This paper presents the research activity of the subcommittee on Hydrogen Embrittlement of the Japan Pressure Vessel Research Council (JPVRC). It describes the superposition effect of hydrogen embrittlement and temper embrittlement in Cr-Mo steels which are used in high temperature and high pressure hydrogen environment like petroleum refining equipments. Three kinds of 2.25Cr-1Mo steels which have different temper embrittlement factors (J-factor) and 2.25Cr-1Mo-0.25V steel were investigated. These steels were prepared with normalizing, tempering and post weld heat treatment (PWHT) which simulated an actual PWHT condition for vessel fabrication. The temper embrittlement was evaluated by step-cooling heat treatment (SCHT). Hydrogen charging was performed by exposure to pressurized hydrogen gas (15MPa) for 48 hours at elevated temperature (550°C). The toughness of the materials in the as-PWHT condition deteriorated with increase of J-factor, and the toughness after SCHT showed the same tendency. 2.25Cr-1Mo-0.25V steel showed no hydrogen embrittlement and no temper embrittlement but the 2.25Cr-1Mo steels showed both types of embrittlements. The degree of hydrogen embrittlement was different between as-PWHT and SCHT conditions and the material toughness after either of the heat treatment followed by hydrogen charging was almost the same. The results mean that the effects of temper and hydrogen embrittlement are nonadditive. SEM observation of fracture surfaces showed increase of intergranular facets both after-SCHT and hydrogen charging. By considering that both the temper embrittlement and hydrogen embrittlement encourage intergranular fracture, it is understandable that both types of the embrittlement are nonadditive and the deterioration of toughness becomes saturated.

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