In Situ Synthesis Of Chromium Carbide Coatings By High Energy Beam And The Wear Resistance

Friction and wear is the main form of material failure. The development of wear-resistant materials or fabrication of wear-resistant layer through surface cladding or surface modification has extended the service life of materials laygely. To improve the wear resistance of materials also contributes to conserve resources as well as reduce energy consumption and environmental pollution, which has an important engineering value and huge socio-economic effects.In this paper, a new type of'four line rotation scanning'vacuum electron beam was proposed; using this electron beam scanning method and CO2 laser, carbides composite surface layer was sucessfully in situ synthesized with Fe/Cr/C (Cr3C2/Fe) alloy powder on 903 steel substrate; through optimization of scanning electron-beam parameters and adjustment of Fe/Cr/C alloy powder ratio, surface composite layers of the different properties were in situ synthesized on the low-alloy steel. The microsructure of each surface composite layer was analyzed with optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM); the hardness and wear-resistant performance of each surface composite layer was evaluated with microhardness tester and tribological tester. Dry sliding wear mechanism of the surface of composite layers was explored; surface composite layers in situ synthesized by the electron beam were also heat-treated to study the changes in its microstructure and the improvement of wear resistance. The results and main conclusion of the study are as follows:Four different kinds of Fe/Cr/C alloy powder were designed according to the Fe-Cr-C ternary phase diagram to study the influence of powder composition on the microstructure and wear resistance of surface composite layer. The microstructure of surface composite layers prepared by different ratio Fe/Cr/C powder mixtures vary significantly. Since the eutectic composition of Fe-Cr-C alloy depends mainly on the carbon content, carbon content has much more influence on the microstructure of surface composite layer compared with chromium content. In a certain range, increasing the carbon content, higher amounts of chromium carbides will be formed.The chromium and carbon content in the surface composite layer can be calculated by considering the dilution of the melting substrate and the powder loss during the scanning. Thus the microstructure of the synthesized layer is predictable using the Fe-Cr-C ternary phase diagram.Samples synthesized with four different ratios of Fe/Cr/C powder mixtures show different microstructure characteristics. With carbon content decreasing, the microstructure of the composite layers is hypereutectic microstructure, eutectic structure, hypoeutectic microstructure and martensite structure respectively. The main features of hypereutectic sample are large primary carbide embeded in ductileγ-Fe/M7C3 eutectic structure; the micro-hardness is 3.3-fold of the substrate. In eutectic structure, the carbides are granular or slender rod-shaped, showing the distribution of chrysanthemum-shaped cluster; the micro-hardness was 3-fold of the substrate. In hypoeutectic microstructure, dendritic austenite first precipated, then eutectic reaction take place to form austenite and carbides; carbide particles are very small; some are connected to form network structure; when the carbon content is too low, carbides does not appear in the surface layer, which is mainly martensite phase. The hardness of the surface composite layer is mainly related to carbide amount. Tthe higher carbide amount, the greater its hardness; the supersaturated carbon atom in martensite, result in the serious distortion of the lattice; its hardness also greatly improved compared to the substrate, reaches 2-fold of the substrate.When using CO2 laser beam to scan pre-placed powder mixture on 903 steel substrate, scanning time is too short for elements to diffuse evenly in melting pool because of the high power of laser beam. The surface composite layer shows some non-uniformity. Dendritic austenite andγ-Fe/M7C3 eutectic structure formed in the high chromium and carbon content region. As a result of fast cooling rate, the carbide particles are extremely fine; in the low chromium and carbon content region, the microstructure is mainly martensite because of the rapid cooling rate. There is overlap on the scanning track, which result in remelting zone and re-heating zone in and near the overlap region. The microstructure of re-heating zone is mainly austenite; its micro-hardness decreases compare with other region on the surface composite layer. Because of the non-uniformity of the microstructure, the wear mechanism is more complicated, abrasive wear and adhesive wear co-exist in the wearing process. The fluctuation of friction coefficient is relatively large.The wear-resistant performance of surface composite layer is directly related to its microstructure. The surface composite layer with excellent wear resistance is mainly due to the large amout of (Cr,Fe)7C3 distributed in the tough austenite phase. In low stress abrasion conditions, carides can effectively prevent the asperity penetrating the surface to form micro-cutting, and the austenitic phase can effectively prevent crack formation and expansion. Different types of carbides play different effects in wear mechnanism. In low stress abrasion conditions, the size of fine eutectic carbides is much larger than the furrows depth caused by asperity. It can effectively resist the scratches and piercing of the abrasive, which enables the surface compoaite layer have a good wear-resistant properties. As to martensite surface layer, although it can effectively resist micro-cutting of abrasive, serious adhesive wear takes place. The wear-resistant performance of martensite layer is worse than the carbides composite layer.Heat treatment of hypereutectic surface composite layer shows that the chromium and carbon atom in austenite are able to diffuse with activation energy at 1000℃and 900℃. More fine secondary carbides are precipitated. Coarse primary carbides undergo no obvious changes in the heat treatment at 1000℃and 900℃. After heat treatment the tough phase is still austenite though its chromium content reduced. The chromium and carbon diffused to fusion line and substrate significantly increased. The microstructure became martensite in the fusion line area. After normalizing at 800℃, the austenite phase in the surface composite layer transform into ferrite phase; a large amount of secondary carbide form due to diffusion of chromium and carbon atom. After heat treatment, the microstructure changed due to chromium and carbon diffusion, which reduce the lattice distortion and form more uniformly distributed secondary carbides. After heat treatment the hardness of all samples decreased slightly, but is more averaged.After heat treatment, the wear resistance of both samples improved. With large amount of fine carbides, whether the soft phase is ferrite or austenite seem to have no difference on the wear performace of the surface composite layer. And the friction coefficient is also very close. In a state of low stress abrasion, hard phase plays a major role in the process of abrasion resistance. The sample heat treatmed at 800℃precipates more amounts of secondary carbides. Its wear resistance increases accordingly. Though the sample heat treatmed at 1000℃contains less amounts of secondary carbides, the reduction of the dislocation accumulation along grain boundary make its wear resistance improve obviously.In summary, basing on the concept of metal matrix composites (MMC), M7C3 carbide surface composite layer is successfully synthesized using scanning electron beam and CO2 laser on low carbon steel. It provides a new way on fabrication of wear-resistant material.