At present, there are many researches on the influence of metallurgical and molding factors on shrinkage cavity and porosity of gray iron castings. The following factors are discussed:
1) Composition and temperature of molten iron
For gray cast iron, the elements related to shrinkage porosity include carbon, sulfur, flow, etc.
Carbon and graphite can promote the graphitization of cast iron. In a certain range, the shrinkage of castings decreases with the increase of CE, while J.F. wallce also confirmed that the shrinkage volume of castings decreases with the increase of CE. However, too high CE is also detrimental to the feeding of castings. Zhang Ruifen, senior engineer of zhycasting, thinks that when the carbon equivalent is between 4.5% and 4.6%, shrinkage free castings can be obtained, but above 4.6% is easy to produce the above defects. As for the cause of this problem, Li Shuhui, senior engineer of zhycasting, thinks that if CE is too high, the fluidity of molten iron will increase, but the deformation resistance of residual molten iron will decrease, resulting in the increase of shrinkage tendency. It is also believed that too high CE will slow down the transmission of molten iron in the riser, lead to premature solidification of the riser neck, lead to the limitation of feeding channel, and lead to the formation of shrinkage porosity in the casting. However, due to the fact that the graphitization precipitation has been fully completed, the shrinkage at the end of solidification is relatively large when the graphitization precipitation is not fully completed (Thin-walled Casting). In this case, the requirement of CE is different from that of heavy wall casting. Du jutuan, senior engineer of zhycasting, thinks that when the modulus is less than half of the main wall thickness of the casting, the shrinkage value is larger than the normal test value.
At the same time, sulfur can reduce the fluidity of molten iron and increase the shrinkage. However, it is also confirmed by some data that with the decrease of sulfur content, the number of eutectic clusters in cast iron decreases sharply, the tendency of white mouth increases, and the tendency of shrinkage cavity and porosity increases. The analysis shows that the content of sulfur and sulfide is low, the number of crystal nucleus is small, the ability of nucleation is low, the A-type graphite is low, and the white mouth is increased. Therefore, for low sulfur hot metal, especially for electric furnace molten iron, a certain content of s also plays a role in the anti shrinkage of castings. Flow is a graphitization promoting element and can improve the fluidity of molten iron. However, the eutectic structure of the alloy can ultimately promote the shrinkage defects of cast iron.
The temperature of molten iron can be divided into two parts: the temperature of molten iron in furnace and the temperature of molten iron in pouring. The temperature of molten iron directly affects the superheat temperature of molten iron. If the superheat temperature is too high, the faster the crystal core of high melting point graphite disappears, the more difficult the graphitization of molten iron will be, and the production rate of shrinkage cavity and shrinkage porosity of casting will be increased. In a certain range, the reduction of pouring humidity can reduce the shrinkage rate of the casting, thus reducing the volume of shrinkage cavity and porosity to a certain extent. However, with the decrease of pouring temperature, the fluidity of molten iron is reduced, so that the feeding capacity of riser is decreased, and porosity is easily produced. So in practice, the choice of pouring temperature is more important.
2) Gating system
In order to avoid shrinkage and porosity of castings, and based on the consideration of cost and preventing other casting defects, two technological principles of sequential solidification and simultaneous solidification are generally followed in the design of casting pouring system. In general, sequential solidification is often used to prevent the casting from producing shrinkage cavity and porosity defects in process design. It usually creates certain conditions to make the position far away from the riser solidify first, and then solidify gradually towards the riser direction, and the alloy in the riser solidifies finally. Thus, the riser is taken as the solidification end point to form the solidification sequence from far to near. In order to strengthen the sequential solidification, in the design of gating system, the top pouring system is often used for castings with simple shape, low height and uniform wall thickness change, and the high temperature slow pouring method is used to increase the temperature gradient to the riser, so as to increase the feeding effect of the riser. However, for the large castings with complex shape and wall thickness changes, the step gating system is used. In the step gating system, the molten iron enters into the mold cavity from the bottom, layer by layer from different internal sprues, so that the temperature of the molten iron at the top of the mold cavity is the highest, resulting in a good directional solidification and feeding effect. Although the principle of sequential solidification can effectively reduce the tendency of shrinkage and porosity of castings, the obvious temperature difference in each part of the castings during solidification leads to large internal stress in the castings during cooling, so that there is a large tendency of hot cracking in the thin wall or thin wall castings. In addition, because of riser feeding, the production cost of castings increases, so for thin-walled castings and some low-grade iron castings, the principle of simultaneous solidification is adopted.
At the same time, solidification requires that all parts of the casting solidify at the same time, eliminating the feeding channel and not setting the riser, so that the shrinkage porosity is mainly distributed on all sections. In order to realize simultaneous solidification, for medium and large-scale thin-walled castings, many and small internal sprues are often used to disperse molten iron evenly into the mold cavity, and the internal sprues are often set at the thin-walled position.
3) Mold conditions
The compactness and sand composition of the mold is an important factor affecting the shrinkage tendency of gray iron castings, and the influencing factor is the mold wall movement. When the mold is poured into molten iron, the mold wall will move outward under the influence of high temperature, molten iron static pressure and eutectic expansion pressure. If the molten iron in the inner gate solidifies and the mold wall continues to move outwards, shrinkage cavity and porosity may appear near the mold wall. Zhang Ruifen and Li Shuhui, senior engineers of zhycasting, confirmed that the higher the mold strength, i.e. the surface hardness and tensile strength, the stronger the feeding capacity of riser, otherwise, the worse. In addition, the use of low moisture sand can reduce the movement of mold wall. However, Zhou Huayue and others think that the expansion of the wall of the dry type is smaller than that of the wet type under the same conditions, and the volume of the shrinkage cavity is also relatively small. However, there is also evidence to prove that the problem of hot cracking is more serious when using castings with high strength of sand mold, especially block resin sand mold. Therefore, for the corresponding castings, in a certain range, choosing the appropriate mold conditions can reduce the tendency of shrinkage cavity and porosity.