Inoculant and inoculation treatment of nodular cast iron

The necessary condition for spheroidization is that magnesium as the main element in spheroidizing agent of nodular cast iron is absorbed by carbon in molten iron to make graphite spherical, while the participation of elements in inoculant is the sufficient condition for spheroidization. Production practice has proved that inoculation treatment is an essential process link. It not only generates more crystal nuclei, but also plays the roles of dehydrogenation, desulfurization, deoxidation and eliminating white mouth. Most inoculants are 75fesi, but other trace elements are often added according to the performance requirements of castings. The effect of the compound inoculant composed of alloy elements is better than that of any single element. The selection principle of its compound is mainly based on the technical requirements of castings. Re + 75fesi compound inoculant is selected in this test.

The success or failure of inoculation treatment is directly related to the spheroidization degree, ball diameter grade and mechanical properties of graphite. Inoculation treatment is usually carried out many times. The main methods are as follows: inoculation in the package, and the inoculant is placed together with the spheroidizing agent of nodular cast iron (one-time inoculation); During the pouring process, it is injected into the mold with the flow or inoculated in the mold (instantaneous secondary inoculation). The particle size of inoculant shall change with the inoculation method, but it shall be dried in advance. The total inoculation amount is 0.8-1.5%, and the first inoculation amount should account for 60-70% of the total inoculation amount. In the process of instantaneous secondary inoculation in this test, when the inoculant is poured into the mold with molten iron, it fails to melt sufficiently and accumulates near the cold iron, affecting the casting quality. Therefore, only one inoculation in the ladle is used. In the whole process of spheroidizing and inoculation treatment of nodular cast iron, the following problems should be paid attention to through the test:

(1) Rapid hot metal discharge, i.e. rapid filling of ladle to reduce temperature loss, reduce alloy oxidation and improve magnesium recovery.

(2) Rapid pouring shall be completed in the shortest possible time to avoid spheroidization recession of nodular cast iron. Generally, pouring shall be completed within 8min.

(3) Reduce the number of rewinding to avoid molten iron oxidation and temperature loss.