High-temperature mechanical behaviors of diffusion-welded Alloy 617

Abstract

The development of a compact-type heat exchanger is one of the critical issues for realizing an efficient gas-to-gas heat exchanging system in a very high-temperature gas-cooled reactor (VHTR). The nickel-based superalloy Alloy 617 (Ni-Cr-Co-Mo, UNS N06617) is considered as the leading material for this application owing to the excellent thermal stability and strength in high temperature ranges. Sixty sheets having dimensions of 200 × 200 mm2 were diffusion-welded under uniaxial compressive pressure of 14.7 MPa at 1423 K (1150 °C) in a high vacuum condition. After 2 h of hot pressing, a post-weld heat treatment (PWHT) was applied to enhance the constituent atomic diffusion across the interface. To evaluate the high-temperature mechanical properties, tensile and stress-rupture tests were employed in this study. In tensile tests, the yield strength of the diffusion weldment is 318 ± 4 MPa at room temperature, and superior yield strengths over the ASME code requirement are maintained up to 1223 K (950 °C). Similarly, the tensile strength reaches 730 ± 15 MPa with ductility of 37.1 ± 1.4% at room temperature. However, both the tensile strength and ductility drop rapidly at elevated temperatures (≥973 K (700 °C)). During stress-rupture testing at a fixed temperature of 1173 K (900 °C) in an ambient air environment, the diffusion weldment shows lower creep properties. A plot of log time-to-rupture vs. log minimum creep rate is illustrated on the basis of the Monkman-Grant (M–G) relationship.