Abstract: Dissimilar welding between austenitic stainless steel Super304H and ferritic-martensitic steel T92 is an important requirement in advanced ultra-supercritical (AUSC) power plants. Fusion welding of alloy Super304H to alloy P92 poses several challenges, right from the choice of filler metal. Premature creep failure of these joints in the weld heat-affected zone (HAZ) of alloy T92 (Type IV cracking) is the most serious concern. Although Type IV cracking phenomenon is widely investigated in alloy T92 and similar creep-resistant ferritic-martensitic steels in literature, the reasons for and the mechanism of Type IV cracking are still to be clearly understood. This study has been undertaken to understand the microstructural evolution of Super304H/T92 dissimilar welds during long-term thermal exposure up to 10000 h at 650 °C. The creep failure of these dissimilar welds has been correlated with the concurrent microstructural changes under creep and thermal exposure. Comprehensive microstructural characterization, including a quantitative microstructural analysis across various zones of dissimilar welds illustrates the accelerated microstructural degradation such as the formation of Laves particles and their coarsening together with coarsening of M23C6 carbides within the HAZ sub-zones of alloy T92 during isothermal exposure. Further, such microstructural deterioration has been found to dictate the service life of these dissimilar welds during creep. Also, Type IV cracking in the region of FGHAZ has been found to be accelerated due to harmful microstructural changes under creep and thermal exposure.
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