Elevated-temperature mechanical stability and transformation behavior of retained austenite in a quenching and partitioning steel

The mechanical stability and transformation behavior of both film and blocky retained austenite (RA) in a quenching and partitioning steel are investigated at 293K, 423K and 573K with X-ray diffraction measurements and transmission electron microscopy. Blocky RA both completely and incompletely transforms to twinned martensite during deformation at 293K and 423K, respectively, and completely transforms to lath martensite during deformation at 573K. At 293K and 423K, only the film RA with widths larger than ~70nm transforms to twinned martensite. However, film RA incompletely transforms to lath martensite at 573K. Hence, RA transformation is non-monotonic with temperature. Significant carbide formation at 573K, and therefore less carbon to stabilize RA, overcomes the increase in austenite stability due to the decrease in the temperature-dependent chemical driving force for the martensite transformation.