Determination of heat input impact on residual stress, microstructural phase transformation and mechanical characteristics of welded S690QL steel joints by Taguchi optimisation approach

This article focuses on optimising the submerged arc welding process to improve the microstructural and mechanical characteristics of welded S690QL steel joints efficiently without preheating, using a Taguchi optimisation approach. Trail runs were performed initially for bead-on-plate studies to optimise the weld cycle ranges by investigating weld characteristics such as microstructural morphology and toughness in the weld zone. Selected weld cycle ranges from bead-on-plate studies were further analysed and optimised through butt weld investigations. The phase transformations in microstructure at the weld and heat-affected zone from tempered martensite, martensite, and bainite to dendritic ferrite, lath bainite, and lath martensitic phase were observed with a change in heat input from 0.91 kJ/mm to 1.37 kJ/mm. The weld residual stresses were tensile at the weld and compressive away from the weld zone, diminishing gradually with an increase in transverse distance from the weld. The overall weld efficiency, weld residual stress concentration and mechanical characteristics of the welded butt joint were improved at a controlled heat input of 1.19 kJ/mm. An increase in heat input beyond the optimised limits influences the extent of the heat-affected zone and the joint's toughness due to the coarsening of the weld microstructure towards the fusion boundary.