A new morphology of retained austenite obtained by intercritical tempering of 13Cr-4Ni martensitic stainless steel

This research investigates the effect of various intercritical tempering in a 13Cr-4Ni stainless steel on the morphology, size, and gamma-stabilizer elements enrichment of the retained austenite, i.e. the austenite retained at room temperature after intercritical heat treatments. A comparative microstructural and chemical composition study was conducted on AISI415 steel using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and micro hardness measurements for three tempering conditions giving the same retained austenite volume fraction (10%). These tempering conditions were: 620 degrees C 1 h, 600 degrees C 4 h, and 580 degrees C 10 h. The intercritical tempering at low temperature does not provide greater gamma-stabilizer elements enrichment of the austenite. In this condition, the morphology of the formed retained austenite can be described as thin hollow particles with short lamellae shapes, which is significantly different than the morphology obtained at higher temperatures; the latter being characterised by thick and long lamellae. The retained austenite produced by tempering at low temperature is surrounded from both internal and external sides by martensite phases. Furthermore, it increases the austenite-martensite interfacial area that contributes to increase the barrier energy to martensitic transformation induced plasticity (TRIP).