the effects of heat treatment conditions on hydrogen attack and creep rupture properties in hydrogen were investigated with extra low carbon 17Ni martensitic steels.The following results were obtained.1) Microstructures of the steels were not clearly etched in the specimens as quenched.2) Tensile strength and reduction of area after hydrogen exposure decreased and fracture occurred more intergranularly with increasing quenching temperature.3) Deterioration of tensile properties after hydrogen exposure was more marked with the specimens tempered at 550 deg C than that with those as quenched for a steel containing 0.004 percent C and vice versa for a steel containing 0.045 percent C.4) Creep rupture properties in hydrogen were better with decreasing quenching temperature. Tempering improved rupture strength and ductility.5) Lower carbon content showed higher strength and ductility in both hydrogen attack and creep rupture tests in hydrogen.6) It was proved that hydrogen decreased prior austenite grain boundary strength itself because quenching at higher temperature lowered rupture strength even in a steel containing only 0.004 percent C.
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