Effect of Prior Cold Rolling and Aging Temperature on PH Martensitic Stainless Steel: Evolution of Microstructure, Micro-Texture and Austenite Stability

A systematic investigation of the evolution of martensitic microstructure, micro-texture, and austenite stability followed by correlation with hardness was carried out as a function of prior cold rolling (PCR) (20-80 % thickness reduction) and aging temperature (510-593 degrees C) for a very low C (0.007 wt.%) Fe-Cr-Ni-Ti-Mo precipitation hardened (PH) martensitic stainless steel. The cold-rolled and aged samples were compared with directly aged samples to emphasize the synergistic effect of PCR and aging temperature. Martensite has been found to have recrystallized to varied extents by this combined effect (severe deformation and higher aging temperature), resulting in variations in size, morphology, and texture of the martensitic/ ferritic microstructure. These in turn affect the formation and the stability of austenite. Austenite stability has been found to be a function of phase composition, strain, and size, which depend on PCR% and the aging temperature. Higher Ni content (which decreases with aging temperature) and fineness (driven by the extent of martensitic recrystallization) of austenite have been found to increase the stability of reverted austenite. Size distribution of the main strengthening precipitate Ni3Ti has been found to be a function of PCR% and aging temperature through the involved mechanisms of faster diffusion, more nucleation sites, and coarsening. Hardness has been significantly influenced by the morphology, size of martensite, and amount of austenite, and size distribution of Ni3Ti precipitates. In the present work, the individual effects of these factors were discerned, explaining the overall variation in hardness as a function of PCR% and aging temperature.