Influence of Carbon on Structure and Stress-Rupture Properties of LW4280W and LW4280LCT High Gamma Prime Nickel Based Superalloys for a Tip Repair of Turbine Blades

The influence of carbides on microstructure and stress-rupture properties of high carbon LW4280W and low carbon LW4280LCT welds produced at ambient temperature GTAW at 649°C (1200°F), 872°C (1600), and 927°C (1700°F) are discussed in the current study. The study was powered by the need of using carbon for enhancing of creep strength of welds despite the well-known negative effect of carbides on high gamma prime superalloys weldability. In that context, a maximum carbon content of 0.12 wt.% has been maintained in many commercially available conditionally weldable superalloys. The current study demonstrates that the recently developed LW4280W superalloy with 0.17 wt.% C capable of producing 58–59 vol.% of gamma prime phase upon aging and the low carbon LW4280LCT comprising < 0.1 wt.% C and forming 54–55 vol.% of gamma prime phase during post weld heat treatment (PWHT) have excellent weldability. The high carbon LW4280W welds demonstrated superior creep strength to LW4280LCT welds despite the interdendritic precipitation of complex plate-like TaHf-based carbides during solidification of a welding pool. Our results show that PWHT resulted in a partial fragmentation of plate-like TaHf-based carbides, believed to enhance creep resistance. Despite having similar tensile properties, the time to rupture of test samples manufactured from LW4280W welds at 927°C (1700°F) under stress of 151.7 MPa (22 KSI) exceeded 156 hours while the LW4280LCT welds fractured in 112 hours. Our study also shows that exposure of heat-treated welds to a potential overheating to 1080°C (1975°F) during takeoff of aircrafts for total of 500 hours had little or no effect on the structure and tensile strength of both superalloys. The example of tip repair of turbine blade manufactured from the single crystal material and recommendations for using of LW4280W and LW4280LCT superalloys are provided.