack along weld fusion lines was so bad that entire lengths of the weld bead could be removed with a few blows of a hammer. The failure occurred simply because the firebox refractory still contained the spilled salt from the previous salt failure. In this case the solution was to replace this blanket insulation each time a new pot is installed. Operating temperature was about 1700-1750°F (930-950°C) using a non-cyanide carburizing salt. In normal operation, oxygen builds up in the salt itself. To prevent the steel workpieces from decarburizing, that salt must be rectified. That is, the oxygen content of the bath must be reduced to low levels. This may be done by introducing methyl chloride (well away from electrodes and metallic pot sidewalls), which converts the alkali oxides back to chlorides. Solid rectifiers such as powdered silicon, silica, ferrosilicon or dicyandiamide are also used. The inorganic rectifiers form a metallic sludge in the bottom of the pot. If the pot is not rectified well and frequently, the oxygen content will shorten the life of the pot by corrosion from the inside. This happens even at oxygen levels which will not harm the steel workpieces.

13-18

Salt Pots, continued This sludge must be removed frequently, perhaps twice daily, lest it act as an insulator, causing the bottom of the pot to overheat. Leaving the sludge in can overheat the bottom to the point that it fails, usually in or near a weld, while the sidewalls are still in good condition. Alloy Casting Institute3 studies of salt baths show that corrosion rates in the sludge itself, even without overheating, are increased by nearly a factor of two.

3/8” (9.5mm) RA309 salt pot bottom

3/4 scale

Rolled Alloys

Report #01-55

The photo above is a classic example of pot failure due to metallic sludge build-up, in this case over two inches (50mm) deep in the bottom. This sample came from a Western US heat treat shop. Neutral salt pots here were failing by leaks at the bottom weld in 4 to 6 weeks of operation, some after as little as 2 weeks. Operation was neutral salt at 1400—1600°F (760—870°C) 24 hours per day, 6 days per week. Pot idled 1200°F (650°C) on Sundays.

Etchant: Oxalic Acid Magnification: 25X Crack near the fusion line, inner weld bead of the RA309 pot above.

13-19

Salt Pots, continued When salt freezes it contracts in volume. If a salt pot is shut down and allowed to freeze solid, that salt will go through a volume increase when remelted on start-up. This increase can be about 3/8 to 1/2 inch per foot (31 to 42 mm/metre) of pot depth. If the pot is full of solid salt, there are a couple of unpleasant possibilities. One is that, as the salt melts first on the bottom, it expands and cracks open the pot, either in a weld or along the knuckle radius of a dished head. Alternately, once sufficient salt has melted, the volume expansion may cause it to explode through the remaining frozen layer on top. If one plans a shut-down, it is a good idea to ladle out most of the salt before it freezes. Floor sweepings can do interesting things to salt pots. Aluminum foil left over from someone’s lunch, for example, will melt and go right through the bottom of the pot. Sulphur from whatever will attack the nickel in the pot, the higher the nickel alloy, the worse the attack. One of our fabricator customers related an incident in which short life of RA309 pots was indeed traced to the practice of disposing of floor sweepings in the heat treat pots at night. Salt Pot Alloy Selection Since the 1930’s the most popular alloys have been the wrought alloys RA309, RA330 and RA600, or the cast grades HT (17Cr 35Ni) and HW (12Cr 60Ni). The higher chromium content of RA310, or of cast HK, is disadvantageous in salt. Both laboratory studies and observations of fixtures are reasonably consistent in showing that the higher nickel grades usually have better resistance to alkali chloride salts. If that were all there were to it, all metallic salt pots would be RA600 or HW, and all electrodes in ceramic pots RA600 or commercially pure nickel. But in practice the majority of metallic pots today are RA330 or RA309, with RA600 distinctly in the minority. Almost all submerged or over-the-top heating electrodes are RA446, with a very, very few being RA330 or RA600. Almost none are pure nickel. It is the case that the various ills that may befall metallic pots obscure any theoretical advantages of higher nickel to the extent that 35% or 13% nickel grades are considered more cost-effective. With respect to fixtures for automated salt lines, performance often follows the alloy’s resistance to chloride salts. Either RA600 or RA330 is, in our opinion, superior to RA309. In some shops RA600 has the advantage over RA330, in others there is no clear difference. In all cases, full penetration welds are necessary. This alloy selection discussion is for pots containing chloride salts, in which some steel piece will be austenitized. Tempering salts, which are mixtures of sodium nitrate and sodium nitrite, may be made either of carbon steel or of 304 stainless. However, contamination by chloride salts will increase corrosion rate. Quenching into a nitrate-nitrite mixture from a chloride high heat pot may considerably shorten the life of the former. 13-20

