NACE NRP-472

Paper No.

417 OVERVIEW OF NACE INTERNATIONAL

STANDARD RP0472

Cathleen Shargay Fluor Daniel, Inc. 3353 Michelson Dr. Irvine, CA 92698

ABSTRACT This paper summarizes the development and objectives of NACE International Standard Recommended Practice RP0472-95, “Methods and Controls to Prevent In-Service Environmental Cracking of Carbon Steel Weldments in Corrosive Petroleum Refining Environments”. Two types of environmental cracking are included; cracking due to hydrogen charging and stress corrosion cracking (SCC). Weldrnents are defined to include the weld deposit, base metal heat affected zones (HAZS) and adjacent base metal zones subject to residual stresses from welding. HISTORY OF THE STANDARD’S DEVELOPMENT The foreword of RP0472 contains a good history of the standard’s development, hence this section is primarily excerpts from that. Most petroleum refining equipment and piping is constructed of carbon steel having a minimum specified tensile strength of up to 485 MPa (70,000 psi), and in nearly every case, the equipment is fabricated by welding. ‘Thewelds for refinery equipment and piping are made to conform to various codes, and according to these codes, these carbon steels are classified as P-1, Group 1 or 2. Hence, in RP0472, they are referred to as “P-1 steels”. Petroleum refineries as well as gas- and oil-processing plants have extensively used P-1 steels for wet hydrogen sulfide (H2S) or “sour” services. They are the basic materials of construction for heat exchangers, pressure vessels, storage tanks, and piping. Decades of successfid service have shown them to be generally resistant to a form of hydrogen stress cracking (HSC) called sulfide stress cracking (SSC). HSC has been shown to occur in high-strength materials, or zones of a hard or high-strength microstructure in an otherwise soft material. In the late 1960s, a number of SSC failures occurred in hard weld deposits in P-1 steel refinery equipment. The high harnesses were found to be primarily caused by submerged arc welding (SAW) with active fluxes and/or additions of alloying elements, both of which resulted in increased hardenability of the weld deposit. High harnesses were also found in gas metal arc welds with high manganese and silicon contents. To detect hard weld deposits caused by improper welding materials or procedures, the petroleum refining industry began requiring hardness testing of production weld deposits under certain conditions and applied a criterion of 200 Brinell (HB) maximum. These requirements were given in previous editions of this standard and in a similar API document discussed below. The hardness of 200 HB is lower than 22 Rockwell C (HRC) which is a recognized level of hardness required for SSC susceptibility and is the equivalent of about 237 HB. Reasons for applying a lower limit were to compensate for nonhomogeneity of some weld deposits and normal variations in production hardness testing using a portable Brinell tester. Copyright @l 999 by NACE International. Requests for permission to publish this manuscript in any form, in pari or in whole must be made in writing to NACE International, Conferences DO.fision,P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

This standard was issued by NACE International under the auspices of Group Committee T-8 on Refining Industry Corrosion, and was prepared by NACE Task Group T-8-7, which was composed of corrosion consultants, corrosion engineers, and other specialists associated with the petroleum refining industry. It was originally prepared in 1972, reafftied in 1974 and revised in 1987 and 1995. API previously published a document, API RP 942, with similar objectives. The API document has been discontinued with the intention of recognizing this NACE standard as the industry consensus standard. RECENT INDUSTRY PROBLEMS In the late 1980s, numerous cracks were discovered in P- 1 steel equipment that met the 200 HB weld deposit hardness limit. Some of these cracks have been identified as another form of hydrogen damage, labeled as stress-oriented hydrogeninduced cracking (SOHIC). These cracks propagated primarily in the HAZS of weldments and were found in both high- and low-hardness HAZS. Also, cases of SSC were reported in HAZS of weldments that met the 200 HB weld deposit hardness limit. In these cases, the HAZ exhibited high harnesses, often greater than 240 HB. However, HAZ hardness limits and testing were outside the scope of the previous editions of this standard, which covered only weld deposit hardness limits. In 1991, NACE Task Group T-8-7 agreed that this standard should be more comprehensive, covering the entire weldment and various inservice cracking mechanisms that can occur in corrosive petroleum environments. SCOPE Materials: This standard covers only carbon steels classified as P- 1, Group 1 or 2. It excludes steels over 485 MPa (70,000 psi) minimum specified tensile strength. Other materials may be vulnerable to cracking, but these materials are outside the scope of this standard. Equipment: included.

