Precipitation reactions in 12Cr–3Ni–3Mn–3Cu–0.15Nb steel

In the current study, a new precipitation hardenable stainless steel was designed with primarily Ni and Mn modification using Thermo-Calc. The solution-annealing heat treatment at 1035 °C for 60 min followed by air quenching resulted in a fully martensitic microstructure. The as-quenched steel responded to the aging heat treatment, resulting in a significant enhancement of hardness and mechanical strength. The aging heat treatment at 500 °C for 120 min provided a good combination of tensile properties. The change in mechanical properties was quite conducive to the aging parameters (i.e., temperature and time) between 400 °C and 600 °C. Transmission electron microscopy and atom probe tomography studies demonstrated that the evolution of Cu-precipitate was the prime reason for the change in hardness and mechanical properties of as-quenched steel. As a result, the newly developed PH-1 steel was comparable to a commercial Cu-precipitate strengthened 17-4 PH steel in terms of mechanical properties. The Cu-precipitate evolved with a concurrent increase of Cu and decrease of Fe contents. APT studies revealed the interfacial segregation of Ni and Mn at the matrix and the Cu-rich precipitate in a peak-aged condition, which evolved as Ni–Mn rich reverse austenite upon over-aging, accompanied by high Mn:Si ratio. Cu precipitate only acted as a heterogeneous nucleation site for reverse austenite formation as there was no Cu atoms partition. •The mechanical properties of newly developed PH-1 (12Cr-3Ni-3Mn-3Cu-0.15Nb) steel is comparable to 17-4 PH steel.•TEM and APT examinations revealed the Cu precipitation process responsible for hardening of PH-1 steel upon aging.•Interfacial segregation of Ni and Mn led to reverse austenite formation in PH-1 steel, supported by high Mn:Si ratio.•FCC Cu precipitates act as a heterogeneous nucleation site for reverse austenite.