The effect of austenite reversion on the microstructure and mechanical properties of a 12Cr-3Ni-3Mn-3Cu-0.15Nb-0.05C maraging stainless steel

A maraging stainless steel was studied by in-situ XRD to monitor the austenite reversion process. The partitioning of Ni aided austenite reversion in this steel was identified from its negative effect on the lattice parameter of the reverse austenite. This chemical enrichment partially retained the reverse austenite against re-transformation to fresh martensite upon cooling to room temperature. The precipitation hardening effect of Cu and martensite tempering controlled the mechanical properties during low temperature aging (i.e., 550-600 degrees C), despite a significant austenite reversion at 600 degrees C. On the other hand, the reverse austenite formation by the austenite reversion process during heating and its subsequent transformation to fresh martensite upon cooling dominated the microstructure for higher aging temperatures (i.e., 650-700 degrees C), and resulted in a serrated prior austenite grain boundary structure. The change in strengthening mechanism from precipitation hardening to martensite strengthening caused a notable change in the tensile flow behavior (especially the yield ratio) with an increase in the aging temperature. As a result, the mechanical strength for the 700 degrees C aging condition approached the asquenched value. This suggests that the aging heat treatment at higher temperature resulted in the reappearance of martensite strengthening due to fresh martensite formation assisted by the austenite reversion process.