PM stainless steels uses in automotive exhausts Several applications for powder metallurgy produced stainless steel exhaust components have been approved and are in production today. Many more stainless steel parts, both ferritic and austenitic, are in the final qualification stages. In this mview, Robert J. Causton, Claudia Mumau and Tina M. Cimino of Ifoeganaes Corp examine the performance and economic criteria employed by automotive companies and exhaust manufacturers in sourcing exhaust coupling flanges and oxygen sensor bosses.
intered powder metallurgy (PM) stainless steel exhaust flanges and oxygen sensor bosses are in volume production for automotive exhaust systemsl. This apparently simple statement conceals considerable process and product development by parts fabricators and powder producers that enables a sintered part to compete with and displace wrought stainless steel components on performance and cost criteria. This paper examines technical aspects of the processing of stainless steel parts by PM processes that underlie this application.
Exhaust flanges The most complete review of the property requirements for PM stainless steel exhaust flanges was provided at the 1997 PM2TEC Conference and Exhibition2. The review covered a wide range of performance criteria including: gas sealing quality; creep resistance; weldability; hot tensile and compressive yield strength; environmental corrosion resistance; thermal fatigue resistance; and cost. The available mechanical property data3 suggest that pressed and sintered PM components would not be suitable for these applica-
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tions. Nonetheless, the PM flange met and exceeded the performance criteria and was accepted for volume production in competition with fully dense wrought stainless steel components. Clearly, the properties of PM stainless steels produced for automotive exhaust flanges must exceed the performance indicated by the MPIF Standard. This article will review some of basic process changes that must be made to produce PM exhaust flanges.
Stainless steel PM Stainless steels offer both an opportunity and considerable challenges to the PM parts producer. Stainless steels possess high value and are relatively difficult to machine compared with other wrought alloys. Thus, the high materials utilization of the PM process and the ability to produce through holes, without machining, offers significant potential cost savings by removing or reducing machining operations if an efficient process route can be devised. However, the physical metallurgy of stainless steels poses significant challenges to devising and operating an efficient process route to produce high quality parts. Stainless steels contain at least 11% chromium to maintain their stainless character. The presence of chromium in the prealloyed atomized powder significantly reduces compressibility4 and demands high quality sintering to prevent oxidation of the part during sintering5,6. Although these challenges are well publicized, their progressive solution leading to production of the PM exhaust flange is an excellent example of process improvement leading to improved performance and new markets. The interaction of process improvement and property development is illustrated by the conversion of mirror mounts, ARS Sensor Rings and exhaust flanges to PM stainless steel. PM stainless steels are typically produced to the MPIF specifications shown in Tables 2 and 3. The compositions are very similar to those of wrought steels and the alloying elements perform similar functions. Thus for the austenitic grades, the higher sulphur content of SS-303 alloys improves their machinability compared with SS-304 or SS-316. The higher carbon and nitrogen contents of the ‘Nl, N2’ alloys increase yield and tensile strength but decrease ductility and corrosion resistance. The basic compositions of the nonaustenitic PM stainless steels are shown in Table 3. Generally, increasing chromium content increases corrosion resistance. Increasing the
Copyright 0 1998, Published
by Elsevier Science Ltd All rights reserved
carbon and nitrogen content of the sintered part increases strength and hardness, but reduces ductility. Although the MPIF compositions are derived from standard wrought grades, powder and parts producers offer modifications to improve the performance of PM parts. These include the use of columbium in SS-409 Cb to ‘stabilize’ welded joints and proprietary alloys with copper and tin additions to improve corrosion resistance. PM
The high alloy content, especially chromium, of stainless steel powders poses several challenges to the production of PM stainless steel parts that can compete with wrought products. Almost all properties of pressed and sintered parts increase as the density of the compact increases. Performance increases as porosity is reduced by compaction and sintering processes. The high alloy content of stainless steels increases the microhardness and work hardening rates of powder particles so as to limit the density that can be achieved by compaction and limits increased densification at higher compaction pressures. Generally, compressibility decreases as alloy content increases, although SS-316L possesses higher compressibilty than SS304L. It appears that the increased molybdenum content of SS-316L stabilizes austenite and increases compressibility. Where necessary, powder manufacturers and parts producers will employ ‘L’ grades. Their lower carbon and nitrogen contents reduce microhardness of the powder particles and increase compressibility. The relatively low compressibility (Table 4) of stainless steel powders produces relatively low green strengths, which can make it difficult to eject powders from the die with-
out damage and to retain edge definition during handling.
