Fracture toughness characterization of hydrogen embrittled Cr–Mo steel

Abstract

Since pressure vessels for petroleum use are operated at high temperature high pressure hydrogen gas, it is a special concern whether a cracks at a stressed region grow by internal hydrogen embrittlement (IHE) environment during shutdown period, i.e., hydrogen absorbed steels exposed to low temperature air ambience. In this study, fracture mechanics tests on hydrogen pre‐charged 2.25Cr–1Mo steels were conducted to clarify how hydrogen assisted cracking occurs and grows at room temperature. In search for loading condition effect, rising load test and constant displacement tests were examined. By imposing of rising loading, the cracks were very low at initiation for the majority of steels tested. It started to grow rapidly at initiation point K_IH followed by continuous propagation at constant speed regardless of an increase in applied K_I level. By changing loading condition from rising load to constant displacement, crack growth soon decreased and no longer increases to give threshold value (K_th) at a given displacement. However, in case of the higher strength steel, crack was very active and continue to propagate in spite of under this static, constant displacement condition where the crack continued to propagate accompanied by the decrease of load to give lower K_th. The study was also focused on the cracking behavior of temper embrittled, old generation made steel. The temper embrittled steels were laboratory made by intentionally adding impurities such as Si, P and Sn to simulate the 1960's steel making practice level. Rising load test results of those temper embrittled steel exhibited that the initiation of cracking K_IH did not make much difference compared to a recently made high toughness level steel. However, the cracking resistibility in R‐curve is very low in toughness whereas new steel exhibited a high resistibility of cracking. Finally, crack growth characteristics are summarized and flaw assessment procedure is proposed.