Phase transformations during heat treatment sequence of a carburized secondary-hardening steel

Microstructural evolutions occurring during tempering of carburized secondary-hardening (CSH) steels have been scarcely studied, contrary to low-alloy steels. Aim of this proposal is to perform in situ HEXRD experiments to address the new metallurgical issues raised by CSH steels. The latter are submitted, after carburizing, to a complex sequence of thermal treatments (annealing, austenitization, cyrogenic treatment, triple tempering). High amount of alloying elements induces various populations of carbides, which impact the metallurgical phenomena during the full sequence of treatments: austenite/martensitic transformations, carbide precipitation/dissolution, carbon redistribution and dislocations recovery. These phenomena occur simultaneously and HEXRD experiments have proved effective to track all of them in situ, in a quantitative way and with one-run experiments. Innovative heat treatments to produce and temper nanostructured bainite will also be studied in this proposal.