Stainless Steels Martensitic PDF
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Stainless Steels: Martensitic$ WM Garrison Jr., Carnegie Mellon University, Pittsburgh, PA, USA MOH Amuda, University of Lagos, Lagos, Nigeria r 2017 Elsevier Inc. All rights reserved.
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Introduction Basi Basicc Meta Metall llur urgy gy an and d Al Allo loyi ying ng of Mart Marten ensi siti ticc St Stai ainl nles esss Stee Steels ls He Heat at Trea Treatm tmen ent, t, As-Q As-Que uenc nche hed d Micr Micros ostr truc uctu ture ress and and Crys Crysta tall llog ogra raph phyy
4 5 6 7 8 9 10 11 Bibliography Further Reading
Toubrit ghittntleme esment s/Snt treof ngMa th R eens lastit ioicnsShta ipinle s less Em Embr le Mart rten itic tain ss Ste teel elss Corro orrosi sion on an and d Str tres esss Co Corr rros osio ion n Crac Cracki king ng Resi Resist stan ance ce Carbi arbide de-S -Str tren engt gthe hene ned d Mar arte tens nsit itic ic Sta tain inle less ss Ste teel elss Prec Precip ipit itat atio ionn-St Stre reng ngth then ened ed Mart Marten ensi siti ticc St Stai ainl nles esss St Stee eels ls Stee Steels ls Comb Combin inin ing g Ca Carb rbid ide e an and d In Inte term rmet etal alli licc St Stre reng ngth then enin ing g We Weld ldab abil iliity of Ma Mart rten ensi siti ticc St Stai ainl nles esss St Stee eels ls Summary
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1 3 3
4 4 4 4 6 7 7 8 8 9
Introd Introduct uction ion
Martensitic stainless steels are basically ternary alloys of iron, chromium, and carbon that possess a martensitic crystal structure in the hardened condition. In the basic composition, there is no nickel. They are ferromagnetic, hardenable by heat treatments, and are generally less corrosion resistant relative to the other classes of stainless steels. Chromium in the steel is in the range of 10.5–18 wt.% with a higher level of carbon than the ferritics; though, the chromium and carbon contents are balanced to ensure a martensitic structure after a complete cycle of heat treatment. Excess carbides may be present to increase wear resistance while elements such as niobium, silicon, tungsten, and vanadium may be added to modify the tempering response after hardening. Small amounts of nickel may equally be added to improve corrosion resistance in some media and to improve toughness. Sulfur or selenium is added to some grades to improve machinability (Davis, ( Davis, 1994 1994). ). These steels are often referred to as ‘air hardening ’ because after austenitization treatment in the furnace, cooling in still air is rapid enough to generate allotropic transformation into martensite. A wide range of strengths is achievable, with yield strengths ranging from 275 MPa in an annealed condition to 1900 MPa in the quenched and tempered condition. High hardness levels are also achievable, promoting metal-to-metal wear and abrasion resistance. In general, the low chromium content together with high carbon content in the martensitic grades lower corrosion resistance compared to the other classes of stainless steels. Thus, they are broadly selected for mild ambient conditions requiring a combination of high strength and corrosion resistance. The low chromium and low alloying element content of the martensitic stainless steels also makes them less costly than the other types. The basic grade, the 410 grade was invented by Harry Brearley in 1913 as the rst ever-produced ‘rustless steel,’ and commercialized and standardized in the 1930s and 1940s, historical applications of martensitic stainless steels (MSS) include cutlery, surgical instruments, scissors, springs, valves, shafts, ball bearings, turbine equipment, and petrochemical equipment. The progression of the basic 410 martensitic stainless steel grade to the various other grades with the presence of additional alloying elements is illustrated in Figure in Figure 1 and matched to specic composition in Table in Table 1 1 and 1.. The hardness range obtainable from common martensitic stainless steel grades at different conditions of heat treatment is shown in Figure in Figure 2 2.. Martensitic stainless steels are used when corrosion resistance and/or oxidation resistance are required in combination with either high strength at low temperatures or creep resistance at elevated temperatures. There are essentially three types of martensitic stainless steels. The rst type comprises those which contain carbon and which are strengthened by iron carbide precipitation when tempered at low temperatures or by alloy carbide precipitation on tempering at higher temperatures (secondary hardening). The second type is the steels which contain low amounts of carbon but which are strengthened by the precipitation of particles of copper or intermetallics on tempering. The third type is those which are strengthened by the precipitation of both alloy carbides and int interm ermeta etalli llics. cs. All thr three ee typ types es have have in com commo mon n a hig high h chr chromi omium um con conten tentt and alloyi alloying ng com combin binati ations ons tha thatt allo allow w
