This paper presents the results of the research activities of the subcommittee on Hydrogen Embrittlement of the Japan Pressure Vessel Research Council (JPVRC). The combined effect of temper embrittlement and hydrogen embrittlement in Cr-Mo steels is discussed. It has been recognized that Cr-Mo steels used widely in the refining and petrochemical industry are quite susceptible to temper embrittlement. Although the synergetic relation between temper embrittlement and hydrogen embrittlement is a matter of major concern, studies regarding this subject are rarely encountered. Task group VIII (TG8) of the JPVRC conducted fracture toughness tests for three kinds of 2.25Cr-1Mo steels and 2.25Cr-1Mo-0.3V steel. These steels were prepared by subjecting them to normalizing, tempering and post-weld heat treatments which simulated actual conditions. Some specimens were embrittled by step cooling (Socal −1 treatment). It was found that the threshold for hydrogen-induced fracture was lowered when the specimen was exposed to pressurized hydrogen gas (15MPa) for 48 hours at 450 °C and there was no marked indication of the synergetic action concerning this embrittlement.

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