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Several problems bred by gray cast iron

来源: 编辑 浏览次数:1321 Release date: 2019-06-06 source: edit views: 1321
1 Determination of incubation amount

  The inoculation amount is generally determined based on the metallographic structure of the product. The metallographic structure to be considered mainly includes the graphite form and length, the presence or absence of cementite, and the content of pearlite. Wet sand is the sand with the fastest cooling speed among all sand molds.The water content is generally around 3.0%, and some reach 4.0% .Therefore, the production of gray iron parts by wet sand is most likely to cause cementite in thin-walled parts, which is unchanged under other conditions. In this case, this becomes the most important basis for judging how much fertility is added. Our company once encountered such a problem. The amount of the same raw iron liquid and the same inoculant on different products varies widely, from 0.15% to 0.9%, and the thin-walled part of the product added with the 0.9% inoculation amount is still Cementite often occurs. Even if the shape of the part is very different, such a large deviation is unreasonable. For this reason, we conducted an on-site investigation. The results are shown in Table 1.
It can be seen from Table 1 that the pouring weight of each type of molten iron of product two is 13 kg less than that of product one. Therefore, the pouring time of the same amount of molten iron must be more than doubled. The weight of each type of molten iron cannot be changed, and no matter what inoculant, it is impossible to keep it from decaying for 37 minutes. The problem lies in the amount of iron produced. If the amount of iron produced per package is reduced and the iron is added twice more, the gestation decline can be avoided; or the gestation can be moved backward, that is, by increasing the amount of current gestation, this problem can also be solved.
Since the tapping and the loading platform of the hoisting material are the same crane, changing the tapping method will inevitably have a greater impact on production and logistics. Therefore, we have chosen to increase the incubation method. According to the introduction of various data, the general inoculation amount of gray cast iron is about 0.2% is sufficient to ensure a good inoculation. We have tried to increase the incubation rate of follow-up inoculation. The test results are shown in Table 2.

  It can be known from Table 2 that after increasing the amount of inoculation following inflow, the amount of inoculant in the iron trough is greatly reduced. Therefore, inflow inoculation is the best way to resolve the decline in incubation. However, the amount of inoculum incubation is too large, and the inoculant may not completely dissolve. Therefore, we finally chose the method of 0.3% + 0.18% of inflow. Relatively wet sand, resin sand has much less chilling of gray iron parts, and the possibility of white mouth is much less. Therefore, the determination of the incubation amount should mainly consider the shape and length of graphite, pearlite content, and so on.
My company's products often have white mouth, which has a great impact on the processing performance of the product. How to prevent and eliminate it has always been the focus of our daily work. There are many reasons for the white mouth. Let's ignore the influence of the shape of the part itself and the moisture of the molding sand, and only conduct some explorations on the molten iron and inoculation. 2.1 Influence of molten iron quality

  It is well known that if the white iron mouth of the original molten iron is deep, the white mouth of the product is also deep. The depth of the original molten iron white mouth is related to the heritability of pig iron white mouth, the amount of scrap steel added, and the level of w (S). During the production process, there was a period of time, due to the tightness of pig iron, we could only use ductile pig iron to produce gray iron castings, which had a particularly low w (Si) amount. Using the original incubation method, the product had many white and deep products. Can only be used to increase the amount of pregnancy to eliminate white mouth.

  Multi-purpose scrap steel will also lead to an increase in product white mouth. The effect of S on pregnancy is relatively clear, w (S) at 0.05% ~ 0.1% is good for pregnancy, and of course it is also beneficial for reducing white mouth; if w (S) is low, you can use ferric sulfur Concentrate method to supplement, but if w (S) is less than 0.05%, there must be a problem? The company used to have a long time, the amount of w (S) in the product was 0.02% ~ 0.04%, also There is no problem of infertility, and the analysis suggests that this may be related to the inoculant used by our company.

  The SiBaCa inoculant used by our company has a good inoculant effect on high and low w (S) amount of molten iron. However, in order to ensure a good incubation, the w (S) amount is still slightly higher. 2.2 Impact of inoculation The existing inoculants with strong ability to eliminate white mouth mainly include inoculants containing Sr, RE, Ca and other elements.Generally, it can ensure that no white mouth appears in the triangle test strip within 15-20 minutes. Product is enough. However, no matter how strong the inoculant has the problem of decay, the key to preventing white mouth is how to cast iron as soon as possible after incubation. Strengthening insemination is undoubtedly the best way.