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Change History : November 2015. M.O.H. Amuda added Abstract, Keywords, expanded the Introduction section with gures and table, expanded text by introducing additional section on ‘ Weldability of martensitic stainless steels,’ and updated the list of references.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02527-3 doi:10.1016/B978-0-12-803581-8.02527-3
1
2 Stainless Steels: Martensitic
http://www.spiusa.com). ). grades of martensitic stainless steel from the 410 grade ((http://www.spiusa.com Figure 1 Development of other grades
http://www.totalmateria.com/page.aspx?ID CheckArticle CheckArticle& &site kts kts& &NM 199 199)) composition position and typical typical use of mart martensitic ensitic gra grades des ((http://www.totalmateria.com/page.aspx?ID Table 1 The com =
AISI Grade
C
Mn
Si
Cr
410
0.15
1
0.5
11.5–13.0
416
0.15
1.25
1
12.0–14.0
Ni
Mo
–
–
–
420
0.15–0.40
1
1
12.0–14.0
–
431
0.2
1
1
15.0–17.0
–
440A
0.60–0.75
1
1
16.0–18.0
–
440B 440C
0.75–0.95 0.95–1.20
1 1
1 1
16.0–18.0 16.0–18.0
– –
0.6
=
P
S
Comments/Applications
0.04
0.03
0.04
0.15
The basic composition. Used for cutlery, steam, and gas turbine blades and buckets, bushings Addition of sulfur for machinability, used for
0.04
0.03
1.25–2.00
0.04
0.03
0.75
0.04
0.03
0.75 0.75
0.04 0.04
0.03 0.03
–
=
screws, gears etc. by 416 Se replaces sulfur selenium Dental and surgical instruments, cutlery Enhanced corrosion resistance, high strength Ball bearings and races, gage blocks, molds and dies, cutlery As 440A, higher hardness As 440B, higher hardness
( http://www.kvastainless.com/stainlessFigure 2 Inuence of heat treatment on hardness prole of different grades of martensitic stainless steel (http://www.kvastainless.com/stainlesssteel.html). steel.html ).
Stainless Steels: Martensitic 3
austenitization at some elevated temperature, and martensite start and nish temperatures which permit quenching from the austenitizing temperature to obtain an almost completely martensitic structure with small amounts of retained austenite. Several review articles discuss the history and physical metallurgy of these steels (Irvine (Irvine et al., 1960; 1960; Irvine and Pickering, 1964; 1964; Pickering, 1976). 1976 ). Pickering ’s ‘Physical metallurgy and the design of steels ’ provides an excellent beginning for reading about these alloys (Pickering, 1978). 1978). ‘ The physical metallurgy of steels’ contains useful information about all classes of stainless steels, including the 1981). ). martensitic stainless steels (Leslie, (Leslie, 1981
2
Basic Me Metallur tallurgy gy and Alloying Alloying of Mart Martensiti ensitic c Stainless Stainless Stee Steels ls
The high temperature equilibrium microstructure of martensitic stainless steel is either entirely or almost wholly austenite (Davis, 1994). 1994 ). At ambient condition, however, it is a mixture of ferrite and carbide. But, this ambient condition microstructure is lost when the material cools back to room temperature upon reheating to austenitic phase. This is because the transformation back to ferrite is very slow rather martensite is formed. Indeed, it is very dif cult to avoid the formation of martensite in these steels. Therefore, with the appropriate chromium (10.5 wt.%) and carbon (0.15 wt.%) contents, martensitic microstructure can be formed and this must meet two requirements. The rst requirement is that the steels contain suf cient chromium to provide for corrosion resistance, which is achieved by the formation of a chromium oxide lm on the surface of the steel. It is generally believed that about 11 wt.% chromium is required for a steel to have stainless characteristics. However, the amounts of chromium required can vary with the composition of the steel, the microstructure of the steel, the extent to which the chromium content of the matrix is reduced owing to the formation of chromium-rich second phase particles such as chromium carbides, and the nature of the environment. The second requirement is that the steel can be austenitized. A binary Fe –Cr alloy containing more than about 10.5 wt.% chromium cannot be austenitized 1977). ). To allow the use of chromium contents of more than 10.5 wt.% austenite stabilizers such as carbon, nitrogen, (Lovejoy, 1977 nickel, and manganese are normally added to these alloys (Lovejoy, (Lovejoy, 1977). 1977). Otherr elem Othe elements ents are norm normally ally added to control the type and volum volumee frac fractions tions of stren strengthe gthening ning particle particless preci precipitat pitated ed on tempering. For example, molybdenum and tungsten are common additions to the carbide strengthened martensitic stainless steels and result in the precipitation of Mo2C (Raynor ( Raynor et et al., 1966) 1966) and W 2C particles (Davenport (Davenport and Honeycombe, 1975). 1975). Elements such as titanium and aluminum are added to the low carbon martensitic stainless steels to promote the precipitation of intermetallics such as NiTi and NiAl on tempering (Pickering, ( Pickering, 1976; 1976; Perry and Jasper, 1977). 