Electrical, microstructural, and mechanical behavior of HSLA X70 pipeline welded by GMAW and FCAW processes

This study proposed the dynamic modeling and control strategy for GMAW and FCAW processes using two parallel-connected inverters in the power supply to weld an HSLA X70 steel pipeline using short-circuiting and spray transfer modes. The obtained results showed a great similarity between the welding current and voltage of the simulation model and the experimental process. A novel technique, the single-sided enveloped spectrum (SSES), proved highly efficient in estimating droplet detachment frequency (DDF). Mesh area calculations confirmed the high arc stability due to optimized welding parameters. The microstructure of a GMAW pass consisted primarily of coarse acicular ferrite, Widmanstätten ferrite, and polygonal ferrite, with minor traces of bainite. In contrast, the FCAW passes exhibited a wider variety of ferrite morphologies. Grain boundary ferrite (GBF) morphology was prevalent, while the capping passes showed a distinct feature: very fine grains of bainite and acicular ferrite. Vickers micro-hardness measurements revealed a significant reduction in hardness within the Heat Affected Zone and Weld Zone of the GMAW process, down to approximately 180 ± 5 HV. Conversely, the capping pass exhibited the highest micro-hardness, reaching around 230 HV. This increase is attributed to the presence of a fine, columnar microstructure composed of Widmanstätten and polygonal ferrite.