Salt Pots, continued References 1. James Kelly, Neutral Salt Pot Alloy Life: Maintenance is the Key, Heat Treating, April 1990 2. A.U. Seybolt, Oxidation of Ni-20Cr Alloy and Stainless Steels in the Presence of Chlorides, Oxidation of Metals, Vol. 2, No. 2, 1970 3. J. H. Jackson and M. H. LaChance, Resistance of Cast Fe-Nf-Cr Alloys to Corrosion in Molten Neutral Heat Treating Salts, Transactions of the ASM Vol. 46, 1954, pp 157-183. Bolts Bolts are commonly used at elevated temperature to withstand a shear load. For example, RA330 threaded rod, nuts and washers are used to assemble high temperature equipment where loose joints are desired to accommodate thermal expansion & contraction during thermal cycling. A good discussion of fasteners in the chemical process industry has been presented by Robert Smallwood1, currently of Det Norske Veritas. At high temperatures relaxation is the primary limitation to the use of threaded fasteners to maintain a clamping load. The most commonly available alloy choice for applications up to 1150 or 1200°F (620-640°C) is RA718, an age hardening nickel base alloy. A286, a less expensive age hardening stainless, is sometimes suggested but has neither the high temperature capability of RA718, nor is it as available in various bar sizes. Above this temperature, to about 1400-1500°F (760-816°C) the choices narrow down to René 41®, WASPALOYTM, or MP-35N®. Much of the published high temperature bolting experience has been with WASPALOY. There are newer alloys that may be technically quite suitable, though of questionable availability. In addition to selecting a strong bolt material it is important to look at the relative expansion coefficients of the alloy to be clamped, and the alloy from which the bolt is made. If the metal to be clamped expands faster than the bolt, that expansion will add to the tensile load in the bolt and may stretch it, so that the assembly is loose once it cools back down. What appear to us as fairly liberal alloy selection suggestions are offered by the Industrial Fasteners Institute as2: Below 450°F (230°C), low alloy steel. 450 to 900°F (230 to 480°C), one of the grades in ASTM A 193. From 900 to 1200°F (480 to 650°C), A286 and 718 . Above 1200°F up to 1600°F (650 to 870°C), René 41 or WASPALOY. 13-21

Bolts, continued Some cautions: Never, NEVER use anti-seize compounds that contain copper, zinc or aluminum anywhere near high temperature equipment. If copper gets carried into an area where the metal is operating above 1981°F (1083°C) it will melt and embrittle or or eat holes through any austenitic alloy it touches. Zinc, or galvanized coatings embrittle austenitics and can also embrittle steel bolts at moderately elevated temperatures. This embrittlement may even occur slightly below the melting point of zinc, 787°F (419°C). Molten aluminum is essentially the Universal Solvent for most alloys. Springs Metals used as springs at elevated temperature are subject to relaxation under load. “A bar loaded to an initial stress of say, 40,000psi (MPa) and then held at a constant strain and temperature may after a time period have a remaining stress of only 30,000psi (MPa). This time-dependent stress reduction of 10,000psi (MPa) is called stress relaxation. The total strain remains fixed but part of the elastic strain is replaced with inelastic strain4”. One intentional example of stress relaxation is the reduction of stress in a fabrication due to a stress relief anneal. From the standpoint of availability, alloy selection is the same as for bolts. RA718 or alloy X750 are practical choices for applications up to 1150-1200°F (620-650°C). Above that, to 1400-1500°F (760-816°C) René 41®, WASPALOYTM, or MP-35N® are the remaining practical choices. A more complete, but approximate, guide to alloy selection would be: Max. Use temperature

Alloys

°F 200

°C 100

music wire, AISI 6150 chromium-vanadium steel

500

260

302 stainless cold drawn, 410, K-500

750

400

17-7PH® condition CH900 (50% cold reduction, plus 900°F (482°C) 1 hour age

1000

540

A286

1200 1500

650 816

RA718, X-750 WASPALOY, René 41

Elastic modulus decreases with temperature, about a 1% drop in modulus for every 100°F (56°C) temperature increase. 13-22

