Influence of Heat Input and Strength Matching on the Microstructure and Mechanical Properties of GMAW Butt-Welded S700MC High-Strength Low-Alloy Steel

High-strength low-alloy (HSLA) steels produced by thermomechanical controlled processing (TMCP) are widely used in structural applications because of their high strength and weldability. However, the performance of welded joints is strongly affected by welding thermal cycles. This study investigated the effects of heat input (0.6, 1.4, and 1.8 kJ/mm) and filler metal strength (matching and undermatching) on the microstructure and mechanical properties of S700MC steel joints produced by metal-cored arc welding (MCAW). Microstructural characterization, hardness measurements, tensile testing, Charpy impact testing, and analysis of variance (ANOVA) were performed. Heat input was identified as the dominant factor controlling heat-affected zone (HAZ) development and mechanical performance. Increasing heat input enlarged the HAZ and reduced hardness through enhanced microstructural recovery. Filler metal strength mainly influenced failure location and joint strength. The lowest heat input (0.6 kJ/mm) provided the highest strength retention, particularly with the matching consumable, but also produced localized hardness peaks approaching 400 HV0.01 at the weld metal (WM)/HAZ interface, reducing ductility and impact toughness. An intermediate heat input (1.4 kJ/mm) produced the best balance between strength and toughness by promoting a more homogeneous microstructure and smoother hardness distribution. These results provide practical guidance for optimizing welding procedures for TMCP HSLA steels.