1 Determination of incubation amount
The inoculation amount is generally determined according to 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 about 3.0%, and some reach 4.0% .Therefore, the production of gray iron parts by wet sand is the 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 and the same inoculant on different products varies greatly, ranging 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 appears. Even if the shape of the parts is very different, such a large deviation is unreasonable. For this reason, we conducted a field survey. 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, increasing the amount of current gestation, which can also solve this problem.
Since the tapping and the top of the hoisting material are the same crane, changing the tapping method will inevitably have a greater impact on production and logistics, so we chose to increase the breeding method following the stream. According to the introduction of various materials, the general inoculation amount of gray cast iron is about 0.2% is sufficient to ensure 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 after inoculation, the amount of inoculant in the iron trough is greatly reduced. Therefore, inoculation is the best way to resolve the decline in incubation. However, the amount of in-stream incubation is too large, and the inoculant may not completely dissolve. Therefore, we finally chose the method of 0.3% + 0.18% out of the iron tank. 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 graphite shape and length, pearlite content, etc.
2 About Baikou
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 us put aside the shape of the part itself and the influence of the molding sand moisture, and only conduct some research 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). In 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 method of incubation, the product had many white mouths and deep. Can only be used to increase the amount of pregnancy to eliminate white mouth.
Multipurpose 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, of course, it is also beneficial for reducing white mouth; if w (S) is low, you can use ferrous 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 poor incubation, 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 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 triangle test strips within 15-20 minutes. Product is enough. However, no matter how strong the inoculant has the problem of decline, the key to preventing white mouth is the question of how to cast iron as soon as possible after inoculation. 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.
3 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 important effects on inoculation. However, the molten iron produced by 75 SiFe decays very quickly, and it is generally not suitable to choose it for large-scale production and assembly line casting 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 added 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 differences in tissues at different thicknesses smaller, and theoretically preventing shrinkage.  . 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 addition 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 , but our company's production practice proves otherwise. A refrigerator compressor part produced by our company occasionally shrinks in thick parts of the product (see Figure 1). We have tried to modify the pouring system to increase the shrinkage of the product and reduce the pouring temperature to 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% with current, the product graphite grade is 5th grade, there is no white mouth in thin-walled parts, but there is still shrinkage in thick parts 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 it also shows that the effect of Sr-containing inoculants on the shrinking effect 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 effect time to about 30 minutes, especially suitable for large castings, and the use of small parts may cause products The pearlite volume is not enough. At present, the Ba-containing inoculant with w (Ba) at 2% ~ 3% is used widely. The addition amount is about 0.3%. The ferrite is basically not declining in 20 minutes. 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 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 does not exceed 0.4%. Rare earth inoculants are not too expensive, only slightly higher than Ba-containing inoculants, and it is a worthy choice.
4 How to control the quality of the inoculant purchased
The quality of the inoculant fluctuates during the production process, and the supplier often sells the good quality and the poor together to us, so how should we control the quality of the inoculant purchased?
4.1 Chemical testing The quality of inoculants can be preliminarily determined by testing 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 Open the bag visually 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 once purchased a batch of inoculants, and the chemical test elements were 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 and passes the above inspections, we also have to test 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 products produced normally. 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 areas, and low mechanical properties. Excessive inoculation will cause problems such as coarse graphite, insufficient pearlite volume, and low mechanical properties. Inoculation overproduction is generally easy 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 incubation amount 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 incubation amount was reduced. The product did not return to normal until 0.4%, which indicates that the over- or under-incubation is different for different inoculants.
Under normal production conditions, poor inoculation does not occur, but when there is a problem with the equipment, 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 still 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, the molten iron in the pouring furnace can only be returned 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 in the moist 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 stove of the pouring 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 Concluding remarks 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 inoculation effect, reduce the amount of inoculant, and stabilize the product. quality.