References 1. R.E. Smallwood, Fastener Problems in the Process Industry, Corrosion 91 Paper No. 161, NACE, Houston, Texas 2. Fastener Standards, 6th Edition, available from: Industrial Fasteners Institute, 1505 East Ohio Building, 1717 East Ninth Street, Cleveland, Ohio 44114 U.S.A. 3. Bruce McLeod, Analyzing Belt Pin Failures, Metal Progress August, 1973, ASM, Metals Park, Ohio 4. Compilation of Stress-Relaxation Data for Engineering Alloys, ASTM DS-60, 1982 ASTM, Philadelphia Pennsylvania

13-23

THUMBNAIL BIOGRAPHIES OF RA ALLOYS To conclude this discussion of heat resisting alloys, let us briefly summarize the chief characteristics of each RA product. RA330 The work horse of the furnace industry, because it does more jobs better and for less money. Has enough chromium for good oxidation resistance, enough nickel for good ductility, appropriate silicon to resist absorption of carbon and nitrogen. Almost always preferred for carburizing atmospheres. Withstands a lot of thermal shock, yet has above-average strength at operating temperatures. Can be cut, bent and welded without troubles. RA333 A superior product that also costs more. The combination of 3% cobalt, 3% molybdenum and 3% tungsten adds high-temperature strength to a base of 45% nickel, 25% chromium and 1% silicon. Field installations have proven it has excellent resistance to carburization, thermal fatigue and distortion in quenching applications. RA 253 MA High strength, excellent oxidation resistance to 2000°F (1093°C). RA 353 MA Twice the strength of RA330 in the 1800-2200°F (980-1200°C) temperature range. Oxidation resistance approximates that of 601 and RA333, but the melting point of RA 353 MA is about 100°F (56°C) higher. Use for muffles, rotary calciners, coal nozzles. RA 602 CA The strongest and most oxidation resistant high temperature alloy we offer. An upgrade over 601, and potential alternate to alloys 617 or 230. Most cost-effective above 1900°F. RA309 Preferred for oxidizing atmospheres under 1900°F (1038°C) where resistance to carburizing or nitriding atmospheres is not necessary. Good resistance to sulfidation. RA310 Good oxidation resistance beyond 2000°F (1093°C) under mildly cyclic conditions. Good sulfidation resistance, generally good hot corrosion resistance. RA600 Lower strength but more ductility. Good oxidation, excellent carburization resistance. Resists hot chlorine gas to 1000°F (538°C) RA601 Stronger and more oxidation resistant than RA600, with good carburization resistance. RA446 Special applications, such as salt bath electrodes, glass molds, copper launders, thermowells, soot blowers, etc., where the hot corrosion resistance of maximum chromium is required. Has the best sulfidation resistance but very, very low strength and ductility.

14-1

CHEMICAL SYMBOLS Al aluminum Ar argon As arsenic B boron C carbon CO carbon monoxide CO2 carbon dioxide CH4 methane Ca calcium Cb columbium (niobium) Ce cerium Cl chlorine (the gas, Cl2) Co cobalt Cr chromium Cu copper F fluorine Fe iron H hydrogen HCl hydrochloric acid He helium H2O water La lanthanum

Mn manganese Mo molybdenum N nitrogen (as the gas, N2) NH3 ammonia Nb niobium (columbium) Ni nickel O oxygen (as the gas, O2) P phosphorus Pb lead S sulfur (sulphur) SO2 sulfur dioxide H2 S hydrogen sulfide H2SO4 sulfuric acid Sb antimony Si silicon Sn tin Ta tantalum Ti titanium V vanadium W tungsten Y yttrium Zr zirconiuim

Hardness is measured by Rockwell or Brinell machines. The Rockwell B scale (Rb, HRB) is most common for our alloys, Rockwell C is for heat treated steel. Brinell is usually abbreviated BHN (Brinell Hardness Number) on mill certifications. Grain size is in ASTM numbers. ASTM 4-7 is about average for RA330. Small numbers (ASTM 0, 2, 3) mean coarse grains. Larger numbers (7, 8, 9) mean finer grain size. Outokumpu and ThyssenKrupp VDM report grain size in micrometers, µm. ASTM 3 to 8 grain size would be 125 to 22 µm, typical for RA 253 MA. Disclaimer Clause: The data and information in this printed matter are believed to be reliable. However, this material is not intended as a substitute for competent professional engineering assistance which is a requisite to any specific application. Rolled Alloys makes no warranty and assumes no legal liability or responsibility for results to be obtained in any particular situation, and shall not be liable for any direct, indirect, special or consequential damages therefrom. This material is subject to revision without prior notice.

