Continuous Hardening During Isothermal Aging at 723 K (450 °C) of a Precipitation Hardening Stainless Steel

The isothermal aging behavior of a cold-rolled precipitation hardening stainless steel has been studied at 723 K (450 °C) for holding times up to 72 hours. The precipitation hardening has been investigated using microhardness Vickers (Hv), thermoelectric power (TEP) measurements, and tensile testing. Microhardness compared to TEP measurements is more sensitive to detect the initial stages of aging. Two precipitation regimes have been observed: the first one related to the formation of Cu-clusters for aging times below 1 hour and a second one associated with formation of Ni-rich precipitates. The results show that the material exhibits an outstanding continuous age strengthening response over the aging time investigated, reaching a hardness of 710 ± 4 H_V1 and an ultimate tensile strength (σ _UTS) of 2.65 ± 0.02 GPa after 72 hours. Engineering stress-plastic strain curves reveal that the strength increases and the ductility decreases as the aging time increases. However, after prolonged holding times (24–72 hours) and, although small, a rise in both the strength and the total elongation is observed. The precipitation kinetics can be well predicted over the entire range of aging times by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Finally, a reliable linear hardness-yield strength correlation has been found, which enables a rapid evaluation of the strength from bulk hardness measurements.