The effect of temperature on sliding wear of self-mated HIPed Stellite 6 in a simulated PWR water environment

Cobalt-based Stellite alloys are widely used in the primary circuit of pressurised water reactors (PWR) to protect valve surfaces against wear and galling in a corrosive environment. In this study, self-mated sliding wear of HIP-consolidated (Hot Isostatically Pressed) Stellite 6 (Co - 27.1 Cr - 1.5 Si - 5.0 W - 0.96 C, in wt%) was investigated. A pin-on-disc apparatus was enclosed in an autoclave for wear testing, which was conducted in lithiated water from room temperature up to 250 °C (a representative PWR environment). Samples were characterized before and after wear testing using mass measurements, profilometry, X-ray diffraction and scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD). The bulk HIPed alloy is predominantly two phase and comprises a cobalt-rich fcc matrix and an M7C3 carbide phase. However, surface grinding prior to wear testing causes a surface layer of the matrix to partially transform to a hcp cobalt-rich phase. The wear (mass loss) is very low below 150 °C but increases by approximately an order of magnitude when the temperature is increased from 150° to 250°C. SEM/EBSD reveals sub-surface damage and partial fcc to hcp transformation of the Co-rich matrix phase to a depth of ~ 15 µm in the disc. However, there is little change in transformation behaviour and depth with temperature and this is not regarded as a significant cause of the increased wear. The order of magnitude increase in wear is instead ascribed to a tribocorrosion mechanism associated with significantly higher corrosion rates at 250 °C than at 150 °C. As the material removal and factors affecting it are found to be significantly dependent on temperature, this work demonstrates the necessity of conducting assessments of materials for use in PWR environments under representative conditions.