14-2

BIBLIOGRAPHY A. L. Marsh, Electric Resistance Element, U.S. Patent No. 811,859 Feb. 6, 1906 F. A. Fahrenwald, Metals for High Temperature, Chemical and Metallurgical Engineering, Vol. 28, p 680—681, April 26, 1923 F. A. Fahrenwald, Some Principles Underlying the Successful Use of Metals at High Temperatures, Proc. ASTM, Vol. 24, p 310—347, 1924 J. D. Corfield, Heat Resisting Alloys and Their Use in the Steel Plant, Iron and Steel Engineer, p 157—194, April, 1929 W.P. Rees, B.D. Burns, and A.J. Cook, Constitution of Iron-Nickel-Chromium Alloys at 650 to 800C, JISI July, 1949 Charles Emery and Paul Goetcheus, Added Life for Brazing Fixtures, Steel, June 27, 1955 Ralph H. Moeller, High-Nickel Alloys for High-Temperature Springs, SPRINGS Magazine, October 1965, Vol. 4, Number 2 H. S. Avery, Cast Heat-Resistant Alloys for High—Temperature Weldments, WRC Bulletin 143, August, 1969 This is the best discussion of heat resistant alloys ever printed. Bruce McLeod, Cracking in Type 309 High Temperature Fabrications and How to Combat It, Industrial Heating, September and October 1972 A. Roy, F. A. Hagen, and J. M. Corwin, Performance of Heat—Resistant Alloys in Emission—Control Systems, SAE Paper No. 740093, Automotive Engineering Congress, Detroit, Michigan February 25—March 1, 1974 James Kelly, Understanding Conditions that Affect Performance of Heat Resisting Alloys, Industrial Heating, March & April, 1979 G. R. Rundell, Evaluation of Heat Resistant Alloys in Composite Fixtures, NACE Paper Number 377, Corrosion 86, March 17—21, 1986 James Kelly, Neutral Salt Pot Alloy Life: Maintenance is the Key, Heat Treating, April, 1990 George Y. Lai, High-Temperature Corrosion of Engineering Alloys, 1990 ASM International Gene Rundell and James McConnell, Oxidation Resistance of Eight Heat-Resistant Alloys at 870o, 980o, 1095o, and 1150oC, Oxidation of Metals, Vol. 36, Nos. 3 / 4, 1991 14-3

Bibliography, continued James C. Kelly, Heat Resistant Alloy Corrosion—More Problems than Solutions, NACE Paper Number 166, Corrosion 91, March 11—15, 1991 James Hamer and James McConnell, Influence of Composition and Microstructure on Performance of Wrought Heat Resisting Alloys, Industrial Heating, April, 1992 James Kelly, Heat Resistant Alloy Performance, Heat Treating, July 1993 J. C. Kelly and J. D. Wilson, Oxidation Rates of Some Heat Resistant Alloys, Heat – Resistant Materials II, Conf. Proc. Of the 2nd International Conference on Heat-Resistant Materials 11—14 September, 1995, Gatlinburg, Tennessee John P. Steward, Flame Straightening Technology, 1981 LaSalle, Quebec James Kelly, Metal dusting in the heat treat industry, Stainless Steel World 1999 Conference, KCI Publishing BV, Zutphen, NL 1999