Refinery vessels, exchangers, storage tanks, piping, valve bodies, and pump and compressor cases are

Types of Welds: All pressure-containing wekhnents or internal attachment weldments to the pressure boundary are included. In addition, wekhnents in some nonpressure-containing equipment, such as storage tanks, are included. External attachment weldments occasionally require special controls in SCC services. Both new fabrication and repair welds are within the scope of this standard. However, the practices are intended to avoid in-service cracking, and do not address cracking that can occur during fabrication, such as delayed hydrogen cracking. Almost all types of weld configurations are included. Some specific exceptions include hot-taps and weld build-ups. Hardness limits and postweld heat treatment (PWHT) requirements for these welds should be reviewed on a case-by-case basis. Welding Processes: Welding processes included in this standard include shielded metal arc welding (SMAW); gas metal arc welding (GMAW), including flux-cored arc welding (FCAW); gas tungsten arc welding (GTAW); and submerged arc welding (SAW). Corrosive Services: The corrosive refinery process environments covered by this standard can be divided into the two categories: w .

Services that could cause cracking due to hydrogen charging, Services that could cause cracking due to alkaline SCC.

The first category includes both the hydrogen cracking mechanisms that occur in high-strength or high-hardness areas, such as SSC or cracking in hydrofluoric acid, and SOHIC which can occur in soft HAZS. Examples of the second category which is services causing alkaline SCC include caustic, alkanolamine solutions containing COZand/or H# and alkaline sour waters containing carbonates. Figure 1 is a simplified schematic showing the interrelationships of the various cracking mechanisms discussed in the standard.

It was not within the scope of this standard to identi~ the specific environments which cause these cracking mechanisms; that is the responsibility of the user. Instead, this document was intended to cover details of the prevention methods. One possible environmentally-induced cracking mechanism in carbon steel weldments that is not addressed by the standard is high temperature hydrogen attack. API 941, “Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants,” gives recommendations on materials selection to avoid this problem. Other types of in-service cracking not addressed by this standard are primarily mechanical in nature. Examples are fatigue, creep, and brittle fracture. SUMMARY OF GUIDELINES A summary of the problems and recommended prevention methods addressed by the standard is: CRACKING MECHANISM 1. Wet H2S Service Hydrogen stress cracking (HSC) or sulfide stress cracking (SSC) 2. Wet H2S Service Hydrogen stress cracking (HSC) or sulfide stress cracking (SSC) 3. Alkaline stress corrosion crackimz

WELDMENT COMPONENT Weld deposit

PREVENTION METHOD Hardness testing of welds - limit of 200 HB

HAZ

Three options: 1. Base metal chemistry controls, 2. PWHT, or 3. HAZ hardness tests during welding procedure qualification. PWHT

Entire weldment

For each of the above mechanisms, application guidelines, test methods and implementation guidelines are given in the standard. The Brinell test method is considered unacceptable for hardness testing of HAZ in production welds, because it creates too large an indentation to obtain hardness values strictly tlom the HAZ. It obtains a bulk hardness value that is not representative of the peak HAZ hardness. SOHIC was treated as a special case, as limiting weldment hardness does not prevent this form of cracking. Reducing weldment hardness and residual stress is believed to reduce the likelihood, so the guidelines given in the standard on these methods may still be helpfid. However, additional steps, such as the use of special clean steels, water washing, corrosion inhibitors, or corrosion-resistant liners, may be needed for some services. An overview of the materials selection, fabrication, PWHT and testing practices that have been applied to new pressure vessels for mitigating SOHIC is provided in NACE Publication 8X194. Many refiners have adopted a practice of controlling carbon steel wekhnent hardness in all refinery services. This helps avoid the use of improper welding materials, welding procedures, or heat treatments. It also minimizes the risk of HSC from external wet atmospheric corrodents, process upsets, or from fiture changes in services. WELD DEPOSIT HARDNESS LIMIT AND TESTING The weld deposit hardness limit specified within RP0472 is 200 HB. The filler metal is required to conform to the various specifications for the ASME Boiler and Pressure Vessel Code, Section II, Part C, and to meet the A-No. 1 chemical composition shown in ASME Section IX. Since experience indicates that the following welding conditions rarely exceed the hardness limit, a waiver on the testing is given to: . ●

SMAW with ASME SFA-5. 1 E60XX or E70XX, and GTAW with ASME SFA-5. 18 ER70S-X (except for any -6, -7 or -G materials).

