The nature of hydrogen-related fracture in X80 pipeline steel with stress concentration

Abstract

The nature of hydrogen-related fracture in X80 pipeline steel with stress concentration was investigated through observation of fracture modes at hydrogen-related crack initiation sites and crystallographic analysis of fracture surfaces. The fracture modes and crack initiation sites were observed using a field emission scanning electron microscope (FE-SEM). The hydrogen-related crack initiation sites of X80 specimens were quasi-cleavage (QC) fracture at the notch tip. In addition, distributions of plastic strain and principal stress near the notch tip were calculated by finite element method (FEM) analysis. Since maximum plastic strain occurred at the notch tip where the initiation of QC fracture was observed, the main factor causing QC fracture was probably plastic strain. In order to clarify the crystallographic features of hydrogen-related fracture surfaces of X80, a trace analysis of the QC fracture surface was carried out using electron backscatter diffraction patterns (EBSD). Since not all points on the fracture surface were parallel to {011} slip planes, hydrogen-related fracture consisted of not only {011} slip planes but also various crystal planes due to cross slip in the body-centered cubic lattice. These findings indicate that the nature of hydrogen-related fracture in X80 is probably associated with plastic strain on various slip planes.