Stress corrosion (SC) crack propagation rate and fracture mode of stress corrosion cracking (SCC) of 3.5 NiCr steels have been determined as a function of P-content (60, 400 ppm) and tempering temperature (250, 350, 450℃). Bolt-loaded wedge opening load (WOL) specimens were used for SCC tests in aqueous 3.5wt.% NaCl solution at 30℃. SC crack propagation rate of the P-undoped steel decreased as the tempering temperature increased. On the other hand, SC crack propagation rate of the P-doped steel decreased in the sequence of 350, 250, 450°C-tempered specimens. This was discussed as being related to the grain boundary weakening due to P segregation and yield strength effects. As stress intensity factor $K_I$ decreased, the corresponding SCC fracture mode changed from microvoid coalescence (MVC) mode to intergranular (IG) mode. Also, the transition in fracture mode from MVC to IG occurred at lower $K_I$ level as the yield strength decreased. The change of the transition in fracture mode with varying $K_I$ and yield strength has been discussed by considering the micromechanisms of SCC and yield strength dependence of critical hydrogen concentration bearing on the respective modes, simultaneously.
