aisi grades of stainless steel for pm applications - GKN an

STAINLESS STEEL AISI GRADES FOR PMAPPLICATIONSChris SchadeHoeganaes CorporationCinnaminson, NJ 08077John SchaberlAncor SpecialtiesRidgway, PA 15853Alan LawleyDrexel UniversityPhiladelphia, PA 19104ABSTRACTApplications requiring stainless steels are growing at a rate of about 5% annually.Opportunities for using powder metallurgy (PM) exist, but additional grades not coveredby MPIF Standard 35 are required. The American Iron and Steel Institute (AISI) hasstandards for a broad range of stainless steels that can be used in many applications, butthe compositions of these grades must be modified to be conducive to manufacture byconventional PM techniques. Several of these grades have been produced as standardpress and sinter powders. The physical properties, mechanical properties andmicrostructures of these various grades are reviewed to serve as a guideline for PM partsmanufacturers and potential applications of these grades are addressed.INTRODUCTIONMPIF Standard 35 1 lists the most common grades of stainless steel used by PM partsmanufacturers. These include austenitic grades such as 303L, 304L and 316L, and ferriticgrades such as 409L, 410L, 430L and 434L. However, with the continued growth ofstainless steel there exists many opportunities for specialized stainless steel grades notcovered by MPIF Standard 35. These include applications requiring enhanced physicalproperties, corrosion resistance, weldability and machinability. There are additionalgrades covered by the American Iron and Steel Institute (AISI) that can be manufacturedby conventional press and sinter powder metallurgy (PM), Figure 1. The AISIdesignation for these alloys is well known with the number series 200 and 300 referringto austenitic stainless steels and the 400 series covering the ferritic and martensiticstainless steels. Letter designations attached to the end of the number series indicatemodifications to the composition. 2 Many societies such as the Society for AutomotiveEngineers (SAE) and the American Society for Testing and Materials (ASTM) use theAISI specification with the latter adding physical property specifications. 3 SAE and

sensitization, particularly in areas adjacent to the weld. Sensitization is the process bywhich chromium combines with carbon to form chromium carbides. The chromium isremoved from areas close to the grain boundaries and leaves these areas depleted ofchromium, with attendant susceptibility to intergranular corrosion. The formation ofchromium carbides is enhanced by temperature and generally occurs in austeniticstainless steels at temperatures between 480 o C and 815 o C (900 o F and 1500 o F). Thecooling rate resulting from the welding process is generally slow which increases thelikelihood of chromium carbide formation. Increased carbon levels due to insufficientlubricant burn-off can also increase the chance of sensitization. In order to avoidpostweld heat-treatment, stabilized grades of austenitic stainless steels have beendeveloped.In order to prevent the chromium from forming carbides, strong carbide-formingelements have been added to the austenitic grades of stainless steel. In AISI grades309Cb, 316Cb and 321L, titanium and niobium were added for this reason. In PMproducts, the use of niobium is preferred because water atomization oxidizes the titanium.Table IV lists the mechanical properties of these niobium-stabilized austenitic PM grades.In general, due to the addition of niobium and the formation of carbides, the ductility andimpact toughness of these grades are slightly lower than those of the non-stabilizedgrades. However, in general, this decline in mechanical properties is small, and can becompensated for by an increase in the level of other elements (such as nickel andchromium). Due to the formation of the carbides, the creep resistance of these grades ofstainless steel is improved.Table IV: Mechanical Properties of PM Austenitic Stainless SteelsImpactGreen SinteredApparentTRSDensity DensityHardnessUTS 0.20% OFFSET ElongationAISI UNS ft.lbs.f (J) (g/cm 3 ) (g/cm 3 ) (ksi) (MPa) (HRB) (ksi) (MPa) (ksi) (MPa) (%)302B S30215 70 94 6.44 6.72 210 1445 56 59 406 32 220 24.8303L S30300 63 84 6.60 6.97 205 1410 48 55 378 28 193 25.4304L S30403 60 80 6.57 6.90 180 1238 55 61 420 35 241 22.4304Cu S30430 64 86 6.73 6.86 129 888 28 55 378 30 206 20.0309Cb S30940 25 34 6.45 6.76 119 819 62 63 433 41 282 14.9310S S31008 38 51 6.42 6.80 139 956 63 61 420 41 282 14.5316L S31603 77 103 6.74 7.13 138 949 56 61 420 32 220 27.0316Cb S31640 73 98 6.56 6.88 137 943 51 51 351 29 200 15.0317L S31703 66 88 6.72 7.01 162 1115 61 70 482 42 289 22.0321L S32100 53 71 6.66 6.90 128 881 57 54 372 32 220 16.2904L N08904 44 59 6.52 6.80 130 894 56 55 378 33 227 14.8Other alloying elements, such as molybdenum, can be added to the austenitic stainlesssteels to improve corrosion resistance. Molybdenum, when added at levels between 2w/o and 4 w/o improves resistance to oxidation, pitting and crevice corrosion. Theaddition of molybdenum also tends to improve both room and high temperatureproperties such as tensile strength and creep. The mechanical properties of AISI 317L arecited in Table IV and the corrosion resistance is illustrated in Figure 3. Currently PMfabricators sinter austenitic grades in a hydrogen/nitrogen atmosphere to increase strength(MPIF Standard 35: grades SS-304N and SS-316N). In so doing, significant chromiumnitride formation occurs, which is detrimental to the overall corrosion resistance of thealloy. The addition of molybdenum is beneficial if both corrosion resistance and strengthare required.

