Injin Sah

Diffusion welding of heat-resistant alloys has attracted interest in the manufacturing of components with complex configurations. Controlling secondary precipitates along the interface is necessary to enhance the quality of the diffusion welding. Surface treatment to increase the solubility product (K_sp) of Ti-rich carbide is proposed to accomplish such an enhancement. The reduction of secondary precipitates along the interface induced grain boundary migration across the interface. The chemical compositions at/near the interface satisfied the material specifications. The mechanical properties of the diffusion weldment were similar to those of Alloy 800H that underwent the same thermo-mechanical processes in the range of 25–700 °C. At 25 °C, the tensile strength was 553 MPa, which satisfied the minimum specified tensile strength described in ASTM B: 409-22. The location of failure was random in the gauge section, and dimples, the evidence of the macroscopic plastic deformation, were observed on the fracture surface.

Plate-type heat exchangers are anticipated to be used in the next-generation nuclear industry, and solid-state diffusion welding is a critical technology for building plate-type heat exchangers with high integrity. In this study, we manufactured a diffusion weldment and evaluated its creep behavior. Microscopic analysis revealed that Al-rich oxides were developed along the interface, significantly impeding grain-boundary movement across the interface. Oxide-containing planar grain boundaries resulted in premature brittle fracture at the interface with less than 9% creep strain under all test conditions. The time to rupture and time to 1% creep strain of the diffusion weldment were less than those of the as-received alloy, while the slopes in double-logarithmic plots were almost identical for both alloys. In a Larson–Miller parameter study, the stress to rupture of the diffusion weldment reached 95.59% of that of the as-received alloy, whereas the stress to 1% creep strain steeply decreased in the low-stress range.

This paper presents a preliminary design and performance analysis of a supercritical CO₂ (SCO₂) heat exchanger for an SCO₂ power generation system. The purpose of designing a SCO₂ heat exchanger is to provide a high-temperature and high-pressure heat exchange core technology for advanced SCO₂ power generation systems. The target outlet temperature and pressure for the SCO₂ heat exchanger were 600 °C and 200 bar, respectively. A tubular type with a staggered tube bundle was selected as the SCO₂ heat exchanger, and liquefied petroleum gas (LPG) and air were selected as heat sources. The design of the heat exchanger was based on the material selection and available tube specification. Preliminary performance evaluation of the SCO₂ heat exchanger was conducted using an in-house code, and three-dimensional flow and thermal stress analysis were performed to verify the tube’s integrity. The simulation results showed that the tubular type heat exchanger can endure high-temperature and high-pressure conditions under an SCO₂ environment.

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.