Mechanical properties of cold worked type 316L stainless steel in high pressure gaseous hydrogen (investigation of materials properties in high pressure gaseous hydrogen-3)

Safety of on-board high-pressure hydrogen fuel tanks and piping systems in hydrogen refueling station is one of the most important subjects for upcoming hydrogen society featured by fuel cell vehicles. Type 316L austenitic stainless steel is known as a material in which the effect of hydrogen on mechanical properties is very small, so JIS SUS316L is recognized as the standard material for 35MPa type on-board fuel tank liner in the Japanese standard JARI-S001. However, solution treated 316L does not always have sufficient 0.2% proof stress, and materials having higher proof stress are strongly needed. One of the solutions is work-hardening of the material, which is conventionally used for piping systems for high pressure gas facilities. In this study, the effect of hydrogen on mechanical properties of 40% cold worked 316L in high-pressure gaseous hydrogen at 45MPa was investigated. Results are as follows: Any significant effect of hydrogen was not recognized in tensile tests using round bar type specimens at room temperature and 85 degrees C. In axial fatigue life tests using sand glass type specimens (stress ratio R=-1) at room temperature, not so large difference was observed on S-N curves in air and in high pressure hydrogen. However, a little influence was observed in fatigue crack growth tests using half inch CT specimens at room temperature (R=0.05). Microstructure observation reveals that any martensitic transformation did not occur. The degradation of fatigue crack growth rate in high pressure gaseous hydrogen is probably caused by the work hardened delta-ferrite which is generally contained in thick materials. However the effect of hydrogen is only limited and 40% cold worked type 316L stainless steel is considered to be used in high pressure hydrogen gas just like solution treated one.