The effect of alloy compositions on the joint strength of carbon steels welded by the extended hot-rolling process

In this study, we investigated the factors that determine the joint strength of carbon steels welded by an extended joining process in air. Our main focus was on the effects of alloying elements (e.g., C, Si, Cr, and Mn, which are more reactive with O than with Fe) on scale adhesion, joint microstructure, and, ultimately, joint strength. Five carbon steels with different compositions of the same set of alloying elements are selected for this investigation. The model steels are hot-rolled, and surface descaling is performed several times during processing. The hot-rolled plates are welded with 1000% deformation at approximately 1000 °C. The joint strengths are evaluated by uniaxial tension testing. The joint microstructures and fractured surfaces are investigated with field emission scanning electron microscopy (FE-SEM). In addition, chemical analysis for phase identification is performed by electron probe X-ray micro-analysis (EPMA) and transmission electron microscopy (TEM). The results of our analyses reveal that Si increases the scale content at the joint by forming Fe2SiO4 along the interfaces between the Fe oxides and alloys and that this phase anchors the scale to the substrate. When both the C and Si contents are increased, Fe2SiO4 constitutes a larger portion of the scale–substrate interface, and the density of the internal oxide particles in the substrate near the joint is markedly increased. The tension test results demonstrate that the joint strength is decreased with increasing contents of C and Si because the joint failure is initiated by brittle fracture of the scale at the joint and the void nucleation sites around the internal oxides.