The relation between aging temperature, microstructure evolution and hardening of Custom 465® stainless steel

The microstructure and hardness of Custom 465® precipitation hardened stainless steel were characterized following 4 hours aging at temperatures ranging from 482 °C to 648 °C. Dilatometry measurements and thermodynamic calculations were used to understand the mechanism of martensite-to-austenite reversion. Three major stages of aging were observed. The first stage is the peak hardening at aging temperatures of 480–510 °C, where both η-Ni₃Ti precipitation and austenite reversion begin independently of each other. The second stage is the initial stage of overaging observed above 538 °C as reduction in hardness is due to a slight increase in both the precipitate dimensions and the reverted austenite volume fraction. The third, progressive overaging stage occurs above 593 °C in which a dramatic hardness decrease is caused by a significant increase in both precipitate dimensions and reverted austenite volume fraction. Transmission Kikuchi diffraction orientation mapping revealed that globular austenite randomly nucleates at martensite grain boundaries, while acicular austenite nucleates along martensite lath boundaries. Thermodynamic calculations indicated that the dissolution of η-Ni₃Ti precipitates does not govern the austenite reversion, although both phases are in competition for Ni. While the thermodynamic calculation predicts increase in the fraction of austenite with increase in aging temperature, the fraction of austenite found at room temperature in aged samples does not vary when the aging temperature, in which austenite reversion occurs, is increased above 593 °C. This is explained by the change in composition of the austenite as a function of aging temperature.