Tensile and Fatigue Properties of 17-4PH Martensitic Stainless Steels in Presence of Hydrogen

Effects of hydrogen on slow-strain-rate tensile (SSRT) and fatigue-life properties of 17-4PH H1150 martensitic stainless steel having an ultimate tensile strength of ~1GPa were investigated. Smooth and circumferentially-notched axisymmetric specimens were used for the SSRT and fatigue-life tests, respectively. The fatigue-life tests were done to investigate the hydrogen effect on fatigue crack growth (FCG) properties. The specimens, tested in air at ambient temperature, were precharged by exposure to hydrogen gas at pressures of 35 and 100 MPa at 270°C for 200 h. The SSRT properties of the H-charged specimens were degraded by hydrogen, showing a relative reduction in area (RRA) of 0.31, accompanied by mixed fracture surfaces composed of quasi-cleavage (QC) and intergranular cracking (IG). The fatigue-life tests, conducted under wide test frequencies ranging from 10-3 Hz to 10 Hz, revealed three distinct characteristics in low- and high-cycle regimes and at the fatigue limit. The fatigue limit was not degraded by hydrogen. In the high-cycle regime, the hydrogen caused FCG acceleration with an upper bound ratio of 30, accompanied by QC surfaces. In the low-cycle regime, the hydrogen caused FCG acceleration with a ratio of ~100, accompanied by QC and IG. The ordinary models such as process competition and superposition models hardly predicted the H-assisted FCG acceleration; therefore, an interaction model, successfully reproducing the experimental FCG acceleration, was newly introduced.