Session: 18-05: Failure Prediction & Life Assessment II

Paper Number: 152421

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

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 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 represent the largest share of power generating capacity for both simple and combined cycle units. The GE 7F technology represents >950 units installed globally to support ~175 GWs 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 hours or 5,000 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 hours 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.

Presenting Author: Alex Bridges EPRI

Presenting Author Biography: Dr. Alex Bridges is a Senior Team Leader at EPRI with expertise in advanced high temperature mechanical testing techniques, analysis of creep behavior and additive manufacturing (AM) of nickel-base superalloys. He currently manages 15 to 20 test programs in the mechanical test lab pertaining to a wide variety of engineering alloys used in extreme environments. He completed his Ph.D. studies at North Carolina State University and his dissertation is titled "Microstructural Evaluation and High Temperature Behavior of an Additive Manufactured Novel Nickel-base Superalloy". His most recent research has been focused on building out evaluation methods for additively manufactured components, working closely with demonstrations of AM component builds in industrial gas turbines and developing databases to increase industry adoption of AM. He also manages several research projects focused on hot-section gas turbine materials, including efforts to enable rotor life extension through material analysis.

Authors:

Alex Bridges EPRI
Kurt Boschmans ENGIE Laborelec