Transitions in carbide morphology in a ternary Fe-C-W steel

Abstract

A ternary steel of composition Fe-0.20C-6.3W (wt pct) was examined with the purpose of characterizing the carbide morphologies that arise during the diffusional decomposition of austenite. Interphase precipitation of M 6 C with ferrite in the 700 ЊC to 800 ЊC temperature range was observed, as previously reported. At higher temperatures (800 ЊC to 920 ЊC), precipitation of M 6 C in austenite in a nodular fashion was also observed, never before seen in this alloy. The carbides inside such nodules exhibited a highly degenerate, quasilamellar morphology, and the evidence suggested a discontinuous precipitation mechanism. Comparison with a calculated phase diagram showed that this quasilamellar carbide precipitation reaction dominated both in the stable and metastable ferrite ϩ austenite ϩ carbide three-phase field, since full equilibrium, for a variety of reasons, is not obtained at the ''short'' reaction times over which this kinetically competitive reaction dominates the microstructural evolution. The formation of carbide-free ferrite preceding the onset of carbide precipitation (found in both cases) is seen to be instrumental in the occurrence of the quasilamellar carbide precipitation reaction, and a simple diffusion model is advanced in support of this view. Finally, the austenite inside these nodules was seen to revert to ferrite at long times, indicating that full equilibrium is not attained at the initial stages of austenite decomposition, in accord with expectation.

About one stable and three metastable eutectic microconstituents in the Fe–W–C system

“…This fact has already been observed by Uhrenius [13]. Usually iron is considered to have negligible solubility in WC [11]. In an earlier investigation concerning an alloy containing 4% Cr (Fig.…”

“…In 1985, a compilation by Rivlin [8] underlined the differences between the findings of different authors. In 1997, Shtansky et al [9] described the mechanism and kinetics of carbide transformation in Fe-W-C steels at 973 K. More recent studies have focused on the decomposition of austenite in Fe-W-C steels [10][11][12].…”