Effect of annealing temperature on microstructure and mechanical properties of a heavy cold-rolled Custom 465 maraging stainless steel

In this study, we investigated the microstructure evolution and mechanical properties of heavy cold-rolled Custom 465 maraging stainless steel subjected to annealing treatments at various temperatures ranging from 300 to 1000 °C. The results indicate that the evolution of microstructure and mechanical behavior can be distinctly divided into three stages as the annealing temperature increases. In the aging stage (Stage A), when annealing temperature is below 550 °C, the synergistic effects of ultrafine lamella structure, high-density dislocations, and newly formed η-Ni₃Ti precipitates contributed to the exceptional mechanical properties, demonstrating ultrahigh yield strength of 2011 MPa, ultimate tensile strength of 2034 MPa, and low uniform elongation of 0.42 % when annealed at 500 °C. In the overaging stage (Stage B), when annealing temperature is within the range of 600–700 °C, the primary microstructural changes observed are the formation of reverted austenite and the coarsening of the η-Ni₃Ti precipitates, which exhibits low work hardening capacity and good elongation. In the recrystallization stage (Stage C), annealing above 750 °C allows for complete austenitization during the holding period, and the formation of fresh martensite. Annealing at 750 °C results in the austenite content of 84 vol%, with the average prior austenite grain size of 1.04 μm, whereas annealing temperature is above 850 °C, the increased prior austenite grain size leads to the formation of fresh martensite and results in mechanical properties similar to those of the undeformed Custom 465. In addition, in the heavy cold-rolled Custom 465, the precipitation phase transitions from η-Ni₃Ti to Laves phase with increasing annealing temperature, with the transformation occurring at 700 °C. Moreover, the ultrafine refinement of prior austenite grain size effectively enhances the stability of austenite. These observations provide valuable insights into the microstructural design and control of ultrahigh-strength steels.