1 Casting defects of large ductile iron parts
Shrinkage hole, shrinkage porosity, slag inclusion, air hole, peeling, deformation and so on are common casting defects in sand casting of large ductile iron. These common casting defects are usually affected by the following factors. For large ductile iron castings, the temperature gradient of molten iron entering the cavity cannot reach the benign state of high and low because of its large contour size, large wall thickness or large hot joint thickness, such as the unreasonable design of the gating system. If a reasonable supporting feeding riser is not set, it is necessary to feed the liquid cooling of the corresponding hot joint. If the rigidity of the casting mold is not enough, it is easy to appear shrinkage cavity and porosity casting defects at some thick hot joints of the casting. Because of its large outline size, if the design of the gating system is unreasonable and the molten iron is not effectively filtered, that is, the gating system can not effectively prevent the slag from entering the casting cavity, it is easy to have slag inclusion casting defects at some thick and large plane of the casting. Its outline size is large, such as the design of the gating system is unreasonable, so that the casting casting time is too long, and the ability of sand mold to resist sand inclusion is weak, it is easy to appear in some large plane of the casting sand inclusion casting defects. Because of its large outline size, such as the design of the gating system is unreasonable, so that the casting casting time is too long and the temperature gradient of liquid iron into the cavity can not reach the benign state of high and low, and the exhaust of the riser is not smooth and the overflow of liquid cold iron at the riser is not sufficient, it is easy to appear porosity casting defects in the upper plane of the casting. Its outline size is large, the mass is large, if the casting system design is not reasonable, the casting temperature distribution is very uneven, the temperature difference is too large, it is easy to make the casting deformation and crack casting defects. In order to overcome and avoid the above defects in large ductile iron castings, the foundry process design of the corresponding castings should be optimized or optimized in the following aspects.
2 Optimal design of gating system
2.1 Optimization design of gating system structure type
In the sand casting production of large ductile iron castings, except a few castings such as plate and ring castings which mostly adopt the relatively simple monolayer injection type, the other castings generally adopt the stepped and layered injection type of the gating system. For the sand casting of large ductile iron castings with high casting position, we think that the reasonable type of casting system should be: the best one is to combine the layered treatment on the basis of the stepped liquid feeding. In this way, a benign temperature field and ideal solidification conditions can be established in the casting cavity to achieve the purpose of obtaining dense castings.
For smaller large ductile iron parts, or the height of the casting position is small, the layered feeding mode of single group gating system is adopted. The “high temperature” molten iron poured into the riser can not only effectively feed the liquid cooling shrinkage of the casting, but also simplify the design of the casting system, especially reduce the riser design, so as to improve the technological yield of the casting. For large ductile cast iron, two groups of gating system stratified injection type; For extra-large ductile iron castings, a three – group gating system of layered injection type. The riser part can obtain high temperature “high temperature” liquid iron to effectively feed the liquid cooling shrinkage of the casting. For heavy and large ductile iron castings, a four – group gating system of layered injection type. The molten iron in the riser part is poured to the highest surface of the riser by the riser system as the “gating system” to obtain good “high temperature” molten iron to effectively feed the liquid cooling and shrinkage of the casting.
2.2 Reasonable determination of effective pouring time
Most casting casting times described refer to the time elapsed between the liquid metal entering the cavity and filling the cavity completely. Most factories still use this traditional casting time theory to guide casting casting time determination. For large ductile cast iron, we think it is reasonable to adopt the new theory of effective casting time to guide the design of casting system. The effective casting time is defined as the time for liquid metal to enter the cavity until the highest profile of the casting is filled.
2.3 Determination of the minimum cross-sectional area of the gating system
Most of the factories adopt the theoretical formula of hydraulics, but the production practice generally reflects that the minimum cross-sectional area of the gating system calculated by the formula is too small, and the principle of large flow rate is advocated. For large ductile cast iron, we think it is more reasonable to adopt the formula to design the minimum cross-sectional area of the gating system.
2.4 Optimal design of section ratio of each component of the gating system
Due to the large amount of molten iron pouring in large ductile cast iron, it is easy to produce oxidized slag inclusions. If the molten iron filling mold is not stable or off flow, it is more likely to produce defects such as oxidized slag inclusions and pores. The gating system must not only have stable and fast filling function, but also have good slag retaining effect. The semi-closed gating system should be preferred for each group of gating system.
3 Appropriate application of filtration technology
Due to the large amount and long pouring time of molten iron in large ductile graphite cast iron, the probability of molten slag in molten iron and oxidized inclusions treated by spheroidization into the cavity is large. The gas brought by molten iron increases the gas content of molten iron, the slag increases the external core of the bubble, and can improve the viscosity of molten iron and increase the gas escape resistance, so as to easily cause the casting to produce pores. In order to overcome the slag inclusion and related porosity defects of large ductile iron castings it is necessary to apply appropriate filtration technology and measures in its pouring system reasonably.
The main measures are to set the casting fiber filter or foam ceramic filter in the appropriate position of the gating system. Some factories have also adopted casting fiber filter and foam ceramic filter overlay filter technology measures.
4 Suitable application of chiller
Ductile cast iron is very different from gray cast iron because of its pasty and solid properties and its unique shrinkage behavior. Therefore, in order to obtain high density ductile cast iron, chiller process measures should be well matched in the molding process, especially for large ductile cast iron with high density requirements, chiller process measures should be applied in large quantities and appropriately. The thickness dimension of the chiller is usually at the set casting position.
5 Reasonable design of riser system
Due to the large amount of molten iron pouring in large ductile iron castings, the liquid cooling shrinkage of molten iron and the amount of feeding are correspondingly large, so the riser system of the ductile iron castings should give priority to the selection of the blank holder riser with strong self-adaptability and feeding ability or the combination of the blank holder riser and the neck shrinking riser.
6 Reasonable determination of pouring process
Take pouring temperature in casting wall thickness and structure complexity, to obtain dense tissue castings, should as far as possible to shorten the molten iron in the casting solidification time, reduce internal interdendritic loose, shall adopt the low pouring temperature, but there is information that ductile iron parts of pouring temperature under 1300 ℃ when the casting defects are prone to slag. In order to overcome the casting defects of large ductile iron castings, the casting system should be reasonably designed to make the “temperature field” of the castings evenly distributed, the temperature difference as small as possible, so that the castings can achieve a more uniform cooling state, and determine the long cooling time of the castings.