The influence of tempering treatment on the microstructure evolution and corrosion resistance of precipitation hardening 17-4 stainless steel

Precipitation hardening (PH) 17-4 stainless steel is first subject to solution annealing and subsequently to a specially designed two-step tempering treatment: at 835°C for 1 h and then at 620°C for 0.5 h up to 8 h. Microstructure after tempering treatments is examined by optical microscopy, electron backscatter diffraction, and transmission electron microscopy. A hardness decrease during tempering at 620°C is found to be linked with the increasing content of reversed austenite. The potentiodynamic polarization test shows that the tempering treatment at 620°C has a negative effect on the pitting potential. The mechanical properties are analyzed in terms of the tensile test, indicating that the softening effect of the increasing reversed austenite has outperformed the hardening effect of Cu-rich precipitates. The environmentally assisted cracking (EAC) evaluation by slow strain rate testing reveals that PH17-4 after the two-step tempering treatment has high EAC resistance in the simulated geothermal power plant environment excluding H₂S.