Microstructural Evaluation of Welded Nickel-Based Superalloy Inconel 740H After Creep Testing

摘要

Inconel 740H (R) alloy is considered a leading candidate for advanced ultra-supercritical steam power generation at temperatures up to 750 degrees C and pressures up to 35 MPa. Stable alloy microstructures for extremely long-term use are needed; however, extended exposure to high temperatures and stresses can lead to microstructural instabilities when the material is joined with other alloys, thereby decreasing component lifetime. It is, therefore, critical to understand the microstructural evolution of weldments of this alloy during high-temperature and high-pressure exposures. This research specifically aims to evaluate the effects of stress and joining processes on the lifetime of Inconel 740H welded with Thermanit 263 filler metal (based on Alloy 263 (TM)). Electron microscopy techniques were used to evaluate the evolution of gamma '-precipitates, grain boundary phases, grain sizes and boundary types, weldment interfaces, and other relevant microstructural characteristics resulting from long-term exposure at elevated temperatures (650-850 degrees C) and external stresses (53-440 MPa) during creep testing. The creep strength of Inconel 740H weldments with Alloy 263 (740H/263) as a filler metal was less than that of Inconel 740H/740H base metal because of the microstructural instability of Alloy 263 at the subject creep temperatures.