Effects of multiple repair welding cycles on the microstructural evolution and mechanical integrity of shipbuilding steel joints

Abstract
This study presents a comprehensive experimental and metallurgical investigation into the effects of multiple repair welding cycles on the structural integrity of ST44 carbon steel butt joints, a material widely used in shipbuilding. Two industrially significant welding processes Shielded Metal Arc Welding (SMAW) and Flux-Cored Arc Welding (FCAW) were employed to evaluate how repeated welding, gouging, and re-welding influence the mechanical performance and microstructural evolution of welded joints. Single V-groove butt joints with a total bevel angle of 60° were prepared, and specimens underwent one to four full repair cycles to simulate realistic industrial repair conditions. Mechanical characterization included tensile testing, Charpy V-notch impact testing, and hardness profiling across the weld metal, heat-affected zone (HAZ), and base metal. Results indicate a progressive deterioration in tensile strength, impact toughness, and hardness uniformity with increasing weld repetitions. The HAZ exhibited significant grain coarsening, widened thermal damage zones, and localized embrittlement after the second and third repair cycles, particularly in SMAW specimens. FCAW joints demonstrated comparatively higher deposition efficiency but showed similar degradation patterns under excessive thermal cycling. Metallographic and SEM examinations revealed microstructural transformations such as pearlite spheroidization, carbide precipitation, and microcrack initiation within the HAZ, especially after repeated reheating. Fractographic analysis identified a transition from ductile dimpled fracture modes in single-pass welds to mixed or quasi-brittle modes in multi-repaired joints. These findings confirm that repetitive repair welding significantly alters the thermal stability and metallurgical behavior of ST44 steel, increasing susceptibility to cracking and reducing overall structural reliability. The study concludes that repair welding should be limited to a maximum of two cycles to maintain acceptable mechanical performance in shipbuilding applications. The results provide essential guidance for shipyards, classification bodies, and welding quality engineers when establishing safe repair thresholds and optimizing welding procedures to ensure long-term service integrity in marine structures.