On the Optimization of Compressibility and Hardenability of Sinter-Hardenable PM Steels
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INTRODUCTION
IN North America, the automotive industry is the largest consumer of components produced by the powder metallurgy (PM) process. This process is well suited for the mass production of near-net-shaped components. In fact, the average north-American car contains approximately 43 pounds[1] of components made by PM. One of the main challenges of PM is that auto makers continually demand that ferrous PM components have improved dynamic mechanical properties for critical applications such as timing gears, clutch plates, connecting rods, etc. This is even more challenging because these high value parts must be produced under ever-increasing pressure to reduce cost. Strategies must therefore be developed to optimize the microstructure and maximize the final density of PM components. Superior mechanical properties are typically obtained with heat treatment, which calls for additional steps in the production sequence, thus increasing the cost adversely and affecting the geometrical conformance of the final part. In this regard, sinter-hardenable powders certainly show serious potential for the development of highperformance PM components. The approach consists of cooling the parts rapidly enough during the final stage of the sintering cycle to avoid the austenite transforming into pearlite and/or bainite, resulting in a fully martensitic NICOLAS GIGUE`RE, Research Engineer, is with the Centre de Me´tallurgie du Que´bec, 3095 Rue Westinghouse, Trois-Rivie`res, QC G9A5E1, Canada. Contact e-mail: [email protected] CARL BLAIS, Professor, is with the Department of Mining, Metallurgical and Materials Engineering, Universite´ Laval, Room 1728, Adrien-Pouliot Builing, Quebec City, QC G1V 0A6, Canada. Manuscript submitted March 26, 2012. Article published online June 25, 2013 4774—VOLUME 44A, OCTOBER 2013
microstructure. Parts are thus quenched at the end of the sintering cycle without the need for reaustenitizing and quenching in a second heat-treatment sequence. The primary advantages of sinter-hardening are[2] no additional heat-treatment steps, reduced distortion which is typically generated by the severity of the oil quench, and no cleaning step after heat treatment. Sinter-hardenability is dictated by the chemistry of the alloy, part density, and the cooling profile of the sintering furnace. Prealloyed chemical elements used to increase hardenability impede compressibility. On the other hand, admixed elements have a less negative effect on compressibility, although they are not as efficient in increasing hardenability, especially in terms of obtaining a homogeneous microstructure. Hence, to fully optimize hardenability and compressibility, a judicious, balanced choice must be made between prealloyed and admixed chemical elements. The aim of this study was to investigate and model the influence of admixing and prealloying on the optimization of the compressibility and hardenability of sinter-hardenable steel powders. Some of the results presented in the current article can be found in Gigue`re.[3] A first desig
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