In many occasions, some people often mention the problem of trace elements in cast iron, which is also the concern of the author. Trace elements in cast iron can be divided into: (1) microalloying elements added for specific purposes; (2) trace elements with negative effects that need to be prevented. The content of the former (mass fraction, the same below) is in the order of ten thousandth of an order of magnitude; the content of the latter needs to be strictly limited, ranging from ten thousandth of an order of magnitude to one millionth of an order of magnitude.
In the middle of last century, trace elements, which are classified as impurity elements or interference elements, have been studied more abroad. Since 1980s, there have been some researches and reports in China. At present, although there is a consensus on the negative effects of trace elements, there are still different understandings on their mechanism of action, and there are still differences on their limits.
The limit of trace elements is based on casting characteristics and casting methods, such as ductile iron or gray iron, thin-walled or thick walled parts, sand casting or other casting. In addition, we should consider whether the microelements coexist with each other.
The graphitization of Sn and Sb independent cast iron is pearlite forming element. High strength gray iron is based on fine pearlite. Sn and Sb are effective microalloying elements. However, Sn and Sb are taboo for ductile iron with ferritic or ferritic matrix. They not only increase the amount of residual pearlite, but also interfere with spheroidization and produce intergranular brittle phase.
Cr, Mo and V are the alloy elements of alloyed grey cast iron and Pearlitic ductile iron, but they need to be limited in Austempered Ductile Iron and low temperature ductile iron. In the appendix of the standard for austempered ductile iron castings (GB / T 24733-2009), the control target values of Cr, V and Mo are: w (CR) < 0.1%, w (V) < 0.1%, w (MO) < 0.3% (only when necessary). In the wind power nodular cast iron, the producer recommends: w (CR) < 0.06%, w (V) < 0.06%, w (MO) < 0.05%. It can be seen that the Cr, V and Mo limits of the above two kinds of ductile iron castings are quite different. In view of the trend of further reduction of Cr, V and Mo in recent years, Cr, V and Mo are not listed in Table 1.
Ti is a trace element with high attention [3]. There are different limits for different purposes. In order to facilitate machining, the amount of W (TI) should be limited to 0.04% (and 0.05%). Ti can interfere with spheroidization directly or indirectly. There are different data about its limit. At one time, TI’s anti spheroidization has been amplified. The limit of Ti in remg nodular cast iron is looser than that in mg nodular cast iron, and within a reasonable range of residual re, the limit of Ti is positively related to the residual re. In general, 0.08% Ti does not cause graphite distortion. Due to the fast cooling speed, the limit of Ti can be extended to 0.1%, even 0.12%. The graphite of heavy wall nodular iron castings is easy to be distorted, and the center of the castings is rich in Ti, P, Mn, Cr, Mo, V, etc. the control of Ti tends to be strict. In order to prevent the early failure of large megawatt wind power casting, the w (TI) content is often lower than 0.035% [the manufacturer of wind power parts recommended in previous years w (TI) < 0.06%]. In the austempered ductile iron, the target value of W (TI) content is less than 0.04%.
The solid solubility of Al in Fe is very large, and it is also a negative segregation element. In low and medium alloy Al Cast Iron, Al promotes eutectic graphitization and eutectoid graphitization, which is the element of ferrite formation. Al is an active element. Adding Al in front of the furnace can remove O and fix n. a small amount of Al in the inoculant is beneficial to strengthen the inoculation. Only the dissolved Al in the molten iron is thought to react with the water in the mold to form H-INDUCED pinholes. Therefore, the author does not list it in Table 1. The author has noticed that there is a word “control target value w (AL) < 0.05%” in the appendix of the standard for austempered ductile iron castings, which is not clear.
The coexistence of microelements, such as sb re, Bi re, te re, Pb re, Ti-N, has a favorable weighting effect, while the coexistence of Bi te pbas, Ti cr v Mo, or more o, N and s, aggravates the adverse effect.
The harmful trace elements in cast iron mainly come from metal charge. Some harmful elements will also be produced in the smelting process and the additives, spheroidizers and inoculants used inside and outside the furnace.
Scrap is a kind of pure charge, with less non-metallic inclusions, low O and s, less P and fine structure. However, there are many kinds of scrap, which may also contain alloy elements that cast iron does not want. It is necessary to analyze the chemical composition and decide whether it can be used or simply choose carbon scrap.
There is no regulation for trace elements in pig iron for casting and w (TI) content in pig iron for nodular cast iron. These two kinds of pig irons can not meet the requirements of continuous improvement of quality and properties of iron castings in many aspects, such as brand, basic element content, trace element control, product consistency and stability. High purity pig iron for casting is a kind of pig iron with low content of Mn, P, s and Ti and low content of specific trace elements. In the standard of high purity pig iron for casting (JB / T 11994-2014), the content of 11 trace elements is specified in Table 2, and the total of 11 elements is divided into two levels: Level 1 ≤ 0.05%, level 2 > 0.05% ~ 0.07%. The standard specifies two grades for Ti in chemical composition: Grade 1 ≤ 0.01%, grade 2 ≤ 0.03%. Compared with table 1, the content of trace elements in Table 2 is significantly lower than the limit of trace impurities in cast iron. This ensures the conditions of heavy load and dynamic load, as well as the strict requirements of large section and large-scale nodular iron castings for furnace burden. In order to ensure sufficient impact value of low temperature ductile iron castings, the contents of P and Mn in high-purity cast iron are strictly regulated: w (P) < 0.02% (0.03%), w (MN) < 0.05% (0.15%). In order to stabilize the spheroidizing effect and reduce the amount of spheroidizing agent, w (s) < 0.015% (and 0.020%) classification is required. The amount of W (SI) in pig iron is also lower than that of pig iron for nodular cast iron.
China’s high-purity pig iron is calibrated according to the needs of relevant iron castings, so it is named “high-purity pig iron for casting”. At present, many enterprises have stable mass production and reasonable price. They have achieved good results in heavy truck, wind power, nuclear power, marine engineering, high-speed train, military industry and supercharger. China Foundry Association and National Foundry Standardization Technical Committee attach great importance to high purity pig iron for foundry. “If the material is strong, the casting will be strong”. High purity pig iron for casting is an indispensable part of the Chinese dream. It is believed that with the new improvement of the equipment level and the new breakthrough of the process technology of the related foundry blast furnace enterprises, the service of casting high-purity pig iron will be better and better!
The effect of trace impurity elements on the matrix structure and graphite of cast iron, as well as the formation of Special equal ways, can significantly weaken the properties of cast iron, which is a big taboo for high-end castings. The factories producing high-end iron castings should have the means to detect trace elements and strictly control the content of trace impurities. The high-quality furnace burden should be used from the source to prevent the disease from entering from the mouth. It is a wise choice to use high-purity cast iron.