Large castings have long production cycle and complex processes. Generally, they have to go through many processes. Reasonable setting of various processes in theis the key factor to determine the final successful casting.
1. Molding process
Most large castings are shaped in the pit. The sand box can be placed in the pit, or it can be shaped directly in the pit without the sand box. China’s large steel castings usually have thick large section and high pressure head. In addition, the high temperature and long duration of thermal action between castings / molds during casting solidification are easy to cause defects such as poor dimensional stability and surface sand sticking. Therefore, China’s large steel castings have high requirements for the high-temperature mechanical properties of the mold and the anti sand sticking ability of the molding sand material.
At present, the molding sand used for large steel castings in China’s heavy machinery industry mainly includes sodium silicate sand (CO2 blowing hardening and organic ester self hardening), resin self hardening sand (furan resin self hardening sand and alkali phenolic resin self hardening sand). Although the raw materials of sodium silicate sand are cheap, the surface quality of castings is poor and the regeneration of old sand is difficult, so it is difficult to meet the needs of high-quality, high-precision and high-performance casting production required by large water turbine generator and thermal generator; Some major large-scale casting enterprises in China have gradually completed the technical transformation of using resin sand process. The surface sand of large steel castings in China generally adopts special sand such as chromite ore. The price of these raw sand is much higher than that of silica sand. Therefore, the separation technology of chromite sand and quartz sand in the old sand regeneration system is also a key technology.
2. Pouring process
The control of pouring process will directly affect the success or failure of pouring and the quality of castings. The pouring process of large castings should meet the requirements of large flow, rapid and stable mold filling. In actual production, some manufacturers use multi package and multi nozzle pouring at the same time. The key points of pouring technology include:
① Strictly control the pouring temperature of molten steel and the temperature of supplementary molten steel;
② Reasonably adopt the drainage method to avoid the drainage sand falling into the mold cavity. At the same time, in order to ensure the rapid and smooth opening of the nozzle, strengthen the baking of ladle and the baking and filling of drainage sand mold;
③ The control of refining time and the consistency of multi nozzle opening pace also need to be strictly controlled.
For extra large steel castings, one of the technical keys is how to ensure the solidification sequence and sufficient feeding of castings, which is an important condition for controlling casting defects and obtaining high-quality Chinese large steel castings. Considering sufficient feeding, it is usually necessary to reasonably set the riser and cooperate with the appropriate sand hanging cold iron process. In the design, the cold iron layout, the thickness of sand hanging layer and other processes have a great impact on the control of solidification sequence. In order to improve the feeding effect of riser, in addition to using thermal insulation riser and high-efficiency covering agent as much as possible, it is also required to pour riser in process. For large castings, on the one hand, the supplementary pouring process can improve the feeding efficiency of riser, on the other hand, it can also solve the problem of insufficient smelting capacity to a certain extent.
3. Casting shrinkage
Mastering the shrinkage and deformation law of Chinese large steel castings during solidification and cooling in the mold is also a key technology to ensure the basic outline and dimensional accuracy of castings. The deformation law and amount of deformation are related to many factors, such as alloy material, casting size, core material and casting structure. At present, most Chinese enterprises still rely on experience to predict and control this aspect.
In short, the casting process of China’s large steel castings is complex and involves many key technologies, which requires a long time of technology accumulation and reserve. At the same time, it also needs to introduce the advanced processes and technologies of foreign enterprises for digestion and absorption.