As we all know, there must be two conditions for the formation of crack. One is that quartz sand undergoes phase change volume expansion at 573 C, which results in cracking on the surface of sand moulds. The other is that the static pressure of high temperature liquid metal is greater than the permeability resistance of liquid metal, which results in the infiltration of hot metal into cracks. That is to say, in the process of heating sand mold with high temperature liquid metal, a great temperature gradient is produced from the outside to the inside of the sand mold, and the thermal stress is generated on the surface. When the high temperature bonding strength of the sand mold is less than this thermal stress, cracks will appear in the sand mold. Whether the high temperature liquid metal can enter the sand mould crack depends mainly on the size of the crack, the wettability of the liquid metal to the surface of the sand mould, the surface tension and the static pressure. Thus, if the thermal strength and thermoplasticity of sand moulds can be improved, the thermal stress and deformation caused by the expansion of silica sand and the burning decomposition of resin can be overcome, the surface cracking of sand moulds can be overcome, or the infiltration cracking of liquid metal can be restrained, the generation of crack defects can be avoided or alleviated via the followings:
1)Controlling the application performance of silica sand: Thermal expansion and phase change expansion of silica sand are one of the main reasons for the cracking of sand moulds, but different types of silica sand have different thermal properties due to their different contents of SiO 2, basic oxides (FeO, CaO, K0, NaO, etc.), grain shape, particle size and distribution, surface microstructures of sand particles, affinity between sand particles and resin, etc. The expansibility is quite different. The thermal expansion stress of Dalin Sand is smaller because of its moderate SiO 2 content, rounded grain shape and thick resin film per unit area; the raw sand with relatively dispersed grain size distribution, such as changing the three-sieve method into four-sieve or five-sieve method, at this time, the grain size of the raw sand varies greatly, and the time of thermal expansion occurs first and then can reduce the macroscopical of the sand mold. Expansion slows down the tendency of thermal cracking. Some irreversible phase transitions have taken place in the phase-change sand calcined at about 900 C. The thermal expansion rate decreases. Lattice distortion and lattice defects appear on the surface of the sand grains, and the activity increases, which improves the adhesion strength between resin and sand grains and has remarkable crack resistance.
2)Choose anti-crack additive: anti-crack additive is a multi-component compound of silicate and metal oxide specially treated. Under normal temperature conditions, it does not affect the service time, fluidity, curing speed, warehousing time and other conventional performance of the mixture, and does not increase the gas production of sand moulds. At high temperature, it can react physicochemically with silicon dioxide to form low melting point glassy multicomponent material, increase the thermal strength and thermoplasticity of sand mould, absorb the volume expansion of quartz sand when phase transformation occurs at 573 C, reduce the tendency of thermal cracking of sand mould, plug cracks, increase the permeability resistance of liquid metal and prevent them from cracking. The crack defect can be effectively solved by adding 5%-10% of the weight of raw sand into the casting when the gas of sand stopping mould diffuses into the casting.
3)Special sand with low expansion rate is selected: zircon sand, chromium ore sand, mullite sand and other non-quartz raw sand have the advantages of small thermal expansion coefficient, large thermal conductivity and thermal storage coefficient, high refractory and low wettability with iron and its oxides. According to the severity of crack defects in different sand moulds, 25-5O% special sand and most chromite sand can be used to solve crack defects. But in practical application, we should pay attention to the difference of density and grain shape between special sand and quartz sand, so we must consider the difference of density and specific surface area and adjust the actual amount of resin added. Generally, the quality relationship between special sand and quartz sand is converted according to the following formula: MTE = m quartz ((p quartz/p) quartz ((s special/s quartz). Among them: m-mass; p-density; s-specific surface area.
4)Selection of resins with good thermoplasticity: Resin is a key factor affecting the normal temperature, thermal strength and thermoplasticity of sand moulds. Due to the difference of the properties of various grades of resin, it is bound to cause different interfacial phenomena between sand and resin film, different resin film strength, different influencing factors and realizing conditions. In order to quantify the thermoplasticity of different resins, it is usually expressed by the duration and deformation of sand moulds which remain non-collapsible under certain loads and temperatures. The positive deformation scale shows the resistance of sand moulds to deformation, while the negative deformation scale shows the expansion stress of sand moulds under thermal action. It is easy to judge the different resins from the deformation amount. The thermal strength and thermoplasticity of resin sand under the same technological conditions can be measured on the high temperature performance tester of resin sand.
5)Selection of chilling and sintering coatings: use chilling coatings to accelerate the cooling of liquid metal, so that liquid metal solidifies before the macro expansion of silica sand has not been completed, or use adiabatic coatings to reduce the heating speed of sand mold, reduce the thermal expansion of silica sand, so that no cracks occur in sand mold, can achieve the purpose of preventing cracks in castings. Yes. The commonly used chilling coatings are antimony coatings. In practical application, it is advisable to apply another layer of conventional coatings on the chilling coatings to avoid pitting structure on the surface of castings, but this chilling coatings are only effective for cracks in small castings. For large castings, sintered coatings with excellent strength, toughness and thermal conductivity should be selected. The expansion coefficient of the coatings should be close to the thermal expansion coefficient of sand moulds, so as to enhance the ability of anti-expansion cracks on the surface of sand moulds.
6)Optimizing moulding, melting and pouring process: In moulding and core-making process, control and optimize the process parameters such as moisture, temperature, pressure and time, for example, the moisture content of raw sand should be less than 0.2%, the sand temperature should not be less than 15 ~40 ~C, and the mixture should be used in service time to ensure that the sand mould has higher strength, smooth surface and compact. In metal smelting, the content of phosphorus (P) should be lowered as far as possible; in pouring, the lower pouring temperature and pressure head should be adopted as far as possible to reduce the degree of thermal expansion of sand moulds and the penetration pressure of metal liquids, while guaranteeing the filling temperature of molten iron which can obtain sound castings without producing defects such as blowhole. Faster casting speed is used to avoid the possibility of expansion cracks in sand moulds baked at high temperature for a long time.