Effects of deformation modes on the susceptibility of delayed fracture of advanced high strength steels

Abstract

The effects of different deformation modes on the susceptibility to delayed fracture in advanced high strength steels (AHSSs) were evaluated using four types of commercially-produced sheet steels (DP780, DP980, TRIP780 and TRIP980). Specimens were strained by either uniaxial tension or bending prior to hydrogen charging and their delayed fracture response was compared with non-deformed samples. Micro hardness testing was performed to assess the effect of deformation on the variation of strength through the thickness of the sheet. Sheet edge effects were minimized by grinding and polishing of the edges and corners. Cleaning of the samples was performed before hydrogen charging. The samples were cathodically charged, using a DC power supply to provide consistent hydrogen charging, and were then transferred to a three-point test setup to evaluate delayed fracture. A simple three-point bend test was used to impose a tensile stress on one of the surfaces. The samples were held in the test set up until fracture occurred in order to assess their susceptibility to delayed fracture as a function of prior deformation mode. Following three point bend tests, scanning electron microscopy fractography and metallography were used to evaluate the fracture mechanisms and crack details. The four steels responded differently to the deformation modes. In general, the steels in this study have an increasing susceptibility to delayed fracture with deformation. No deformation showed the least susceptibility, followed by uniaxial tension, and then bending. The surface region of the bent samples had a higher local effective strain that created a high dislocation density on the surface, making it more susceptible to delayed fracture. After the bending deformation the two surfaces were each tested. The samples tested from the concave down surface (surface in contact with the die) were more susceptible to delayed fracture than those samples tested from the concave up surface (surface not in contact with the die). The concave down surface was compressed on bending and stretched on unbending with additional tension occurring upon delayed fracture testing. The concave up surfaces were stretched during bending followed by compression on unbending then stretched during delayed fracture testing. Steels with an ultimate tensile strength of 980 MPa were more susceptible to delayed fracture, as compared to the 780 MPa strength steels. The TRIP980 steel was more susceptible to delayed fracture than the DP980 steel when strained with uniaxial tension or by bending. However, the DP780 steel was more susceptible to delayed fracture than the TRIP780 steel, for both uniaxial and bending deformation. The TRIP780 had some retained austenite after deformation, whereas the TRIP980 showed no evidence of retained austenite after deformation.