Microstructure and phase analyses of Stellite 6 plus 6 wt.% Mo alloy

The alloy Stellite-6 was cast with 6% Mo. Then, some pieces of this alloy were cooled in water after annealing at 950°C for 2 h. A piece of the water-cooled samples was cooled in liquid nitrogen again. Later, all of the samples were investigated using metallographic and x-ray diffraction techniques and thermal (DSC and DTA) analysis methods. The hardnesses were also measured as 40, 46 and 49 HRC for samples cast, cooled in water and cooled in liquid nitrogen, respectively. In addition to μ, R and σ phases, it was determined that various metal carbides such as MC, M 7C 3, M 23C 6, M 6C and some other inter-metallic phases may exist in these samples. It was identified by x-ray diffraction that a phase transformation from α phase with fee (face centered cubic) structure to ϵ phase with hcp (hexagonal close-packed) structure occurred in the sample cooled in water, the amount of the ϵ phase (martensite phase) increasing in the sample cooled in the liquid nitrogen, and the hardnesses of these samples also increasing. In addition, from the DTA curves it was seen that a second allotropic transformation of cobalt from α phase to ϵ phase had taken place in the temperature range between the Curie temperature (1121°C) and 1223°C for all of the samples.