The Research On Cast Iron Surface NiCr Alloy-the Green Mold Casting Infiltration Technology And Microstructures And Heat Resistance Properties Of The Alloy Layers

With the development of modern industry, rigid requirements are put forward to the properties of mechanical products. They require the products to have long-term and stable running under wicked working conditions of high pressure and high speed. This shows that products with higher wear-resistance or heat-resistance are needed. Sometimes, choosing expensive metal or whole-product alloy can also satisfy the requirements of surface performance, but it is not realistic when considering its cost. Because reducing the cost of materials is also one of the missions which are put forward by the mechanical industry to the material workers. Moreover, the materials which can satisfy the need of whole product and surface together can not be found in most conditions. So, studying and developing the surface protection and surface intensification technology have an important meaning to improve the life and reliability of parts, to improve the properties and quality of mechanical equipments, even to increase the economy benefit, to enhance the competition of the product, to promote the development of the high-technique, to economize raw materials and energy sources.In recent years, workers devoting to the materials have made unremitting efforts to find new materials. And they have developed many material surface alloy technologies to produce new composite material which can be used in the special working condition, especially in the condition of wear-resistance, heat-resistance, and corrosion-resistance, such as plasma-spray, laser cladding, puddle welding technology, plating, vapor-precipitation, and so on. These techniques although have improved the surface performance and prolonged its life span, but there are some problems such as higher cost, longer cycle. Some need special equipments, some can make the parts distortion, and some need to consider whether the matrix materials can be welded. All these conditions block the development of surface engineering technology.But cast-in-place surface alloying which is a surface intensification technology has been more and more noticed by people because it has a special technology. And it is favored by engineering workers because its merits such as high alloy degree, decreasing working procedure, shortening produce cycle and decreasing cost. But the devoting to the research is not enough, the theory of casting has not been fully understood, and a perfect theory has not been built so far. The products have some problems such as the instabilities of surface alloying layer, inclusions and gas porosities. So it still needs to discuss the casting technology and casting theory, and we can study its rules, getting high-quality, no disfigurement casting products.This paper has made the heat-resistance coatings successfully on the surface of gray cast iron by green mold casting technology. And many experiments have been done in comparisons. They contain using different amount of binders will result in different effects on the surface quality of the casting; when changing spread position of the coatings on mold cavity, different microstructure morphology of the specimen will be got; when the addition amount of alloying elements are different , we observe the microstructure morphology to know the best ratio of alloying elements. At last, oxidation weight increase method and thermal fatigue test have been taken to measure the oxidation-resistance and fatigue-resistance properties of the coatings. The test results are as follows:1. Observing the macro surface quality of the specimens with different amounts of binders, we find that the surface of the coatings are leveling and have no cracks or slag inclusion when the addition amount of binders is 3%. When the spreading position of the coatings is different, through the observing and analyzing of optical microscope, scanning electron microscope, different microstructures of alloy layers will be got. Among them, the best is to spread the coatings on the bottom surface of the mold cavity. Because that, the coatings which are spread there have a compact, homogeneous structure, their alloy layers are thicker and have no defects. Spreading on the side surface, the quality of the alloy layers is better, the coatings and their substrate have a metallurgical bonding, and no defects have been found. Spreading on the top surface, the quality of the coatings is bad, and defects of slag inclusion have been found.2. By analysis the interface between the coatings and substrate through X Ray Diffraction, it has been found that the alloying elements added in the coatings form compounds at the interface. Then using Energy Spectrum to analyze the outer layer and diffusion layer, finding that no matter the outer layer or diffusion layer all have the alloying elements exist, moreover, in the diffusion layer, the amount of Ni/Cr is lower to the outer layer; the amount of Fe is higher than the amount added in the coatings when spreading. This shows that in the course of infiltration layer formation, between the coatings and substrate, alloying elements have diffused because there are different concentrations.3. The coatings which have been made on the surface of the cast iron have a better oxidation-resistance performance known by oxidation weight increase test. The oxidation-resistance ability of the single coatings is 1.6 times of the common cast iron. In the course of coatings oxide, all kinds of oxidation films have made their protective role. Heating the coatings at 950℃, and keeping the temperature for 5 hours, then cooling with furnace temperature. The results show that, oxide scale shedding from the surface of the cast iron; the surface of the coatings is more compact, it is presumed that may be the NiO formed on the coatings'surface have a protective effect.4. The test of thermal fatigue shows that the coatings have a better fatigue-resistance property because that the oxide films formed at the early stage of thermal cycling improve the fatigue-resistance property. As the times of the thermal cycling increase, the oxide films become thicker, the internal stress become bigger because that different thermal expansion coefficient is exist, moreover, oxide films change to inhomogeneous owning to the cold-thermal cycling stress, the coatings continuously chap an shed due to the alternating thermal stress.In summary, the tests studied by the modern research method shows that: using the common green mold casting technology can make heat-resistance coatings on the gray cast iron, and the quality is better, infiltration layer is thicker, also the coatings have better oxidation-resistance and fatigue-resistance performance. This paper has chose the gray cast iron as matrix materials due to its low cost, common green mold casting technology, also made the coatings on the part of the cast iron needing to increase the heat-resistance property. If this technology can be applied on the real production of mechanical products, there will be a good economic benefit, decrease the cost, shorten the producing cycle, and prolong the life-span of the products.