The Research On Microstructures And Performances Of Cast Iron Surface Alloying Layer With EPC Casting-penetrating

At present, with the continuous improvement of materials manufacture technology and research capability, new materials emerge endlessly. but in industrial production, the metal material is still indispensable, especially iron and steel materials. However, the metal material, which works for many hours in high temperature environment, is susceptible to oxidation and fatigue damage. The whole alloying methods can improve the oxidation resistance properties of metal materials, but it wastes alloy elements. Because the oxidation and fatigue damage of lots of parts working under high temperature take place on the surface, so only improving the oxidation resistance and fatigue properties of the surface, can effectively extend the service life of parts.EPC casting-penetrating surface alloying process can improve some surface performance of parts, and has been widely used in production. The process combines the traditional casting and metallurgy technology, and achieves the process of surface alloying in the forming process of the casting, full using of the hot liquid metal heat and latent heat of crystallization, it can not only greatly improve the life of the casting, but also reduce the energy consumption and simplify the process. There are many advantages, such as no special equipment, simple process, short production cycle, low cost, saving materials, convenient forming, thick surface alloyed layer, etc.In this study, Fe-Cr-Mo, Fe-Ni-Cr-Mo, Cr-Mo and Fe-Si-Ni-Cr-Mo 4 kinds of alloy components and 18 kinds of high temperature oxidation resistance alloyed layer were successfully prepared on the surface of gray cast iron with EPC casting-penetrating. Fe, Si, Ni, Cr and Mo act as the main components of alloy coating, a certain amount of rare earth and borax added into the alloy coating, surface alloyed layers with good quality and thickness ranging between 0.5mm-2mm.The microstructures, element diffusion and new phase formation in the sinter layer and diffusion layer were observed and analyzed by modern materials analysis methods, such as optical microscopy, X-ray diffraction, scanning electron microscopy, etc. Following conclusions can be inferred from these anlysis: there is not a obvious boundary between alloy layer and the substrate, the diffusion between liquid iron and molten elements occurred strongly in the diffusion layer; A large number of solid solutions and alloy carbides formed in this region, which infers the chemical metallurgical reaction occurred between that molten iron and alloy coating. The bonding of alloyed layer and the substrate is metallurgical; Microhardness testing results of alloyed layer organizations in different regions of the substrate show that diffusion phenomenon occurred between the alloy coating and the substrate.Thermal fatigue and oxidation damage are the main failure modes for the casting working at high temperature environment, so these two properties must be tested. After oxidation 100 hours at 800℃and 50 hours at 900℃, the oxidation weight gain speed of all the samples with alloyed layer was less than that of iron specimen. And the performance of Fe-Si-Ni-Cr-Mo series high temperature oxidation resistance coatings is better than the other three series. The oxidation weight gain speed of the best sample (Fe-10Si-5Ni-20Cr-5Mo) is only one third of that of the cast iron specimen. After thermal fatigue cycle 2000 times at 700℃, the surface crack number of the sample with alloyed layer is smaller than that of cast iron sample, and surface cracks are shorter and narrower. The oxide layer of cast iron sample seriously fell off the cast iron base.In summary, the high temperature oxidation resistance performance of alloyed layer relates with not only composition and content of the alloy elements, but also with the thickness, the thicker of alloyed layer, the stronger of the high temperature oxidation resistance performance; the bonding strength of alloyed layer and the substrate determines the thermal fatigue property, and the width of the diffusion layer determines the bonding strength of alloyed layer and the substrate.