(a) (b) (c)Figure 3. Representative appearance of salt spray specimens: (a.) 304L, (b.) 316L, (c.) 317L.More highly alloyed 300 series stainless steels are available that are designed to resistoxidation at high temperatures while maintaining a high degree of tensile strength andcreep resistance. These alloys rely on the formation of the chromium oxide film forprotection from corrosion, but the additional nickel and silicon in these alloys helps toform a more ductile scale, which increases its adherence to the base metal. The adherentscale is particularly important when service conditions involve cyclic temperatures. Theproperties of several of these PM grades (302B, 304L and 310L) are listed in Table IVand the relative oxidation resistance of these grades is shown in Figure 4.1.41.21304L302B310L% of Initial Weight0.80.60.40.20600 700 800 900 1000 1100 1200Temperature ( o C)Figure 4. Oxidation of 300 series stainless steels.

As with the other categories of stainless steels, there exists “super austenitics” whereincreased levels of nickel, copper, and molybdenum provide superior or specializedcorrosion resistance. However, for the PM grades of these alloys the increased alloycontent has a negative impact on powder compressibility and therefore on overall density.In consequence, care has to be taken to ensure that the increased corrosion resistance andenhanced mechanical properties gained by the increase in alloy content are not offset by areduction in achievable density. Type 904 stainless steel is considered a “superaustenitic” stainless steel.Martensitic Stainless SteelsMartensitic steels in the 400 series are similar to the ferritic stainless steels in that theycontain chromium in the range of 11 w/o to 18 w/o but also contain other elements suchas nickel Table V. The martensitic stainless steels are magnetic and are generally used inapplications where hardness and/or wear resistance is required. When heat-treated theycan achieve high strength and when tempered, they can exhibit some ductility.Essentially, these steels achieve mechanical properties comparable with those of a heattreatablelow alloy steel but with enhanced corrosion resistance, although their corrosionresistance is the lowest of any of the stainless steel categories. MPIF standard 35recognizes SS-410-90HT as a martensitic alloy. For this grade, the sintering atmospherecontains a high level of nitrogen, and the alloys form high temperature austenite, whichtransforms to martensite on cooling.Table V: Composition of PM Martensitic Stainless Steels (w/o)AISI UNS C S Si Cr Ni Cu Mo Nb414 M S414000.03 .030 1.0 12.0 1.25 .50 .50Max. Max. Max. 15.0 2.50 Max. Max.---414 M S414260.03 .030 1.0 12.0 4.0 1.5 1.5Max. Max. Max. 15.0 7.0 2.0 2.0---415 M S415000.03 .030 1.0 11.5 3.5 .50 .5Max. Max. Max. 14.0 5.5 Max. 1.0---420 M S42000.15 .030 1.0 12.0 .50 .50 .50.30 Max. Max. 14.0 Max. Max. Max.---440B MS440030.75 .030 1.0 16.0 .50 .50 .750.95 Max. Max. 18.0 Max. Max. Max.---440C MS440040.95 .030 1.0 16.0 .50 .50 .751.20 Max. Max. 18.0 Max. Max. Max.---410LCu J911510.15 .030 1.5 11.5 1.0 3.0 .50Max. Max. Max. 14.0 Max. 5.0 Max.---M Designates material made from a mix of a base powder and additives such as nickel, graphite, copper andmolybdenum.Other AISI martensitic grades of stainless steel, such as 420,440A, 440B and 440C, canbe processed by adding graphite to ferritic grades of stainless steels such as 410L and430L. Table VI gives the properties of 420L, 440B and 440C made by this approach.The level of carbon added to the alloy dictates the mechanical properties of themartensitic stainless steel. The higher the carbon content, the larger the extent of