14-4

HISTORY Austenitic heat resistant alloys and stainless steels as we know them today were invented by Benno Strauss1 of Friedrich Krupp before World War I. Our 35Ni 19Cr alloy RA330 may trace its roots to Nichrotherm® 4, containing 35% nickel and 13-14% chromium, introduced to Germany in 1910 for high temperature applications1. U.S. patents for Strauss’ alloys were issued on June 25, 1913. What we now call 310 was developed by Adolf Fry, also at Krupp, in 1926. The electrical resistance wire Nichrome®, nominally 80Ni 20Cr, and the European alloy Nimonic® 75, nominal 76Ni 20Cr, would appear to be developments of A. L. Marsh’s U.S. Patent No. 811,859, Feb. 6, 1906, for a 15-25% Cr, balance nickel electrical resistance alloy. Rolled Alloys’ verbal history says that in the early 1930’s the Misco sales manager, John Johnson, loaded an ingot of the cast alloy HT, at that time 35Ni 15Cr, in the trunk of his Buick. He drove it from Detroit to Lockport, New York, to be rolled to the wrought alloy trademarked Misco Metal. The cost included new springs for the Buick & a couple of replacement mill rolls for Simonds Saw & Steel Co. who did the rolling. We have a folder from The Simonds Saw & Steel Company, dated 1934, which includes data and microstructures for a wrought 15% Cr 35% Ni alloy. The Rolled Products Division of Michigan Steel Casting Company initially developed the market for rolled Misco Metal in the heat treating industry. When Rolled Alloys was founded as an independent company in 1953, this alloy was renamed RA330. In 1958 Rolled Alloys lowered the carbon to 0.08% max and, to maintain the strength, raised chromium to the present 19% Cr. The RA330 silicon range was tightened at that time, to 1.00-1.50%. In that same year work began at Simonds, in conjunction with Rolled Alloys, on the stronger and more carburization resistant grade, RA333. All of the ASTM specifications for RA330 were written by Rolled Alloys technical personnel, and shepherded through the committee meetings. In 1975, after several years of creeprupture and tensile testing, along with Rolled Alloys’ attendance at numerous committee meetings RA330 was approved by AMSE Case 1654-1 for use to 800°F (427°C). A few years later RA330 was approved for use to 1650°F (899°C). Reference 1. The Sorby Centennial Symposium On The History Of Metallurgy, Volume 27, edited by Cyril Stanley Smith, Cleveland, Ohio October 22-23, 1963 14-5

TRADEMARKS RA330 and RA333 are registered trademarks of Rolled Alloys, Incorporated 153 MA, 253 MA and 353 MA are registered trademarks of Outokumpu AB 602 CA is a registered trademark of ThyssenKrupp VDM AL-6XN is a registered trademark of ATI Properties, Inc. 20Cb-3 is a registered trademark of Carpenter Technology Corporation Haynes, Hastelloy, 214, 239, HR-120 and HR-160 are registered trademarks of Haynes International Kanthal is a registered trademark of Kanthal AB Nimonic, Inconel, Incoloy, Monel, MA956 and 800HT are registered trademarks of Special Metals, Incorporated Refrasil is a registered trademark of SGL Carbon Group, Business Unit Fibers and Composites René 41 is a registered trademark of Teledyne Industries Incorporated Stellite is a a registered trademark of Deloro Stellite, Incorporated MO-RE, 22H and Supertherm are registered trademarks of Duraloy Technologies, Inc. (MO stands for Marty Ornitz, RE for Ray English)

Thermax and Thermalloy are registered trademarks of ElectroAlloys Corporation WASPALOY is a trademark of United Technologies Corporation 17-4PH and 18SR are registered trademarks of AK Steel Corporation

14-6

COMPARISON – German & European Standards with American Grade

UNS No. Werkstoff Nr.

DIN Designation

EN Number

ferritic stainless 405 S40500 1.4002 X6CrAl13 410 S41000 1.4006 X12Cr13, X10Cr13 410 S41000 1.4024 X15Cr13 410S S41008 1.4000 X6Cr13 416 S41600 1.4005 X 12 CrS 13 430 S43000 1.4016 X6Cr17 446 S44600 1.4763 X8Cr24 duplex stainless 2304 S32304 --2205 S31803 1.4462 X2CrNiMoN22-5-3 2205 S32205 1.4462 X2CrNiMoN22-5-3 2507 S32750 1.4410 X2CrNiMoN25-7-4 austenitic stainless 201 (stainless) S20100 1.4372 X12CrMnNiN 17-7-5 303 S30300 1.4305 X8CrNiS18-9 304L S30403 1.4307 X2CrNi18-9 304 S30400 1.4301 X 5 CrNi 18 10 (X4CrNi18-10) 304H S30409 1.4301 X 5 CrNi 18 10 (X4CrNi18-10) 316 S31600 1.4401 X 5 CrNiMo 17 12 2 316L S31603 1.4404 X2CrNiMo17-12-2 316Ti S31635 1.4571 X6CrNiMo17-12-2 317L S31703 1.4438 X2CrNiMo18-15-4 321 S32100 1.4541, 1.4878 X6CrNiTi18-10, X12CrNiTi18-9 321H S32109 1.4541, 1.4878 X6CrNiTi18-10, X12CrNiTi18-9 347 S34700 1.4550 X6CrNiNb18-10 heat resistant alloys ® 153 MA S30415 1.4891 X 4 CrNiSiN 18 10 --1.4828 X15CrNiSi20-12 ® RA 253 MA S30815 1.4893 (EN: X9CrNiSiNCe21-11-2) 309S S30908 1.4833 X12CrNi24-12, X 7 CrNi 23 14 309 S30900 1.4833 X12CrNi24-12, X 7 CrNi 23 14 ® RA85H S30615 --310S S31008 1.4845 X8CrNi25-21 310H S31009 1.4845 X8CrNi25-21 310 S31000 1.4845 X12CrNi25-21 314 S31400 1.4841 X15CrNiSi25-20 800 N08800 1.4876 X10NiCrAlTi32-20 800H N08810 1.4876 X10NiCrAlTi32-20 ® TM 800HT /AT N08811 (1.4959 similar) (X8NiCrAlTi32-21, similar) ® Incoloy DS - -similar to - -1.4864 - -similar to - -X12NiCrSi36 16 ® RA330 N08330 -(EN: X10NiCrSi35-19) ® RA 353 MA S35315 -(EN: X6NiCrSiNCe35-25) 45 TM N06045 2.4889 NiCr28FeSiCe ® RA333 N06333 2.4608 NiCr26MoW X N06002 2.4665 NiCr 22 Fe 18 Mo 617 N06617 2.4663 NiCr23Co12Mo 601 N06601 2.4851 NiCr 23 Fe 602CA N06025 2.4633 NiCr25FeAlY 603GT N06603 2.4647 NiCr25FeAlYC 600 N06600 2.4816 NiCr 15 Fe ® Nimonic 75 N06075 2.4951 NiCr 20 Ti