1977). Molybdenum, tungsten, titanium, and aluminum are ferrite stabilizers which reduce the size of the austenite phase eld and austenite stabilizers are sometimes required to compensate for the additions of the ferrite stabilizers used to form strengthening precipitates. The alloying additions used for corrosion and oxidation resistance, to achieve high strength and to achieve austenitization, must also permit a fully martensitic structure on quenching from the austenitizing temperature. This means that the martensite start and nish temperatures cannot be too low. The effects of alloying additions on the martensite start and nish temperatures have been extensively studied for low alloy and high alloy steels both with and without carbon and chromium. All elements except 1978;; Magnee et al., 1974). 1974). While cobalt raises the martensite start cobalt depress the martensite start temperatures (Pickering, (Pickering, 1978 1964)) and to an even temperature in some systems it depresses the martensite start temperature in Fe–Cr alloys (Hammond, (Hammond, 1964 greater degree in Fe–Cr –C alloys (Coutsouradis, (Coutsouradis, 1961 1961), ), although the effect of cobalt on depressing the martensite start temperature is never as great as that of other substitutional alloying additions such as chromium, nickel, and molybdenum.
3
Heat Tre Treatmen atment, t, As-Qu As-Quenche enched d Microstructu Microstructures res and Cryst Crystallogr allography aphy
The martensitic stainless steels are austenitized, cooled to room temperature, possibly refrigerated at some temperature below room temperature to minimize retained austenite content, and then tempered to achieve the desired strength and toughness. A critical aspect of the heat treatment of these steels is the cooling rate employed while quenching from the austenitizing temperature. First, if the cooling rate is too rapid there is the possibility of quench cracking of the carbon steels. Quench cracking is a pro proble blem m in all steels steels con contai tainin ningg car carbon bon but it is a partic particula ularr pro proble blem m in steels steels conta containi ining ng carbon carbon and large large amo amount untss of chromium. chrom ium. This is becau because se the volume expan expansion sion associated with the austenit austenitee to mart martensit ensitee transform transformation ation increas increases es with 1958). ). Therefore, water and oil quenching of carbon-containing marincreasing carbon and chromium contents (Kenneford, ( Kenneford, 1958 tensitic stainless steels should be approached with caution. However, slow cooling of the carbon-containing martensitic stainless steels can lead to large amounts of retained austenite in the as-quenched structure, which is also undesirable. The effects of the rate of quenching on the microstructure and mechanical properties has not been investigated to the same extent for the low carbon precipitation-strengthened stainless steels. Because of their low carbon contents these materials should not be as susceptible to quench cracking as the carbon-containing martensitic stainless steels. The as-quenched microstructure of most martensitic stainless steels are similar. They will contain inclusions, as do all steels, ne second-phase particles inherited from the austenitizing temperature, and will largely consist of lath or dislocated martensite
4 Stainless Steels: Martensitic
rather than plate martensite. There will be thin lms of retained austenite between martensite laths, between martensite packets and at the former austenite grain boundaries if lath martensite is formed. Signi cant amounts of plate or twinned martensite should be observed only in the steels containing more than about 0.30 wt.% carbon. There will be regions of retained austenite between the martensite plates if plate martensite is formed. Two types of martensitic structure have been identied in stainless steels (Neraghi, ( Neraghi, 2009; p. 5). 5). The ferromagnetic, bodycentered cente red cubic (BCC) (BCC) a’-martensite and non-ferromagnetic, hexagonal closed-packed (HCP) e-martensite. The ferromagnetic martensite in steels usually has a BCC or body centered tetragonal (BCT) structure but in case of stainless steels due to the relatively low content of interstitials, the martensite is often referred to as BCC rather than BCT, therefore, the e-martensite is not quite common. It appears the e-mar -martensi tensite te with HCP cryst crystallogr allography aphy do occur in auste austenite nite.. The crystallog crystallography raphy of the martensi martensite te transformation is usually described by the habit plane and orientation relationships between the martensite and the parent (2012) reported that this relationship austenite phase which for martensite do change with chemical composition. Shibata et al. (2012) reported alternate between a Greninger – Troiano relationship and Kurdjumov –Sachs relationship particularly near the interphase boundary.
