Understanding the dissolution mechanism of fused tungsten carbides in Ni-based alloys: An experimental approach

Ni-based alloys reinforced with fused tungsten carbides (FTCs) become more and more important as hardfacing material in the mining and oil drilling industry. The reason for the high abrasive wear resistance of FTCs lies in the microstructure, characterized by an eutectoid morphology of mono-tungsten carbide (WC) and di-tungsten carbide (W2C). The benefit is a combination of high hardness and fracture toughness. However, FTCs are thermally instable and dissolute in case of high thermal loads during welding. An indicator for the dissolution of FTCs is the formation of a degradation seam along the interface to the surrounding Ni-based alloy. However, the formation mechanism of the degradation seam has not been fully understood and is diversely discussed. By means of an experimental approach, the formation of the degradation seam was reproduced experimentally and proven for the first time. The degradation is initiated by the selective dissolution of the phase W2C. Due to W and C dissolved in the melt, WC precipitates in the Ni-based alloy. Fused tungsten carbides act as nucleus and promote the precipitation of WC. The precipitation of WC leads to the formation of the typical degradation seam.