Valves are widely used in chemical, food, medicine, electric power and other industries. According to the different working conditions, technological requirements of the conveying system and conveying medium, the materials, specifications and models of the valves are also different, and there are many types. With the continuous progress of modern production technology, the quality requirements for valves are also getting higher and higher. Due to the particularity of the valve structure, various casting defects will occur during the casting process, such as air holes, shrinkage holes, shrinkage porosity and other quality defects, which will eventually lead to leakage of the valve. According to statistics, most of the reasons for valve leakage are caused by the two casting defects of shrinkage cavity and shrinkage porosity generated during the casting process. In this paper, the quality defects such as shrinkage holes and shrinkage porosity generated in the casting process of the valve are analyzed, the reasons for their formation are analyzed, and the preventive measures are discussed to improve the inherent quality of valve casting products.
Defects such as shrinkage holes and shrinkage porosity are the main reasons for valve leakage. In the process of casting production, some casting defects are formed, which lead to leakage of the valve during use, affecting its normal use. The reasons for valve quality defects in the casting process are studied, and the preventive measures are discussed to improve the casting quality and meet the production process needs of the industry.
1 Causes of shrinkage cavities, shrinkage porosity and porosity defects
We all know that in the cooling process of molten steel in the mold after casting, there are three stages of shrinkage: the first stage is liquid shrinkage. As the temperature of molten steel in the mold decreases, molten steel is produced before nucleation and crystallization shrinkage; the second stage is solidification shrinkage, the shrinkage that occurs after the molten steel nucleates until it is completely solidified; the third stage is solid-state shrinkage, after the molten steel is completely solidified, the body shrinks as the temperature of the casting decreases. After research and analysis, the shrinkage cavity and shrinkage porosity of castings are mainly formed in the process of solidification and shrinkage of castings. In the casting process, the setting of the gating system and the feeding riser is unreasonable, and when the hot joint of the casting cannot be fed in time, shrinkage holes or shrinkage porosity will be formed at the hot joint of the casting. For the valve, because of its complex structure , There are many hot nodes, and at the same time, the hot nodes are not conducive to the setting of the riser, so shrinkage holes and shrinkage porosity are the casting defects that are more likely to occur in the valve.
During the solidification process of castings, the degree of liquid shrinkage is related to the pouring temperature. The higher the pouring temperature, the greater the volume expansion of molten steel, and vice versa. Usually, on the premise of ensuring filling, the pouring temperature should be reduced as much as possible. The solidification shrinkage is mainly affected by the alloy composition. For example, with the rest of the ingredients being the same, the larger the carbon and silicon content, the smaller the shrinkage, and the larger the manganese and sulfur content, the larger the shrinkage. Solid state shrinkage is the shrinkage that occurs with the decrease of temperature after the casting is completely solidified, and has no effect on the occurrence of shrinkage cavities and shrinkage porosity casting defects. After research, the shrinkage cavity and shrinkage porosity of castings are mainly formed in the solidification shrinkage stage.
2 The parts where shrinkage cavities and shrinkage porosity defects are prone to occur
The last cooling part of the casting, usually the hot node of the casting, is the place where shrinkage cavities and shrinkage porosity defects are more likely to occur. For the valve, the hot junctions located at the boss of the valve body casting and the connection between the casting flange and the pipe body, due to the poor heat dissipation conditions of these parts, have defects when they are finally solidified.
3 Preventive measures
In order to completely solve the problem of quality defects such as shrinkage holes and shrinkage porosity during the casting process of the valve, it is necessary to start with the casting process design, select the casting process plan, and optimize the casting process design. Specific solutions are as follows:
3.1 Optimizing the riser design
In the original casting process method: the designed riser adopts the open riser, and the high-efficiency heating and thermal insulation covering agent is added after pouring. Due to the relatively wide root of the riser, there is no feeding channel for molten steel, which affects the normal feeding, resulting in shrinkage holes and shrinkage porosity defects in the poured valve castings. The optimal design of the riser is still to set the riser on the flange of the valve, and the high-efficiency heating and heat preservation riser is preferred. After pouring, the riser burns and emits a lot of heat, which heats the molten steel in the riser for a second time, promotes sequential solidification, and increases the feeding ability of the riser to the casting. The shrinkage holes and shrinkage defects existing on the flange are introduced into the riser to solve the quality defect of the valve.
3.2 Selection of cast chilled iron
In the lower part of the casting mold, the outer cold iron is arranged on the flange plate, the boss and the middle of the valve body to shorten the effective feeding distance of the riser and avoid the shrinkage area in the middle of the valve body, which affects the quality of the valve.
3.3 Design of the welt
In the upper mold of the casting mold, a welt is set on the flange under the thermal insulation riser to strengthen the sequential solidification of the casting, and at the same time to ensure that the molten steel in the riser has a feeding channel to effectively feed the hot joint of the valve.
3.4 Optimization of the inner runner position
The location of the ingate has a great influence on the occurrence of casting defects. For the valve, the inner runner should be opened on both the flange and the boss, and the inner runner should be opened on the lower mold. During the pouring process, the casting should be filled smoothly and avoid the occurrence of sand flushing and slag inclusion defects.
3.5 Optimization of pouring process
The parameters involved in the pouring process are mainly pouring temperature, pouring time, pouring speed, slag removal, and air-entraining operation requirements. Low pouring temperature will result in unsatisfactory pouring and cold insulation defects. If pouring temperature is too high, sand sticking defects will occur. Therefore, during the pouring process, the pouring temperature and pouring speed must be controlled in strict accordance with the process requirements, as well as the requirements for slag removal and air entrainment to avoid other castings. defect.
By taking the above measures, the problem of shrinkage cavity and shrinkage porosity of valve castings has been basically solved, and the quality of valve products has been significantly improved.
To sum up, there are two parts of the steel casting valve body that are prone to shrinkage holes and shrinkage porosity: the connection between the flange and the valve body, and the valve body boss. In view of these defects, corresponding preventive measures have been taken, including: optimization of casting riser design, selection of cold iron, standardization of pouring process, etc. In addition, in order to prevent shrinkage holes, shrinkage porosity and other casting defects, the production process must be strictly implemented in the production process, so as to improve the overall quality of valve casting products.