Failure analysis of jet turbine engine high pressure nozzle material made of cobalt based super alloy

In this study, we carried out the failure analysis based on metallographic research of a cobalt superalloy material which had been used in jet turbine engine. The failure analysis was made by chemical composition analysis, hardness test and microstructural researches after metallographic sample preparation on the material which had operated about 30000 hours and damaged. Macro cracks which can be seen by visual inspection have been detected on the cobalt based superalloy nozzle material’s some surfaces which had exposed to high temperature and high pressure fuel-air mixture. After the metallographic sample preparation and optical and electron microscope investigation, it has been determined that the main reason of the failure is coarse and continuous morphology M7C3 carbide precipitations on the grain boundaries and as a result of that, stress corrosion cracking has moved through that grain boundaries which have been embrittled and weakened against high temperature corrosion. Also, after consideration of the chemical composition on the damaged area of material, it has been detected that, owing to the usage of carbon content fuel during operation, carbon diffusion at high temperature into the material had occurred. So, with high carbon content in the material, the type and morphology of the carbides had changed and caused the failure.