Stress Assisted Grain Boundary Oxidation Behavior of IN706 Material Used in Gas Turbines

Abstract

Abstract. The electric power sector is currently facing major industry challenges around decarbonization efforts, and gas turbines will continue to play a crucial role for the energy transition. Major efforts have included improving the efficiency of gas turbines through alternative part design enabled by advanced manufacturing and leveraging low carbon fuels, such as hydrogen. Additionally, many industrial gas turbines are now facing original equipment manufacturer (OEM) end-of-life design criteria, which has become an emerging concern in recent years. This has led to a growing need for strategies around life extension in major components, such as the rotor wheel and spacer. Specifically, the F class gas turbine technology, developed in the 1980s, represents the largest share of power generating capacity for both simple and combined cycle units. The General Electric 7F technology represents >950 units installed globally to support ∼175 GW of power. In this model, an IN706 iron–nickel base alloy is used for first stage wheel and spacer. These components have been shown to be the major life limiting parts of the rotor and are typically required to be replaced after 144,000 h or 5000 starts. The focus of this research was to evaluate the major damage mechanism that occurs in IN706 rotor components, which is stress assisted grain boundary oxidation (SAGBO). A detailed microstructure evaluation revealed that no damage existed in a retired stage 1 wheel and spacer after ∼120,000 h of operating service in ideal operating conditions. This service exposed material was further leveraged to develop a novel mechanical testing method to assess the susceptibility to SAGBO exposed to various temperature and stress profiles using a notch-bar geometry. Results were compared to standard round bar creep tests to show that an increase in temperature >500 °C resulted in an increase in notch sensitivity. Post-test microstructural assessments were performed to confirm SAGBO was the primary damage mechanism under temperature and loading conditions comparable to the cooling slot and lock tab regions in actual rotor wheel components. Findings from this study will help enable improved rotor life extension strategies in gas turbines utilizing IN706 material.