PM miww mount The mirror mount that attaches the rear view mirror to the car windscreen was one of the first high volume automotive applications of PM stainless steel parts. It is typical of many ‘first generation’ PM components in that the PM process produces an accurate part more efficiently and at lower cost than competing processes. The major technical requirement of the part is that its coefficient of thermal expansion should match that of the windscreen glass. The application makes little demand upon the corrosion resistance of the stainless steel nor its mechanical properties. The property targets are achieved with the ferritic stainless steels, such as SS-410, SS-430 or SS-434. The performance requirements do not justify the higher cost of the austenitic steels. In principle, the part can be produced by standard PM compaction and sintered at conventional sintering temperatures of 1X20-1150°C. Thus, the part could be sintered on belt furnaces under conventional sintering atmospheres at relatively low cost. Its volume production can be considered as the first step enabling powder and parts makers to develop efficient process routes to compact and sinter stainless steels consistently.
PM ASS sensor The PM stainless steel ABS sensor, or tone wheel, can be considered as a second generation PM stainless steel component. The part design requires accurate shape, consistent soft magnetic properties and corrosion resistance during the life of the vehicle. These design requirements represent a considerable advance over the mirror mount in green strength, and corrosion resistance. The re-
MPR May 1998 23
quirement for soft magnetic properties dictates that a PM ferritic stainless steel be used. The improvements to processing that lead to improved part performance are illustrated below. Green strength: The detail and precision of the AEISsensor ring requires that the powder possess an improved green strength over the powders used for mirror mounts. One means to achieve this is for the parts producer to employ an annealed powder. Annealing improves green strength and can improve compressibility by reducing nitrogen contents. Annealing is an extra step in the production of stainless steel powders and thus increases final cost over that of as-atomized powders. In the case where the part producer uses press-ready premixes supplied by the powder producer, recent improvements in premix technology offer an improvement in green strength without an extra process step’. These improvements, particularly ‘Ancor GS-6000’, enable an atomized stainless steel powder, such as ‘Ancor 434L’, to develop significantly higher combinations of green strength and green density. The improvement in green strength, particularly at compaction
24 MPR May 1998
pressures of 550 and 690 MPa (40 and 50 tsi), is significantly higher than can be attained by annealing. Sintering conditions: The primary problem in processing PM stainless steels is the stability of chromium oxide with regard to the sintering atmosphere5v6. Reduction of chromium oxide requires high sintering temperatures greater than 1200°C and low dew points estimated at -20 to -40°C. This quality of sintering is above that used for most conventional PM parts. However, if not adhered to, the chromium in the stainless steel will act as a ‘getter’ and remove moisture and oxygen from the sintering atmosphere. Chromium also enhances the solubility of nitrogen in the iron powder. Thus if sintered in a nitrogen based atmosphere, the nitrogen content of the sintered parts will increase, reducing both magnetic properties and corrosion resistance. Production of high quality requires high sintering temperatures, a pure hydrogen content of low dew point or vacuum. One benefit of the high sintering temperature is that all sinking reactions are enhanced so that pores are closed during sintering, increasing part density and properties. Magnetic properties: Magnetic properties are extremely sensitive to microstructure, in particular levels of residual impurities, such as carbon, nitrogen and oxygen, that impede the motion of magnetic domain walls. Several reviews have indicated the beneficial effects upon soft magnetic properties of high temperature sintering stainless steels, in a hydrogen atmosphere416. Although different sensor designs require different magnetic properties, the Al3S sensor ring can be considered as a sofi magnetic application. This requires low coercive force, low remittance, high permeability and high maximum induction. These are all favoured by the change to hydrogen atmospheres and high temperature sintering. Although the data (Table 5) are for SS41OL, similar trends are observed for other grades including SS-430L and SS-434L (Table 6). Corrosion Resistance: The ARS sensor requires resistance to atmospheric salt corrosions,g. Improved corrosion resistance of stainless steels