Valves are widely used in chemical, food, medicine, electric power and other industries. According to the operating conditions, the process requirements of the conveying system and the conveying medium, the materials and specifications of the valves are different, and there are many types. With the continuous progress of modern production technology, the quality requirements for valves are getting higher and higher.
Due to the special structure of the valve, various casting defects will occur during the casting process, such as porosity, shrinkage cavity, shrinkage and other quality defects, which will eventually lead to valve leakage. According to statistics, most of the reasons for valve leakage are caused by shrinkage cavity and shrinkage porosity during the casting process. This article analyzes the causes of shrinkage and porosity caused by the valve in the casting process, and discusses its preventive measures to improve the inherent quality of valve casting products.
1 Causes of shrinkage, shrinkage and porosity
We all know that during the cooling process of the molten steel in the mold after casting, there are three stages of shrinkage: the first stage is liquid shrinkage. As the temperature of the molten steel in the mold decreases, the molten steel is produced before nucleation and crystallization Shrinkage; the second stage is solidification shrinkage, which occurs after the molten steel nucleates and crystallizes before it is completely solidified; the third stage is solid state shrinkage, which is the body shrinkage formed as the temperature of the casting decreases after the molten steel is completely solidified. After research and analysis, the shrinkage cavity and porosity defects of the casting are mainly formed during the solidification and shrinkage of the casting. When in the casting process, the gating system and the feeding riser are set unreasonably, and the hot joints of the castings cannot be fed in time, shrinkage holes or shrinkage porosity will be formed at the hot joints of the castings. For the valve, because of its complex structure , There are many hot nodes, and the hot nodes are not conducive to the setting of the riser, so shrinkage cavity and shrinkage porosity are the most likely casting defects of the valve.
In the solidification process of the casting, the degree of liquid shrinkage is related to the pouring temperature. The higher the pouring temperature, the greater the volumetric expansion of molten steel, and vice versa. Under normal circumstances, under the premise of ensuring the filling, the pouring temperature should be reduced as much as possible. The solidification shrinkage is mainly affected by the alloy composition. For example, when the other components are the same, if the content of carbon and silicon is larger, the shrinkage will be smaller, and when the content of manganese and sulfur is relatively large, the shrinkage will be larger. Solid state shrinkage is the shrinkage that occurs with the decrease in temperature after the casting is completely solidified, and has no effect on the appearance of shrinkage cavities and shrinkage porosity casting defects. After research, the shrinkage cavity and shrinkage porosity casting defects of castings are mainly formed in the solidification and shrinkage stage.
2 Locations where shrinkage and porosity are likely to occur
The last cooling part of the casting is usually the hot node of the casting, which is the most prone to shrinkage and shrinkage defects. For valves, the hot junctions located at the junction of the valve body casting boss and the casting flange and the pipe body, due to the poor heat dissipation conditions of these parts, defects appear when they are finally solidified.
3 Preventive measures
In order to completely solve the problem of quality defects such as shrinkage cavity and shrinkage porosity in the casting process of the valve, it is first necessary to start with the casting process design, select the casting process plan, and optimize the casting process design. There are several specific solutions:
3.1 Optimize the riser design
In the original casting process: the designed riser adopts the exposed riser, and the high-efficiency heating and heat preservation covering agent is added after pouring. Because the root of the riser is relatively wide, the molten steel has no feeding channel, which affects the normal feeding, causing the pouring valve castings to still show shrinkage and shrinkage defects. The optimized design of the riser is still to set the riser on the flange of the valve, and the high-efficiency heat-preserving riser is preferred. After pouring, the riser burns and emits a lot of heat, which reheats the molten steel in the riser, promotes sequential solidification, and increases the feeder's ability to feed the castings. The shrinkage holes and shrinkage defects on the flange are introduced into the riser to solve the quality defects of the valve.
3.2 Selection of cast cold iron
In the lower mold, the flange, boss and the middle of the valve body are provided with external cooling iron to shorten the effective feeding distance of the riser and avoid the shrinkage zone in the middle of the valve body, which affects the quality of the valve.
3.3 Welt design
The flange under the heat preservation riser is provided with welts to strengthen the sequential solidification of the castings, while ensuring 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 position of the inner runner
The position of the gate has a great influence on the occurrence of casting defects. For the valve, an 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 castings should be smoothly filled to avoid sand washing 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 cause dissatisfaction and cold barrier defects. Too high pouring temperature will produce sticky sand defects. Therefore, the pouring temperature and pouring speed must be strictly controlled in accordance with the process requirements during the pouring process to avoid other castings defect.
Through these measures, the shrinkage and porosity of valve castings have been basically solved, and the quality of valve products has been significantly improved.
In summary, there are two parts of the steel casting valve body that are more prone to shrinkage and shrinkage defects: the connection between the flange and the valve body, and the valve body boss. In response to these defects, corresponding preventive measures have been taken, including: optimizing the design of the casting riser, the selection of chilled iron, and the standardization of the casting process. In addition, in order to prevent shrinkage cavities, shrinkage porosity and other casting defects, the production process must be strictly implemented during the production process, so as to improve the overall quality of valve castings.