4
Toug Toughness/ hness/Stre Strength ngth Relationsh Relationships ips
When the room temperature strength and some measure of toughness, such as the Charpy impact energy, are plotted as a function of tempering temperature a minimum in toughness is observed which is usually coincident with the onset of precipitation hardening, harde ning, whethe whetherr the precipita precipitation tion strength strengthening ening is by parti particles cles of inte intermet rmetallics allics or by ne alloy carbides. The minimum toughness is often, but not always, associated with cleavage or quasi-cleavage fracture, indicating that the microstructures with the minimum in the room temperature toughness have ductile-to-brittle transition temperatures above room temperature. The toughness minimum appears to be simply associated with the precipitation of ne, sharable precipitates in the low-carbon precipitation strengthened steels (Garrison ( Garrison and Brooks, 1991). 1991). In the carbide-strengthened steels the embrittlement may be due not only to the precipitation of very ne alloy carbides but changes in the mechanical stability of the retained austenite and the precipitation of interlath carbides associated with the decomposition of retained austenite on tempering. The decomposition of retained austenite occurs at much higher tempering temperatures in the carbon-containing stainless steels than it does in low alloy steels (Garrison (Garrison et al., 1983). 1983). Thus, the embrittlement trough observed for the carbide-strengthened stainless steels may partly be owing to tempered martensite embrittlement (Horn ( Horn and Ritchie, 1978), 1978), which has been extensively studied in low-alloy steels.
5
Embr Embrittle ittlement ment of Martensitic Martensitic Stainless Stainless Steels Steels
In addition to the embrittlement behavior noted above, alloys of the Fe–Cr system can be embrittled by the precipitation of certain phases (Leslie, (Leslie, 1981). 1981). These precipitation processes exhibit typical carbon-curve behavior but are not usually observed in martensitic stainless steels during normal tempering. Precipitation of a 0 , s , and χ phases are associated with the embrittlement of these alloys. There is a miscibility gap in the Fe –Cr system which leads to the separation of the solid solution of chromium in iron into two solutions, the iron-rich phase a and the chromium-rich phase a 0 . Both a and a 0 are b.c.c. phases. Embrittlement by a 0 is favored by additions of aluminum, molybdenum, titanium, silicon, niobium, and phosphorus. The s phase formation is favored by high chromium contents and additions of molybdenum, silicon, titanium, and phosphorus. The χ phase phase formation is also favored by high chromium contents and by additions of molybdenum.
6
Corr Corrosion osion an and d Stress Corros Corrosion ion Cra Cracking cking Resista Resistance nce
The corrosion and oxidation resistances of high chromium steels are owing to the formation of a scale of chromium oxide on the surface. However, the effectiveness of a particular grade with respect to corrosion or oxidation resistance depends very much on the environment. Oxidation resistance of these alloys are discussed in the Handbook of Stainless Steels (Morris, 1977) 1977) and ‘Damage mechanisms and life assessment of high temperature components ’ ( Viswanathan, ( Viswanathan, 1989). 1989 ). Stress-corrosion cracking (SCC) can be regarded as ‘the brittle or quasiquasi-britt brittle le fract fracture ure of a mate material rial under the conjo conjoint int actions of a stre stress ss and a corro corrosive sive environm environment, ent, 1981). ). The SCC characteristics of martensitic and neither of which would cause fracture acting alone or consecutively ’ ( Truman, ( Truman, 1981 ferritic stainless steels are reviewed by Truman Truman (1981) (1981).. In general, the resistance of the martensitic stainless steels to SCC appears to be outstanding compared to high-strength martensitic steels of low chromium content and it appears that martensitic stainless steels can be used in situations where austenitic stainless steels would fail.
7
Carb Carbide-S ide-Streng trengthene thened d Martensiti Martensitic c Stainless Stainless Steels Steels
Broadly speaking the steels in this category can be divided into three groups (Table (Table 2). 2). The rst group is the low to medium carbon (0.10–0.30 wt.%) alloys containing about 12 wt.% chromium. The second group of carbide-strengthened martensitic stainless
Stainless Steels: Martensitic 5
Compositionss in wt.% of carbide strengthened martensitic sta stainless inless steels Table 2 Composition Alloy
C
Cr
410 416 422 440A 440B 440C D2
0.10 0.15 0.23 0.60/0.75 0.75/0.95 0.95/1.20 1.50
12 12 12 17 17 17 12
D7
2.35
12
Ni
Mo
V
W
Mn
Si
– – 0.22 – – – 1.0
1 .0 – – – –
1.0 1.25 1.0 1 .0 1. 1 .0 1. 1 .0 1. 0 .3 0.