chromium carbide formation, and the higher the strength and apparent hardness of thealloy.Table VI: Mechanical Properties of PM Martensitic Stainless SteelsImpactGreen SinteredApparentTRSDensity DensityHardnessUTS 0.20% OFFSET ElongationAISI UNS ft.lbs.f (J) (g/cm 3 ) (g/cm 3 ) (ksi) (MPa) (HRB) (ksi) (MPa) (ksi) (MPa) (%)410LCu J91151 13 17 6.43 6.84 249 1713 95 116 798 89 612 2.7414 S41400 50 67 6.55 6.95 223 1534 90 98 674 75 516 3.7414 S41426 21 28 6.62 7.00 209 1438 89 100 688 76 523 2.5415 M S41500 26 35 6.69 7.03 253 1741 96 108 743 80 550 2.9415 PA S41500 11 15 6.17 6.52 178 1225 83 98 674 73 502 2.3420 S42000 24 32 6.68 6.86 172 1183 67 57 392 37 255 2.2440B S44003 20 27 6.58 6.82 134 922 93 67 461 42 289 2.0440B HT S44003 3 4 6.58 6.76 128 881 29/c 74 509 70 482 0.6440C S44004 13 17 5.56 6.91 169 1163 94 79 544 61 420 2.2440C HT S44004 3 4 5.56 6.90 155 1066 37/c 65 447 62 427 0.6MAISI 415 = Powder Mix, AISI PA = Prealloy.A major drawback to carbon-containing martensitic stainless steels is their relatively poorcorrosion resistance and ductility. Low carbon-containing martensitic stainless steels canbe produced by adding nickel, molybdenum and copper to form martensitic stainlesssteels with improved toughness and corrosion resistance. Table V cites the compositionof several martensitic stainless steels made by adding nickel, copper and molybdenum.Nickel and copper are austenite formers, while molybdenum improves properties viasolid solution strengthening; this element is responsible for improving high temperatureproperties. While the apparent hardness of these alloys is slightly inferior to that of heattreatedcarbon-containing martensitic stainless steels, other mechanical properties such astensile strength, toughness and ductility are superior. These grades of stainless steel alsoexhibit superior corrosion resistance which reflects the absence of carbide formation andhence sensitization, as shown in Figure 5.(a) (b) (c)(d)(e)Figure 5. Representative appearance of salt spray specimen s: (a.) 440B, (b.) 440C, (c.) 410Lcu,(d.) super-martensitic admixed, (e.) super-martensitic prealloyed

