Microstructural Evolution and ISO-Based Weld Quality in MAG and Laser Welding of HC420LA Steel Under Different Heat Inputs

In this study, HC420LA steel plates joined by gas metal arc welding (MAG), manual laser welding (ML), and robotic laser welding (RL) were comparatively examined under heat input (HI) levels obtained from an active production line exhibiting weld defects. The effect of HI and welding method on mechanical properties, microstructural evolution, phase characteristics, and weld integrity was evaluated using tensile and hardness tests, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Weld imperfections were evaluated according to ISO 5817:2023(E) for MAG welds and BS EN ISO 13919-1:2019 for laser welds, and the corresponding quality levels were determined. The highest tensile strength (568 MPa) and elongation (23%) were achieved in RL welds at the lowest HI value (0.108 kJ/mm), with fracture occurring outside the weld region, indicating superior joint integrity and mechanical compatibility with the base metal. In contrast, MAG and ML welds exhibited a non-linear relationship between HI and ductility and tensile strength. ML welds showed higher hardness and reduced ductility due to the formation of harder transformation products, while MAG welds demonstrated a non-linear response associated with heat-affected zone (HAZ) coarsening. Heterogeneous phase distribution XRD analysis confirmed the presence of α-Fe-based phases and secondary alloyed structures, while EDS analyses revealed a relatively homogeneous distribution of the principal alloying elements within the weld regions and provided supporting evidence for the Mn3O4 oxide phase identified in the RL welds. SEM observations further demonstrated distinct microstructural transitions across the fusion zone (FZ) and HAZ, reflecting the influence of the welding process and heat input on weld evolution. The assessment of weld imperfections according to the relevant ISO standards showed that ML and RL welds satisfied Quality Level B, whereas MAG welds exhibited quality levels ranging from B to D, depending on the evaluated imperfection. These results indicate that equivalent HI values do not guarantee comparable weld quality or mechanical performance across different welding processes. The study provides insight into the relationship between heat input, weld quality, microstructural evolution, phase constitution, and mechanical performance in HSLA steels.