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 and welding along with subsequent heat-treatments. These repair processes may lead to recrystallization in the SX materials, and the presence and extent 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 microstructural changes which may be used to the repair/rejuvenation process. 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 was particularly effective in identifying changes in recrystallization due to various processing methods. 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.