1.0 1.0 0.75 1.0 1.0 1.0 0.25
4.0
–
0 .4 0.
0.4
– – 0.75 – – – –
– 0.60 1.0 0.75 0.75 0.75 1.0
–
1.0
steels are those containing much higher levels of carbon (0.60–1.2 wt.%) and rather large amounts of chromium (16–18 wt.%). The nal group of carbide-strengthened martensitic stainless steels are the high-chromium cold work die steels such as D2 and D7. This rst group can be divided further into four classes. The rst class would be alloys of this type which contain no (or very small amounts) of strong carbide-forming elements or cobalt. The 410 alloy would be typical of this class. The second class would be steels of this type which contain molybdenum but no other strong carbide-forming elements or cobalt. The alloy 416 would be typical of this class. The third class would include steels of this type which contain molybdenum and other strong carbide-forming elements but no cobalt. The alloy 422 is an example of such a material. The nal class would be such steels containing strong carbide-forming elements such as molybdenum and tungsten and vanadium and cobalt; these compositions often are strengthened by both alloy carbide and intermetallic precipitation. A major driving force for the development of the low to medium carbon martensitic stainless steels has been the improvement of high temperature properties, especially creep resistance, and high temperature ductility (Briggs (Briggs and Parker, 1965 1965). ). The simplest of the commercially available carbon-strength carbon-strengthened ened martensitic stainless steels is the 410 alloy. While there are many modications of the 410 alloy, the tempering characteristics of the alloys are similar to that of a 0.1C/12Cr composition. The tempering curve (a plot of room temperature hardness as a function of tempering temperature for a xed tempering time) of the 0.10C/12Cr alloy is essentially at for tempering temperatures from 200 to 500 C, although a small secondary hardening peak 1977). ). For tempering temperatures greater than 500 C the hardness drops might be observed on tempering at 500 C (Lovejoy, 1977 rapidly with increasing tempering temperature. In the as-quenched condition and for low tempering temperatures the carbides within the laths are M3C, where the M is iron and chromium and the chromium occupies a maximum of about 18% of the metal sites. As the tempering temperature is increased the M3C particles are replaced by M7C3. Chromium occupies about 50% of the 1965). ). M7C3 can either form by nucleation at defects or it can form by in situ metal sites in M7C3 ( Woodhead and Quarrell, 1965 nucleation at M3C particles. For tempering temperatures greater than about 500 C, M 23C6 forms at lath and other boundaries and grows at the expense of the M 7C3 particles (Irvine (Irvine et al., 1960). 1960). Modications of the simple 0.10C/12Cr alloy, typied by the alloy 410, include additions of the strong carbide-forming elements molybdenum, tungsten, and vanadium, the addition of silicon and cobalt. In some materials there are small additions of niobium. The primary aims of these additions, except for the niobium, have been to increase the hardness after tempering at higher temperatures or to introduce particle distributions on tempering which will resist coarsening at higher temperatures and thus provide improved creep performance. 1
1
1
1
The additions of molybdenum and/or tungsten in suf cient amounts can result in the precipitation of M 2C particles, as molybdenum can form Mo 2C (Raynor ( Raynor et et al., 1966) 1966) and tungsten can form W 2C (Davenport and Honeycombe, 1975). 1975). M2C can 1965). ). The precipitation of M2C have up to at least 30% of the metal sites occupied by chromium atoms ( Woodhead ( Woodhead and Quarrell, 1965 can lead to a pronounced secondary hardening peak and to higher hardness at tempering temperatures above 500 C. When vanadium is added to a 0.10C/12Cr steel a vanadium carbide will be formed on tempering and this will have an effect on 1966). Also, tempering response qualitatively similar to those obtained by adding molybdenum or tungsten (Raynor (Raynor et al., 1966). vanadium additions to steels to which molybdenum has also been added can have a benecial effect on tempering response as vanadium partitions strongly to M2C and this partitioning has the effect of making the particles of M2C more resistant to coarsening (Raynor (Raynor et et al., 1966). 