Twinning-Induced Plasticity Steel for the Automotive Industry: Design Stress for Gas Tungsten Arc Welded Parts

Abstract

We investigated the effect of gas tungsten arc welding, GTAW, on the mechanical properties of 1 mm thick sheets of a TWIP Fe-22Mn-0.6C-1.5Al (wt.%) steel, using optical microscopy, transmission electron microscopy (TEM), x-ray diffraction, energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and microhardness These sheets had undergone two distinct processes before welding: cold-rolling followed by annealing at 750 ºC (CR-750) and warm-rolling at 600 ºC followed by an 850 ºC anneal (WR-850). To ensure viable weldability, the welding process parameters, were adjusted, and then face and root bending tests were carried out. These tests verified that no surface defects were present in the weld bead. An analysis of chemical composition showed only low chemical segregation present in the different characteristic zones of the weld, which controlled the value of the stacking fault energy. Consequently, austenite phase stability was maintained, and dislocation glide and twinning were the main deformation mechanisms, for both sheet processing conditions. During uniaxial tensile tests, the sheets CR-750-W and WR-850-W(after welding) exhibited yield strengths of 240 and 200 MPa, 21 and 28 percent deformation, and 772 and 783 MPa maximum tensile strengths, respectively. The deterioration of the mechanical properties was due to heating during welding, which resulted in softening in both the heat-affected and welded zones due to the presence of coarse and columnar dendritic grains. SEM observations showed that the specimens failed by ductile fracture after localized deformation and necking. All of the plastic deformation concentrated in the welded zone, giving rise to a high density of dislocations and mechanical twinning, as observed by TEM and high microhardness values. The main contribution of this investigation, on the industrial level, was the measurement of the design stresses for welded joints in thin sheets of TWIP steel. It is intended that these steels will be used for vehicle chassis parts.