Phase transformation mechanism during heat treatment of Co-free maraging steel produced by twin-roll strip casting

The strip casts of cobalt-free maraging steel were fabricated using a twin-roll strip casting simulator, and its characteristics of sub-rapid solidification were studied. Subsequently, the confocal laser scanning microscope (CLSM) was employed to in situ observe the phase transformation during the heat treatment of maraging steel strip cast such as austenitization, solution treatment, and aging processes. It was found that due to the high cooling rate during the twin-roll strip casting process, the sub-rapid solidified strip cast possessed a full lath martensitic structure, weak macrosegregation, and evident microsegregation with a dendritic morphology. During austenitization of strip cast, the austenite grain size increased with the austenitization temperature. After holding at 1250 degrees C for 250 s, the austenite grain size at the high temperature owned a high similarity to the prior austenite grain size of the strip cast, which effectively duplicates the microstructure of the strip cast after sub-rapid solidification. During the solution treatment process, the martensitic structure of the strip cast also underwent austenitic transformation, subsequently transformed into martensite again after quenching. Due to the low reheating temperature during solution treatment, the austenite grain size was refined, resulting in the fine martensitic microstructure after quenching. During the aging process of strip cast, some of martensite transformed into fine austenite, which was located in the interdendritic region and remained stable after air cooling, resulting in the dual-phase microstructure of martensite and austenite. The solute segregation of Ni and Mo elements during the sub-rapid solidification of strip cast caused the enrichment of Ni and Mo elements in the interdendritic region, which can expand the austenite phase region and thus enhance the stability of austenite, leading to the formation of austenite in the interdendritic region after aging treatment.