Effects of Heat Treatments on Microstructures and Strength of Cold Sprayed IN738LC Ni-Base Superalloy

Hot section parts of combined cycle gas turbines are susceptible to degradation due to high temperature creep, crack formation by thermal stress, and high temperature oxidation, etc. Thus, regularly repairing or replacing the hot section parts such as gas turbine blades is inevitable. For this purpose, revolutionary and advanced repair technologies for gas turbines have been developed to enhance reliability of the repaired parts and reduce the maintenance cost of the gas turbines. The cold spraying process, which has been studied as not only a new coating technology but also as a process for obtaining a thick deposition layer, is proposed as a potential repairing solution. The process results in little or no oxidation of the spray materials, so the surfaces stay clean, which in turn enables superior bonding. Since the operating temperature is relatively low, the particles do not melt and the shrinkage on cooling is very low. In our previous study, repairing of the Ni base superalloy turbine blades by the cold spray was studied. As a result, mechanical property and microstructure of an as-sprayed cold spray coating have been unsatisfactory compared with a bulk material. Therefore, in order to improve the material characteristics, heat treatments were conducted to the coatings. However, the optimum heat-treatment condition has not been obtained yet. In this study, the effects of heat treatment were clarified, and then the optimum heat treatment condition was investigated for the cold sprayed Ni base superalloy IN738LC coatings.