The processes leading to hydrogen-related fracture in X80 pipeline steel with stress concentration have been investigated comprehensively through observations of fracture surfaces and subsidiary cracks, a stress analysis, crack initiation and propagation analyses and a crystallographic analysis of fracture surfaces. Fracture morphology showed quasi-cleavage (QC) fracture under various amounts of hydrogen. It was found that QC cracks initiated in the area ranging from the notch tip to 100 µm inside based on interrupted tensile tests just before fracture strength with hydrogen charging. Moreover, fracture surface topography analysis (FRAS-TA) revealed that QC cracks initiated at the notch tip. A finite element analysis indicated that the equivalent plastic strain was maximum at the crack initiation site at the notch tip. In addition, a backscattered electron image showed that nanovoids of 50-250 µm in diameter were present near the initiation site. Regarding the crack propagation process, field emission scanning electron microscopy (FE-SEM), electron backscattered diffraction (EBSD) and FRASTA results indicated that some microcracks in ferrite grains coalesced stepwise and propagated. Trace analyses using EBSD revealed that the QC fracture surface consisted of {011} slip planes, {001} cleavage planes and non-specific index planes. These findings indicate that QC fracture initiates at the notch tip due to the interaction between dislocations and hydrogen associated with local plastic deformation, and propagates stepwise by coalescence through vacancies, nanovoids and microcracks on various planes associated with/without plastic deformation in ferrite grains.

The processes leading to hydrogen-related fracture in X80 pipeline steel with a stress concentration have been investigated comprehensively through observations of fracture surfaces and subsidiary cracks, stress analyses, crack initiation and propagation analyses and crystallographic analyses of fracture surfaces. Fracture morphology showed quasi-cleavage (QC) fracture under various amounts of hydrogen. It was found that QC cracks initiated in hydrogen-charged specimens in an area ranging from the notch tip to 100 μm inside based on interrupted tensile tests until just before fracture strength. A fracture surface topography analysis (FRASTA) revealed that QC cracks initiated at the notch tip. A finite element analysis indicated that the equivalent plastic strain was maximum at the crack initiation site at the notch tip. A backscattered electron image showed that nanovoids of 50-250 nm in diameter were present near the initiation site. Regarding the crack propagation process, field emission scanning electron microscopy (FE-SEM), electron backscattered diffraction (EBSD) and FRASTA results indicated that some microcracks in ferrite grains coalesced in a stepwise manner and propagated. Trace analyses using EBSD revealed that the QC fracture surface consisted of {011} slip planes, {001} cleavage planes and non-specific index planes. These findings indicate that QC fracture initiated at the notch tip due to the interaction between dislocations and hydrogen associated with local plastic deformation, and propagated in a stepwise manner by coalescence through vacancies, nanovoids and microcracks on various planes associated with/without plastic deformation in ferrite grains.

Cr-Mo casting steels are widely used as casings and valves in thermal power plants. Creep fatigue damage gradually proceeds in these high-temperature components during operation resulting in creep voids and micro crack initiation in which repair welding is considered to be applied for life extension. In this study, a temper bead welded joint of Cr-Mo casting steel as well as fine and coarse grain simulated materials were prepared. Creep and fatigue, creep-fatigue tests were performed using specimens taken from base and weld metals of the welded joint, the simulated materials and cross welded specimens. Material constants of creep deformation and cyclic stress-strain properties for constituted materials of the welded joint were obtained from the creep and fatigue tests. Minimum creep strain rate of the base metal at the same stress was the highest than that of other materials. Plastic deformation resistance is higher in the order of the base metal, fine grain, coarse and weld metal. Creep-fatigue life of the welded joint, which failed at the base metal 2mm away from the boundary between the base metal and fine grain, was shorter than that of the uniform base metal specimen indicating that the temper bead welding may reduce the creep-fatigue life. Finite element analysis of the cross welded specimen showed that elastic-plastic and creep strains accumulated at the base metal near the boundary are higher than those at other regions causing life reduction and failure at the base metal. The creep-fatigue life prediction by using the analysis results indicated that the nonlinear damage accumulation model gave more accurate life prediction results than the time fraction and ductility exhaustion rules.

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