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

A Physics-Based Mean-Field Model for Ferrite Recovery and Recrystallization

An original mean field model for the nucleation and the growth of new recrystallized grains during annealing treatments of deformed, low-carbon ferritic steels is proposed in this paper. The model was calibrated on two steels extensively studied in the literature under both isothermal annealing and continuous heating schedules. It permits one to predict not only recrystallization kinetics but also advanced microstructural features (such as dislocation density, dislocation cell size and grain size) during complex heat treatments. Once calibrated, the model was applied to the case of a third ferrite/pearlite steel and was shown to accurately capture the effect of cold-rolling ratio on the recrystallization kinetics

In Situ Determination of Phase Stress States in an Unstable Medium Manganese Duplex Steel Studied by High-Energy X-ray Diffraction

Duplex medium Mn steels are high-potential advanced high-strength steels (AHSS) for automotive construction. Their excellent forming properties stem from the specific stress partitioning between their constituting phases during deformation, namely the ferritic matrix, unstable retained austenite, and strain-induced fresh martensite. The stability of the retained austenite and the 3D stress tensors of each phase are determined simultaneously in this work by in situ high energy X-ray diffraction on synchrotron beamline during a tensile test. The role of internal stresses inherited from the manufacturing stage are highlighted for the first time as well as new insights to understand the origin of the serrations shown by these alloys

Dislocation densities in a low-carbon steel during martensite transformation determined by in situ high energy X-Ray diffraction

The evolution of the dislocation densities in martensite and in austenite during the quench of a low-carbon (0.215 wt% C) steel is investigated in situ by the mean of a High Energy X-Ray Diffraction experiment on a synchrotron beamline. The line configuration offers an excellent time resolution well adapted to the studied martensitic transformation kinetics. The mean density of dislocations in martensite increases as the trans­formation proceeds confirming that dislocations are not homogeneously distributed between the laths in agreement with some recent post-mortem observations. The resulting spatial distribution of dislocations and the associated strain-hardening support the views assuming that lath martensite is a heterogeneous microstructureand behaves as a “multiphase” aggregate. In austenite, the increase in dislocation densities is even more sig­nificant meaning that austenite in martensite is also a hard phase, contradicting some recent theories attributing to films of retained austenite a major role in the plasticity of martensite

Recovery of severely deformed ferrite studied by in situ high energy X-ray diffraction

Recovery of severely deformed ferrite was followed in situ by High Energy X Ray Diffraction during heating and isothermal holding experiments. Dislocation densities during annealing were determined by a modified Williamson Hall method. The deduced recovery kinetics was compared to post-mortem hardness measurements. A temperature dependent saturation of recovery was exhibited during holding. Dislocation density drop and saturation behavior cannot be reproduced simultaneously by the classical physically based models

Numerical investigations of the effects of substitutional elements on the interface conditions during partitioning in quenching and partitioning steels

International audienceIn quenched and partitioned steels, carbon partitioning is considered to be driven by a constraint para-equilibrium at the martensite/austenite interface. Using Thermo-Calc calculations, we investigated the effect of non-partitioned elements on the resulting interface condition. Among all tested elements, only aluminum and chromium have significant effects. From this numerical study, a practical composition- and temperature-dependent relationship describing interface tie lines was derived and calibrated for Fe-C-2.5Mn-1.5Si-X wt pct alloys (X = Cr or Al)