1.4002 1.4006 -1.4000 1.4005 1.4016 -1.4362 1.4462 -1.4410 1.4372 1.4305 1.4307 1.4301 -1.4401 1.4404 1.4571 1.4438 1.4541 --1.4818 -1.4835 1.4833 --1.4845 -------1.4886 1.4854 ----------

weld filler metals—SG designates bare wire, EL is for covered electrodes

RA333 X FM 602 CA FM 617 FM 718

-(ERNiCrMo-2) -(ERNiCrCoMo-1) (ERNiFeCr-2)

2.4608 2.4613 2.4649 2.4627 2.4667

NiCr26MoW SG-NiCr21Fe18Mo SG-NiCr25FeAlY SG-NiCr22Co12Mo SG-NiCr19NbMoTi

(AWS spec)

14-7

------

COMPARISON—German & European Standards with American, continued Grade

UNS No.

Werkstoff Nr.

DIN Designation

EN Number

S17400 S66286 N07750 N07718 N07263 N07041 N07001

1.4548 1.4980 2.4669 2.4668 2.4650 2.4973 2.4654

X5CrNiCuNb17-4-4 X5CrNiTi26-15 NiCr15Fe7TiAl NiCr19NbMo NiCo 20 Cr 20 MoTi NiCr19CoMo NiCr 19 Co 14 Mo 4 Ti

1.4542 -------

R30188 R30605

2.4683 2.4964

CoCr22NiW CoCr 20 W 15 Ni

---

X1NiCrMoCu 25 20 5 X 1 NiCrMoCu 25 20 6 -X1NiCrMoCu31-27-4 X1NiCrMoCu32-28-7 X1CrNiMoCuN33-32-1 -NiCr20CuMo NiCr21Mo -NiCr 22 Mo 6 Cu NiCr 22 Mo 7 Cu NiCr22Mo9Nb NiMo 16 Cr 16 Ti NiMo 16 Cr 15 W NiCr21Mo14W NiCr29Fe -NiMo 28 -NiMo29Cr ?? ?? Ni 99.2 LC-Ni 99 NiCu30Fe 2.4375 CuNi10Fe1Mn

-----------------------------

age hardening alloys ®

17-4PH A-286 X-750 718 C-263 René 41 TM WASPALOY cobalt alloys 188 L-605

corrosion resistant alloys

904L 1925hMo ® AL-6XN ® Sanicro 28 3127hMo 3033 ® 20Cb-3 3620Nb 825 G-30 G G-3 625 C-4 C-276 C-22 690 B B-2 B-3 B-4 B-10 FM B-10 200 (nickel) 201 (nickel) 400 K-500 90-10 Cu-Ni

N08904 N08926 N08367 N08028 N08031 R20033 N08020 N08020 N08825 N06030 N06007 N06985 N06625 N06455 N10276 N06022 N06690 N10001 N10665 N10675 N10629 N10624 -N02200 N02201 N04400