  In our experience, as long as the amount of inoculation is about 0.15%, for most products, the white mouth problem can be basically solved.

  About inoculants

  There are many types of inoculants sold in the market. You should choose inoculants that are cost-effective and suitable for you.

  3.1 75SiFe 75 SiFe is the most commonly used inoculant. Among them, Al and Ca have an important effect on the inoculation effect. However, the 75 SiFe-produced molten iron decays very quickly, and it is generally not suitable to choose it for large-scale production and assembly line furnace production. In order to effectively promote the formation of A-type graphite and prevent the occurrence of white mouth at thin walls, the amount of 75 SiFe must be increased, sometimes reaching 1.0%, but the addition of 1.0% not only increases the cost, but also may cause the casting to shrink. Therefore, it is not recommended as the preferred inoculant.

  3.2 Sr-containing inoculants Sr-containing inoculants have a strong ability to eliminate white mouth, which is conducive to improving the shape and distribution of graphite in thin-walled castings, making the difference in tissues at different thicknesses smaller, and theoretically preventing shrinkage. [1] . Sr-containing inoculants have high requirements for the amount of aluminum w (Al) and w (Ca), because the inoculant effect of Al and Ca is exactly the opposite of that of Sr. Generally, w (Al) ≤0.1% and w (Ca) ≤0.3%. To achieve such w (Al) and w (Ca) amounts, SiFe must be refined, which will inevitably increase the production cost of Sr-containing inoculants, so Sr-containing inoculants are more expensive, almost twice as much as 75SiFe. However, the amount of Sr-containing inoculant is about one-third of 75SiFe, so in general, compared with 75SiFe, the incubation effect is improved, which is also economical. There is information that Sr-containing inoculants can solve the problem of product shrinkage [1], but our company's production practice proves otherwise. A refrigerator compressor part produced by our company occasionally loosens in thick parts of the product (see Figure 1). We have tried to modify the pouring system to increase the shrinkage of the product, reduce the pouring temperature and reduce the liquid shrinkage of the product, but the effect is not obvious. Try using Sr-containing inoculant, adding 0.25% of iron in the iron trough + 0.15% of current flow, graphite grade of the product is 5th grade, there is no white mouth in the thin-walled part, but there is still shrinkage in the thick part of the anatomy. The number of co-crystal groups of the product is about 200 / mm2, which is indeed less than the 500 or more / Si2 of SiBaCa, indicating that there is no problem with Sr-containing inoculants, and at the same time, it shows that the effect of Sr-containing inoculants on the shrinkage is relatively limited. high.

  3.3 Ba-containing inoculants

  The most important role of Ba-containing inoculants is to slow down the gestational decline. It has a strong ability to promote graphitization, which can improve the shape and distribution of graphite in thin-walled castings, thereby reducing the hardness difference between thick and thin parts. w (Ba) SiBa inoculant at 20% to 30%, can significantly reduce the white mouth tendency of cast iron, can maintain the incubation time to about 30 minutes, especially suitable for large castings, and the use of small parts may lead to products Pearl volume is not enough. At present, the Ba-containing inoculant with w (Ba) at 2% ~ 3% is added widely, and the addition amount is about 0.3%. The ferrite is not declining in 20 minutes, and its price is only more than 1,000 yuan more than 75SiFe. About one-third of 75SiFe is an inoculant worthy of widespread promotion.

  3.4 Rare earth compound inoculants Rare earth inoculants are generally not used alone, and they are often combined with Ba and Ca to form a rare earth-Ba-Ca inoculant. On the basis of retaining the advantages of Ba-containing inoculants, the deoxidizing ability is stronger, and the high w (S ) Iron is particularly effective. However, the amount of this inoculant should not be too high, otherwise the w (RE residue) in the product is too high, which may cause the cast iron to become too cold during crystallization and the cementite structure may appear. The dosage must be strictly controlled during use, and the maximum dosage should not exceed 0.4%. Rare earth inoculants are not too expensive, only slightly higher than Ba-containing inoculants, and are a worthwhile choice.

