Optimization and metallurgical characterization of double-pulse metal inert gas welded P91 steel

The principal aim of this research was to optimize the welding parameters in the double-pulse metal inert gas welding of P91 alloy steel. Welding was carried out using a P91 filler wire of 1.2 mm diameter. The plates were welded in butt joint configuration and all trials were conducted in accordance with the L9 Taguchi array. Welding parameters such as wire feed speed, frequency, and amplitude were considered for controlling weld quality. Bead geometry, tensile strength, impact strength, and hardness were the factors considered for studying the weld characteristics. The technique for order preference by similarity to an ideal solution multi-objective function was used to identify the best welding parameter combination. Electron backscatter diffraction analysis was also done on the weld and it was observed that the optimized weld (sample 6) had a higher percentage (22.8%) of high-angle grain boundaries: > 15° than the weld (sample 7) that had the least favorable properties (16.8%). A greater percentage of high-angle grain boundaries imparts higher strength and hardness as they act as barriers to dislocation motion. On comparing the mechanical characteristics (which were heavily influenced by the microstructure) of samples 6 and 7, it was found that sample 6 had a much higher toughness value of 48 J as opposed to 8 J for sample 7. The bead width of sample 6 was also lower by a factor of 42.5%. Although both the hardness and tensile strength were reported to be lower for sample 6 (by 3.86% and 20.34%, respectively), the overall characteristics of weld sample 6 were more closely aligned with boiler application requirements.