Significance of Martensite Reversion and Austenite Stability to the Mechanical Properties and Transformation-Induced Plasticity Effect of Austenitic Stainless Steels

The phenomena occurring during continuous heating of cold-rolled AISI 304L and AISI 316L stainless steels from room temperature up to 1150 °C were studied. X-ray diffraction (XRD), scanning electron microscopy (SEM), hardness measurement, and tensile testing were used for characterization. The XRD analysis revealed that the AISI 304L stainless steel was more susceptible to the strain-induced martensitic transformation during cold rolling, and the martensite reversion kinetics during annealing was faster in this stainless steel. The latter was related to the effect of molybdenum in AISI 316L stainless steel, and it was rationalized based on the concept of continuous heating reversion temperature. Due to the presence of the retained austenite in AISI 316L stainless steel, the strength fell more slowly during continuous heating and the equiaxed microstructure was obtained at higher temperatures after recrystallization of the retained austenite. The latter resulted in the micron-size grain sizes in the AISI 316L stainless steel while ultrafine-grained (UFG) microstructure was obtained for the AISI 304L stainless steel as revealed by SEM images. In general, higher ultimate tensile strength (UTS) and total elongation values were observed for the AISI 304L stainless steel. These were related to factors other than the grain size strengthening (as presented by the Hall–Petch law), where the transformation-induced plasticity (TRIP) effect was found to be a major parameter affecting the tensile strength and ductility of AISI 304L stainless steel and this effect became more pronounced at coarser grain sizes. However, the TRIP effect was marginal in the case of AISI 316L stainless steel. This revealed the importance of the stability of the austenite phase in determining the mechanical properties of austenitic stainless steels.