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

• Published in: Tribology international Vol. 158; p. 106899
• Main Authors: Carrington, M.J., Daure, J.L., Ratia-Hanby, V.L., Zhang, D., Shipway, P.H., Stewart, D.A., McCartney, D.G.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.06.2021; Elsevier BV
• Subjects: Chromium; Corrosion; Corrosion resistance; Corrosive wear; Deformation effects; Degradation; Electron microscopy; Hard surfacing; Hardfacing; Martensite; Microscopy; Particulates; Plastic deformation; Pressurized water reactors; Primary circuits; Sliding contact; Stellite; Strain localization; Synergistic effect; Temperature dependence; Transmission electron microscopy; Tribocorrosion; Tribology; Tungsten; Twinning; Wear rate; Wear resistance; Tribocorrosion; Transmission electron microscopy; Hardfacing; Sliding contact
• ISSN: 0301-679X; 1879-2464
• DOI: 10.1016/j.triboint.2021.106899

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 °C. In this study, pin-on-disc type sliding contact tribocorrosion testing was conducted on HIPed Stellite 6 at 20 °C and 250 °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 °C to 250 °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 γ Co-based matrix to ε-martensite. Closer to the sliding contact, the ε-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 °C is attributed to a temperature dependent increase in corrosion enhanced nanowear. The degradation mechanisms revealed are important for the design of future hardfacings. •Material removal via tribocorrosion increases significantly with temperature.•The general mechanisms of degradation largely remain insensitive to temperature.•Sliding contact generates an ε-martensite nanocrystalline subsurface microstructure.•Plastic strain localisation is minimised and wear is confined to the nanoscale.