Changes in crack initiation behavior of delayed fracture by applied stress and martensite transformed from retained austenite for ultra-high tensile strength steel sheet

Abstract

Changes in the initiation behavior of delayed fracture cracks have been investigated based on quantitative cross-sectional observation near the fracture surface in the previously reported sustained tensile-loading test during hydrogen charging of ultra-high strength steel sheets containing a small amount of retained austenite. The morphology of the delayed fracture surface depends on the applied stress and pre-deformation. With increasing applied stress, the number of delayed fracture cracks decreased in the martensite in the cross section immediately below the fracture surface, whereas the number at the prior austenite grain boundary tends to increase. Upon pre-deformation sufficient to transform from retained austenite to martensite, the number of cracks in the martensite increased at each applied stress in spite of the short time to delayed fracture. Even without fracture at low applied stress (200 MPa), short cracks exist in the martensite, but no long cracks are observed. The long cracks in the martensite increases with increasing applied stress or pre-deformation. The long cracks presumably result from connection of short cracks in the martensite. The long cracks probably enhance transgranular fracture. The results of the present study indicate that applied stress or pre-deformation affects delayed fracture behavior by changing the crack initiation and propagation behavior. With increasing applied stress in the elastic deformation region, the crack suppression effect of bainitic ferrite is likely to weaken, resulting in more transgranular fracture. In addition, the preferential crack initiation in deformation-induced martensite formed by plastic deformation probably shortens the time to delayed fracture.