Molybdenum (Mo) has been used as an alloying element in powder metallurgy (PM) steels for several decades, starting in the 1960's with early sponge-based diffusion alloys and later with Fe-Mo-Ni prealloys for powder forging applications. The first Fe-Mo steel powders were commercially introduced in 1990. A 0.85 wt.% Mo alloy (Ancorsteel 85 HP) was presented by Hoeganaes Corporation at the Metal Powder Industries Federation (MPIF) 1990 Powder Metallurgy Conference & Exhibition in Pittsburgh1 and a 1.5 wt.% Mo alloy was introduced by Hoganas AB at the PM'90 World Conference on Powder Metallurgy that same year.2 Other powder manufacturers quickly introduced competing grades as the importance and usefulness of these alloy systems became apparent. In North America, the Mo grades were combined with admixed Ni and/or Cu additions for highly compressible powders used in hybrid alloys for as-sintered, heat-treated, and sinter-hardened applications. In Europe, diffusion alloys based on 1.5 wt.% Mo were introduced in the early 90's and remain the primary sinter-hardening grades in the region to date. It is interesting that the initial preferences established by the grade introductions in 1990 continue today, with the greater use of the 0.85 wt.% Mo alloy in North America and the higher use of the 1.5 wt.% Mo in Europe.More recently, Fe-Mo grades with lower Mo contents have been introduced with 0.3 wt.% and 0.5 wt.% Mo. The 0.5 wt.% Mo was primarily introduced as a base to produce binder-treated premixes (FLN2C-4005 and FLN4C-4005) with equivalent chemical composition to the diffusion alloys FD-0205 and FD-0405. It has also been used as a base for modified diffusion alloys of this same composition. With the large increase in Mo prices in the 2000's, interest increased in lower Mo alloys for heat-treated grades, and both the recently introduced 0.3 wt.% Mo~3 and the existing 0.5 wt.% Mo materials gained acceptance in these roles. Table I shows the different Fe-Mo prealloy grades available with the corresponding MPIF designation and the year they first appeared in the standard, either as a prealloyed material, or part of a hybrid or diffusion alloy. The importance of these prealloyed steels in the industry cannot be overstated. The subsequent sections of this article describe the technical benefits of molybdenum in steel and its use in PM alloys.
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