As with other categories of stainless steels, super-martensitic stainless steels can beformed by adding high levels of nickel, copper and molybdenum. For PM alloys theprealloyed materials are usually low in compressibility but can exhibit superior corrosionresistance due to their high alloy content.Precipitation-Hardening Stainless SteelsPrecipitation hardening stainless steels are not defined by their microstructure, but ratherby strengthening mechanism. These grades may have austenitic, semi-austenitic ormartensitic microstructures and can be hardened by aging at moderately elevatedtemperatures, 480 o C to 620 o C (900 o F to 1150 o F). The strengthening effect is due tothe formation of intermetallic precipitates from elements such as copper or aluminum.These alloys generally have high strength and high apparent hardness while exhibitingsuperior corrosion resistance compared with martensitic stainless steels. Heat treatmentscan be used to vary the properties of the alloys and involve short times (1 h) attemperatures ranging from 480 o C to 620 o C (900 o F to 1150 o F). The aging treatment cantake place in either air or in nitrogen, depending on the surface appearance required.However, these alloys should not be subjected to welding or in service temperaturesabove the heat-treatment temperature because strength can be lost due to overaging.The AISI designation for these alloys is the 600 series of stainless steels, but most aremore commonly known by their alloy name, for example, 15-5PH, 17-4PH and17-7PH.The aluminum-containing precipitation hardening alloys are difficult to process by thePM route due to their high affinity for nitride formation and the difficulty in reducingaluminum oxide during sintering.Table VII and VIII give the chemical compositions and mechanical properties of severalprecipitation hardening alloys produced by conventional PM techniques. 17-4PH is amartensitic grade in which ductility and toughness are generally higher than in thecarbon-containing martenstitc grades. The mechanical properties of 17-4PH can beincreased by 15% by aging at 538 o C (1000 o F) for 1 h. Applications for this alloy existin the food, chemical and aerospace industries.Table VII: Composition of PM Precipitation Hardening Stainless Steels (w/o)AISIUNS C S Si Cr Ni Cu Mo17-4PHS174000.03 .030 1.0 15.0 3.0 3.0 .50Max. Max. Max. 17.0 5.0 5.0 Max.410LCu J91151.15 .030 1.0 12.0 .50 .50 .50.30 Max. Max. 14.0 Max. Max. Max.633 M S350000.07 .030 1.0 16.0 4.0 .50 2.500.11 Max. Max. 17.0 5.0 Max. 3.25Nb.15.45------633 is a semi-austenitic precipitation hardening stainless steel offering improvedcorrosion resistance compared with martensitic precipitation hardening alloys. Thesealloys are used for parts requiring high strength at moderately elevated temperatures.Depending on the aging treatment, the ductility and toughness of this alloy can approach

those of the austenitic stainless steels. The microstructure of the alloy is a mixture ofaustenite, martensite and small quantities of ferrite.Table VIII: Mechanical Properties of PM Precipitation Hardening Stainless SteelsImpactGreen SinteredApparentTRSDensity DensityHardnessUTS 0.20% OFFSET ElongationAISI UNS ft.lbs.f (J) (g/cm 3 ) (g/cm 3 ) (ksi) (MPa) (HRB) (ksi) (MPa) (ksi) (MPa) (%)17-4PH S17400 10 13 6.39 6.66 220 1514 81 92 633 60 413 2.817-4PH Aged S17400 6 8 6.39 6.66 273 1878 93 119 819 100 688 1.7410LCu J91151 21 28 6.50 6.98 219 1507 88 99 681 76 523 3410LCu Aged J91151 22 29 6.50 6.98 245 1686 94 113 777 94 647 3.5633 S35000 65 87 6.57 7.23 218 1500 94 111 764 63 433 4.5633 AGED S35000 67 90 6.57 7.24 265 1823 96 113 777 85 585 8.7Usage of the precipitation-hardening alloys is generally limited by the high cost of thealloying elements. Recently, a lower cost PM precipitation hardening alloy has beenintroduced based on UNS J91151 (a cast grade). 10 This alloy has only 13 w/o chromiumand utilizes the precipitation of copper to provide a low cost-high strength alloy withmoderate corrosion resistance. Table VIII shows that the mechanical properties approachthose of 17-4PH, while still maintaining a level of corrosion resistance that is better thanthat of the high carbon martensitic grades.Duplex Stainless SteelsTable IX: Composition of PM Duplex/Dual Phase Stainless Steels (w/o)AISI UNS C S Si Cr Ni Cu Mo NbDuplex- 329 M S329000.08 .030 1.0 23.0 2.5 .50 1.00Max. Max. Max. 28.0 5.0 Max. 2.00---Duplex- 2205 S322050.03 .030 1.0 21.0 4.5 3.0 2.50Max. Max. Max. 23.0 6.5 5.0 3.50---Duracor/3Cr12 S41003.15 .030 1.0 10.5 1.50 .50 .50.30 Max. Max. 12.5 Max. Max. Max.---Technically, duplex steels are stainless steels that contain two phases. 3 Duplex stainlesssteels are more-accurately defined as alloys containing a mixed microstructure of ferriteand austenite. New alloys being developed that contain mixtures of ferrite and martensiteare generally termed dual-phase. 11 Compositions of PM Duplex/Dual Phase stainlesssteels are listed in Table IX. A major advantage of these stainless steel grades is that eachphase imparts improved properties to the alloy.

