Studies Of Plasma Spray Metal-Ceramic Compounds Coatings On Q245r Steels

Plasma spraying is an important surface treatment technology, with the characteristics of high clear gas mixture, thermal enthalpy and reaction velocity, in-situ rapid solidification and few produce steps. The coating is strengthening and repairing the surface of materials. At the same time, material surface will get particular properties, so plasma spraying technology is widely used in petrochemical pipeline, space aircraft industries, motor manufacturing and ship industries.The Cr₃C₂-NiCr coating was prepared through the plasma spraying equipment. Orthogonal test was applied to analyze and determine the optimum technical condition at first. In terms of spraying the different contents of NiCr in the working coatings, the corrosion resistance of the uncoated substrate and 3 kinds of coating(Cr₃C₂-20wt%NiCr,Cr₃C₂-25wt%NiCr,Cr₃C₂-30wt%NiCr) were tested by salt spray at normal temperature; along with the high temperature oxidation resistance and erosion resistance experiments. Microstructure and mechanical properties of Cr₃C₂-NiCr coating by plasma spraying were studied by means of XRD, microscope and mechanical properties testing, the mechanism of high temperature oxidation was discussed as well.It was shown that the best processing parameters are I=650 A,L=110 mm,first gas flow=67.1 L/min,second gas flow=27.2 L/min,carrier gas flow=6.14 L/min,powder feeding rate=32.6 g/min,speed of nozzle is 100mm/s.The laminate structure coating has typical mechanical bond with the substrate. The salt spray test, polarization curve and EIS shows that sealed coating has a high corrosion resistance than the unsealed and bare steel. When increase the content of NiCr in the top layer, the Ecorr increased while the Icorr declined, thus enhanced the corrosion resistance of the coatings. High temperature oxidation test shows that the phase of NiCr alloy was oxygenized at first, the formation of Ni-oxides and Cr-oxides will induce the free Cr reacted with O at the grain boundary which causes the decarburization of the chromium carbide phase and formed a dense layer on the surface. The layer main consists of Ni, Cr and O which prevents effectively the substrates from oxygenization. The result of high temperature corrosion test with salt coating shows that the corrosion resistant ability of Cr₃C₂-NiCr coating is improved clearly. No spallation or cracks were fund in the surface during the oxidation test and corrosion test. The tests also show that the higher NiCr contents, the better high temperature oxidation resistance and corrosion resistance the coatings will have.