Isothermal Transformation of γ-Co to ε-Co in Stellite 6 Coatings

Stellite 6 alloy coatings were produced by the plasma arc cladding process, and the as-cladded samples were treated with isothermal aging at 700 °C for up to 1000 hours to evaluate the effect of long-term isothermal aging on microstructural evolution. The results show that the microstructure of the cladding consists of γ-Co solid solution and carbide-based eutectics. During aging, γ-Co gradually transformed to ε-Co, and the volume fraction of ε-Co reached to about 75 pct as the aging time was 1000 hours. On other hand, the M23C6 fine particles were found to gradually precipitate in the solid solution during aging. Approximately pairwise proportional relationships were found among the fraction of precipitated M23C6 carbides, the volume fraction of martensite transformation, and the stacking fault energy, and they were verified by the statistical results of phase fraction and change of SF possibility calculated through X-ray diffraction patterns. A kinetic model for martensite transformation was also established to explain the change of martensite volume fraction during the aging period and to predict the transformation degree during high-temperature aging.