Mechanical and microstructural properties of autogenous arc welds of 2 GPa-strength hot-press-forming steel

Abstract Currently, the strength of hot-press-forming (HPF) steel has been increased to produce lightweight vehicles, and in this study, the first arc welding investigation on a 2.0 GPa-strength HPF steel was conducted. HPF steel sheets with thicknesses of 1.1 mm were butt-welded by gas tungsten arc welding without a filler metal, with the heat input controlled in the range of 140–260 J/mm. The weld metal, coarse-grained heat-affected zone (HAZ), and fine-grained HAZ had martensitic microstructures and contained small amounts of bainite only under the highest heat input (260 J/mm). The softened zone was composed of an intercritical HAZ (ICHAZ) (with polygonal ferrite and martensite/bainite) and a subcritical HAZ (with tempered martensite). The ICHAZ softened the most during welding and fractured during tensile testing. The strengths of the arc welds were in the range of 1141–1264 MPa, which were 57–63% strength of the base metal. The ICHAZ softening was revealed to originate from carbon diffusion during the martensite to austenite transformation, which is unavoidable in arc welding and should be considered in the design of a welded structure.