Study On Mathematical Model Of The Thickness And Properties Of TD Chromizing Coatings

Die and mechanical parts are widely used in all aspects of our life.Many high-tech products have emerged with the development of modern industry,while the properties of die and mechanical parts is difficult to meet the requirements of production of high-end materials.Salt bath chromizing has the advantage of stable craft,simple operation,simple instrument and good treatment effect.The chromium carbide coating obtained by TD technology has high hardness and strong oxidation resistance property.But the research of microstructure and property of chromium carbide coatings is insufficient.The microstructure and properties of the chromium carbide coatings were further studied in the paper.Firstly,nine kinds of steels with carbon content range form 0.5%to 1.2%were selected.And the steels were borax salt bath chromizing in 2 hours,4hours and 6 hours at the temperature of 900?,respectively.The microstructure of the coating were analyzed by SEM,OM and XRD.The properties of the coating were studied by the test of anti-molten aluminum,anti-molten copper,anti-washout and thermal fatigue,respectively.The main and valuable conclusions are as followings:?1?The chromium carbide coating on the surface of the steels were dense and uniform,which is made up of Cr₇C₃??Cr,Fe?₇C₃??Cr,Fe?₂₃C₆ and C₂₃C₆.The thickness of the coating ranges from 1.07?m to 12.43?m.And the surface hardness of the coating greatly increases in the range of 1492HV_0.01-1848HV_0.01.?2?The mathematical model of the thickness can be described by the equationl²?28?Kt.The tend of the constant K basically follows a linear increase law with the increase of the dissolved carbon content.?3?After anti-molten aluminum test,the weight loss of 5CrNiMo TD sample and Cr12 TD sample is less than that of its matrix steel,and the weight loss of5CrNiMo TD sample is more obvious than that of Cr12TD sample.The higher the high temperature performance of the matrix steel is,the more the improvement of erosion resistance of the TD sample is.The corrosive mechanism of matrix steel in molten aluminum alloy involves the Fe phase being etched by aluminum to form iron-aluminum compounds.The chromium carbide coating retards the reaction of the aluminum liquid with the steel substrate and delays the occurrence of the soldering phenomenon.?4?After anti-molten copper test,cracks appeared on Cr12 steel surface and no cracks appeared on the surface of Cr12 steel TD sample.The corrosive mechanism of Cr12 steel in molten copper alloy involves the substrate is oxidized and corroded at high temperature.The chromium carbide coating retards the reaction between the copper liquid and the steel matrix,and delays the time of cracks and holes in the surface.?5?The washout rate of Cr12 TD sample and Cr12 steel is lower first and then rise.The washout resistance of salt bath chromizing sample is better than the steel at the same temperature.?6?Severe shedding were found in the corner of the Cr12 TD sample and Cr12after 500 times thermal fatigue cycles.And big sunken was found in TD sample.The coating of the Cr12 TD sample has disappeared.The surface cracks of Cr12 TD sample are less than those of Cr12 after 900 thermal fatigue cycles,but shedding region is found in the surface.The thermal fatigue resistance of the TD sample is enhanced.The cracks initiation lies in the region with high chromium content.The chromium can form the hard and brittle inclusion in the steel,which is prone to crack initiation at high temperatures.