Investigating the heat input effect of the GTAW process upon the microstructure and HAZ extension of HSLA-100 steel weld joints using thermal cycles

In the present research, the welding of HSLA-100 steel was accomplished with three different heat inputs using the GTAW procedure. The microstructure of the disparate joint zones was examined utilizing an optical microscope and scanning electron microscope. To assess the impact of thermal cycles upon the extension and changes in the heat-affected zone, the thermocouples were placed at distinct distances from the fusion line. The results revealed that the weld metal microstructure consists of acicular ferrite and some polygonal and quasi-polygonal ferrites with the M/A constituent. An epitaxial growth was observed in the fusion line in all the heat inputs. It was determined that the heat-affected zone microstructure comprises two regions, namely a coarse-grained heat-affected zone with a granular and lath bainite microstructure and a fine-grained heat-affected zone with the prevailing microstructure of granular Bainite. The results indicated that the increasing of the heat input leads to a decrease in the amount of acicular ferrite and M/A constituent volume fraction in the weld metal and an increase in lath bainite in the coarse-grained zone. It was found that by increasing the heat input from 0.78kJ/mm to 1.42 kJ/mm, the grain size increases from 29.2 μm to 76.5 μm and from 15.7μm to 32.5μm in the coarse-grained and fine-grained zones, respectively. The thermal cycles evaluation pertinent to the implanting of a thermocouple disclosed that the heat-affected zone width is extended from 3.6 mm to 4.5mm by increasing the heat input from 0.78 kJ/mm to 1.42 kJ/mm.