Creep Behavior of Welds in a Nickel-Based Superalloy

The next generation of coal fired power plants, termed ‘Advanced Ultra Supercritical’ (A-USC), is designed to operate at higher temperatures (1300-1400°F) and higher pressures (4500-5000psi) than current generation plants. As a result, A-USC boilers will exhibit efficiency improvements as well as emissions reductions. Alloy IN740H has shown promise for application in A-USC systems because of its high temperature strength and corrosion resistance. However, accelerated creep failures of Inconel 740H weldments have necessitated an in-depth analysis of the microstructural development of the alloy’s base metal, heat affected zone, and fusion zone after high temperature service. A study is being conducted to understand the microstructural development of Inconel 740H base metal and weld metal after creep testing. Scanning electron microscopy has been used to assess changes in the γ’ phase, including size and distribution, across the regions of Inconel 740H weldments. The segregation of substitutional alloying elements may be a major contributing factor to the observed reduction in creep life for Inconel 740H weldments. Therefore, the distribution of alloying elements across dendrites in gas metal arc and gas tungsten arc welds has been determined through X-ray energy dispersive spectrometry. These concentration profiles have been incorporated into thermodynamic and kinetic models to develop a series of viable homogenization heat treatments. Results acquired to date suggest that the presence of a γ’ denuded zone may be responsible for the accelerated creep failures of the fusion zone.