Characterization of Nickel Diffusion and its Effect on the Microstructure of Nickel PM Steels
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ONE of the many advantages that the powder metallurgy (PM) process offers is its flexibility toward final chemical composition. In the case of PM steels, alloying elements can be admixed, prealloyed (or both) to the base powder to yield a rather wide variety of microstructures upon sintering.[1] Admixing pure elements to powder mixes can cause the formation of heterogeneous microstructures in sintered parts. This is mostly the case when substitution solid solution elements are admixed. It is well known that substitution elements have lower diffusion rates than interstitial elements.[2] Moreover, diffusion coefficients of an element vary with alloy chemistry. Lower diffusion rates, at a given sintering temperature, can lead to significant heterogeneity in the microstructure of PM parts.[3] Nickel is one of the most popular admixed elements in PM steels and is one of the chemical elements that increases the most the strength of PM steels as highlighted (FN-) in Figure 1. Nevertheless, admixed nickel is well known to produce nickel-rich areas (NRA) that are usually found near the original locations of nickel particles that did not disperse (or diffuse) uniformly in the steel matrix during sintering.[5] Typically, between 1 and 4 wt pct of BERNARD TOUGAS, Project Leader, is with the Department of Mining, Metallurgical and Materials Engineering, Universite´ Laval, 1065 De la Me´decine, Quebec, QC, G1V 0A6, Canada, and also with Centre de Me´tallurgie du Que´bec (CMQ); 3095 Westinghouse, TroisRivie`res, QC, Canada. Contact email: [email protected] CARL BLAIS, Professor, is with the Department of Mining, Metallurgical and Materials Engineering, Universite` Laval. FRANC¸OIS CHAGNON, Principal Scientist, is with Rio Tinto Metal Powders, 1655 rte Marie-Victorin, Sorel-Tracy, QC, J3R 4R4, Canada. SYLVAIN PELLETIER, General Manager Automotive Portfolio, is with National Research Council of Canada, 75 boul. de Mortagne, Boucherville, QC, J4B 6Y4, Canada. Manuscript submitted December 9, 2011. Article published online September 27, 2012 754—VOLUME 44A, FEBRUARY 2013
nickel is added to PM steel powder mixes to increase hardenability and consequently mechanical properties.[4] However, the presence of NRA, which contain much more nickel than the targeted alloy chemistry, generally leads to the formation of retained austenite. As it was recently pointed out,[5] the literature on the use of nickel in PM steels is somewhat confusing with regards to admixed nickel distribution in the iron matrix during sintering. This observation highlights the fact that the mechanisms of nickel diffusion in PM steels are still misunderstood by many researchers. The present work is aimed at characterizing the principal diffusion mechanisms of nickel and their influence on PM steel microstructures and properties.
II.
VOLUME DIFFUSION
Uniformly distributed admixed nickel is rarely obtained in iron-nickel (FN) steel series for conventional sintering conditions [1394 K (1121C)—30 minutes]. Nickel heterogeneity comes from its low volume (lattice) diffu
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