The solution to shrink the defect is an important factor affecting the cost of the product, which directly affects the process yield and production efficiency of the casting. The feeding effect of the casting has a direct relationship with the material and structure of the casting. Although the gray iron castings are easier to feed according to the solidification characteristics of the material, the ductile iron castings are difficult to replenish, but if there is no feeding passage, even the gray cast iron is difficult to achieve. For example, the brake disc of the HT250 material has a simple structure, and since there is no feeding passage, a shrinkage defect is often generated; and if a good feeding passage is provided, the ductile iron crankshaft is less likely to cause a shrinkage defect. Common methods for eliminating or reducing shrinkage defects are generally: feeder feeding, smelting and pouring process adjustment control, cold iron chilling, and using cooling ribs, heat sinks and high thermal conductivity sand, core sand local accelerated cooling. The following are examples of the author's use of these methods to solve shrinkage shrinkage defects.
1 riser feed
Riser feeding is a preferred method and the most widely used method. When designing the gating system, the modulus relationship between the casting, the riser neck and the riser must be properly selected to achieve the purpose of feeding with the riser; however, due to the complexity of the shape of the casting, the theoretical calculation and the actual situation are larger. Differences, the size of the riser and the neck of the neck are constantly adjusted to achieve the desired goal.
At present, insulated risers and hot risers have been widely used. Insulation riser and hot riser not only reduce or eliminate shrinkage defects, but also improve the process yield rate by more than 10%, especially in the production of large and large pieces of heat preservation or heat riser.
When the company produces car crankshafts and forklift steering axle castings, it uses heat-insulating risers. When it is difficult to solve the problem of shrinkage (holes), the recommended method is to use heat-insulated risers. For example, when producing a spherical iron bearing cap for a car engine, it is difficult to eliminate due to shrinkage. The casting of the casting body is only 30%, and the use of the insulated riser increases the yield to over 50%, which has a relatively high economy. Sex.
2 Adjustment and control of smelting and casting process
If the casting has only a small microscopic shrinkage or microscopic shrinkage, it can be considered by adjusting and controlling the smelting casting process. It should be noted that this method is only suitable for shrinkage or microscopic shrinkage. The x-ray inspection does not find that each casting has such a defect, that is, when the proportion of the casting having the shrinkage defect is not high.
2.1 Control of chemical composition
(1) Appropriately improve CE. The CE is high, and the graphitization expansion ability is strong during the solidification process of the molten iron, which can play a certain role in the feeding of the casting; however, the method of improving the CE can only be adopted if the casting performance satisfies the technical requirements.
(2) The amount of ω (Mg residual) should be strictly controlled for the ductile iron. In order to obtain a better spheroidization rate, the amount of ω (Mg residual) is generally higher than 0.03%, and since Mg tends to increase the shrinkage tendency of the molten iron, the amount of ω (Mg residual) should be as low as possible from the viewpoint of preventing shrinkage. Taking into account the difficulty of process control, the control range generally used in actual production is 0.035%~0.05%.
(3) Pay attention to the influence of Sn. In general, Sn promotes pearlite by 10 times that of Cu. In order to obtain the same amount of pearlite, the cost of using Sn is lower than that of using Cu. However, the tendency of Sn to increase shrinkage is obvious, which tends to cause shrinkage. Or microscopic shrinkage defects. When the crankshaft castings of the author add Cu, there is no problem in quality. However, when Sn is added, the shrinkage defects of the crankshaft connecting rod and the neck are found after machining and grinding. This shows that the influence of Sn on the shrinkage of the casting is larger than that of Cu. .
2.2 Selection of inoculants
The use of a suitable inoculant can also improve the tendency to shrink, especially in improving microscopic shrinkage. For example, the inoculation with sulphur and oxygen can reduce the micro-retraction to some extent. For example, when the steering gear housing produced by our company uses ordinary ordinary ferrosilicon inoculant, there is microscopic shrinkage, which leads to failure of fatigue test. After the use of sulfur-oxygen inoculant, micro-contraction is improved, fatigue test Meet the user's technical requirements. Figure 1 reflects the tissue difference between a thiored inoculant and a non-sulfur oxygen inoculant.