1.4539 1.4529 -1.4563 1.4562 1.4591 -2.4660 2.4858 -2.4618 2.4619 2.4856 2.4610 2.4819 2.4602 2.4642 -2.4617 -2.4600 2.4710 2.4702 2.4066 2.4068 2.4360 N05500 C70600 2.0872

NiCu 30 Al

weld filler metals—SG designates bare wire, EL is for covered electrodes

70-30Cu-Ni K-500 C-276 C-276 C-22 C-22 625 112 82 182

ERCuNi 2.0837 -2.4373 ERNiCrMo-4 EniCrMo-4 ERNiCrMo-10 ENiCrMo-10 ERNiCrMo-3 ENiCrMo-3 ERNiCr-3 2.4806 ENiCrFe-3 2.4620

SG-CuNi30Fe -SG-NiCu 30 Al -2.4886 SG-NiMo16Cr16W 2.4887 EL-NiMo15Cr15W 2.4635 SG-NiCr21Mo14W 2.4638 EL-NiCr20Mo14W -2.4831 SG-NiCr21Mo9Nb 2.4621 EL-NiCr20Mo9Nb SG-NiCr20Nb -EL-NiCr16FeMn --

----

UNS chemistries generally overlap the German standards shown but they are NOT identical. When the customer requires DIN certification of stock material, it can be re-certified by the producing mill. Two exceptions are AL-6XN and 20Cb-3, as they have no direct German equivalents. RA330 does now have an EN spec, designation X10NiCrSi35-19, EN number 1.4886. DIN 50049 3.1.B is a general quality specification which can apply to any alloy. The producing mill can certify to this specification, or Rolled Alloys can provide a certificate of conformance. Many of the EN (European Harmonized Standards) numbers and designations are the same as DIN, others are not.

14-8

CORPORATE OFFICE ROLLED ALLOYS 125 WEST STERNS ROAD TEMPERANCE, MICHIGAN 48182 1-800-521-0332 1-734-847-0561 FAX: 1-734-847-6917 E-MAIL: [email protected] www.ROLLEDALLOYS.COM

NORTH AMERICAN LOCATIONS: CALIFORNIA, CONNECTICUT, ILLINOIS, OHIO, OKLAHOMA, TEXAS ALBERTA, ONTARIO

GLOBAL LOCATIONS: CHINA, FRANCE, GERMANY, THE NETHERLANDS, SCOTLAND, SINGAPORE, SPAIN, UNITED KINGDOM

Thermal Expansion, continued Temperature Range °F 70-200 -400 -600 -800 -1000 -1200 -1400 -1600 -1800 -2000

Total Thermal Expansion, inches/foot

SA-387 RA446 RA321

RA309

RA 253 MA RA310 RA 353 MA RA330 RA333 RA601 RA600 RA 602 CA

0.0104 0.0281 0.0471 -0.870 ------

0.0137 0.0356 0.0591 -0.108 --0.185 ---

0.0141 0.0370 0.0610 0.0859 0.111 0.137 0.164 0.193 0.224 --

0.00874 0.0225 -0.0526 0.0681 0.0854 0.102 0.123 0.152 --

0.0145 0.0372 0.0604 0.0876 0.115 0.144 0.174 0.204 0.237 --

0.0131 0.0348 0.0569 0.0806 0.106 0.133 0.160 0.186 0.214 0.245

0.0134 0.0345 0.0566 0.0796 0.104 0.129 0.154 0.181 0.209 --

0.0129 0.0341 0.0566 0.0797 0.104 --0.180 0.208 --

0.0109 ---0.0960 0.122 0.148 0.173 0.201 --

0.0119 0.0317 0.0516 0.0727 0.0949 0.120 0.147 0.175 0.204 0.236

0.0115 0.0305 0.0502 0.0710 0.0937 0.117 0.142 0.167 0.193 --

0.0103 0.0297 0.0496 0.0710 0.0915 0.115 0.144 0.174 0.201 0.227

The more general way to calculate thermal expansion is to use the mean coefficients of thermal expansion, such as those given on the next page. Multiply the length in inches, times the difference between room temperature and operating temperature, times the expansion coefficient. Note that these coefficients are all multiplied by 10-6, which is the same as dividing by one million. For that 20 ft long RA330 muffle operating 1800°F 982°C) this is: 20 ft X 12 inches/foot X (1800-70F) X 10.0x10-6 = 240 inch X 1730F X 10x10-6 = 4.152 inches. To convert these numbers to the metric system, multiply by 83.33 to get millimeters expansion per meter of length

