STUDIES ON DISSIMILAR WELD METAL JOINTS FOR ADVANCED ULTRA SUPERCRITICAL POWER PLANT APPLICATIONS

Abstract

Advanced Ultra Supercritical (AUSC) thermal power plants represent cutting-edge technologythat operates at exceptionally high temperatures and pressures, surpassing the capabilities of traditional supercritical plants. These AUSC plants offer increased thermal efficiency and promise to substantially reduce carbon dioxide (CO2) emissions within the coal-sourced electricity generation. Through their advanced design and operation, the AUSC power plants could mitigate CO2 emissions by maximizing the conversion of fuel into energy while minimizing the environmental impact. Traditionally employed austenitic stainless steel or ferritic steel components fall short of satisfying the stringentdemands imposed by high-temperature operating conditions ofAUSCplants. These conventionalmaterials, primarily steels, exhibit limitations in their capacity to withstand temperatures beyond 700℃ and pressures exceeding 350 bars. These materials' intrinsic mechanical properties and structural integrity degrade under such extreme thermal and pressure stresses, leading to compromised performance, reduced lifespan, and increased susceptibility to corrosion and creep deformation. Creep is the gradual deformation that occurs over time under sustained mechanicalstress and elevated temperatures. To effectively address these challenges, the use of nickel alloys has emerged as a promising alternative. Nickel alloys offer superior mechanical strength, oxidation resistance, and thermal stability at elevated temperatures, aligning more closely with the demanding conditions of AUSCplants. However, while nickel alloys render enhanced performance, they also entail premium costs due to the complexity of their production and processing. This cost factor prompts the exploration of hybrid solutions involving dissimilar metal joints, enabling the judicious use of high-cost nickel alloy components only where they are necessary while employing more cost-efficient ASS in areas with lower temperature and pressure requirements. For instance,when making boiler components, austenitic steels can be utilized for superheaters and reheatersoperating at temperatures up to 650℃, while nickel alloys, suited for their stability at over 700℃, can be employed in thin and thick section components. This thoughtful combination of materials through dissimilar welding thus strikes a balance between performance and economicfeasibility, making it a pragmatic approach for achieving optimal functionality in AUSC plants. Nonetheless, welding dissimilar metals, such as steel and nickel alloy, presents a multitude of challenges stemming from inherent disparities in their properties and structures. While base metals possess wrought structures with varying elemental compositions, the weld bead features a cast structure, necessitating meticulous fusion techniques. The heat-affected zone (HAZ) containing intermetallic compounds adds complexity to the fusion. Although most austenitic stainless steels are weldable, they are susceptible to distortion due to excessive thermal expansion, leading to cracking and exacerbated corrosion. The pronounced discrepancy betweencoefficients of thermal expansion prompts the formation of stresses at the interface, leading to thermal fatigue—a precursor to creep failure, which is particularly concerning in high- temperature applications. In the context of dissimilar welds, the dissimilarity in thermal expansion coefficients amplifies the likelihood of thermal fatigue, ultimately accelerating the onset of creep deformation and failure. A noteworthy issue arises when substituting stainless steel filler for nickel alloy in the weld. While the initial welds may pass standard tests, theysuccumb to corrosion within a few months due to material incompatibility. Adding to the complexities, carbon migration can adversely impact both mechanical and metallurgical properties, compromising joint integrity. The use of multi-pass welding (closure weld) is often suggested to overcome these issues. Nevertheless, dissimilar metal welds continue to struggle with creep-related problems during operation. Especially, multi-pass welds encounter exacerbated creep deformation due to the inherent material heterogeneity, coupled with the presence of defects and weld residual stresses.Thereby, a profound understanding of creep deformation is essential for comprehensive damageassessment. While extensive research covers traditional steel dissimilar metal welds and failures,there is a gap in knowledge regarding newer 9Cr-1Mo steels like P91 when combined with Inconel 617 (IN 617). Ongoing failures with these dissimilar combinations lack full explanation, with suspected factors including varying thermal expansion, residual stresses, and microstructural changes caused by interdiffusion across weld interfaces.