V - Epc Infiltration Casting Process Of The Preparation Of Tungsten Carbide / Iron Matrix Surface Composites, Organization, Performance And Interface Research

In this study, WC particles reinforced iron matrix composites were prepared successfully by V-EPC infiltrating casting process combined with the theory of surface alloying and surface compositing. The fabrication process not only overcome the disadvantages (Due to hardness of reinforcing phase being not high or matrix not supporting reinforcing phase effectively, the prepared materials all have not good wear resistance) from surface alloying or surface compositing by V-EPC infiltrating casting method, but also avoid the shortcomings (imperfect surface quality, low precision, high content of gas porosity and slag inclusion, little possibility to fabricate components with complex geometry ) from surface alloying and surface compositing by the other infiltrating casting method.The relationship between microstructure and properties of iron matrix composites has been studied systematically. The results show that, with the increase in the volume fraction of WC particles, the hardness of WC particles remains constant, and the hardness of the matrices between WC particles decreases slowly (HV 524-667). With the increase in the volume fraction of WC particles, the erosion resistance of composites first increased until reached the maximum when the volume fraction of WC was 36%, then decreased. With the increase in the volume fraction of WC particles, the dry three-body abrasive wear resistance of the composites first increased until reached the maximum when the volume fraction of WC was 27%, then decreased, and was 1.5-5.2 times higher than that of the high chromium cast iron. The increase of wear resistance of the composites is the result of the protecting effect of the surface WC particles on the matrix and the supporting effect of the matrix to WC particles through good interface bond. And a mixed mode of material removal involving both cutting/ploughing and fatigue may be occurring.The interface between WC particles and the matrices in the composites prepared by infiltrating casting were simulated and studied by DTA tests. The results show that the interface belongs to interface with reaction and diffusion, and the forming process is summarized in four steps as follows. The first step is that the high C chromium iron is melted by the substrate melting entry in the preform and the WC particles are surrounded by the melting. The second step is that the WC particles with micro-cracks are divided into a great deal of bits. The third step is the WC particle dissolution and protrudeing of WC surface being liquated. The last step is that the WC dissolved and liquated is separated out to form the interface when the system is cooled.Combined to thermodynamic calculations, dissolution of WC particles in the matrix and the effects of the matrices on the dissolution of the WC particles were investigated, which directed the theory of controlling interface between reinforcing phase and matrix. The results show that WC particles begin to be dissolved in the HT300 matrix as the temperature is about 1281℃. When the system temperature arrive at a temperature point, resolution reaction of WC (2WC=W₂C+C) begin and diffusion of elements can accelerate the dissolution of WC particles. The dissolution and diffusion rate decrease with decreasing temperature. Finally, the eutectic phase of WC and W₂C is precipitated out from the C and W supersaturated position. The residual C and W indwell in the matrix in carbides containing high tungsten content (Fe₃W₃C- Fe₄W₂C and Fe₆W₆C) and being dissolved in the matrix directly. It is considered that the Cr in the matrices can promote dissolution of WC particles and the C can prevent that.The dynamics model of fabricating composite with V-EPC method is established. And using substance transmission and heat transmission of substrate melting, forming process of composite layer was discussed. The results show that, the forming of composite layer is the result of harmonious effects of substance transmission and heat transmission, with the decrease of WC volume fraction, the effect of heat transmission is enhanced and that of substance transmission is decreased. Compared with coating preform method, the effect of heat transmission is larger in the fabricating composite with suppressing preform method.