After spheroidizing treatment, spheroidizing elements will make nodular cast iron have a greater tendency to white. At this time, inoculation treatment has become an important link in the production of nodular cast iron. Inoculation treatment can not only eliminate white cast iron and reduce the sensitivity of casting wall thickness to structure, but also increase the number of graphite balls, improve the roundness of graphite balls and reduce intergranular segregation, which plays an obvious role in improving the properties of nodular cast iron.
The inoculant is mainly ferrosilicon with silicon content of 70% – 75%. Due to the high silicon content, the final silicon content of castings is often adjusted and controlled by the amount of inoculant. It can be seen from the analysis that the influence of silicon on the low-temperature impact toughness of nodular cast iron has two sides. Therefore, when producing wind power accessories, the amount of inoculant should be based on the principle of suitability. On the basis of ensuring the inoculation effect, it should not be added too much, otherwise it will only have an adverse impact on the low-temperature impact toughness.
Similar to spheroidizing agent of nodular cast iron, there are also compound inoculants specially used in the production of wind power accessories in China. In addition to silicon, the chemical composition generally contains an appropriate amount of calcium, bismuth, barium and other elements, in which the content of calcium is 0.5% – 1.1%, the content of barium is 3% – 5%, and the content of bismuth is 0.002% – 0.01%. The addition of these elements can improve the nucleation process and inoculation effect. Reduce inoculation decay and improve the microstructure and mechanical properties of nodular cast iron.
The inoculant can be added by primary inoculation and secondary inoculation. The inoculant can be wrapped in the nodular cast iron once, and the remaining inoculant can also be put into the nodular cast iron bath together with the nodular cast iron. Among them, what is added at the bottom of the ladle is also called ladle bottom inoculation, and what is added in the tapping tank is also called flow inoculation. Secondary inoculation, also known as instantaneous inoculation, is added in a variety of ways, mainly including external inoculation, sprue cup inoculation, ferrosilicon rod ladle inoculation, floating silicon inoculation, wire insertion inoculation and in mold inoculation.
In addition to the type and adding method of nodular cast iron inoculant, the inoculation treatment effect also has a certain relationship with the adding time. The closer the adding time is to the pouring, the better the inoculation effect will be. Therefore, instantaneous inoculation can strengthen the inoculation effect, improve the metallographic structure and improve the mechanical properties.
In the production of nodular iron castings for wind power, in order to obtain ideal inoculation effect, special compound inoculant is selected and multiple inoculation method is adopted to promote the formation and refinement of ferrite in the tissue, which is conducive to the improvement of impact toughness of nodular iron. It is the best pregnancy treatment process in production at present.