The effect of heating rate on the martensite to austenite transformation in Fe-Ni-C alloys

Abstract

The martensite to austenite ( α′ → γ) transformation was studied as a function of heating rate in a series of Fe-Ni-C alloys with carbon contents of 0.004, 0.05, 0.3 and 0.6 wt. %. Observations of microstructure, fine structure, surface relief and the transformation temperatures determined from thermal arrest data for heating rates between about 3 and 28,000°C/sec showed that the α′ → γ transformation can occur by two different, often competing mechanisms. The mechanism which predominates at the higher heating rates in the three higher-C alloys (and over the entire range of rates in the 0.004C alloy) results in second generation γ plates with high dislocation densities and is accompanied by the sharp tilt surface relief characteristic of a martensitic-type transformation. At lower heating rates the α′ → γ transformation in the three higher-C alloys occurs largely by a thermally activated mechanism as indicated by a significant initial increase in the transformation finish temperature, A f , with heating rate and confirmed by the observation of newly-formed γ areas with equiaxed morphology, relatively low dislocation density and lack of sharp tilt relief. The thermally activated transformation apparently involves short-range diffusion across a curved, incoherent interface.