Comparison between PH17-4 steel and PH13-8Mo steel

Hydrogen embrittlement behaviors of PH17-4 steel and PH13-8Mo steel used as steam turbine last stage blades were investigated by means of slow strain rate tensile tests and hydrogen permeation tests. 

The results indicate that PH13-8Mo steel exhibits higher strength level mainly due to higher precipitate strengthening and lath strengthening compared with PH17-4 steel in the absence of hydrogen. Moreover, pre-hydrogen charging does not affect tensile strength of PH17-4 steel, but decreases tensile strength of PH13-8Mo steel. The plasticity of PH17-4 steel and PH13-8Mo steel is degraded after hydrogen charging and the deterioration is accompanied with considerable change in fracture mode. 

The loss of plasticity of PH13-8Mo steel is higher than that of PH17-4 steel, indicating that PH13-8Mo steel exhibits lower resistance to hydrogen embrittlement, which is mainly related to higher hydrogen diffusion behaviors and higher strength level of PH13-8Mo steel. Additionally, PH17-4 steel exhibits lower apparent hydrogen diffusion coefficient and higher apparent hydrogen solubility in comparison with those of PH13-8Mo steel, which implies that incoherent Cu-rich precipitates in PH17-4 steel possess higher capability to trap hydrogen atoms than that of coherent NiAl precipitates in PH13-8Mo steel.