Failure Analysis on the Diaphragm of a High Pressure Hydrogen Transducer

The failure analysis of a high pressure hydrogen transducer which was actually used in a test equipment of 70MPa hydrogen gas exposure vessel was carried out. A crack existed at the root of flange of diaphragm in the transducer and a small amount of hydrogen leaked through the crack. EDSD analysis and Vickers hardness measurement confirmed that a diaphragm material was a precipitation hardened stainless steel SUS630. SEM observation showed that intergranular fracture surfaces with the size of about 20μm were formed at the root of flange and that dimple, quasi-cleavage and intergranular fracture were sequentially observed away from the flange root. The stress analysis and hydrogen invasion analysis of diaphragm were conducted. Using the plate of SUS630, the hydrogen diffusion coefficient and solubility, and the threshold stress intensity factor of hydrogen assisted cracking under static loading were measured. The diaphragm was fixed to the pressure transducer by bolt end. The elastic principal stress at the flange root by bolt end was about three times higher than the tensile strength of 1358MPa of the diaphragm. It was presumed that the 20-μm-long intergranular crack initiated at the flange root by the high principal stress and propagated by hydrogen embrittlement.