Microstructural and microchemical aspects of the solid-state decomposition of delta ferrite in austenitic stainless steels

The solid-state decomposition of delta-ferrite in a 20-10 stainless steel has been followed using optical microscopy, transmission electron microscopy, and high-resolution STEM microanalysis. As the quenching rate was increased, the formation of Widmanstätten austenite was replaced by a plane-front diffusional transformation, with near local equilibrium at the austenite-ferrite interface. Retained ferrite rims, observed at the impingement points of austenite grains, were used to characterize the nature of this transformation. Recent analyses of the thermodynamics of the massive transformation reaction are used to discuss the results.