Root cause analysis of turboprop engine Inconel 713LC turbine blades failure

In this paper, the root cause analysis of failed Inconel 713LC superalloy blades with approximately 3500 flight hours since new from the stage Ι of power turbine is presented. A two-stage axial-flow power turbine driving the propeller of an aircraft turboprop engine through an internal shaft, during take-off at altitude of 600 feet experienced an in-flight shutdown. At the time of the incident the rotational speed of power turbine was at 90 % torque rotating at approximately 1,300 rpm, having operating temperatures above 950 °C. The failure occurred after 120 operating hours from the last engine overhaul inspection. To examine the root of this catastrophic failure, both fractographic and metallographic examinations were performed by various means. It was found that four successive blades of the stage Ι turbine disk initially fractured by fatigue mechanism and then by overload. Fatigue initiation sites were observed mainly at the leading edge of those four blades, where coating degradation appears due to a synergism between thermal fatigue cracking and corrosion that extends through the coating and into the blade base material. Microstructural analysis performed at these four blades revealed solutioning and coarsening of the nickel superalloy gamma prime phase and intergranular creep voids, indicating long-term exposure to high temperature. The remaining 62 blades were subsequently fractured by overload due to impact damage from the fragments. From these examinations, it is concluded that the fatigue cracks on the four turbine blades initiated from the combination of thermal fatigue and corrosion. Subsequently, the cracks propagated by a mix of creep and fatigue due to coating and microstructure degradation caused by overheating.