Solidification structure of stellite overlayer formed on spheroidal graphite cast iron by plasma transferred arc process

The solidification structures of the overlayers of stellite 1, 6 and 21 cobalt base alloys after overlaid on spheroidal graphite cast iron by plasma transferred arc process were studied in this investigation. The results of low overlaying current (I = 60 A) indicated that the stellite 1 overlayer is dendriteless with large amount of M₇C₃ and M₂₃C₆ carbides, which is different from the dendritic structure of the stellite 6 and 21 overlayers. The interdendritic precipitates of the stellite 6 overlayer are M₇C₃ and M₂₃C₆, whereas those of the stellite 21 overlayer are σ phase and very fine M₂₃C₆ carbide. An interfacial layer exists between the overlayer and the base material. Dilution of the overlay material by the base material is responsible for the formation of this layer. The overlayers are rich in iron content when the overlaying current is high (I = 100 A). Under this situation, the solidification structure of the stellite 1 overlayer is dendriteless while that of the stellite 6 and 21 is dendritic. Regardless of this microstructure difference, all of the overlayers consist of M₇C₃ carbide in blocky and colony shape, and M₂₃C₆ carbide in fishbone shape. Owing to the dilution effect, the matrix of the overlayers is α-Fe with lamellar M₇C₃ carbide.