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.

This paper is the first microscopic observation of the entire cracking process in delayed fracture of highstrength steel bolt after long-term atmospheric exposure. A sufficiently fresh fracture surface exhibits the initiation of the propagating crack in a thin zone beneath the screw groove, resulting from the merging of multiple cracks nucleated therein. The fracture morphology is initially intergranular, exhibiting the threedimensional shape of prior austenite grains, but the stress and strain states at the nucleation sites are not uniquely specified. The fracture morphology alters as the crack extends from intergranular to quasicleavage and fine dimples, associated with increasing stress intensity under a constant-displacement condition. The change from inter-to trans-granular fracture is continuous, implying affinity among different morphologies associated with the increased density and the distribution of potential crack nucleation sites in the crack front. The crack propagation in the quasi-cleavage and fine dimple regions is step-wise of about 50 µm per step. Recent studies about the function of hydrogen in embrittlement are referred to in respect of the accumulation of strain-induced damage. The enhanced generation of strain-induced vacancies is the presumable function of hydrogen compatible with the present findings.

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