results from the presence of a coherent surface film of chromium oxide, the absence of chromium carbides that lead to chromium depletion and a smooth surface. In principle, the process changes that improve soft magnetic response should improve the corrosion resistance of PM stainless steels by reducing porosity and removing carbon and nitrogen. These basic factors have been confirmed and extended in a recent study of the corrosion behavior of ferritic stainless steelsg. The reference showed that the anodic polarization curve for high temperature (1290°C) sintered PM 434L test pieces showed a distinct passive region. The existence of this passive region implies that the PM steel should not corrode under the test conditions as long as the surface oxide film is maintained. This region is much less distinct
on the curve for PM 434L sintered at lower temperatures (1200°C) and that for PM 409Cb sintered at 1200°C. Although the passive region was not clearly developed for the PM 434L sintered at lower temperature, its polarization curve was similar to the ‘quasi passive’ behaviour of wrought 409. This scientific study was supported by corrosion tests under laboratory conditions following ASTM Standard 763. Measurements showed that the rate of intergranular corrosion of the PM 4341, sintered at 1200°C was less than that of wrought 434L (Table 7). The PM 409Cb did not possess such good response. It is possible that when sintered at 12OO”C,the columbium was still combined as oxide and could not reduce the rate of intergranular corrosion, The study also showed that the intergranular corrosion rate of a welded test piece was lower than that of a welded wrought 409 test piece. This is significant given prior concern over the weldability of PM components. Salt spray tests showed that the corrosion rates of the PM 434L in both as-sintered and welded condition were similar to those of wrought 409 (Table 8). The laboratory corrosion tests showed that the corrosion resistance of the PM alloys, particularly the high temperature sintered PM 434L, was equivalent to that of the wrought 409 alloy. A recent references extended laboratory results to examination of in-service parts. The results confirmed the laboratory finding that the corrosion resistance of high temperature sintered PM ABS sensor rings produced from SS-410L and a modified SS-434L were comparable to those of wrought alloys. The PM ABS sensor ring offers significant advantages in materials utilization over potential cast or machined stainless steels. The corrosion resistance of ferritic stainless steels enables the PM part to be used without the coating or sealing techniques required for competing PM magnetic alloys, such as plain iron or iron phosphorus alloys. The performance requirements of the part indicate that a SS-410L ferritic stainless steel produced by high temperature sintering represents an optimum material choice for many applications, although the part performance requires more controlled sintering than the mirror mount to achieve consistent performance.
The recent transition to stainless steel exhaust systems offers significant opportunities for the application of PM stainless steel components, such as exhaust flanges. The performance requirements for these parts present a more difficult challenge than AEKSsensors. They require a combination of high temperature oxidation resistance, corrosion resistance, resistance to thermal cycling and some degree of stress rupture and creep resistance. These requirements can be met by a range of high temperature corrosion resistant alloys including: ferritic and austenitic stainless
steels and the more highly alloyed iron chromium and nickel alloys. The optimum choice depends upon a balance of system design, performance and economics. The performance requirements require higher density levels than can be produced by single compaction. Thus, a high temperature sintering route, as used for high quality ASS sensors, that produces densification and pore closure is required. The preferred alloy systems include derivatives of 434L or the austenitic 316L. Mechanical properties: The high temperature sintering practice required for high quality AElS sensors increases part density significantly. Consequently, the mechanical properties of high temperature sintered stainless steel are significantly higher than those sintered at conventional sintering temperature&9*10.The data show that increasing sintering temperature from 1120 to 1260°C increases UTS by 50% for the ferritic grades 410L and 434L. The columbium-stabilized grades, 409Cb and 434 Cb show even larger increases (Table 9). MPR May 1998 25
Further improvements in mechanical properties may require an extension in the processing and sintering of austenitic grades or the introduction of enhanced sintering techniques to increase density and performance beyond current levels.
