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 situHEXRD, combined with dilatometry and SEM observations, and by considering the same steel composition andaustenitization conditions. The martensitic microstructure presents a mixed lath-plate morphology with largescatter of sizes whereas the bainite microstructure shows finer laths with more uniform sizes. Recently introducedmethods are used to track in situ by HEXRD, in one single experiment, the phase fractions, the distributionof the carbon and the evolution of the dislocation densities. The study of nanobainite revealed that about twothirds of the carbon partitions from the ferrite to precipitate into transition iron carbides or to enrich theaustenite. Both processes occur very fast after the formation of each nanobainite lath, but the ferrite remainslargely 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 lesssignificant. No carbides were detected, contrary to the nanobainite. The carbon mass balance is analyzed in thelight of these new results and previous investigations on similar systems