| With the correct compound designing, a composite layer with thickness of 1.5to 6 millimeters was formed on the surface of casting by means of Cast-sintering technique.Microstructure of the composite layer was analyzed with help of optical microscope, scanning electronic microscope(SEM), electronic probe microanalysis(EPMA)and X-ray diffraction(XRD).The compacting of the billet and the forming mechanism of the carbide reinforcement were studied based on thermodynamics and kinetics of reaction, sintering theory and the result of DTA. At the same time, the wear-resistance of the composite layer was studied under condition of dry sliding friction.The result of those studies as follow:(1) An composite layer of definite thickness on the casting was gained by cast-sintering technique, and it has well wear-resistance.(2) The reinforcement of the composite layer is TiC or VC, which was exiguous and is well distributed. Martensite and Austenitic form the matrix ofthe composite.(Cr,Fe)7C3 is also a kind of reinforcement.(3)The combine of thecomposite layer and master-alloy is metallurgic combination and has a very high combining-intensity. And that the interface is very clear without bad resultant.(4) The reinforcement VC was formed in the carbonizing reaction V+C→VC, the free energy of the reaction is minus at the temperature of molten iron poured into mold, this proved that the reinforcement VC can be formed in-situ spontaneously.(5)The carbonizing reaction V+C→VC begins carry at the temperature about 900 ℃, the reaction's speed was slow because the reaction here was carried hi solid state, so it can't be carried completely. The speed became fast at the temperature 1050℃. The reaction could carry completely when billets was kept in vacuum tube for 3 hours' at 1050℃ temperature, under the striking of great quantity of heat when the molten iron poured into the mold, the reaction was blazed rapidly and continued at a high speed. The diffusible speed of V and C was very fast because the billet was heated over 1300℃ in a moment, so the reaction can complete in a few minutes. The sintering type of the billet is a solid state sintering at the temperature 1050℃, the sintering contacting's speed was slow, even keeping temperature for 3 hours', billet's sintering compacting degree was not high. Sintering speed increased greatly with the help of great quantity of heat of the molten iron, the great quantity of heat sent out from the carbonizing reaction and a little quantity of liquid, this sintering type is an instantaneous sintering, the sintering compacting can carry completely in a few minutes, the degree of sintering compacting is very high.(6)The VC-Fe surface composite has high rigidity, it's rigidity exceeds 60HRC, under condition of dry sliding friction and overloading. The VC-Fe surface composite shows high wear-resistance, it's wear-resistance is 4.20 times as that of chilling respectively. With the content of V increasing, it's rigidity and wear-resistance increases continually. More VC grains gained, and well distributed, better wear-resistance the VC-Fe surface composite will show.(7)Surface cermet composite was formed by the means of Cast-sintering technique, with the help of adding WC grains and the quantity of heat sent out from the reaction V+C→VC, the carbide cermet quality percent exceeds 60%. The experiments showed that WC and VC can dissolve mutually in the carbonizing reaction. When WC's content exceeds VC's, VC willdissolve into WC and make WC's crystal lattice bring aberrance and become WC1-x, contrarily when VC's content exceeds WC's, WC will dissolve into VC and also make VC's crystal lattice bring aberrance. The surface cermet composite has high rigidity and well wear-resistance. |
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