These weld deposits do not require hardness testing unless otherwise specified by the user. This waiver maybe applied even if a different consumable is used for the root pass. Some of the testing requirements given in the standard are: . ●

. ● ● ●

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Testing shall be taken with a portable Brinell hardness tester. Test technique guidelines are given in an Appendix in the standard. Other hardness testing techniques may be employed if approved by the user. Testing shall be done on the side contacted by the process whenever possible, For vessel or tank butt welds, one test per ten foot of seam with a minimum of one location per seam is required. One test shall be made on each nozzle flange-to-neck and nozzle neck-to-shell (or -head) weld. For piping welds, a percentage of welds shall be tested (5% minimum is suggested). Testing of fillet welds should be done when feasible (with the testing frequency being similar to the butt welds). Each welding procedure used shall be tested. Hard welds shall be either heat treated or removed. HAZ HARDNESS CONTROL METHODS

The intent of the standard is that only one of the following three methods be used, and that they are each equally effective methods of controlling the HAZ hardness. Some refiners have chosen to apply two or three of these control methods to some equipment or piping, especially in the most severe services. 1. Base Metal Chemistry Controls Control of base materials chemistry of all production materials for a given application can be an effective method of controlling the HAZ microstructure and hardness. This is generally done by controlling the: ● ● ●

carbon equivalent (CE * %C + ‘YOMn/6 + (’??Ni+ %Cu)/1 5 + (%Cr + ?40M0+ %V)/5) total content of unspecified elements, and microalloying element additions.

There is not an industry consensus on limits for these chemistry controls, hence there were no limits given in this standard. Examples of possible limits are provided by NACE Publication 8X 194. 2. Postweld Heat Treatment PWHT almost always reduces both weld deposit and HAZ harnesses to acceptable levels. When PWHT is being done to control hardness (in contrast to other services, where it is used to reduce residual stresses from welding), the following are some of the applicable issues and concerns: ● ● ●

●

a one-hour minimum hold time should be used, the lower temperatures allowed by ASME with longer hold times, should not be used, a PWHT procedure should be developed with the type of heating process, the number and locations of thermocouples, support details, heat-up and cool-down rates, maximum allowable temperature differentials, soak time, etc. PWHT is not practical for some welds on valves, pumps, compressor casings, etc. 3. Use Of HAZ Hardness Testing During Welding Procedure Qualification

When preproduction welding procedure qualification HAZ hardness testing is used, the maximum allowable HAZ hardness shall be 248 HV (70.5 HR 15N). Additional requirements are needed to ensure that the qualification hardness tests are representative of the production wekhnents. Materials and welding conditions used for the procedure qualification tests should be equivalent to what will be used for the equipment or piping. This can be done by applying the following additional essential variables, as appropriate:

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production materials should be the same ASME specification or grade as the qualification materials, maximum CE should not exceed the value for the qualification materials, maximum content of each deliberately added microalloying element (Cb, V, Ti and B) should not exceed the corresponding value on the test sample, heat input during production should not vary by more than +25/-10% from the heat input used during qualification, for SAW, the flux and wire, and for FCAW, the wire used for production welding should be the same brand and type as that used in the qualification tests, for GMAW and FCAW, the wire size used for production welding should be the same as that used during qualification, for SMAW, GTAW and SAW, only one size variation between the electrode or filler metal size fkom that used for the qualification tests should be permitted, preheat and interpass temperatures used during production should be greater than or equal to that used during the qualification, for welds that will not be preheated, the maximum thickness allowed on the production materials should be equal to the test sample thickness, if a temper bead technique is used during qualification, a temper bead should be placed on the cap pass of production welds, for production fillet welds, the qualification hardness survey should be performed on a single- or two-pass fillet weld sample. for fillet welds, position should bean essential variable, but tests on an overhead fillet weld shall qualify all other fillet positions. Figures showing the typical hardness test locations for butt and fillet welds are given in the standard. MITIGATING ALKALINE STRESS CORROSION CRACKING (ASCC) Three conditions must generally be simultaneously present for ASCC to occur: a specific environment, a susceptible material, and tensile stresses.

