Martensite and nanobainite transformations in a low alloyed steel studied by in situ high energy synchrotron diffraction

Martensitic and nanobainite transformations are studied in situ in a low alloyed, high-Si steel by using in situ HEXRD, combined with dilatometry and SEM observations, and by considering the same steel composition and austenitization conditions. The martensitic microstructure presents a mixed lath-plate morphology with large scatter of sizes whereas the bainite microstructure shows finer laths with more uniform sizes. Recently introduced methods are used to track in situ by HEXRD, in one single experiment, the phase fractions, the distribution of the carbon and the evolution of the dislocation densities. The study of nanobainite revealed that about two thirds of the carbon partitions from the ferrite to precipitate into transition iron carbides or to enrich the austenite. Both processes occur very fast after the formation of each nanobainite lath, but the ferrite remains largely supersaturated in carbon. The dislocation density increases inside each new forming bainitic ferrite lath. It then decreases when recovery becomes preponderant, as described with a recovery model from the literature. After the martensitic transformation, the retained austenite ends up with high hydrostatic compressive stresses. Dislocation densities are higher than in nanobainite and probably more heterogeneous, because recovery is less significant. No carbides were detected, contrary to the nanobainite. The carbon mass balance is analyzed in the light of these new results and previous investigations on similar systems.