Trace elements are interfering spheroidizing elements, but due to the neutralization effect of rare earth elements added in production, adding an appropriate amount of trace elements under the condition of containing rare earth will still achieve good results. The common trace elements are bismuth, antimony, titanium, tin and so on.
1.Bismuth
Bismuth can reduce the size of graphite balls in nodular cast iron, increase the number of graphite balls, promote the formation of ferrite structure, and improve the low-temperature impact toughness of nodular cast iron. It is found that adding trace bismuth into molten iron can significantly reduce the surface tension of molten iron and the interfacial tension between graphite and molten iron, but there is no tin compound in graphite and matrix, and there is no bismuth in graphite core. After adding bismuth, iBismuth can reduce the size of graphite balls in nodular cast iron, increase the number of graphite balls, promote the formation of ferrite structure, and improve the low-temperature impact toughness of nodular cast iron. It is found that adding trace bismuth into molten iron can significantly reduce the surface tension of molten iron and the interfacial tension between graphite and molten iron, but there is no tin compound in graphite and matrix, and there is no bismuth in graphite core. After adding bismuth, it will be adsorbed at the interface between graphite and molten iron, block the dislocation steps of graphite balls and hinder the diffusion of carbon atoms, so as to refine graphite balls. In the sample with a diameter of 300mm, the total number of graphite balls obtained by adding a small amount of bismuth is 6 times that without bismuth, and the effect is quite obvious. Adding 0.01% bismuth to yttrium based heavy rare earth treated molten iron can improve the morphology of graphite ball and improve the tensile strength of nodular cast iron, and adding a small amount of bismuth to the inoculant can also significantly improve the inoculation effect. Excessive bismuth will prevent the formation side of the graphite ball shell, distort the graphite ball in contact with the molten iron for a long time, deteriorate the morphology of the graphite ball, and have an adverse impact on the mechanical properties of nodular cast iron. Therefore, the amount of bismuth should be controlled below 0.01%.
2.Antimony
Adding a small amount of antimony to nodular cast iron will not only not cause the distortion of graphite ball, but also inhibit the formation of fragmented graphite and make the graphite ball fine and round. Studies have shown that when the hot metal temperature is high, antimony will appear abnormal graphite and deteriorate the mechanical properties. Therefore, antimony is suitable to be added at a lower temperature of 1350-1370 ℃. Adding 0.002% – 0.01% antimony into large section nodular cast iron can improve the roundness and quantity of graphite balls.
3.Titanium
Titanium has strong reduction ability, which can reduce other trace elements in molten iron, destroy the spheroidization of graphite and increase the sensitivity of section. When the content is more than 0.1%, the graphite ball is distorted, resulting in the decrease of elongation, impact toughness and tensile strength.
4.Tin
Tin has an obvious effect on the mechanical properties of nodular cast iron. When the addition amount is 0.06% – 0.1%, the content of pearlite can be significantly increased and the tensile strength of nodular cast iron can be improved. When the addition amount exceeds 0.08%, tin will segregate at the grain boundary to produce brittle phase, improve the low-temperature brittle transition temperature and reduce the low-temperature impact toughness. When the addition amount exceeds, the embrittlement phenomenon will be very obvious.
Through the analysis and research of the above chemical elements, it can be seen that the low-temperature impact toughness of nodular cast iron is quite sensitive to the content of chemical elements. Even a small amount of difference may have a significant impact on the low-temperature impact toughness of nodular cast iron. Therefore, when producing nodular cast iron for wind power accessories, it is particularly important to control the chemical composition of molten iron. When selecting raw materials, it is very necessary to have accurate and stable chemical composition and less impurity content. Only in this way can we provide good basic conditions for the subsequent production of wind power accessories and ensure that the mechanical properties of accessories meet the standard requirements.