Development of Functionally Graded Transition Joints to Enable Dissimilar Metal Welds

The primary factors affecting the service life of dissimilar metal welds, in particular those between ferritic and austenitic steels, include a sharp difference in thermal expansion behavior and carbon migration across the weld. Prolonged exposure to high temperatures, either during post-weld heat treatment or during service, causes carbon diffusion driven by the chemical potential gradient between the BCC and FCC phases, which leads to the formation of a carbon-depleted zone near the fusion zone. This project will develop dissimilar metal welds using functionally graded transition joints via additive manufacturing that will exhibit high resistance to creep and fatigue as well as high microstructural stability. The primary focus of the project will be on joining ferritic steels to austenitic steels, in particular alloys that are relevant in coal-fired power plants. A key aspect of the proposed R&D activities will be determining optimum compositional profiles of the transition joint, which will be achieved using computational materials science and engineering, with additional focus on avoiding sharp changes in chemical potential across the joint region.