Investigation into creep strength of INCONEL® alloy 740H® thin-walled welded tubing for CSP applications

Research was conducted to improve the economics of critical components, such as receivers and piping, for Concentrating Solar Power (CSP) plants. Such components are expected to require the use of heat-resistant nickel-based alloys due to the elevated operating temperatures in designs carrying molten salt or supercritical CO₂. INCONEL® alloy 740H® (alloy 740H) was investigated as an alternative to HAYNES® 230® alloy as it possesses superior high-temperature strength which can lead to overall reductions in material cost. A key challenge is understanding how autogenous seam welding with and without re-drawing can be used to manufacture thin-wall tubing for CSP receivers and heat-exchangers to further reduce costs over traditional seamless production routes. Alloy 740H welded tube was successfully fabricated and re-drawn to several relevant tube sizes. Since traditional mechanical testing samples could not be removed from the thin-wall tubing, full-sized welded tubes were used for tensile, fatigue, and vessel testing (internally pressurized creep-rupture) which was critical to understanding the performance of the weld and the manufactured product forms. The generated vessel test data exhibited a creep strength reduction when compared to wrought product with no clear trend with temperature or test duration. It was found that re-drawing the welded tubes improved the creep strength to approximately 85% of the wrought material performance. Post-test detailed characterization found that nano-sized carbides formed during the laser seam-welding process remained stable after multiple solution-annealing steps, restricted grain growth, and impacted the time-dependent performance. This paper will focus on the time-dependent behavior of the examined welded and drawn tubes, supporting metallographic evidence, and give perspective on future considerations for using alloy 740H in CSP components.

Research supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Solar Energy Technologies Office Award Number DE-EE0008367