Influence of Sheared Edge on Hydrogen Embrittlement Resistance in an Ultra-high Strength Steel Sheet

Abstract

Automotive parts made from steel sheets normally have sheared edges, which have been reported to decrease the hydrogen embrittlement (HE) resistance of ultra-high strength steel (UHSS) sheets. However, the mechanism on the detrimental effect of the sheared edge on HE resistance is not yet clearly understood. In this study, the influence of the edge condition in UHSS sheets on the HE property was investigated using a 1 180 MPa grade steel sheet. The HE resistance of specimens with the edges ground or as-sheared was evaluated by the U-bend method. Two types of as-sheared specimens, which had been bent so that either the burnished surface or the fracture surface was the outer side, were prepared. The specimens with the ground edges did not fracture under any conditions. The fracture stress of the fracture surface specimens was significantly lower than that of the burnished surface specimens. Microcracks were observed at the edge of the specimens except for the ground specimens, and larger microcracks were observed in the fracture surface specimens. Fracture stress drastically decreased as the microcrack length increased. The threshold of the stress intensity factor K decreased with increasing diffusible hydrogen content. When the threshold stress intensity factor at each diffusible hydrogen content was defined as KH, the fracture condition was described as K > KH. The reason why the fracture stress in the as-sheared specimen decreased was considered to be that K increased due to the microcracks introduced by bending.

Effect of Residual Stress on Hydrogen Embrittlement at Sheared Edge

“…It is widely known that a significant residual stress is generated on the fracture surface of the sheared edge 8) . Therefore, hydrogen embrittlement was previously considered to occur solely because of residual stress, even if no load stress was applied after shearing 9,10) , suggesting that the residual stress accounts for a large part of the stress factor. However, the residual stress, unlike the load stress, is not intentionally set.…”

Effects of Stress and Plastic Strain on Hydrogen Embrittlement Fracture of a U-bent Martensitic Steel Sheet

“…1) Especially, the hydrogen embrittlement susceptibility of the press-formed advanced high-strength steels (AHSSs) of the tensile strength of 1 500 MPa grade applied for the automotive structural part is predicted to be high. To characterize hydrogen embrittlement susceptibility of the automotive parts of press-formed high-strength steel sheets, the hole-expanding, 2) disk pressured, 3) deep drawn, [4][5][6][7] stretch-formed 8) and U-bent [9][10][11][12][13][14][15][16] specimens have been adopted. In general, in the evaluation method for hydrogen embrittlement properties using a U-bent specimen, a rectangular steel sheet is first bent in a U-shape to introduce a plastic strain, and then the feet of the U-bent steel sheet are tightened by a bolt and nut to apply elastic strain (i.e.…”

Stress Mapping on Ultrahigh Strength Steel Cut Edges

“…In the case of cutting procedures such as slitting and punching, the cut edge quality must be examined and given consideration since poor edge quality with sites having high levels of induced strain and stress can serve as initiating points for cracks in the steel. [ 1 ] Investigations on how different processes influence the delayed fracture of cut edges are typically measured using methods such as 4‐point bending and U‐bend testing, as, for example, in Yoshino et al and Kröger et al [ 4,5 ] In these measurements, the resistance to delayed fracture is measured when applying strain (additional to the residual that is introduced in the manufacturing) as well as the time to fracture. By these methods, the cutting procedure can then be optimized for a specific material case.…”