TRANSITIONS IN CARBIDE MORPHOLOGY IN A TERNARY FE-C-W STEEL

A ternary steel of composition Fe-0.20C-6.3W (wt pet) was examined wit h the purpose of characterizing the carbide morphologies that arise du ring the diffusional decomposition of austenite. Interphase precipitat ion of M6C with ferrite in the 700 degrees C to 800 degrees C temperat ure range was observed, as previously reported. At higher temperatures (800 degrees C to 920 degrees C), precipitation of M6C in austenite i n a nodular fashion was also observed, never before seen in this alloy . The carbides inside such nodules exhibited a highly degenerate, quas ilamellar morphology, and the evidence suggested a discontinuous preci pitation 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 obtai ned at the ''short'' reaction times over which this kinetically compet itive 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 t he quasilamellar carbide precipitation reaction, and a simple diffusio n model is advanced in support of this view. Finally, the austenite in side these nodules was seen to revert to ferrite at long times, indica ting that full equilibrium is not attained at the initial stages of au stenite decomposition, in accord with expectation.