Application of High Temperature Sand Coating on Small and Medium Cast Steel Parts

Abstract: According to the characteristics of film sand, this paper presents the quality problems encountered in the steel casting sand produced by the author's company, analyzes the causes of them, and adopts various formulas and processes for different practical situations so that steel castings can be produced that meet the requirements of the film sand process, which is especially suitable for investment casting of steel castings.

Keywords: film sand; steel casting; investment casting; investment casting

Cast steel parts have many excellent properties, such as carbon steel, low alloy steel, high strength, high toughness and good casting, welding characteristics; and special steel can meet wear-resistant, heat-resistant, corrosion-resistant, low-temperature resistance and other special working conditions. Therefore, cast steel parts are widely used in heavy, mining, transportation, power plant, machinery, aerospace and many other industries. At present, in terms of the demand for cast steel parts, or medium and large castings mainly, and the requirements of mass production, small and medium castings are relatively small, but the quantity is larger, and belonging to thin-walled, precision, complex castings, so it plays a very important role in various industrial fields.

At present, in the production of small and medium-sized steel cast parts, the technological methods are quite a lot, and the most diverse, and each of them has its own characteristics, each with its own advantages and disadvantages. According to the high melting point of cast steel, liquid steel oxidation, poor steel fluidity, shrinkage, cast steel parts are prone to undercasting, cold segregation, shrinkage and weakening, cracks and sand and other casting defects in the characteristics of the search for the best small and medium cast steel parts should be the world's casting workers urgently need to solve the technical key. From China's actual casting production production process is applied in the production process has been applied in the following three kinds of methods: clay wet sand, resin self-curing sand and fusion casting method.

(1) Clay Wet Sand

Clay wet sand, the biggest limitation is that the strength of sand is not high, resulting in the molding method is easy to make castings produce some casting defects. Such as sand traps, sand holes, sand holes, air holes, shrinkage holes, surface sand buildup, cracks, box swelling, etc., the number of defects that must be welded to deal with a relatively large amount of cleaning costs are high. Therefore, based on the world situation, it seems that clay wet sand production of steel parts is not much, with automatic molding production line production is even less, so that at present, clay wet sand is mainly used for the production of cast iron parts.

(2) Self-curing resin sand

Since 1973, China began to explore the application of furan resin self-curing sand, due to the modeling process with castings of high dimensional accuracy and surface quality (dimensional accuracy CT10 ~ 12, surface roughness Ra25 ~ 50um), core routing is good, foundry sand cleaning is easy, old dry sand regeneration is easy to reuse, reuse rate is high (more than 90%) and other advantages. At present, China's large and medium-sized steel castings on more progress and wide application. However, due to the large coefficient of thermal expansion of sand resin, high temperature yielding poorly, cast steel parts are easy to produce cracks. The binder system contains S, N and other harmful elements, easily cause the surface carburization of cast steel, sulfur penetration, surface micro cracks; sand outgassing is large, the casting is prone to porosity defects or N caused by subcutaneous porosity; sand mixing, modeling, pouring, sand, cleaning process produces S0, C0 and other harmful gases, polluting the working environment. Therefore, the sand process using furan resin to produce cast steel parts is not an ideal choice, especially for the production of thin-walled, structurally complex pumps, valves, locomotives, small and medium-sized cast steel parts, which is even more undesirable.

(3) Investment casting method

According to the product requirements and production process of precision casting, can be divided into two categories: the first is the use of high quality mold material, high pressure mold, silica Sol and silicate ethyl acetate shell, steam dewaxing, rapid atmospheric or vacuum melting and pouring process, that is, international general melt casting process. It can produce CT4~6 level accuracy castings, surface roughness Ra0.8~6.3um, mainly used to produce aircraft and gas turbine blades and other high quality investment castings: or general stainless steel, low alloy steel, carbon steel and other investment castings. This type of castings requires a higher level of technology and production costs, in China, its overall proportion is small. The second category is using low quality paraffin stearate molding material, low pressure mold making, water glass shell, hot water dewaxing, slow atmospheric melting and pouring process. It can produce castings with accuracy level CT7~9 and surface roughness Ra2.5~25μm, mainly used for carbon and low alloy steel parts with low quality requirements. With shorter production cycle, low prices of raw materials and auxiliary materials, less investment in equipment, it is widely used in domestic industries such as heavy vehicles, tractors, machinery, metallurgical equipment, machine tools, pumps and valves, etc. At present, it is used to replace sand mold casting in China. It is the main modeling method to replace sand mold casting in the production of small precision and complex steel castings in China. However, there are casting processing allowances, energy consumption and raw material consumption, long production cycle, heavy environmental pollution, low production efficiency and other problems.

In view of the current casting production in the production of the above three kinds of production of small precision casting process methods, there are certain shortcomings, the authors of this article, according to the hot box core furan resin sand box mobility is poor, cold box core sand box mobility is good, but out of the box strength is low, and proposed the use of resin-coated sand shell casting process, to produce small precision casting process castings.

Shell casting is developed by Germany in the 1940s, but in the domestic research on its late. Compared with ordinary sand casting shell casting has less investment in equipment, covers an area of small, convenient production, high production efficiency, casting with less sand, less dust at the production site, less noise, and thus less pollution of the environment, castings with high surface finish, high dimensional accuracy, etc., in our country's automotive industry, cast iron parts on the casting has been widely used. In order to improve the heat resistance of the film sand, China has adopted special raw sand (such as zircon sand, chromite sand, etc.) processed into special film sand to meet the production needs. However, zircon sand, chromite sand and other special raw sand resources are scarce, expensive, relying on imports: Moreover, the surface of the cast steel parts, especially in the casting of the hole (core head) at the surface and corners, the emergence of flow marks or wrinkled skin casting defects, which seriously affect the surface quality of the casting, restricting the application of it in the production. According to the high pouring temperature, poor metal fluidity and large casting shrinkage of small cast steel parts produced by coated sand, we modified the phenolic resin to improve its heat resistance, improve its thermal properties at the same time, and add some special auxiliary materials to slow down the decomposition of the resin, inhibit the decomposition of the heat generated by the sand, to further improve the fluidity of coated sand, to improve the high-temperature strength and its stability, and to maintain low Gas, thus developed a quartz sand-based special high-temperature resistant coated sand, which has excellent high-temperature performance and other comprehensive casting performance, can replace the expensive zirconium sand coated sand to produce a variety of complex small precision steel castings, significantly reducing the cost of improving the yield and quality of castings. The high-temperature compressive strength of the high-temperature resistant coated sand is increased from 0.1~0.2MPa to more than 0.8MPa, the heat-resistant time is increased from 60~110s to more than 150s, and the room-temperature tensile strength is more than 3.5MPa, the heat-resistant strength is more than 2.0MPa, and the gassing quantity is less than 18m1/g (see Table 1).