(a)(b)Figure 6. Representative microstructures of (a) duplex stainless steel: (b) dual-phase stainless steel.Duplex stainless steels are ferritic stainless steels (Figure 6(a)) containing chromium andmolybdenum to which austenite formers (primarily nickel) have been added to ensurethat austenite is present at room temperature. Duplex stainless steels have severaladvantages over the austenitic grades including high strength, acceptable toughness, andsuperior corrosion resistance, particularly to chloride stress corrosion cracking. Themechanical properties of a duplex stainless steel (2205) are shown in Table X.Table X: Mechanical Properties of PM Duplex/Dual Phase Stainless SteelsGreen SinteredApparentImpactTRS UTS 0.20% OFFSET ElongationDensity DensityHardnessAISI UNS ft.lbs.f (J) (g/cm 3 ) (g/cm 3 ) (ksi) (MPa) (HRB) (ksi) (MPa) (ksi) (MPa) (%)Duplex- 329 M S32900 46 62 6.27 7.07 179 1232 50 76 523 67 461 7.7Duplex- 2205 S32205 82 110 6.48 7.20 202 1390 81 84 578 62 427 10.8Duracor/3Cr12 S41003 49 66 6.62 7.34 278 1913 88 107 736 82 564 3.1Dual phase stainless steels vary in composition but are generally non-austenitic (Figure6(b)) and magnetic, containing 11 w/o Cr. The chemistry of the alloy is balanced byferrite formers and austenite formers. The austenite transforms to martensite upon coolingresulting in a mixture of ferrite and martensite. Because of the low cost of the alloy it isused as a replacement for plain carbon steels where increased corrosion resistance isneeded. The martensite in the alloy allows the material to be used in applicationsrequiring strength and wear resistance. The properties of a PM version of this stainlesssteel (S41003) are cited in Table X.FATIGUE BEHAVIOR OF PM STAİNLESS STEELSFatigue tests were performed on some of the high strength PM alloys developed. The results ofthese tests,in terms of the 90% survival limit, are compared with those of other stainless steelfatigue data by Shah et al. 12 in Figure 7. The latter study study compared the fatigue strength ofvarious stainless steels as a function of tensile strength.

Fatigue Endurance Limit (KSI)504540353025201510500 100 200 300 400 500 600 700 800 900430L434L410L409LE434N430N2Tensile Strength (MPa)DUPLEX409LNi409LNi-HC40 50 60 70 80 90 100 110 120 130 140420Tensile Strength (KSI)410HT17-4PHDual Phase410LCuAged410LCuMartensiticFigure 7. Fatigue endurance limit (90% survival) as a function of tensile strength.The excellent fatigue response of these alloys is attributed to their high tensile strength.In general, fatigue crack propagation rates in PM steels are high and the fatigue limit isdictated by crack initiation rather than by crack propagation. Resistance to crackinitiation increases as the tensile strength increases. All the PM alloys included in Figure7 have high tensile strengths, and therefore high fatigue endurance limits. It appears thatthe addition of copper, nickel, and molybdenum, results in harder martensite, which has apositive effect on fatigue strength.3452952451951459545-5Fatique Endurance Limit MPa)CONCLUSIONS• Many AISI grades of stainless steel can be made via conventional wateratomization and press and sinter PM. These grades are not currently covered byMPIF Standard 35, but provide a range of properties and corrosion resistance thatcan lead to increased opportunities for PM parts producers.• These PM grades can be made as prealloys, or admixed nickel, copper andmolybdenum powders can be added to the base stainless steel.• In ferritic grades, higher levels of chromium, niobium and sulfur can lead toimproved mechanical properties, corrosion resistance and machinability.• The addition of carbon to low chromium PM alloys results in martensitic stainlesssteels with increased strength and apparent hardness. Additions of nickel, copper

