Keizo Ohnishi

Effect of Hydrogen Gas Pressure on the Mechanical Properties of Low Alloy Steel for Hydrogen Pressure Vessels

To provide engineering data useful in design, manufacture and operation of hydrogen storage vessels in hydrogen refueling stations, fatigue test machine equipped with high-pressure hydrogen autoclave was introduced. The effect of steel’s strength level, temperature effect, fracture toughness and pressure effect were evaluated in gaseous hydrogen environment. When steel’s strength level exceeds around 930MPa to 1000MPa, the elongation and notch tensile properties deleteriously degraded. The elongation reduction by the effect of hydrogen increased with lowering the temperature. The same sensitivity to temperature on crack growth behavior was observed. However, it was shown that the gaseous hydrogen environment only affect the slow stable crack growth but did not affect the critical flaw growth of the steel at low temperature, i.e. fast fracture. The pressure dependence of notch tensile strength ranging from 0.1MPa to 75MPa hydrogen pressure shows approximately 1/2 power dependence.Copyright

Effect of Surface Machining on the Fatigue Life of Low Alloy Steel for Hydrogen Pressure Vessels

The effect of surface machining on fatigue life in high pressure hydrogen gas was investigated. The test was conducted under the elastic range under 45MPa gaseous hydrogen environment by the ground specimen which were machined so that the surface roughness to be Rmax = 19μm(Mark: 19s), 26μm(26s) and 93μm(93s) and by the polished specimen which are prepared so that the surface roughness to be Rmax = 1μm(1s), 3.6μm(3.6s) and 10μm(10s). The hydrogen fatigue life of ground specimens was considerably reduced with increasing surface roughness as compared to the fatigue life in air at the same surface condition. On the other hand, for the annealed conditions of the ground specimen, the reduction by hydrogen effect was fairly small. The residual stress for the ground specimen at the surface rises sharply in tension while the residual stress for the annealed specimen was nearly equal to zero. We have shown that the hydrogen fatigue damage can be evaluated by obtaining the information about residual stress on surface, stress concentration by maximum surface roughness and the threshold stress intensity SH above which hydrogen fatigue damage occurs.Copyright

Internal Pressure Fatigue Test of Cr-Mo Steel in 45MPa Hydrogen Environment

Internal pressure fatigue properties of Cr-Mo steel (JIS G 4052, SCM435H) in high-pressure hydrogen or nitrogen atmosphere were investigated by the internal pressure fatigue test using a specially designed apparatus. Internal pressure was varied in the range from 0 to 40.5MPa, and test temperature was 20°C. The cycle length of internal pressure was varied up to 1000 seconds. We obtained the following results. 1. The number of fracture cycles in 45MPa hydrogen atmosphere was much less than those of in the 45MPa nitrogen atmosphere. 2. The number of fracture cycles was not depended on the cycle length. 3. In the fracture surface region where quasi-cleavage and intergranular surfaces are coexisting, the crack growth behavior is different from that of in the quasi-cleavage region.Copyright