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Effect of upset pressure on the weld zone microstructure, texture evolution and its effect on toughness were investigated. It was observed that an increase in upset pressure leads to a decrease in the heat-affected zone width and also the grain size as a result of recrystallisation. It was also observed that insufficient upset pressure leads to porosity in the joints. Electron backscattered diffraction study on the weld zone showed that an increase in the upset pressure resulted in a sharp rise in (110)[001] and (110)[1 10] component. Shear deformation at the plate edges due to the constrained flow of material led to the evolution of Goss fibre component parallel to the fracture plane. The presence of Goss fibre components that are generated during welding as a result of upsetting operation played a critical role in decreasing the toughness at the weld zone compared to the parent material.

The effect of electrode negativity (EN) ratio, current and welding speed on bead profile and arc stability in case of constant current square wave power source were investigated by conducting bead on plate (BOP) welding experiments on API X80 steel. In addition to this, effects of heat input on prior austenite grain (PAG) size, microstructure and hardness of coarse grain heat affected zone (CGHAZ) were also studied. Deleterious hump angle created due to EN ratio variation, decreased with increase in EN ratio and welding current. Average PAG size and spacing between bainite laths increased from 17.09 µm to 62.66 µm and 0.67 µm to 50 µm respectively with increase in heat input from 0.69 kJ/mm to 3.6 kJ/mm, which altered CGHAZ hardness. Arc stability and metal transfer mode for different EN ratios and currents were analyzed by means of probability density distribution (PDD) graphs of current and voltage transients recorded using a digital storage oscilloscope. PDD graph show better arc stability at higher currents and during the electrode positive (EP) part of current waveform.