Hydrogen Embrittlement of Hole-Expanded TRIP-aided Martensitic Steel Sheet

Abstract

The effects of stress, plastic strain, and hydrogen on hydrogen embrittlement fracture of hole-expanded transformation-induced plasticity-aided martensitic steel were investigated. The hydrogen embrittlement properties were evaluated by means of cathodic hydrogen charging to the hole-expanded specimen. The residual stress and plastic strain distributions in the hole-expanded specimens were analyzed using finite element analysis. The hydrogen content was measured using a thermal desorption spectrometer. Hydrogen embrittlement cracking occurred approximately 3 mm from the hole edge in the radial direction. As the crack propagated, it diverged in the circumferential direction. The fracture morphology primarily consisted of a mixture of intergranular and quasi-cleavage fractures. The tensile stress in the circumferential direction at the position where the hydrogen embrittlement crack was initiated was the highest, and the tensile stress in the radial direction and hydrostatic stress were also high. The hydrogen content in the vicinity of the hole edge of the hole-expanded specimen was the highest owing to the large amount of plastic strain applied by hole punching and hole expanding whereas the hydrogen content at the positions where the hydrogen embrittlement crack was propagated was not very high. Thus, the highest tensile stress in the circumferential direction is the controlling factor in the location of the crack initiation site and the direction of the initial crack and its growth during the initial phase. The high hydrostatic stress that causes hydrogen accumulation could also assist the crack initiation.