yvonne@nydcasting.com 
High strength and shrinkage have always been a contradiction. The production of high-strength castings has a large shrinkage tendency. To solve the shrinkage problem of materials, the general principle is to have a higher carbon-silicon equivalent. The process of high carbon-silicon equivalent plus alloying has a smaller shrinkage tendency than the process of low carbon-silicon equivalent plus alloying. Therefore, new technologies to improve performance should be developed under the premise of selecting high carbon-silicon content.
The specific measures to reduce shrinkage can be considered from the following aspects:
⑴ The process measures to promote graphitization are the best measures to reduce the shrinkage of molten iron.
In electric furnace smelting, the application of carburizing technology is the key technology to solve the shrinkage of molten iron. Since the graphite precipitation during the solidification of molten iron produces graphitization expansion, good graphitization will reduce the shrinkage tendency of molten iron. Therefore, carburizing technology is the best process. Since the addition of carburizer improves the graphitization ability of molten iron, the shrinkage tendency of molten iron is smaller when the process of full scrap steel smelting and carburizer is adopted. This is a very important change of concept. The traditional concept is that adding more scrap steel will increase the shrinkage tendency of molten iron. In this way, we are prone to a misunderstanding. We are unwilling to use more scrap steel, but prefer to use more pig iron. The disadvantage of using more pig iron is that there are many coarse hypereutectic graphite in pig iron. This coarse graphite is hereditary. If it is smelted at low temperature, the coarse graphite is difficult to eliminate. The coarse graphite is inherited from the liquid to the solid state, which weakens the expansion effect that should be produced by the precipitation of graphite during the solidification process. Therefore, the shrinkage tendency of the molten iron during the solidification process is increased, and the coarse graphite will inevitably reduce the performance of the material.
Therefore, compared with the scrap steel carburization process, the disadvantages of using a large amount of pig iron are: ① Low strength performance. The same composition has been compared and the performance is half a grade lower. ② Whether the carburization process can obtain a good graphitization effect and reduce the shrinkage of molten iron depends mainly on the carburizer: ① The carburizer must be a carburizer that has been graphitized at high temperature. Only after high-temperature graphitization treatment can carbon atoms change from the original disordered arrangement to a flake arrangement, and flake graphite can become the best core for graphite nucleation and promote graphitization. ②Good carburizers have very low sulfur content, and w(S) less than 0.03% is an important indicator. For cupola smelting: high-temperature smelting is the most critical technical indicator, and high-temperature smelting can effectively eliminate the hereditary nature of coarse graphite in pig iron. High-temperature smelting can increase the carburizing rate and reduce the amount of pig iron added to the ingredients. The carbon obtained by carburizing has good activity and has a better graphitization effect than the carbon brought by adding more pig iron. This is reflected in the casting, that is, the graphite has a better shape and a more uniform distribution. A good graphite shape will improve the performance of the material, including cutting performance, and a good graphitization effect can reduce the shrinkage tendency of the molten iron.
⑵Increase the silicon content of the original molten iron and control the inoculation amount.
Part of the silicon in gray cast iron is silicon in the original molten iron, and part is silicon brought in by inoculation. Many people like to have a low silicon content in the original iron liquid, and then use a large amount of inoculation to inoculate. This practice is not scientific: a large amount of inoculation is not advisable, which will increase the shrinkage tendency. Inoculation is to increase the number of crystal nuclei and promote graphitization. A small amount of inoculation (0.2% to 0.4%) can achieve this goal. From the perspective of process control, the inoculation amount should be relatively stable and should not change too much. This requires that the silicon content of the original iron liquid should also be relatively stable. Increasing the silicon content of the original iron liquid can not only reduce the tendency of white cast iron and shrinkage, but also play the role of silicon solid solution strengthening the matrix, and the performance will not be reduced. At present, the more scientific approach is to increase the silicon content of the gray cast iron original iron liquid, and the inoculation amount is controlled at about 0.3%. In this way, the solid solution strengthening effect of silicon can be played, which is beneficial to improving strength and reducing casting shrinkage.
⑶ The alloying method has a great influence on the shrinkage of the iron liquid.
Alloying can effectively improve the performance of cast iron. The alloying elements we commonly use are chromium, molybdenum, copper, tin, and nickel. Chromium can effectively improve the performance of gray cast iron, and the performance will continue to improve as the amount added increases. Chromium has a relatively large tendency to white cast iron, which is the most worrying issue for everyone. If the amount added is too large, carbides will appear. As for how to control the upper limit of the chromium amount, different chromium addition processes have different upper limits. If chromium is added to the original molten iron, its upper limit should not exceed 0.35%. Increasing the amount of chromium in the original molten iron will increase the tendency of white cast iron and shrinkage, which is very harmful. Another chromium addition process is not to increase the chromium of the original molten iron, but to add chromium to the molten iron ladle and flush it in by flushing. This process will greatly reduce the tendency of white cast iron and shrinkage. Compared with the previous process, the same amount of chromium will reduce the tendency of white cast iron and shrinkage by more than half. In this chromium addition method, the upper limit of chromium can be controlled to 0.45%. The characteristics of molybdenum are very similar to those of chromium, so no specific description will be given. Since molybdenum is expensive, adding molybdenum will greatly increase the cost. Therefore, as little molybdenum as possible should be added, and more chromium should be added. Adding chromium and molybdenum by flushing is an effective measure to reduce alloying shrinkage.
⑷ The influence of molten iron pouring temperature on shrinkage.
It is common experience that molten iron tends to shrink more when the temperature is high. It is very important to control the pouring temperature within a reasonable range. If the pouring temperature is 20-30℃ higher than the reasonable temperature specified by the process, the shrinkage tendency will increase significantly. In production, we should pay attention to such a phenomenon. Electric furnaces without automatic heat preservation function may increase the temperature of molten iron. The pouring temperature of the first bag of molten iron will be lower, and then the temperature will become higher and higher. If it is not controlled, shrinkage waste may be produced. In production, the first bag of molten iron should be hot, and the hot bag should be used again. Moreover, the pouring temperature of the first bag of molten iron should be controlled at the lower limit, not at the upper limit, to prevent the temperature from rising continuously. Controlling the pouring temperature in electric furnace smelting is a key measure to prevent shrinkage waste from castings.
⑸ The oxidation tendency of molten iron cannot be ignored: large oxidation and large shrinkage.
The large oxidation tendency of molten iron is very harmful and will also increase the shrinkage tendency.