The Study On HVOF Sprayed Cr₃C₂-NiCr Coating

In this paper, HVOF sprayed Cr₃C₂-NiCr coating on the valve rod of a steam turbinewere studied. Three kinds of coating on the valve rod, included surface intact coating,bubble-like coating and shedding coating, were investigated. Some new Cr₃C₂-NiCrcoating were deposited with the material and process as same as coating on the valve rodof the turbine. The effects of thermal shock, high temperature and high humidity, surfacestress on crack producing and extending of Cr₃C₂-NiCr coating were examined by thermalshock, high temperature vapor corrosion and surface pressure experiments. The changesabout composition, organization, cracks and hardness of both failure coating and newsimulation coating were investigated by OM, SEM, EDS, XRD and microhardness tester.The failure mechanism of Cr₃C₂-NiCr coating on the valve rod of turbine was analyzed.By observing the HVOF sprayed Cr₃C₂-NiCr failure coating on valve rod of theturbine, it was found, shedding face is mainly located in the intermediate position of thecoating, shedding area is30mm30mm-100mm100mm, and there are many cracksparallel to the surface in the shedding coating. There are severe oxidation at fall off surface,crack surface and interal local of the shedding coating, oxides are Cr2O3, NiO and NiCr2O4.The fracture morphology of original section is similar with fresh section, but different withthe fall off surface. The cross section of surface intact coating on valve rod have cracks thatmainly located in the Cr₃C₂phase. Secondary phase Cr₃C₂particles were precipitated fromthe grey area of NiCr binder phase. The bubble-like coating on valve rod were binded bythe NiCr binder phase, but the Cr₃C₂is broken.Through thermal shock experiment, it was found, only the cracks perpendicular to thesurface are produce by heat shock. The generation and extension of cracks are mainly inCr₃C₂phase, NiCr bonding phase has an inhibitory effect on the crack extension. The crackdensity in coating changes through in turn incubation stage, accelerated increase stage andstable stage with the increase of the times of thermal shock.Through the high temperature vapor corrosion experiment and interaction experimentof thermal shock and high temperature vapor corrosion, it was found, the oxidation ofcoating surface is accelerated by high temperature steam environment. The oxides areCr2O3, NiO and NiCr2O4. Secondary phase Cr₃C₂micro particles were precipitated from the grey area of NiCr bonding phase and distribut uniformly in NiCr bonding phase due tolong-term effects of high temperatures. The effect of high temperature water vaporcorrosion on crack perpendicular to the surface isn’t obvious.Through the surface pressure experiment, it was found, the damage style of HVOFsprayed Cr₃C₂-NiCr coating on the pressure condition include three styles: clastation ofhole, clastation of carbon loss carbide, clastation of Cr₃C₂hard particle, and they occurredmore difficult in turn. The hardness of coatings was decreased when Cr₃C₂hard particlefragmentate. The damage of Cr₃C₂-NiCr coating is accelerated by circulating pressure. Inthe case of the Cr₃C₂hard phase particles and NiCr binding phase distribute with bandparallel to the surface, the Cr₃C₂hard particle fragmentate along the band, so cracksparallel to the surface.