Session: 18-02 Service of Gas Turbine Components - Life Assessment & Repair Processes

Paper Number: 82537

82537 - Characterization Methods to Evaluate Recrystallization and Weld-Repair in Single Crystal Nickel-Base Superalloys 

Single crystal (SX) nickel-base superalloys have been extensively used for the highest temperature gas turbines blades. In power generation turbines, these alloys are often exposed to high temperatures and stresses for extended periods of time and microstructural evolution occurs. Rejuvenation heat treatments and numerous repair methods can be successfully utilized to extend the life of components manufactured from SX alloys. These repair methods may include surface modifications such as shot-peening, welding, and heat-treatments. These repair processes may lead to recrystallization in the SX materials, and the presence of grain boundaries can have deleterious effects on local high-temperature mechanical properties.

The research in this paper focuses on characterization methods for evaluating recrystallization in SX nickel-base superalloys during the repair process. Micro hardness mapping, micro x-ray fluorescence (µXRF) and several different scanning electron microscopy (SEM) tools were used to evaluate recrystallization and microstructural features in both weld repairs and shot peened samples subjected to various heat-treatments. Simulated weld repairs consisted of both SX castings in Rene N4 and Rene N5 that were welded with Haynes 230 and Rene 80 filler metals using a gas tungsten arch welding process. The effect of recrystallization from shot peening was also evaluated on both alloys. Additionally, various samples were given either a full solution, partial solution or stress relief heat treatment to evaluate the effects.

The findings in this research show that several different characterization tools can be effectively used to evaluate recrystallization in SX nickel-base superalloys. Micro hardness and µXRF mapping showed local changes in hardness and chemical composition across the weld metal, heat affected zone and base metal as a function of weld metal selection. Electron backscattered diffraction SEM based techniques were effective in identifying changes in recrystallization due to various welding processes.  The research shows that filler metal selection, shot-peening control, and heat-treatment conditions all need to be carefully considered in the development of repair processes to minimize recrystallization effects.

Presenting Author: Alex Bridges Electric Power Research Institute

Presenting Author Biography: Alex Bridges is an Research Engineer at the Electric Power Research Institute (EPRI).<br/><br/>Alex graduated from the University of North Carolina at Charlotte with a Bachelor of Science degree in Mechanical Engineering in 2017. Alex then began his career at EPRI in the Materials and Repair research group. His primary research focus is in the high temperature behavior of power plant materials, development of non-traditional testing techniques, microstructural characterization and additive manufacturing of nickel-base superalloys. Alex also manages the high temperature mechanical testing laboratory, equipped with 60+ test frames supporting 8-10 research projects. <br/><br/>Since 2017, Alex has been the primary or contributing author to more than 15 manuscripts including journal publications, conference proceedings and EPRI technical reports. Recent research efforts have been in the development of comprehensive material property datasheets, large-scale mechanical testing to improve non-destructive examination techniques and improving characterization techniques for single crystal gas turbine weld repairs. <br/><br/>Alex is currently pursuing a Doctor of Philosophy in Materials Science at the University of North Carolina State while maintaining full time employment at EPRI. His thesis is focused on the microstructural evaluation and mechanical properties of a novel additively manufactured nickel-base superalloy.

Authors:

Alex Bridges Electric Power Research Institute
John Shingledecker Electric Power Research Institute
Eeva Griscom Electric Power Research Institute
Hans Van Esch TEServices
Stijn Pietersen TEServices
Robert Zuber Mechanical Dynamics & Analysis
Wayne Greaves Sulzer