Microstructural characterisation of subsurface deformation and the degradation of Stellite 6 induced by self-mated sliding contact in a simulated PWR environment

Stellite 6 (Co-29.5%Cr-5%W-1.2%C in wt%) is traditionally used as a hardfacing material in the primary circuit of pressurised water reactors (PWRs) due to its good corrosion and wear resistance in water at up to 300 degrees C. In this study, pin-on-disc type sliding contact tribocorrosion testing was conducted on HIPed Stellite 6 at 20 degrees C and 250 degrees C using a bespoke tribometer to simulate a primary circuit environment. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize, for the first time, the material affected by tribocorrosion. Whilst the material loss increases by 16-39 times when the test temperature is increased from 20 degrees C to 250 degrees C, the mechanisms of degradation and deformation remain largely unchanged. Furthest from the sliding contact, strain is principally accommodated by the deformation-induced transformation of the gamma Co-based matrix to epsilon-martensite. Closer to the sliding contact, the epsilon-martensite phase accommodates further strain via twinning and dislocation slip. At the sliding contact the intense deformation generates a nanocrystalline structure. The tribologically affected material is resistant to plastic strain localisation; this confines wear to the nanoscale where the synergistic effects of chemical degradation and mechanical deformation permit the removal of nanoscale particulates (corrosion enhanced nanowear (tribocorrosion)). The increased wear rate at 250 degrees C is attributed to a temperature dependent increase in corrosion enhanced nanowear. The degradation mechanisms revealed are important for the design of future hardfacings.

Automated summary

The study conducted pin-on-disc type sliding contact tribocorrosion testing on HIPed Stellite 6 at different temperatures and characterized the material affected by tribocorrosion using various microscopy techniques. As the temperature increased, material loss also increased, but the deformation and degradation mechanisms remained largely unchanged.