At present, the most commonly used spheroidizing agent for nodular cast iron in China is rare earth silicon iron magnesium alloy. Practice has proved that the use of this kind of spheroidizing agent is difficult to meet the spheroidizing level required by wind power accessories, and the stability is not good. Therefore, in the production of wind power accessories, special high-efficiency spheroidizing agents are generally used, including magnesium content of 5% – 10%, silicon content of 40% – 47%, calcium content of 3% and yttrium content of 3%. On the basis of rare earth Si FE Mg alloy, adding an appropriate amount of calcium can promote the spheroidization of nodular cast iron and inhibit the tendency of white cast iron. The heavy rare earth element yttrium can eliminate the degeneration of graphite balls in slow cooling castings and reduce the distortion of graphite balls in the heart of castings. In order to ensure the spheroidizing effect, a small amount of rare earth and magnesium elements need to be left in the castings after spheroidizing treatment. The content of residual rare earth is 0.02% – 0.03%, and the content of residual magnesium is 0.03% – 0.04%.
In addition to the type of spheroidizing agent, the spheroidizing method of nodular cast iron will also have a direct impact on the spheroidizing effect, which needs to be reasonably selected. There are many ways to add spheroidizing agent to nodular cast iron, but the best way is to add the spheroidizing agent into molten iron smoothly to ensure high absorption rate of spheroidizing elements of nodular cast iron and reduce environmental pollution. The common spheroidizing methods mainly include punching method, pressure magnesium addition method, in mold spheroidizing treatment method, subcontracting method, wire feeding method, etc.
The commonly used spheroidizing method of nodular cast iron in China is the punching method, and its process flow has been quite mature. The advantage of rush in method is that it has low requirements for equipment conditions, does not need to add additional equipment, and the operation process is simple and convenient. The disadvantage is that the absorption rate of spheroidizing agent is low, and a large amount of soot and flash are produced during the reaction of spheroidizing agent of nodular cast iron, resulting in poor production environment. In order to improve the production environment and improve the absorption rate of spheroidizing agent of nodular cast iron, the capping method is improved and developed on the basis of the punching method in China. By installing a cover tundish on the spheroidizing ladle, the flash and white smoke of magnesium in the reaction process are effectively reduced, the absorption rate of spheroidizing elements is improved, the amount of spheroidizing agent of nodular cast iron is saved, and the production environment is improved. Because the equipment cost increased by the capping method is not much, it can significantly improve the spheroidizing effect, and is being recognized and adopted by more and more people. Although other spheroidizing methods of nodular cast iron, such as pressure magnesium addition method, in mold spheroidizing treatment method, subcontracting method and wire feeding method, have higher absorption rate than the punching method, they are not widely used in production because the equipment cost will increase significantly.
To sum up, combined with the domestic casting production level, it is the best spheroidizing treatment process of nodular cast iron to select special high-efficiency spheroidizing agent and add it by punching method or capping method in the production of wind power accessories.