Heat treatment tests for reduction of delta phase in Alloy 718

Large hot-rolled rings made of Alloy 718 were used as semi-finished product for turbomachinery components. Samples taken from one such ring exhibited non-uniform mechanical properties, partly below specified limits. Too high a volume fraction of delta phase in the microstructure was identified as the metallurgical root cause of failure. The case study presented in this contribution shows that such deviation from specified material properties does indeed constitute component failure, since the subject raw forging could not be used as is for fabrication of rotating equipment for gas turbine engines. To salvage the subject part and avoid having to scrap it at high cost, successful heat treatment tests were performed in the laboratory with the aim of eliminating delta phase to an extent, sufficient for property recovery. Heat treatment parameters were derived and recommended to the client. They were successfully applied before final machining and the subject hot-rolled ring was salvaged.