The role of the strengthening phases on the HAZ liquation cracking in a cast Ni-based superalloy used in industrial gas turbines

This work presents the influence of microstructural constituents on liquation crack formation in the cast Ni-based superalloy, Rene 108. The investigation was divided into three parts: characterisation of the material's microstructure in pre-weld condition, hot ductility studies and analysis of liquation cracking induced by the gas tungsten arc welding process. Using advanced electron microscopy techniques it is shown that the base material in pre-weld condition is characterised by a complex microstructure. The phases identified in Rene 108 include gamma matrix, gamma' precipitates, MC and M23C6 carbides, and M5B3 borides. Based on Gleeble testing, it was found that Rene 108 is characterised by high strength at elevated temperatures with a maximum of 1107 MPa at 975 degrees C. As a result of constitutional liquation, the superalloy's strength and ductility were significantly reduced. The nil strength temperature was equal to 1292 degrees C, while the nil ductility temperature was 1225 degrees C. The low ductility recovery rate (32.1), ratio of ductility recovery (36.2) and hot cracking factor (R-f = 0.05) values confirmed the low weldability of Rene 108. In the heat-affected zone (HAZ) induced by welding, constitutional liquation of mainly gamma' precipitates, with a contribution of M23C6 carbides and M5B3 borides, was observed. The thin non-equilibrium liquid film, which formed along high-angle grain boundaries, led to crack initiation and their further propagation during cooling. The eutectic gamma-gamma' re-solidification products are visible on the crack edges.