Factors Influencing Propensity for Stress Relaxation Cracking in Inconel^® Alloy 740H^® and Practical Guidance for Applications

Inconel^® alloy 740H^® (UNS N07740) was the first age-hardenable nickel-based alloy approved by the ASME Boiler and Pressure Vessel Code for use in pressure-boundary applications. In recent years, advanced energy systems such as supercritical CO₂ power cycles have utilized alloy 740H in large demonstration projects driven by the requirement for higher fluid temperatures and pressures. Stress relaxation cracking (SRxC) following post weld heat treatment (PWHT), also known as strain age cracking (SAC), has been identified in a limited number of weldments during these industrial builds resulting in focused research to further clarify factors influencing this cracking tendency. This paper will summarize some of the findings from shop and field fabrication leading to successful welds and characteristics of observed SRxC. Laboratory experiments supported by microstructural characterization will be presented to highlight the importance of variables such as strain, material starting condition, and PWHT temperatures. Finally, the results will be summarized within the context of practical guidance for industry to successfully weld the material in boiler, heat exchanger, and piping applications.