Nine main reasons for the excessive carbon content of lost foam castings
Keywords：
lost foam casting
castings
carbon content
parameter design
process control
2021-11-24 13:11
42812
小中大
Keywords: lost foam casting, castings, carbon content, parameter design, process control Abstract: In view of the reasons that may cause excessive carbon content in the production of low carbon steel castings by lost foam die, the technical parameters of the production process of low carbon steel castings by lost foam die are modified and adjusted, and a reasonable and reliable production operation mode of low carbon steel castings pouring technology condition setting system is created, so as to ensure the technical requirements of the production of qualified low carbon steel castings by lost foam die.
The main reasons for excessive carbon content in castings are as follows 01 The mold material selection of casting is unreasonable. In the selection of foam material, one is high carbon content. Second, the density of foam is too high. As a result, the carbon content in the thermal decomposition of the casting model during the pouring process is high, which makes the free carbon content in the liquid phase and fog of the casting high during the pouring and filling process, and increases the carburizing probability of the cast steel. 02 Foamed plastic with low carbon content or preformed beads were selected to make the casting pattern. At present, the carbon content of polystyrene foam EPS produced in China is 92 %. The carbon content of STMMAC is 69.6 %. Available polymethyl methacrylate EPMMA with carbon content of 60.0 %. The carbon content of molten steel will increase by 0.1 % ~ 0.3 % in the casting process by using EPS material to make casting mold. When using EPMMA or STMMA material to make casting mold, the carbon increase caused by mold material in the casting process is less than 0.05 %. 03 Select the appropriate foam density for casting mold production. When other defects caused by the quality of foam plastic do not appear when the technical requirements of mold production temperature and casting production pouring are ensured, the smaller the foam density of the casting mold is and the less the foam plastic is, the better it is to reduce the carburizing and carbon deposition of the casting. 04 Improve the quality of casting mold. When the casting pattern can be made as a whole, it should not be made by combination, and the bonding surface of the casting pattern should be minimized. It is necessary to ensure that the bonding interface of the sample is smooth and smooth, minimize the amount of adhesive used for bonding, and reduce the carbon content of the thermal decomposition product of the adhesive. 05 Select low carbon or carbon - free adhesive. That is, special adhesive for negative pressure casting should be used for pattern combination bonding, rather than low-quality ordinary adhesive with high carbon content. And in the sample combination bonding, to ensure the adhesive bonding temperature and bonding strength, to minimize the amount of adhesive, so as to reduce the thermal decomposition products of adhesive. 06 Select and determine the reasonable gating system. For the design of casting casting process, it is necessary to accelerate the gasification of foam plastics in the casting process as much as possible, reduce and stagger the contact and reaction time between liquid phase and solid phase in the thermal decomposition product, so as to reduce or avoid the occurrence of carburizing of castings. 07 Select and determine the appropriate casting temperature and casting speed. Because the same casting such as pouring process modeling is different, the actual filling temperature is completely different when pouring molten steel at the same temperature. If the pouring temperature increases and the pouring speed increases, the thermal decomposition of the casting mold will be accelerated and it is not easy to completely gasify, so that the amount of thermal decomposition products in the liquid phase increases. At the same time, because the gap between the molten steel and the mold is small, the thermal decomposition products in the liquid phase are often extruded into the mold coating layer and the metal liquid, or the cold angle and dead angle of the molten steel flow, resulting in an increase in the contact surface, carbon concentration and carburizing amount. At the same time, it should be noted that if the casting pouring process is not reasonable, the pouring temperature of molten steel is too high and the pouring speed is too fast, it will cause the production accidents of gas blowing and back spraying. 08 Flame retardants were added to the model to prevent the pyrolysis and combustion of the model at high temperature, so that it did not produce or produce less carbon-containing solid products. Such as adding flame retardant 0.5 % ~ 3 % chlorinated paraffin, triphosphate, pentabromodiphenyl ether, antimony trioxide, etc. At the same time, 0.2 % – 0.5 % dibenzoyl peroxide and dilauroyl peroxide were added to accelerate the transformation of the flame retardant pattern into gas, thereby reducing the occurrence of carburizing conditions and conditions in the casting pouring process. 09 Anti-seepage carbon material can be added in the brushing process of model coating layer. Some anti-carburizing catalysts, such as alkali metal salts and limestone powder, were added in the preparation process of model coating to make the model decompose enough CO and CO2 gas in the coating layer after pouring for carbon absorption, so as to prevent casting carburizing. Or adding oxidant in the coating, the C and H2 gas after thermal decomposition of the mold is transformed into neutral gas, so that the C and H after thermal decomposition are reduced to infiltrate into the casting, resulting in carbon or hydrogen embrittlement.
The following key points should be paid special attention to when adjusting the parameters of production operation control technology for the production of low-carbon steel castings by LFC : 01 Using intermediate frequency smelting, it is necessary to strictly control the burden calculation of smelting carbon steel and the operation of actual burden, selection and feeding. The calculation of ingredients is the key to ensure the melting of molten steel with qualified composition and the casting of high-quality steel castings with minimum gas and inclusions. The accuracy of ingredients, selection and feeding is the fundamental guarantee for the quality of casting products. Therefore, it is necessary to carry out a strict separate inspection system for recycled steel scrap. In particular, the scraps of alloy steel and unclear materials must be removed to ensure that the composition of smelting ingredients meets the requirements of casting process technical parameters, which is the first priority in the operation control of low carbon steel castings produced by LFC. 02 Strict control of molding sand recycling quality. To ensure that the production and use of molding sand meet the requirements of process technical parameters, especially the dust content in molding sand must be strictly controlled to prevent the air permeability of the casting sand box from decreasing due to dust, resulting in that the thermal decomposition products of the casting model cannot be discharged from the casting cavity in time, thereby increasing the carbon increase probability of the casting. 03 The vacuum degree of pouring sand box should be improved when low carbon steel castings are produced. The molding sand box should adopt the structure of vacuum suction on the box wall. Vacuum suction on the box wall during pouring can accelerate the release of thermal decomposition products from the coating to the outside of the mold cavity, thereby reducing the concentration and contact time of the thermal decomposition products of the model, and reducing or avoiding the carburizing and carbon deposition of the cast steel. When the sand particle size is between 20 mesh and 40 mesh, 0.03 MPa – 0.06 MPa is suitable for casting steel. If the negative pressure is too large, it will cause the occurrence of sand sticking and other defects in the casting. 04 For the pouring of low-carbon steel castings, the bottom-injection pouring method should be used as far as possible to make the filling flow of molten steel smooth, and the thermal decomposition products of the model can smoothly enter the slag collection chamber or riser, so as to reduce and reduce the contact reaction time of liquid and solid phases in the thermal decomposition products of the model, and reduce and eliminate the carbon increase probability. The pouring of low carbon steel castings by rain pouring system will increase the working conditions and conditions of carburization, carburizing and carbon deposition of castings, resulting in serious defects in castings and not suitable for use.
Related downloads
Analysis of nine main reasons for carbon content exceeding standard in lost foam casting and summary of operation points.pdf
