Cyclic behaviors of diffusion welded alloy 800H

Diffusion welding creates monolithic joints with complex interiors, such as in the printed circuit heat exchanger (PCHE) for high-temperature reactors. PCHEs are mainly subjected to low cycle fatigue, which is caused by thermal contraction and expansion during the start-up/shutdown. To utilize PCHEs in high-temperature reactors, it is essential to understand the cyclic behaviors of the diffusion weldment (DFW). In this study fatigue tests were conducted on Alloy 800H in its as-received (AR) state with the diffusion weldment at 650 °C and 760 °C and a total strain range of 0.5–1.2 %. The heat absorbed during the diffusion welding process softened the DFW, allowing it to accommodate large plastic strains. Fatigue tests revealed that the fatigue life of the DFW was longer than that of the AR where the plastic strain dominates the fatigue life. However, the fatigue life of the DFW was shorter than that of the AR where the materials resisted the cyclic behavior based on their strengths. The location of crack initiation was outside the diffusion welding interface, indicating that the cyclic behaviors of the diffusion weldment depend on the thermal history of the manufacturing process.