Acknowledgements Increasing sintering temperature and density have an even more significant effect upon impact energy. Impact energy, measured by unnotched Charpy test, increases from approximately 9.5 Nm when sintered at 1120°C to approximately 82 Nm when sintered at 1260°C. The improvements in mechanical properties of high temperature sintered parts translate to hot tensile properties. Increasing the test temperature naturally decreases the hot tensile strength of PM stainless steels. However, at 87O”C, the values compare well with those quoted for wrought stainless steels (Table 10). Overall, the test results show that by utilizing high temperature sintering, PM stainless steel components are produced with a combination of properties that compete favourably with wrought stainless steels. It appears that PM SS-434L materials possess the optimum balance of properties for several current applications, although some applications favour austenitic grades. Weldability: Weldability of PM stainless is a critical element in the development of automotive exhaust system components. Various welding trials l1 have confirmed the relative influence part density has on the overall integrity of the weldment. With rare exception, lower density parts generally result in greater percentages of weld porosity, whereas parts above 7.0 g.cmy3provide sound, high strength welds. It should also be noted that processing conditions, particularly those which influence residual interstitial levels, play an important role in determining weld quality. Lower percentages of interstitial elements (C, N, 0) minimize the formation of undesirable constituents and help prevent weld cracking. In view of the foregoing considerations, acceptable weld parameters (GMAW) and select filler metals have been developed to ensure stainless PM parts can be successfully welded and consistently satisfy the demanding automotive exhaust system requirements.
Further development The competition between material process routes is not static. As wrought technologies continue to develop to meet the cost and performance challenges of both PM and new exhaust systems, PM technologies will have to develop further. It is clear that the production of large complex components requires further improvements in the combination of strength and Reprinted with permission from SA E paper No. 9803 13 green density of PM stainless steel powders. 01998 Society of Automotive This could result from further improvements to annealing and premix technologies. Engineers, Inc.
26 MPR May 1996
The authors wish to thank Hoeganaes Corp for supporting this work. They acknowledge the contributions made by H.G. Rutz and others.
References (1) ‘Stainless Steel PM Part of the Year Award’, Znt. Journal of Powder Metallurgy, 33 (51, (19971, pp. 25-31. (2) Pamela Lee, ‘Requirements for Stainless Steel PM Materials in Automotive Exhaust System Applications’, PM2TEC 97 Conference, Chicago (1997). (3) MPZF Standard 35: Materials Standards for PM Structural Parts, 1997 Edition, Metal Powder Industries Federation, USA. (4) H. Kopech, H. Rutz, P. A. depoutiloff, ‘Effects of Powder Properties and Processing on Soft Magnetic Performance of 400 Series Stainless Steels’, Advances In Powder Metallurgy and Particulate Materials, (edited by A. Lawley and A. Swanson), Metal Powder Industries Federation, USA, (1993). (5) Howard Sanderow ted.), New Perspectives in Powder Metallurgy: High Temperature Sintering, Metal Powder Industries Federation, USA, (1990). (6) Chaman Lall, ‘Fundamentals of High Temperature Sintering as Applied to High Performance Stainless Steels and Soft Magnetic Materials: Parts I and II’, Zndustrial Heating, (November and December 1991). (7) Sydney H. Luk et al., ‘Processing Experience of Green Strength Enhanced Material Systems’, Advances In Powder Metallurgy and Particulate Materials, (1997), (compiled by R.A. McKotch and R. Webb), Metal Powder Industries Federation, USA. (8) Suresh Shah, et al., ‘On the Real Life Performance of Sintered Stainless Steel ABS Sensor Rings’, SAE Paper 970423. (9) M. Baran, H. Kopech, T, Haberberger et al., ‘PM Ferritic Stainless Steels for Exhaust System Components’, SAE Paper 9708282. (10) A.J. Rawlings, H.M. Kopech and H.G. Rutz, ‘The Effect of Service Temperature on the Properties of Ferritic Stainless Steels’, Advances In Powder Metallurgy and Particulate Materials, (compiled by R.A. McKotch and R. Webb), Metal Powder Industries Federation, USA, (1997). (11) J.J. Hamill, Jr, F.R. Manley, D.E. Nelson, ‘Fusion Welding of PM Components for Automotive Applications’, SAE Paper 930490. Contact: !Fim Hale, Marketing
Hoeganes Corp River Road and Taylors Lane Riverton, NJ 08077, USA, tel: +l-609-829-2220; fax: +l-609-786-2574.