Observations in the High-Temperature Creep Performance of Diffusion Bonds for sCO2 Heat Exchangers

Abstract

Abstract In support of understanding the long-term performance of high-temperature diffusion bonded heat exchangers for sCO2 power cycle application, a commercially produced alloy 316 diffusion bonded block was characterized and subjected to uniaxial creep testing in air at temperatures of 600 to 750°C with varying sample sizes with some tests durations exceeding 7,000 hours. The results show a range of different behaviors when comparing between samples sizes and in comparison to wrought alloy 316. At lower testing temperatures, some sample sizes did not meet average rupture life predictions while others did. At intermediate and higher temperatures, sample size effects were not as prevalent. While measured ductility was large for the standard and larger samples, unique observations from post-test metallurgical analysis identified non-uniform strain distributions leading to sample ovality. Additionally, failures occurred on the bond lines with oxidation potentially impacting longer-term and higher-temperature tests. Overall, these results demonstrate that careful selection of sample size and test conditions is required for engineering studies on diffusion bond performance to ensure a relevant assessment of component performance. Furthermore, the results show microstructural texture effects in the starting sheet may persist after diffusion bonding and therefore should be explored in more detail as part of future research. Finally, pre and post-test characterization suggest that procedures to quantify grain growth and bond-line features (voids and oxides) which are not measured using the standard acceptance tests may be needed for further advancement of the technology.