Sliding wear of a self-mated thermally sprayed chromium oxide coating in a simulated PWR water environment

Bearing surfaces in the primary circuit of pressurized water reactors (PWR) are prone to damage due to aggressive chemical and tribological conditions under which they operate, and a wide range of materials have been examined in this regard. One of the most promising candidates is chromium oxide in the form of a thermally spayed coating, and in this work, the behaviour of a commercially available Cr2O3 coating in self-mated sliding was considered. Tests consisted of a number of start-stop cycles of sliding between a crowned pin and a rotating disc in a water environment in an autoclave in an attempt to simulate the most aggressive phase of bearing run-up and run-down. Wear and damage mechanisms were examined at temperatures from ambient up to 250 C (a representative PWR environment). Samples were characterized before and after wear testing using mass measurements, profilometry, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM) and X-ray photoelectron spectroscopy (XPS). Across the temperature range, wear was mild, with no evidence of coating delamination. A five-fold increase in wear was observed between 80 C and 250 C (with wear depths of generally less than 8 µm being observed on the disc samples even at the higher temperature), despite there being only very small changes in hardness of the coating over the same temperature range. Debris was observed on the wear tracks following testing, with the evidence together suggesting that this debris was a very fine-grained mixture of Cr2O3 and amorphous -CrOOH, a corrosion product of Cr2O3

Microstructural characterisation of Tristelle 5183 (Fe-21%Cr-10%Ni-7.5%Nb-5%Si-2%C in wt%) alloy powder produced by gas atomisation

Nitrogen gas atomised powders of the hardfacing alloy Tristelle 5183 (Fe-21%Cr-10%Ni-7%Nb-5%Si-2%C in wt%) were sieved into different particle size ranges and their microstructures have been investigated. Powder particles larger than approximately 53 μm are composed of dendritic fcc γ-Fe as the principal phase with smaller quantities of: α-Fe, an interdendritic silicide phase isostructural to Fe5Ni3Si2, and Nb(C,N). Particles 10 μm) sized Nb(C,N) particles, that are seen in all powder size fractions, pre-existed in the melt prior to atomisation, whereas micron-sized Nb(C,N) particles that are found within α-Fe, γ-Fe or silicide are the primary solidification phase. Nanoscale Nb(C,N) also formed interdendritically in the last stages of solidification. Compared with a mould cast sample, a significant difference is the suppression of M7C3 formation in all powder size ranges. The increasing quantities of α-Fe and silicide in smaller sized powder particles is consistent with increased undercooling prior to nucleation permitting metastable phase formation

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 pressurized 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 and wear was measured in 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 hcp Co-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 behavior and depth with temperature and this is not regarded as 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 so dependent on temperature, this work demonstrates the necessity of conducting assessments of materials for use in PWR environments under representative conditions

The evolution of subsurface deformation and tribological degradation of a multiphase Fe-based hardfacing induced by sliding contact

Multiphase Fe-based hardfacing alloys, for example Tristelle 5183 Fe-21%Cr-10%Ni7.5%Nb-5%Si-2%C in wt.%, are extensively used for tribological applications, including valves, bearings and drive mechanisms, where two surfaces are unavoidably subjected to loaded sliding contact within engineering systems. In this study, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize, for the first time, the tribologically affected material induced by the self-mated sliding contact of HIPed Tristelle 5183. This provided novel insight into the deformation modes which permit the accumulation of the high levels of subsurface strain required for plasticity dominated (adhesive) wear in a commercial hardfacing. In the subsurface regions furthest from the sliding contact, plastic deformation is accommodated by deformation induced martensitic transformation to e-martensite and α0-martensite, twinning, the generation of planar dislocation arrangements (generated by planar slip) and the generation of dislocation tangles. Closer to the sliding contact, the subsurface becomes unstable, and nanocrystallisation driven by grain boundary mediated deformation mechanisms and crystallographic slip completely engulf the near surface microstructure. It is postulated that nanocrystalisation within the subsurface is a needed in order to accommodate the extremely high strains required in order to permit tribological degradation via plasticity dominated wear. The extrusion of metallic slivers via plastic ratcheting generates ductile shear cracksgoverned by plastic strain, and the failure of these slivers generates plate/flake-like wear debris