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 M6C with ferrite in the 700 °C to 800 °C temperature range was observed, as previously reported. At higher temperatures (800 °C to 920 °C), precipitation of M6C 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 + austenite + 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.