and molybdenum produce low carbon martensitic stainless steels with increasedtoughness and corrosion resistance.• Niobium, when added to PM austenitic stainless steels, improves weldability.Increases in the molybdenum content of austenitic stainless steels can increasestrength and enhance corrosion resistance. Increases in chromium, nickel andsilicon levels enhance oxidation resistance.• Several precipitation hardening alloys with a range of mechanical properties,microstructure and attendant corrosion resistance can be produced byconventional PM processes.• Mixed microstructure stainless steels exhibiting excellent mechanical propertiesand corrosion resistance can be produced by conventional PM processes.• Fatigue response of the high strength PM alloys is a function of their tensilestrength.REFERENCES1. Materials Standards for PM Structural Parts, 2007, Metal Powder IndustriesFederation, Princeton, NJ.2. J.R. Davis, Alloying: Understanding the Basics, 2001, ASM International, MaterialsPark OH.3. J. Beddoes and J. G. Parr, Introduction to Stainless Steels, 1999, ASM International,Materials Park, OH.4. J.D. Redmond, Metals and Alloys in the Unified Numbering System, 10 th Edition,2004, Joint Publication of the Society of Automotive Engineers, Inc. and theAmerican Society for Testing and Materials.5. J.E. Bringas, Stainless Steel Data Book, 1992, CASTI Publishing Inc., Edmonton,Alberta, Canada.6. H. Cobb, Steel Products Manual- Stainless Steels, 1999, Iron and Steel Society,Warrendale, PA.7. T.R. Albee, P. DePoutiloff, G. Ramsey and G.E. Regan, “Enhanced Powder MetalMaterials for Exhaust System Applications,” SAE Paper No. 970281, SAEInternational, Warrendale, PA, USA.1997.8. T. Hubbard, K. Couchman and C. Lall, “Performance of Stainless Steel PM Materialsin elevated Temperature Applications,” SAE Paper No. 970422, SAE International,Warrendale, PA, USA.1997.9. R.J. Causton, T. Cimino-Corey, and C.T. Schade “Improved Stainless Steel ProcessRoutes,” Advances in Powder Metallurgy and Particulate Materials – 2003, compiledby R. Lawcock and M. Wright, Metal Powder Industries Federation, Princeton, NJ,2003, part 2 pp.1-13.10. A. Lawley, R.Doherty, P. Stears and C.T. Schade, “Precipitation Hardening StainlessSteels,” Advances in Powder Metallurgy and Particulate Materials – 2006, compiled

y W. R. Gasbarre and J.W. von Arx, Metal Powder Industries Federation, Princeton,NJ, 2006, part 7 pp.141-153.11. A. Lawley, E. Wagner, and C.T. Schade, “Development of a High-Strength-Dual-Phase P/M Stainless Steel,” Advances in Powder Metallurgy and ParticulateMaterials – 2005, compiled by C. Ruas and T. Tomlin, Metal Powder IndustriesFederation, Princeton, NJ, 2005, part 7 pp.78-89.12. S.O. Shah, J.R. McMillen, P.K. Samal and L.F. Pease, “Mechanical Properties ofHigh Temperature Sintered P/M 409LE and 409LNi Stainless Steels Utilized in theManufacturing of Exhaust Flanges and Oxygen Sensor Bosses,” SAE Paper No.2003-01-0451, SAE International, Warrendale, PA 15096, USA, 2003.