The role of the microstructural changes during induction preheating on the HAZ liquation cracking susceptibility of Ni-based superalloy

This work presents the influence of high-frequency induction preheating (900, 1000, 1100 °C) on liquation crack formation in the René 108 Ni-based superalloy. The investigation was divided into two parts: (1) characterization of the material's microstructure after preheating and (2) determining the influence of preheating on liquation cracking during autogenous gas tungsten arc welding. During preheating, the dissolution of γ′ precipitates showed accelerated progress with increase in temperatures. This dissolution involved the continuous thinning of each precipitate, as well as more intricate mechanisms, such as splitting. The mean size of the secondary γ′ decreased from 0.32 to 0.26 μm. In the heat-affected zone (HAZ) induced by welding, constitutional liquation of mainly γ' precipitates, with a contribution of M23C6 carbides and M5B3 borides, was observed. The formation of a thin non-equilibrium liquid film along high-angle grain boundaries led to the crack initiation and their further propagation during cooling. The eutectic γ–γ' re-solidification products were visible on the crack edges independently of preheat temperature. Preheating at 900 °C decreased the length and amount of liquation cracks, while preheating at 1100 °C allowed to prevent them due to the liquid-healing effect.