9-6

MEAN COEFFICIENTS OF THERMAL EXPANSION ALLOY

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

304

9.6

--

--

--

9.9

--

--

--

10.2

--

10.4

--

--

--

--

--

--

--

--

316

8.9

--

--

--

9.0

--

--

--

9.7

--

10.3

--

--

11.1

--

--

--

--

--

2205

7.2

7.3

7.5

7.7

--

--

--

--

--

--

--

--

--

--

--

--

--

--

--

RA321

9.3

--

9.4

--

9.5

--

10.0

--

10.3

10.5

10.6

--

10.9

--

11.1

--

11.4

--

--

RA309

8.8

8.9

9.0

9.2

9.3

9.4

--

--

9.7

--

--

--

--

10.0

10.1

--

--

--

--

RA310

8.4

8.6

8.8

--

8.95

--

9.2

--

9.5

--

9.8

--

10.05

--

10.15

--

10.3

--

10.6

SA-387

6.7

--

7.1

--

7.4

--

--

--

7.8

--

--

--

--

--

--

--

--

--

--

RA 253 MA

9.06

--

9.34

--

9.59

--

9.81

--

9.97

--

10.14

--

10.3

--

10.5

--

10.8

--

--

410

®

5.5

--

--

--

--

--

--

--

--

--

6.5

--

--

--

--

--

--

--

--

®

8.3

8.4

8.6

8.7

8.9

9.0

--

9.2

9.3

9.4

9.6

--

--

9.7

9.8

9.9

10.0

--

--

®

7.95

--

8.29

--

8.56

--

8.80

--

8.98

--

9.24

--

9.52

--

9.72

--

9.87

--

--

RA 353 MA

8.48

--

8.68

--

8.88

--

9.07

--

9.27

--

9.46

--

9.66

--

9.86

--

10.05

--

--

RA800AT

7.9

--

8.8

--

9.0

--

9.2

--

9.4

--

9.6

--

9.9

--

10.2

--

--

--

--

RA446

5.6

--

5.7

5.8

--

5.9

6.0

--

6.1

--

6.3

--

6.4

--

6.7

6.9

7.3

--

--

RA600

7.4

--

7.7

--

7.9

--

8.1

--

8.4

--

8.6

--

8.9

--

9.1

--

9.3

--

--

RA601

7.6

--

8.01

--

8.11

--

8.3

--

8.5

--

8.87

--

9.19

--

9.51

--

9.82

--

10.18

RA 602 CA

6.6

--

7.5

--

7.8

--

8.1

--

8.2

--

8.5

--

9.0

--

9.5

--

9.7

--

9.8

RA333

7.0

--

--

8.0

--

--

--

--

8.6

--

9.0

--

9.3

9.3

9.4

9.5

9.7

--

--

HH

--

--

--

--

--

--

--

--

9.5

--

9.7

--

9.9

--

10.2

--

10.5

--

10.7

HK

--

--

--

--

--

--

--

--

9.4

--

9.6

--

9.8

--

10.0

--

10.2

--

10.4

HT

7.9

--

8.14

--

8.37

--

8.61

--

8.85

--

9.09

--

9.33

--

9.56

--

9.8

--

--

--

--

--

--

--

--

9.2

--

9.5

--

9.8

--

10.0

--

10.3

--

10.6

E-BRITE

5.17

5.3

5.44

5.56

5.67

--

--

--

--

6.09

6.22

6.4

6.57

6.72

6.85

6.88

7.1

--

--

825

RA330

HR-120

®

®

®

HP ®

10.04

7.8

--

8.3

--

8.5

--

8.7

--

8.8

--

9.1

--

9.5

--

9.7

--

--

--

--

®

8.2

8.3

8.4

--

8.65

--

--

8.9

8.95

--

9.15

--

9.3

9.4

9.5

--

--

--

--

AL-6XN

®

7.9

8.3

8.37

8.42

8.6

8.7

8.8

8.85

8.96

--

9.3

--

--

--

--

--

--

--

--

TiGr 2

4.8

--

--

--

5.1

--

--

--

5.4

5.6

--

--

--

--

--

--

--

--

20Cb-3

-6

NOTE: All coefficients are reported as inch/inch °F x 10 , room temp to indicated temp. Multiply by 1.8 for metric units.