Characteristics of molding process for large steel castings

1. Application of resin sand process

Ordinary sodium silicate sand process has been used in the production of large steel castings in China since the 1970s. However, with the vigorous development of power, transportation, petrochemical, basic machinery, national defense and other industries, users have higher and higher requirements for product quality. Some of the original casting processes have been difficult to meet the requirements of high-quality castings, and it is difficult to meet the quality requirements of castings only by subsequent finishing methods. In addition, this not only increases the manufacturing cost of castings, but also prolongs the production cycle of castings, and has been difficult to meet the needs of customers. Because the production of large steel castings has its own characteristics and laws, the products produced by various enterprises are different, and there are different views on the selection of molding process due to the different habits and understanding of molding process and the differences in the emphasis of various processes. However, with the increasingly stringent requirements of the state on environmental protection, the discharge of a large number of casting waste sand has seriously polluted the environment and has become a very serious problem in the foundry industry. At present, many large-scale steel casting production enterprises have carried out technological transformation. According to the actual situation at home and abroad and the specific situation of enterprise products, different molding processes have been adopted respectively. Generally speaking, they are mainly divided into three categories: ester hardened sodium silicate sand process, furan resin sand process and alkaline phenolic resin sand process.

Sodium silicate sand is widely used in cast steel production because of its high fire resistance, good working conditions, low cost, fast hardening speed, non-toxic and tasteless production site, good high temperature concession, and good environmental protection effect. With the increase of railway speed, it is widely used in the production of railway castings, such as bolster and side frame castings. Although great progress has been made in the research and application of ester hardened sodium silicate sand in recent years, and sodium silicate is also a harmless binder with low cost, sodium silicate sand has so far failed to solve the problems of easy deliquescence, difficult regeneration of old sand, easy collapse of large pieces, poor collapsibility and so on in the production process, so that it can be used as resin sand from sand mixing, molding, pouring The promotion and application of regenerative closed cycle production in the cast steel industry is still limited.

Furan resin sand is the most widely used, most mature self hardening sand. Because of its simple process operation, high dimensional accuracy, good internal quality, smooth surface, easy regeneration of old sand, high utilization rate of old sand and low comprehensive cost relative to castings, furan resin sand is widely used in gray iron, ductile iron, cast steel, nonferrous metal and other castings, and is the preferred process for technical transformation. However, due to the poor high temperature concession of furan resin sand, some thin-walled parts and steel castings with uneven wall thickness are prone to hot cracking, which is its disadvantage. Factories with such castings in production should be careful in their selection.

Alkaline phenolic resin sand is developed to overcome some shortcomings of furan resin sand. Because it has the advantages of good high-temperature plasticity, high dimensional accuracy of castings, good dimensional stability, no s and N curing agent, no n pores and pinhole defects in production, and small hot cracking tendency of castings, it is more and more widely used in the production of steel castings, especially alloy steel castings and large steel castings. The obvious advantages of this kind of resin are that it improves the hot cracking problem of thin-walled steel castings, especially stainless steel thin-walled castings, but its disadvantages are also obvious, such as: the regeneration of old sand is more difficult than furan resin sand, the strength at room temperature is lower, the amount of resin added is larger, and the use cost is significantly higher than furan resin sand.

Although the cost of resin sand is higher than that of sodium silicate sand, and there are relatively many environmental problems involved, the development of resin sand not only improves the surface quality and casting quality, but also has a high utilization rate of old sand, and the old sand is easy to regenerate, which makes the casting production process form a closed cycle, and the discharge of waste sand to the minimum. At present, it is widely used in the steel casting industry at home and abroad, especially in the production of large steel castings.

2. Main process characteristics of large steel castings production

Compared with iron castings, the casting performance of steel castings is relatively poor. Molten steel is easy to oxidize, with low fluidity and slag inclusion; Linear contraction and volume contraction are relatively large. Especially when V, Mo, Cr and Ti alloy elements are contained, they form oxides with high melting point in the steel, making the liquid steel sticky and poor fluidity. When their content is high, it is easy to make the surface quality of the casting worse, the non-metallic inclusions in the steel increase, and even wrinkle and cold shut appear. Therefore, it is necessary to choose an appropriate high pouring temperature and faster pouring speed, especially large steel castings. Therefore, large steel castings generally use chromite sand as surface sand and quartz sand as back sand to adapt to its high pouring temperature.

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