The effect of thermomechanical welding on the microstructure and mechanical properties of S700MC steel welds

Abstract

Abstract Grain refinement by plastic deformation attempts to compensate for the loss of mechanical properties of welded joints during conventional TIG welding. We performed thermomechanical welding (TMW) tests on the S700MC steel with welding combinations of the TIG arc heating and frequent hammering over three target cooling times (t8/5 = 5s, 15s and 25s). Additionally, we analysed the effect of initial microstructures on the weld joint quality by testing three materials conditions: hot-rolled (as-received), and cold-rolled with 10% and 30% thickness reductions. We separately studied the effects of plastic deformation, and the mechanical vibration on the grain refinement. Optical microscopy, electron backscattered diffraction and Vickers hardness were used to characterise the weld microstructure heterogeneity. We correlated the weld width and depth, and the mean grain size as the function of cooling time t8/5. The results show that the weld dimensions increase as increasing the t8/5. The weld microstructures transformed from the mixed martensite and bainite into the mixed ferrite and bainite with increasing mean grain size as increasing the t8/5 time. The TMW welds exhibit smaller grains compared to TIG welds due to the coupled effects of the mechanical vibration and plastic deformation. The mechanical vibration contributes to weld metal homogenisation, accelerating TiN precipitation in the fusion zone. The proposed TMW process can refine the weld microstructures of S700MC steel, enhancing its mechanical properties.