Abstract
The role of hydrogen on intergranular (IG) fracture in hydrogen-assisted fatigue crack growth (HAFCG) of a pure iron at low stress intensity was discussed in terms of the microscopic deformation structures near crack propagation paths. The main cause of IG fracture was assumed to be the hydrogen-enhanced dislocation structure evolution and subsequent microvoids formation along the grain boundaries. Additionally, the impact of such IG cracking on the macroscopic FCG rate was evaluated according to the dependency of IG fracture propensity on the hydrogen gas pressure. It was first demonstrated that the increased hydrogen pressure results in the larger area fraction of IG and corresponding faster FCG rate. Moreover, gaseous hydrogen environment also had a positive influence on the FCG rate due to the absence of oxygen and water vapor. The macroscopic crack propagation rate was controlled by the competition process of said positive and negative effects.
Highlights
For the storage and transportation of compressed gaseous hydrogen in the forthcoming hydrogen energybased society, hydrogen-assisted fatigue crack growth (HAFCG) in structural steels is of significant concern with regards to the safe design of high-pressure components such as pressure vessels or pipelines [1,2]
Whereas the large FCG acceleration in Stage II regime was accompanied by transgranular (TG) quasi-cleavage (QC) type fracture surfaces, brittle appearance intergranular (IG) features were observed in Stage I, contrary to the fact that there was no significant change of macroscopic FCG rate in this FCG regime
Based on crystallographic orientation maps obtained via electron backscattered diffraction (EBSD), we evaluated the amount of plastic deformation around the crack propagation paths by using two types of strain analyses, i.e. grain reference orientation deviation (GROD) and kernel average misorientation (KAM)
Summary
For the storage and transportation of compressed gaseous hydrogen in the forthcoming hydrogen energybased society, hydrogen-assisted fatigue crack growth (HAFCG) in structural steels is of significant concern with regards to the safe design of high-pressure components such as pressure vessels or pipelines [1,2]. For elucidating the detailed fatigue crack growth (FCG) behaviour of iron and steels with ferritic, i.e. bodycentered cubic (BCC) crystal structure under the presence of hydrogen, we conducted FCG tests of a commercially pure iron in gaseous hydrogen environment with various pressures at room temperature. Whereas the large FCG acceleration in Stage II regime was accompanied by transgranular (TG) quasi-cleavage (QC) type fracture surfaces, brittle appearance intergranular (IG) features were observed in Stage I, contrary to the fact that there was no significant change of macroscopic FCG rate in this FCG regime. The role of hydrogen in triggering the fracture along grain boundaries (GB) is discussed based on the amount of plastic deformation and dislocation structures underneath the IG fracture surfaces
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