Activated Tungsten Inert Gas Weld Characteristics of P91 Joint for Advanced Ultra Supercritical Power Plant Applications

Activated Tungsten Inert Gas (A-TIG) welding, a variant of tungsten inert gas (TIG) welding, was used for welding P91. In Generation IV power plants, P91 welds are prone to premature failure due to the presence and formation of brittle phases and creep at high temperatures. When performing A-TIG welding, the flux composition plays a role in the reversal of the Marangoni flow in the weld pool, which ultimately determines the level of penetration achieved. A-TIG gave a complete penetration of 8 mm in the P91 weld. The weldment exhibited non-uniform growth of microstructures with varying grain sizes and precipitates, resulting in variation in mechanical properties. The weld fusion zone (WFZ) had a martensitic structure. The standard flat tensile test specimens were found to fail in the base metal and the fine-grain heat affected zone (FGHAZ) region. The sub-size flat tensile test specimen gave a high strength of 863 ± 10 MPa, failing in the WFZ. The high temperature tensile test specimens had the tensile strength of 512 ± 10 MPa (at 450 °C) and 469 ± 10 MPa (at 450 °C). In both the high temperature tensile test specimens, failure occurred in the base metal region. The impact toughness was recorded at 76 ± 15 Joules due to the presence of untempered martensite in the AW state, and in the PWHT state, the impact toughness increased up to 98 ± 15 Joules. In the AW state, the coarse grain heat affected zone (CGHAZ) region was observed with a maximum microhardness of 450 ± 5 HV and WFZ 460 ± 5 HV. The post-weld heat treatment (PWHT) was successfully performed to temper the martensite and impart some ductility to the weld. The A-TIG weld had sufficient benchmark strength, and the study successfully concluded its aim.