Reasons for the high carbon content of lost foam steel castings
The carbon content of the surface, body or local surface of the steel castings produced by the lost foam casting process will exceed the value of the casting product quality requirements due to the material of the casting pattern, smelting ingredients and casting molding process. Causes casting defects such as carburization, carburization, and excessive carbon. After careful study and analysis, in the production process of steel castings using lost foam casting, the main reasons for the excessive carbon content of the castings are as follows:
1. The selection of casting material is unreasonable. In the selection of casting white mold material, one is high carbon content; second, the density of white mold is too high. As a result, the casting pattern has a high carbon content during thermal decomposition during the pouring process, and the free carbon content of the casting during the pouring filling process is high, resulting in an increase in the carburizing probability of the steel casting.
2. The carbon content in the production and smelting ingredients of steel castings is not strictly controlled. In particular, the carbon content in various scrap steels and whether there are other alloy components in various scrap steels is not clear, because if alloy steel scraps are mixed into smelting ingredients for production smelting, alloying elements will promote the formation of carbides , Thereby increasing the solubility of carbon in molten iron, firstly, it will cause difficulties in oxidative decarburization of molten steel, and secondly, it will produce obvious carbon increasing effect.
3. There are many bonding surfaces for castings. Or the quality of the pattern assembly is poor, and the combined surface is not smooth and flat, causing the pattern to use more glue during assembly and bonding, and the carbon content of the thermal decomposition product increases. This promotes the increase of the carburizing probability of the casting during the pouring process.
4. The choice of white mold adhesive is unreasonable. One is that the material quality and content of the adhesive are not required, and the adhesive with high carbon content is used; the second is the poor bonding ability of the selected adhesive, resulting in the amount of glue used when the white mold is combined. Large; increase the carbon content of the thermal decomposition products of the adhesive during the pouring process, and increase the carburizing probability of the steel castings.
5. The pouring filling design in the casting system setting is unreasonable. Due to the low carbon content of steel castings in the production of low-carbon steel castings, if the pouring filling method of the castings is not designed properly, the probability of carbon in the thermal decomposition products of the casting pattern diffusing into the castings will increase. And cause carburization and carburization.
6. The coating layer of the casting pattern and the molding sand in the casting sand box have poor air permeability. During the production and pouring process of the casting, the pattern and pattern combination adhesive encounter the thermal decomposition products of the molten steel during the thermal decomposition of the pattern, and the casting cavity cannot be quickly discharged, creating the disadvantages of carburization and carbon deposition.
7. The setting of the casting system of the steel casting is unreasonable. Especially the configuration of the vacuum system and the casting flask or the casting process modeling is unreasonable, resulting in the casting process, the vacuum in the casting flask is unequal, the deviation is too large, or the actual vacuum is insufficient, and the vacuum pressure gauge The display is an error value that meets the requirements of the technical parameters, so that the thermal decomposition products of the appearance cannot be quickly and quickly discharged from the casting cavity, resulting in carburization or carbon deposition of the casting.
8. The setting of casting conditions for castings is unreasonable. If the pouring filling process is set too long, the temperature of the molten steel during the pouring process is low, especially where the wall thickness of the casting is increased, the solidification speed of the molten steel is slow, resulting in a longer liquid-solid residence time, which promotes The action time of the molten steel and the thermal decomposition products of the pattern increases, which increases the carburization and carbon deposition of the steel castings.
The following 6 points should be paid special attention to in production operation control.
1. Using intermediate frequency smelting, it is necessary to strictly control the calculation of the ingredients for smelting carbon steel and the actual ingredients, selection and feeding operations. Because the calculation of the ingredients is the key to ensure that the molten steel with qualified composition and the least gas and inclusions are melted to produce high-quality steel casting products. The accuracy of ingredients, selection and feeding is the fundamental guarantee for the quality of casting products. Therefore, a strict sub-inspection system must be carried out for recycled scrap. In particular, it is necessary to remove alloy steel scraps and scraps with unclear materials to ensure that the composition of the smelting ingredients meets the requirements of the casting process technical parameters, which is the first priority in the operation and control of the production of low carbon steel castings in the lost foam.
2. Select the appropriate foam density to make the casting pattern. When ensuring the technical requirements of the pattern making temperature and no other defects caused by the quality of the foam during the production and pouring of the casting, the smaller the foam density and the less the foam, the better to reduce the carburization of the casting.
3. Use adhesives with low or no carbon content. That is, the special adhesive for negative pressure casting and bonding should be used for pattern combination bonding, instead of low-quality ordinary glue with high carbon content for bonding. And when the pattern is combined and bonded, when ensuring the bonding temperature and bonding strength of the glue, the amount of adhesive should be minimized to reduce the thermal decomposition products of the adhesive.
4. Select and determine a reasonable pouring system. For the casting process design of castings, it is necessary to make the castings have the effect of accelerating the gasification of the foam during the casting process, and minimize and stagger the contact and reaction time of the liquid and solid phases in the thermal decomposition products, thereby reducing or avoiding Carburization of the casting occurs.
5. Select and determine the appropriate pouring temperature and pouring speed of the casting. Due to the different shapes of the same casting such as the casting process, the actual filling temperature of the casting is completely different when the molten steel is poured at the same temperature. If the pouring temperature is increased, the pouring speed will also increase, which will result in faster thermal decomposition of the casting pattern and not easy to complete gasification, which will increase the amount of thermal decomposition products in the liquid phase. At the same time, because the gap between the molten steel and the pattern is small, the liquid After being squeezed out of the gap, the thermal decomposition product is often squeezed between the pattern coating layer and the molten metal, or the cold corners or dead corners where the molten steel flows, causing the contact surface to increase, the carbon concentration and the amount of carburizing will also increase. At the same time, pay special attention to the fact that if the casting process is unreasonable, the molten steel pouring temperature is too high and the pouring speed is too fast, it will cause gas out and back spray production accidents.
6. When low-carbon steel castings are produced and poured, the vacuum of the casting flask must be increased. The moulding sand box should adopt the structure of box wall vacuuming. When pouring, the box wall vacuuming can accelerate the escape of thermal decomposition products to the outside of the mold cavity, thereby reducing the concentration and contact time of thermal decomposition products of the pattern, reducing or avoiding cast steel Carburization and carbon deposition of parts. When the size of molding sand is 20-40 mesh, 0.03-0.06MPa is suitable when casting steel castings. If the negative pressure is too large, it will cause sand sticking and other defects in the casting.
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