Investigating the effects of high and low heat input on the mechanical properties of API 5L L415Q welds using GTAW and SMAW processes

Weld quality is an important parameter in ensuring the overall safety and integrity of pipelines made from high-strength steel used for the transmission of oil and natural gas. In multi-layer pipeline welding, poor heat input control can result in excessive reinforcement, lack of fusion, unsatisfactory microstructural changes, and poor mechanical properties. Hence, in the current research, the effects of Low Heat Input (LHI) and High Heat Input (HHI) on the shape, thermal behaviour, and mechanical properties of API 5 L L415Q pipeline steel were analyzed via welding using the GTAW-SMAW process. The GTAW process is used for the root pass, while the SMAW process is used for the hot, fill, and cap passes. A combination of Taguchi experimental design and desirability-based multi-response optimization was used to determine the optimal values of welding current, welding voltage, welding travel speed, heat input, and reinforcement properties for individual passes. Welding parameters were verified through tensile tests, impact energy tests, hardness tests, bend tests, macroexamination, non-destructive testing, and field-scale application of the developed welding technique. Both LHI and HHI welding conditions produced welded joints that met the API pipeline welding specifications. LHI welding conditions offered better penetration consistency and a more uniform hardness distribution, since the thermal cycle was lower and the cooling rate was higher. In contrast, HHI welding conditions yielded higher impact energy in the weld metal at the fusion line and in the heat-affected zones, due to variations in the thermal cycle. The heat-input optimization strategy also resulted in reduced grain coarsening and a balanced metallurgical balance between strength and toughness. Moreover, industrial-scale validation of 250 pipeline girth welds has indicated consistent welding quality, acceptable geometry, and the absence of critical weld defects on radiographic testing.