Microstructure and Tensile Properties of Diffusion Bonded Austenitic Fe-Base Alloys—Before and After Exposure to High Temperature Supercritical-CO2

Abstract

Austenitic Fe-base alloys, SS 316H and Alloy 800HT, were diffusion bonded for use in compact-type heat exchangers in supercritical-carbon dioxide (S-CO2) Brayton cycles. For diffusion bonded 316H, grain boundary migration across the bond-line was observed despite the formation of some Cr-rich carbide, and its tensile properties were similar to those of as-received 316H. However, diffusion bonded Alloy 800HT exhibited severely degraded elongation compared to as-received 800HT due to the formation of continuous Ti-rich carbides along the bond-line. Post-bond heat treatment (PBHT) was found to improve elongation at fracture for diffusion bonded alloys. However, a subsequent corrosion test in S-CO2 at 600 °C (20 MPa) for 1000 h resulted in a loss of elongation. This was much more severe for PBHT-ed 800HT due to the formation of Cr-rich carbides at the bond-line. Meanwhile, it was found that the effect of ageing on loss of elongation during high temperature exposure was greater than that of S-CO2 environment.

Corrosion Behaviors of Heat-Resisting Alloys in High Temperature Carbon Dioxide

“…This explains why the strengthening effect increases with increasing temperature. Furthermore, some studies [29,65,125,126] have reported that the mechanical performance of base materials or welded samples or diffusion-bonded joints of austenitic steel 316SS and 800HT were greatly compromised after exposure to high temperature S-CO 2 . The fracture mode gradually changed from transgranular fracture to intergranular fracture at room temperature due to the carburization in high temperature S-CO 2 .…”

Compatibility of Different Commercial Alloys in High-Temperature, Supercritical Carbon Dioxide

“…Supercritical carbon dioxide (SCO 2 ) is considered as one of the prime coolant candidates for next-generation power plants, as it has advantages of higher efficiency, smaller component sizes, fewer components, and a simpler cycle layout, etc. [1,2]. In the SCO 2 Brayton cycle system, material selection of the key components such as heat exchangers and pipes is one of the important problems of long-term, safe operation [3][4][5][6].…”

“…Moreover, Oleksak et al [16] further suggested that O 2 , H 2 O, and SO 2 had negligible effects on the oxidation of ferritic Fe alloys, while SO 2 could significantly enhance the rate of oxidation for austenitic Fe alloys. Fourthly, studies have examined the integrity of welding joints, including examinations of the diffusion-bonded technique [17] towards the printed circuit heat exchanger (PCHE) [1,18], the fusion-weld technique [2], and the typical gas tungsten arc welding (GTAW) technique [19]. Fifthly, studies have examined the preparation of the coating on the metal substrate, such as the deposition of a Cu coating [20], a Si coating [21], an Al coating [22,23], a Cr coating [24,25], and a NiAl coating [26].…”