1966). 1973) and silicon (Irvine, (Irvine, 1962 1962)) do not form carbides but when added to steels they have signicant Cobalt (Speich (Speich et al., 1973) effects on carbide precipitation. It has been suggested that cobalt has this effect because it increases the activity of carbon in ferrite and this increased carbon activity would increase the driving force for the formation of M2C, and hence increase the nucleation rate of M 2C particles. Also, because cobalt reduces the rate of dislocation recovery and the greater dislocation density will result in more and ner M2C particles, since the particles of M 2C are believed to nucleate at dislocations (Speich ( Speich et al., 1973). 1973). Niobium additions are sometimes made in small amounts to these steels and these additions have the effects of re ning the 1
prior austenite of lowering ened the ductile-to-brittle transition temperature. The second grain groupsize of and carbide-strength carbide-strengthened martensitic stainless steels are those containing much higher levels of carbon (0.60–1.2 wt.%) and rather large amounts of chromium (16 –18 wt.%). Examples of these steels are 440A, 440B, and 440C
6 Stainless Steels: Martensitic
(Roberts and Cary, 1980). 1980). These steels have a higher carbon content in order to achieve more hardness and to ensure the existence of primary carbides after austenitizing. Both the greater hardness and the primary carbides in the structure promote wear resistance. Higher chromium is required for these steels because much of the chromium is consumed by the formation of the primary carbides. The nal group of carbide-strengthened martensitic stainless steels are the high-chromium cold work die steels such as D2 and D7 (Roberts ( Roberts and Cary, 1980). 1980). They combine both high hardness with large volume fractions of primary carbides to promote wear resistance. Given the amounts of chromium in the steels and the amounts of chromium which should be in the primary carbides it is doubtful if these steels have the corrosion resistance one would expect from a stainless steel. Both the amounts of primary carbides and amounts of retained austenite present in D2, D7, and the alloys 440A, 440B, and 440C are very sensitive to the austenitizing temperature used and the cooling rate from the austenitizing temperature.
8
Prec Precipita ipitationtion-Stren Strengthene gthened d Martensitic Martensitic Stainless Stainless Steels Steels
Martensitic precipitation-strengthened stainless steels are low-carbon martensitic stainless steels which are strengthened by the precipitation of second-phase particles, other than alloy carbides, during tempering of the steel. A number of different approaches 1977). ). Such steels can be strengthened by to strengthening such steels have been investigated (Pickering, (Pickering, 1978; 1978; Perry and Jasper, 1977 the precipitation of pure copper; the steel PH15-5 is an example of a martensitic stainless steel strengthened by the precipitation of 1982). Such steels can copper particles. Precipitation of the compound NiAl is used to strengthen the steel PH13-8 ( Taillard ( Taillard et al., 1982). be strengthened by the precipitation of NiTi and Ni3 Ti. The precipitation of Ni3Be has been reported in steels modi ed by additions of beryllium (Smolinski, (Smolinski, 1966 1966;; Crawford and Contractor, 1969 1969). ). Japanese workers have reported the precipitation of a G phase having the composition Ni 16 Ti6Si7 in mart martensit ensitic ic stain stainless less steels ((Hoshino 1966). The nal strengthHoshino and Utsunomiya, 1966). ening system reported in the literature is strengthening by the R-phase. R-phase is a complex compound which is precipitated in 1960; Dyson and Keown, 1969 1969). ). martensitic steels containing chromium, molybdenum, and cobalt (Komura ( Komura et al., 1960; While there has been no attempt to exploit strengthening by G-phase or Ni3Be precipitation in commercial alloys, strengthening by copper precipitation, NiAl precipitation, NiTi or Ni 3 Ti precipitation, and R-phase precipitation have been used in alloys Magnee et al., 1974). 