By means of high-energy synchrotron X-ray diffraction, the interplay between martensite and retained austenite phases in steel during the application of stress has been analyzed. Martensite properties were varied through controlled reheating heat treatments in a low carbon Quenched and Partitioned (Q&P) steel consisting of retained austenite and martensite. The reheating treatments significantly altered martensite strength while keeping the same fractions of retained austenite as the non-reheated Q&P microstructures, resulting in different degrees of stress partitioning and work hardening of the individual microconstituents. Results of this study show that the strength ratio between the different phases in the microstructure plays a crucial role in the onset and rate of mechanically induced decomposition of retained austenite. Consequently, the strength ratio between phases controls the yielding and work-hardening of the material.

The mechanical and thermal stability of austenite in multiphase advanced high strength steels are influenced by the surrounding microstructure. The mechanisms underlying and the relations between thermal and mechanical stability are still dubious due to the difficulty of isolating other factors influencing austenite stability. In this work, martensite/austenite microstructures were created with the only significant difference being the degree of tempering of the martensite matrix. Hence, the effect of tempering in martensite is isolated from other factors influencing the stability of austenite. The thermal stability during heating of retained austenite was evaluated by monitoring phase fractions as a function of controlled temperature employing both dilatometry and magnetometry measurements. The mechanical stability was studied by performing interrupted tensile tests and determining the remaining austenite fraction at different levels of strain. The thermal stability of this remaining austenite after interrupted tests was studied by subsequent reheating of strained specimens. The results are evidence for the first time that thermal recovery of martensite during reheating assists austenite decomposition through shrinkage and softening of martensite caused by a reduction of dislocation density and carbon content in solid solution. This softening of martensite also leads to a subsequent reduction of austenite mechanical stability. Additionally, remaining austenite after pre-straining at room temperature was thermally less stable than before pre-straining, demonstrating that plastic deformation has opposing effects on thermal and mechanical stability.

The role of retained austenite on tensile behavior in quenched and partitioned (Q&P) steels has been studied extensively, but the deformation behavior of martensite, which comprises the majority of Q&P microstructures, has received less attention. In this investigation, martensite properties were varied through heat treatment in a low carbon Q&P steel consisting of retained austenite and martensite. Additional conditions were produced by reheating the Q&P steel to 450 °C for 30 min or to 700 °C followed immediately by quenching. The reheated microstructures contained similar fractions of retained austenite as the non-reheated Q&P microstructures, but reheating tempered the martensite, thereby decreasing martensite dislocation density. The reheated conditions had a lower yield stress and initial work hardening rate than the non-reheated Q&P condition. However, the reheated conditions had a greater work hardening rate at larger strains and greater uniform strain due to less stable retained austenite. Furthermore, the tensile strength of the condition reheated to 450 °C was nearly equal to the non-reheated condition. In addition to retained austenite to martensite transformation, the early stage work hardening rate of martensite is critical to ductility and is dependent on martensite dislocation density, which can be decreased through tempering.