  4 How to control the quality of inoculants purchased

  The quality of the inoculant fluctuates during the production process, and the supplier often sells the good and poor quality to us, so how should we control the quality of the inoculant purchased?

  4.1 Chemical detection The quality of inoculants can be preliminarily determined by detecting the mass fractions of Si, Al, Ca, Ba, and rare earths in the inoculant. However, the detection of these elements is more complicated and requires a longer time. Generally foundries do not have this. Ability, therefore, the following method is more practical.

  4.2 Visually inspect the bag to check whether the color of the inoculant is normal? Is there too much black slag? Is it too thick or too thin that does not meet the particle size requirements? These problems occur most often. Be sure to open the bag after each purchase. Our company has purchased a batch of inoculants, and the chemical test elements are qualified, and then put into use, resulting in more than 20 furnace products with white mouth and graphite abnormalities. After inspection, it was found that there were too many black residues in the inoculant, which caused the darkening of the inoculant.

  4.3 On-site trial After each batch of inoculant arrives at the factory, after passing the above inspections, we will also try on-site 3 ~ 5 furnaces, and analyze the graphite, white mouth, pearlite and other tissues of the product through metallographic analysis, and compare them with the normal production products. Immediately discontinue when abnormality is found, which can avoid scrapping of batch products.

4.4 Supplier Selection When selecting a supplier, generally choose a supplier with a larger scale and a certain reputation, so that the quality is more guaranteed. Some suppliers do not have their own smelting furnaces or chemical inspection methods for their products. They just buy large pieces of raw materials from others for crushing. Such suppliers have weak quality assurance capabilities.

  5 Harm of infertility or excess

  Poor inoculation will result in abnormal graphite, white mouth in thin-walled parts, and low mechanical properties. Excessive inoculation will cause problems such as coarse graphite, insufficient pearlite volume, and low mechanical properties. Inoculation overproduction is generally prone to occur during the process of trial production, and generally does not occur under normal production conditions. In 2006, our company replaced the 75SiFe inoculant with a SiBa inoculant. Although the inoculum has been reduced from 1.0% to 0.6% during the process test, the customer responded that the graphite was coarse in the thick parts of the product after processing, until the inoculum was reduced. The product did not return to normal until 0.4%, which indicates that over- or under-incubation is different for different inoculants.

  Under normal production conditions, poor inoculation does not occur, but when equipment problems occur, especially in the production line process, the process of pouring molten iron with a pouring furnace often appears infertile. Because when the equipment malfunctions, the pouring furnace also stores the molten iron that has been conceived but has not been poured. With the prolongation of the failure time, these molten irons will cause infertility due to the decline in incubation. When the amount of molten iron is not too much, we generally adopt the method of adding more inoculants to the re-inferred iron; if the amount of molten iron is too large, we can only return the molten iron in the pouring furnace to the furnace for processing.

  6 Storage and drying of inoculants

  The inoculant must be placed in a dry and ventilated place, and the inoculant that has regained moisture must be dried before use. From March to May each year, it is the rainy season in the south. The more airy the place is, the more humid the air becomes. Therefore, you should be more careful when using inoculants in this season. The inoculant added to the tapping trough or transfer bag does not need to be dried, just put the inoculant in the tapping trough or transfer bag a few minutes before tapping, and use the residual heat to dry the inoculant. Pay special attention to The inoculant added to the transport package should not be premature, otherwise the inoculant may be oxidized or stick to the bottom of the package, affecting the incubation effect. The stream inoculation device and the pouring machine are together, and the stream inoculating agent can be naturally dried on the furnace floor of the casting machine for 1 to 2 hours in advance every day, and then added to the stream inoculation device. Generally, no special drying is required. In some factories, compressed air is used to drive the inoculant. It is necessary to pay attention to the drying of the compressed air. Otherwise, the moisture of the compressed air may affect the inoculation.

  7 Conclusion The inoculation of gray cast iron plays a vital role in product quality. Therefore, the type of inoculant should be carefully selected, and special attention should be paid to the use method. As far as possible, the inoculation can be increased to increase the effect of inoculation, reduce the amount of inoculant, and stabilize the product quality.

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