(a) Microfibrillation with ordinary ferrosilicon
(b) Inoculation with a deoxygenated inoculant (no micro-retraction)
Fig.1 Effect of inoculant on microscopic shrinkage of steering gear block
2.3 pouring temperature selection
The advantages and disadvantages of the pouring temperature can not be generalized. Because the casting structure and the feeding process are different, the influence of the pouring temperature is also different.
Under normal circumstances, if the feeder is used for feeding, the higher pouring temperature can increase the feeding effect of the feeder. If the feeder is not used, the lower pouring temperature should be selected.
The choice of casting temperature is directly related to the size and wall thickness of the casting. The pouring temperature of the brake caliper castings is suitable for 1380~1430 °C; the pouring temperature of the thin-walled parts of the exhaust pipe should be suitably higher, generally 1390~ 1450 °C; the temperature of the crankshaft thick parts can be lower, but Generally can not be lower than 1360 °C. In the process of mass production, a reasonable pouring temperature control range should be selected through practical data statistics.
The problem of feeding compensation through the smelting and pouring process adjustment should be carried out under the premise of satisfying other technical requirements of the casting. For example, adjusting the pouring temperature should take into account the effect on the inoculation effect, the tendency of the casting porosity, etc.; the adjustment of the composition should take into account the influence on the mechanical properties.
3 cold iron chilling
The principle of the cold iron method to solve the shrinkage cavity defect is to quickly cool the hot joint portion. Generally, cold iron can only displace the defect, that is, the defect is moved to a non-critical part, and the defect cannot be eliminated. Unless there is enough feeding channel to achieve sequential solidification, the defect can be moved to the final solidified riser. The application of cold iron is also common. The use of cold iron can reduce the riser and increase the yield of the process.
The steering gear cylinder produced by the company used to use the body heating riser to eliminate the internal shrinkage defects. Later, the cold iron was used to achieve the same effect, and the quality was more stable. The reason is that when the riser is used for the riser, the heat is concentrated at the portion where the riser is connected to the casting body, and the solidification is late, resulting in low hardness of the casting and poor graphite morphology, as shown in Fig. 2. The company's exhaust pipe and magnetic suspension connectors are well used in the cold iron solution to solve the internal shrinkage quality problem.
The cold iron has a fast chilling speed, so the effect of solving the shrinkage defects is better, but the pouring temperature and the inoculation conditions are also relatively high. If the gestation is not good or the pouring temperature is too low, the contact with the cold iron is prone to seepage. Carbon body.
Figure 2 Steering engine block is changed from hot riser to cold iron
4 Accelerate cooling with cooling ribs or fins
The working principle of the cold rib is similar to that of the cold iron, but it can eliminate the disadvantage of the cost increase caused by the cold iron manufacturing. A thin rod with a diameter of 5~8 mm or a heat-dissipating sheet with a thickness of 2~5 mm is arranged on the mold near the hot joint portion which is prone to defects. After pouring, the thin round rod or sheet is rapidly solidified, and the heat is quickly dissipated to drive the hot joint portion. The solidification speed is increased, which is equivalent to placing a piece of cold iron at this position, shifting or eliminating the shrinkage. Figure 3 is a schematic illustration of the use of cold ribs or fins, and a photograph of the use of cold ribs in the exhaust pipe casting.
Figure 3 Using cold ribs to eliminate the shrinkage of the exhaust pipe
5 Accelerated cooling with sand and core sand with high thermal conductivity
The use of a shape and core material with a high heat storage coefficient and a high thermal conductivity in the hot joint portion is also one of methods for accelerating the cooling rate of the hot joint portion, transferring or eliminating the shrinkage.
In actual production, chromite ore and zircon sand are used more, and chromite or zircon sand can be mixed into molding sand or core sand for use in hot joints. The author uses chromite sand core to prevent shrinkage defects in the castings such as the center hole of the cylinder head of the steering gear and the neck of the crankshaft. The steering wheel cylinder head shown in Figure 4 solves the problem of shrinkage of the center hole with a chromite sand core.
Figure 4 steering cylinder head and its chromite sand core
There are many ways to solve the quality defects of the casting shrinkage, and the appropriate method should be selected according to the structure of the casting and the actual situation of the foundry. At the same time, we must also consider the impact on cost, process flow, as well as the quality requirements and use characteristics of castings. There is no need to spend too much cost to eliminate inconsequential shrinkage defects, but it must cost to eliminate shrinkage defects at key locations, which can cause serious quality consequences and even lead to car recalls.