Abstract:
WC-Co-Cr coatings were deposited on some hydroturbine 13Cr4Ni and 16Cr5Ni steels by the
detonation-gun spray process. An in-depth characterization of the as-sprayed coating was done
using X-ray diffraction (XRD) and scanning electron microscopy (SEM)/energy-dispersive
X-ray spectroscopy (EDS) techniques. Microhardness and porosity measurements were also
made. The coating was found to have a typical splat-like morphology with some indications of
unmelted carbide particles. The XRD results showed the presence of WC as the primary phase
along with W2C and Co6W6C as secondary phases. Furthermore, the slurry erosion behavior of
the coatings was investigated to ascertain the usefulness of the coatings to reduce the slurry
erosion of the steels. The effect of four operating factors viz. the velocity, impact angle, concentration,
and particle size on the slurry erosion of coated and bare steels has been studied
using a high-speed jet-type test rig. The sand used as an erodent was collected from a power
plant to replicate the actual turbine conditions. It has been observed that the given cermet
coating can enhance the erosion resistance of the steel. Velocity was found to be the most
significant factor affecting the erosion behavior of the coating, whereas it was the erodent
particle size in the case of uncoated steel. As evidenced from the SEM images, the platelet
mechanism of erosion seemed to be the prominent one, causing the removal of material from the
surface of the steel, whereas for the coating, the formation and interlinking of cracks resulted in
the removal of material.