1974). The compositions of which are currently being made or which have had at least a brief commercial existence ((Magnee some examples of these alloys are given in Table 2 and 2 and mechanical properties of these alloys after conventional heat treatment are given in Table 3. 3. Commercial alloys which have employed copper precipitation include PH15-5, PH17-4, and Custom 450 (Table 4). threee alloy alloyss reach a peak yield stre strength ngth of about 1250 MPa MPa.. The steel PH13 PH13-8, -8, strengt strengthene hened d by precipita precipitation tion of 4). These thre NiAl, reaches a peak yield strength of about 1450 MPa. Custom 465, strengthened by precipitation of Ni 3 Ti, achieves a peak yield strength of about 1600 MPa (Carpenter ( Carpenter Specialty Alloys, 1998). 1998). There have been a number of steels which have employed R-phase Compositionss in wt.% of martensiti martensiticc precipitatio precipitation n strengthe strengthened ned stainles stainlesss steels Table 3 Composition Alloy
C
Cr
Ni
PH15-5 Custom 450 PH17-4 PH13-8 Custom 465 Custom 455 Pyromet X-15 Pyromet X-23
0.04 0.04 0.07 0.03 0.02 0.05 0.01 0.03
15 11.5 16.5 12.6 11.8 11.5 15 10
4.7 8.5 4.0 7.9 11 8.5 – 7
Mo
Co
– – – 1.7 1 0.5 2.9 5.5
– – – – – – 20 10
Cu
3.0 1.5 3.5 – – 2.0 – –
Al
– – – 1.0 – – – –
Ti
0.20Nb 0.7Nb 0.3Nb þ Ta – 1.7 1.1 – –
off selected martensitic p precipitation recipitation strengthened stainless ste steels els Table 4 Properties o Alloy
Tempering temperature ( C)
PH15-5 Custom 450 PH17-4 PH13-8 Custom 465 Custom 455 Pyromet X-15 Pyromet X-23
496 510 550 510 510 510 550 510
1
Yi Yiel eld d stre streng ngth th (MPa (MPa))
UT UTS S (M (MPa Pa))
1213 1269 1117 1448 1603 1516 1482 1634
1317 1289 1158 1551 1738 1585 1620 1779
Fr Frac actu ture re toug toughn hnes esss (M (MPa Pa m 1/2 )
– – – – 93 – – 77
Charpy (J) 79 55 54 41 – 19 20 24
Stainless Steels: Martensitic 7
Compositionss in wt.% of martensitic stai stainless nless steels combining combining intermetallic an and d alloy carbide streng strengthening thening Table 5 Composition Alloy
C
Cr
Ni
AFC77 AFC260 HSL 180 CSS-42L
0.15 0.08 0.20 0.13
14.5 15.5 12.5 13.8
– 2.0 1.0 2.1
Mo
Co
5 4.3 2.0 4.7
13.5 13.0 15.5 12.5
V
Nb
– – – 0.60
– – – 0.04
martensitic artensitic stainless stainless steels comb combining ining interm intermetallic etallic and allo alloyy carbide stren strengthening gthening Table 6 Properties of m All Alloy
Temp emper eriing te tem mperat eratu ure ( C)
AFC77
371 593 427 538 385 496
1
AFC260 HSL 180 CSS-42L
Yiel Yield d stre streng ngth th (MPa (MPa))
UT UTS S (MPa (MPa))
Frac Fractu ture re toug toughn hnes esss (M (MPa Pa m 1/2 )
Charpy (J)
1379 1476 1296 1572 1340 1200
1758 2000 1544 1751 1800 1764
68 25 101 67 121 –
24 12 – – 43 65
1974). These steels include Pyromet X-15, Pyromet X-23, and the British steel D-70. All of these strengthening (Magnee (Magnee et al., 1974). steels, with the exception of Custom 465, were developed in the 1960s and early 1970s. None of the alloys using R-phase strengthen stren gthening ing are curr currently ently produce produced. d. Asaya Asayama ma has inves investigat tigated ed the effe effects cts of comp compositio osition n on the toughnes toughnesss of R-ph R-phase ase 1976,, 1978). 1978). Custom 450, Custom 465, Pyromet X-15, and Pyromet X-23 are trade-marks of strengthened steels ( Asayama, ( Asayama, 1976 Carpenter Technology.
9
Stee Steels ls Combin Combining ing Carb Carbide ide and Int Intermet ermetallic allic Strengt Strengthenin hening g
There have been attempts to combine secondary hardening with strengthening by other particle types ((Tables 5 and 6 Tables 5 and 6). ). This work 1963)) and involved the combined precipitation of NiAl and alloy carbides (secondary was pioneered in the UK (Pickering, 1963 1963; Webster, 1971 1971), ), which take advantage of hardening). Crucible developed the alloys AFC77 and AFC260 (Kasak ( Kasak et al., 1963; secondary hardening and the precipitation hardening obtained when large amounts of chromium, molybdenum, and cobalt are simultaneously present (R-phase strengthening). The steels AFC77 and AFC260 were developed in the 1970s and are not produced today. However, two steels which would appear to combine secondary hardening and R-phase strengthening have recently been introduced. HSL 180 was developed by Sumitomo Precision Products and Hitachi Metals and CSS-42L was developed by Timken Latrobe Steel for bearing applications.
