Effect of fracture mode on acoustic emission behavior in the hydrogen embrittled low-alloy steel

Hydrogen embrittlement (HE) of steels and other alloys is a long-standing scientific and engineering problem causing sudden premature failures of industrial components. Although several mechanisms of hydrogen-assisted cracking (HAC) have been proposed, there is still neither commonly accepted agreement on the exact role of hydrogen in this process nor the reliable way to recognize the operating mechanism of HAC. In the present paper, we endeavor to demonstrate that the acoustic emission (AE) technique is a promising tool for in-situ investigating of HAC mechanisms and distinguishing between them. The experiments were performed using the low-alloy steel grade 09G2S obtained in the low-strength as-received state and the high-strength state after severe plastic deformation by equal channel angular pressing (ECAP). The specimens of both kinds were tensile tested before and after hydrogen charging as well as after removing diffusible hydrogen for evaluation of the influence of hydrogen induced cracks (HICs). It is established that HICs, which are formed during hydrogen charging, do not affect the mechanical properties, fracture surface topology and the AE behavior in both as-received and ECAPed specimens, provided that diffusible hydrogen is removed from the steel. Two different kinds of HAC were observed. The first operated in the as-received low-strength steel; it produced weak AE response and was featured by the formation of fisheyes defects with quasi-cleavage morphology. The second type of HAC was observed in the ECAPed high-strength steel; it generated high-energy AE signals and was featured by the formation of mixed tearing/cleavage morphology on the fracture surface. Obtained results are discussed and explained from the viewpoint of the known mechanisms of HAC including hydrogen-enhanced localized plasticity (HELP), hydrogen enhanced decohesion (HEDE) and others.