Redistribution of alloying elements during tempering of a nanocrystalline steel

Redistribution of alloying elements during tempering of a novel nanocrystalline steel consisting of a mixture of lower bainitic ferrite and carbon-enriched retained austenite has been analyzed by atom probe tomography. Three physical processes, namely redistribution of substitutional solute across the austenite/bainitic ferrite interface before retained austenite decomposition, redistribution of solute across the lower bainite cementite/ferrite interface, and the precipitation of transition carbides and cementite, have been observed. Results suggest that retained austenite decomposes during tempering before full equilibrium is reached at the interface. Moreover, cementite precipitates from supersaturated ferrite via a paraequilibrium transformation mechanism.