Crack Initiation and Propagation Behavior of Hydrogen-induced Quasi-cleavage Fracture in X80 Pipeline Steel with Stress Concentration

Abstract

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.

Sulfide Stress Cracking (SSC) of Low Alloy Linepipe Steels in Low H₂S Content Sour Environment

“…), but recently the latter HESIV theory is particularly influential, and it is understood that hydrogen promotes the formation of atomic vacancy damage (nanovoid) accompanying plastic deformation. [40][41][42][43]45,46) The fracture mechanism of SSC crack initiation samples observed in this study is described in detail in the next section.…”

“…28,44) On the other hand, as mentioned in the previous section, the HE mechanism is understood as hydrogen promotes the formation of atomic vacancy damage (nanovoid) accompanying plastic deformation. [40][41][42][43]45,46) It is considered that the local plastic instability (formation and connection of nanovoids) is promoted by hydrogen accumulation with the stress field expansion by the local stress concentration including plastic deformation. 45) The fact that the SSC crack initiated only in the sample 15W with the hard LB microstructure suggests that hydrogen diffused and accumulated in the region where the stress increased due to the stress and strain concentration at the bottom tip of the corroded pit and groove, and when the amount of hydrogen exceeded a certain value, the local plastic instability condition was satisfied in the presence of hydrogen.…”