Defining the environmental conditions which can lead to ASCC on P- 1 steels was outside the scope of this standard. They vary by the alkali concentrations, temperatures, other contaminants, etc. Residual stresses from welding and/or forming are the most common sources of the tensile stress necessary for cracking. Residual stresses are usually highest in the HAZ, but can sometimes extend up to 50 mm (2 in) away from the weld deposit. Hence, ASCC is most common in the HAZ, but in severe cases can occur in a band next to the weld which is up to two inches wide. Cracks are typically oriented parallel to the weld (perpendicular to the residual stresses). Weldment harnesses usually have no effect on ASCC susceptibility. There are some services, in which both ASCC and HSC are concerns. In these cases, controls to prevent both mechanisms are typically applied, with the HSC requiring weldment hardness controls. PWHT is the most commonly used method to prevent ASCC. It reduces the residual welding stresses, which removes the tensile stresses necessary for ASCC. When PWHT is being done to reduce residual stresses (in contrast to when PWHT is used to reduce weldment harnesses), the following are some of the applicable issues and concerns: ● ●

. ●

a one-hour minimum hold time should be used. the lower temperatures allowed by ASME with longer hold times, should not be used. heating bands wider than required by codes are typically necessary on large diameter piping (>25 cm or 10 in). A band width that has been used successfully is >25 cm or 10 in. all welds and HAZS require PWHT, including all pressure-containing welds, internal attachment welds, nozzle reinforcing pad welds, temporary fabrication attachment welds, arc strikes, etc. Most external attachment welds also require PWHT. However, on thick-wall components, an evaluation can be done of whether the residual stresses will extend through the wall, and if not, the PWHT can be waived.

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After PWHT, any fabrication step which reintroduces high stresses, such as weld repairs, straightening, etc, should be postweld heat treated again. SUMMARY

NACE RP0472 addresses both cracking due to hydrogen charging (HSC, SSC, SOHIC, etc) and cracking due to residual stresses (ASCC). For the former, hardness controls are typically used and are applied to both the weld deposit and the HAZS. Harnesses of weld deposits can be measured on the actual production welds, but there are no reliable production test methods for HAZ hardness. Hence, the HAZ hardness is controlled by: 1. Base metal chemistry controls, 2. PWHT, or 3. HAZ hardness tests during welding procedure qualification with additional essential variables. Alkaline stress corrosion cracking is generally controlled by PWHT. Numerous special requirements for each control procedure are given in the document.

ENVIRONMENTAL CRACKING OF CARBON STEEL

I

(’

-) HYDROGEN STRESS CRACKING (HSC) [A,B)

\

-)

●H

@

NON-SULFIDE ENVIRONMENTS SUCH AS: Hydrofluoric

0“

Acid

●H

●

r

A

SULFIDE STRESS CRACKING

L--

INDUCED CRACKING (HIC)

(Ssc)(’)

See Footnote

ENVIRONMENTS SUCH AS:

(A,C)

1

STRESSORIENTED HYDROGENINDUCED CRACKING (SOHIC) (c)

B

---m-

Interrelationships

m---m-

-Caustic -Alkanolamine solutions containing C02 and/or H2S -Alkaline sour waters containing carbonates

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FIGURE 1 of the Various Cracking Mechanisms

(A) Refer to the NACE Glossary of Corrosion-Related Terms for definitions (including “stress corrosion cracking”). (B) The forms of environmental cracking included within the dotted line are commonly referred to as “wet H2S cracking” when they occur in wet sulfide environments. (C) This form of environmental cracking can also occur in non-sulfide environments such as hydrofluoric acid.