Abstract

High-Strength Low-Alloy (HSLA) steel is a preferred material for pipeline construction due to its high strength, toughness, and resistance to environmental degradation. However, welding HSLA steel introduces significant challenges, particularly concerning the structural integrity, mechanical performance, and long-term durability of butt-fusion welds. The welding process produces fusion and heat-affected zones with microstructures that differ significantly from the parent material, often leading to graded structures, residual stresses, hardness variations, and susceptibility to brittle fracture or corrosion. This study investigates the influence of key welding parameters—including heat input, welding sequence, weld bead geometry, and post-weld heat treatment (PWHT)—on the microstructure, mechanical properties, and corrosion behaviour of HSLA pipeline steel butt-fusion welds. A series of controlled experiments were conducted in which standard butt-weld joints were fabricated under varying welding conditions. Tensile testing revealed that the base HSLA steel had an ultimate tensile strength of 426.62 MPa, yield strength of 404.27 MPa, and elongation at break of 32.5%. Optimized welded joints exhibited improved ultimate tensile strength of approximately 460 MPa. Impact toughness more than doubled, increasing from 30.4 J in the untreated material to 75.78 J in PWHT-treated welds. Hardness measurements indicated peak values of 653.3 HV in high-hardness HAZ regions, which decreased to 310.3 HV after PWHT, balancing strength with ductility. Electrochemical corrosion testing demonstrated that PWHT improved performance, with open circuit potential shifting from −0.58 V to −0.21 V and current density reducing from 2.2 × 10⁻⁴ A/cm² to 1.0 × 10⁻⁴ A/cm². Microstructural analysis confirmed the transition from ferrite–pearlite in the base material to acicular ferrite in the as-welded HAZ and tempered martensite after PWHT. Multi-response optimization using Taguchi signal-to-noise ratios and Grey Relational Analysis identified the optimal welding condition as Experiment 25 of the L25 Taguchi array, combining a 180 A welding current, 26 V arc voltage, 140 mm/min travel speed, and a heat input of approximately 1.8 kJ/mm, achieving a Grey Relational Grade of ~0.88. The results demonstrate that controlled heat input, appropriate multi-pass welding sequences, and effective post-weld heat treatment are essential for achieving HSLA welds with balanced mechanical properties, reduced susceptibility to cracking, and improved corrosion resistance. This study provides quantitative guidance for pipeline welding, enabling safer, more reliable, and cost-efficient fabrication while advancing the understanding of how welding parameters influence the microstructure and performance of HSLA steel butt-fusion welds. The findings are directly applicable to industrial pipeline fabrication and have implications for optimizing welding practices to prevent joint failure in service.