10
Weldabilit Weldabilityy of Ma Martensi rtensitic tic Stainle Stainless ss Steels Steels
The relatively high carbon contents of the martensitic grade compared to the other classes making welding dif cult in these grades. These steels are specically sensitive to both weld region and heat affected zone cracking mainly as a result of the localized stress associated associ ated with the volum volumee chang changee accom accompanyi panying ng mart martensit ensitic ic tran transform sformation ation.. This is furt further her increased with the presence of hydrogen in the weld resulting in hydrogen induced cracking. Therefore, there must be great caution when welding them. In most cases, a regime of preheat and/or post weld heat treatment is applied to the steels to prevent incidence of weld cracking and produce sound weld. Preheating and interpass temperature in the range of 200 –300 C is recommended for most martensitic stainless steels combined with low hydrogen welding processes. Grades with over 0.2 wt.%C usually require post weld treatment 2001). to soften and toughen the weld. Preheating temperature is dependent on the detailed composition of the steel ( Béres et al., 2001). Furthermore, a range of matching electrodes are available to maintain the corrosion and mechanical properties of the weld with those of parent metal; most of these matching electrodes contain small addition of nickel to prevent the formation of ferrite in the weld. It must be stated that since nickel lowers the temperature at which martensite transforms to austenite, post weld heat treatment of welds made with such ller metals should not exceed 750 C otherwise untempered martensite will form in the weld 1
1
as it to ambient condition. Acools standard heat treatment cycle to control weld microstructure and properties in martensitic stainless steel involves cooling the weld to below 100 C to achieve complete transformation of both the weld region and heat affected zone to martensite. This is 1
8 Stainless Steels: Martensitic
followed by controlled post weld heating to about 700 C for 1–4 h to minimize stresses from temperature variations then cooling to ambient conditions. 1
11 Summ Summar aryy The metallurgy and characteristics of martensitic stainless steels have been reviewed in this article although not exhaustive, yet it presented a general overview of the steel. There are three types of martensitic stainless steels. The rst type consists of alloys containing carbon which are strengthened by the precipitation of alloy carbides. Alloys of this type fall into two main groups. The
–
rst corrosion are alloys resistance. with carbon levels of 0.10 wt.%, primarily designed forthese creepalloys resistance additionprimarily to oxidation and The second group0.30 consists ofwhich alloys were with high carbon contents; were in developed for wear resistance. The second type of martensitic stainless steel is low carbon alloys strengthened by the precipitation of intermetallics. The most widely used alloys of this type are PH13-8 and PH15-5. However, none of the alloys of these two types have suf cient toughness at high strength levels to be used in critical applications requiring both high strength and high toughness. The alloy development leading to the third type of martensitic stainless steels, those alloys combining strengthening by alloy carbide and intermetallic strengthening, was motivated by a need for stainless steels with reasonable toughness at high strength levels. The weldability of the steel iiss poor in relation to other classes of stainless steel due to its higher carbon content but preheat and post weld heat treatment offer opportunity for improving the weldability combined with the use of appropriate matching electrode.
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The Physical Metallurgy of Steels. New York, NY: McGraw-Hill. Lovejoy, P.T., 1977. Structure and constitution of wrought martensitic stainless steels. In: Peckner, D., Bernstein, I.M. (Eds.), Handbook of Stainless Steels. New York, NY: McGraw-Hill (Chapter 6). Magnee, A., Drapier, J.M., Dumont, J., Coutsouradis, D., Habraken, L., 1974. Cobalt-containing High Strength Steels. Brussells: Centre D’Information du Cobalt. Morris, L.A., 1977. Resistance to corrosion in gaseous atmospheres. In: Peckner, D., Bernstein, I.M. (Eds.), Handbook of Stainless Steels. New York, NY: McGraw-Hill (Chapter 17). Neraghi,, R., 2009. Marten Neraghi Martensitic sitic transformation transformation in austeni austenitic tic stainless steel. Master’s Dissertation. Swede: Royal Institute of Technology, p. 5. Perry, D.C., Jasper, J.C., 1977. Struct Structure ure and constit constitution ution of wrought precipitationprecipitation-hardena hardenable ble stainless steels. 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Stainless Steels: Martensitic 9
Further Reading Martensitic Stainless Steel, 2015. Available at: http://www.totalmateria.com/page.aspx?ID=CheckArticle& Martensitic http://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts site=kts& &NM=199 NM=199 (accessed 09.02.16). Stainless Stainl ess Steel: Versatile, Versatile, reliable, reliable, functional functional and economical, 2015. Available Available at: at: http://www.spiusa.com/stainlesssteel_overview.php http://www.spiusa.com/stainlesssteel_overview.php (accessed 08.02.16). http://www.kvastainless.com/stainless-steel.html (accessed 08.02.16). Why Chose Stainless, 2015. Available at: at: http://www.kvastainless.com/stainless-steel.html
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