The alloying treatment of cast iron can be traced back to the copper bushing factory 1930s and 1940s. The alloying treatment has made a qualitative leap in the performance of cast iron. At the same time, some special-purpose cast irons such as wear resistance, corrosion resistance and heat resistance have been born. The use of inoculation to produce cast iron was also produced during this period. In the late 1940s, cast iron with spherical graphite replaced the usual flake graphite cast iron. We call this type of cast iron nodular graphite cast iron. Classification of spheroidizing elements and anti-spheroidizing elements spheroidizing elements are generally divided into three groups according to their spheroidizing effect. The first group: Mg, Y, Ce, La, Pr, Sm, Dy, Ho, Er. The second group: Ba, Li, Cs, Rb, Sr, Th, K, Na. The third group: Al, Zn, Cd, Sn. The first group has the strongest spheroidizing ability, the second group is second, and the third group is the weakest. When magnesium is used as the spheroidizing element, the third group of elements tends to produce anti-spheroidizing effect. Anti-spheroidizing elements: sulfur and oxygen are common anti-spheroidizing elements in cast iron. In addition, Ti, Al, B, As, Pb, Sn, Sb, Bi, Te, Se, etc. are common anti-spheroidizing elements in molten iron. . The attached table is classified according to its mechanism of action. How to choose spheroidizing agent Spheroidizing agent and inoculant are the most important materials in the spheroidizing process. In addition to the stable quality, choosing the proper spheroidizing agent also needs to consider the following factors. Spheroidization temperature: If the spheroidization temperature is> 1480 ℃, the spheroidization reaction will be more violent, which will result in lower magnesium absorption rate. In order to stabilize the spheroidization reaction, a spheroidizer with a relatively high calcium content can be selected. If the spheroidization temperature is <1480 ° C, then a spheroidizing agent with a relatively low calcium content can be used. Treatment package size: If the height-to-diameter ratio of the treatment package is 1: 1, the magnesium absorption rate will be reduced due to the loss of magnesium vapor. It is recommended to use a spheroidizing agent with a higher calcium content. If the height-to-diameter ratio of the treatment package is 2: 1, the spheroidization reaction will be relatively stable, the magnesium vapor will diffuse into the molten iron, and the magnesium absorption rate will be improved. Spheroidization process: If the capping method is not used, the smoke generated by the spheroidization reaction will enter the atmosphere and produce dazzling white light. In order to make the spheroidization reaction stable, a spheroidizing agent with low magnesium and high calcium can be used. If the capping method is used, the molten iron will not splash, and the fumes generated are less. A high-magnesium and low-calcium spheroidizer can be used to reduce the amount of addition and reduce the cost of spheroidization. Treatment weight: If the weight of molten iron is less than 500kg, you can use a spheroidizing agent with a smaller particle size. It is recommended to use a spheroidizing agent with a particle size of 12mm or less. If the weight of molten iron is 500 ~ 1000kg, you can use a spheroidizing agent with a larger particle size, such as a spheroidizing agent with a particle size of 3 ~ 25mm. If the weight of the molten iron is greater than 1000kg, you can use 4 ~ 32mm nodulizer. Silicon content: If the process yield of the cast product is low or the scrap rate is high, I want to smelt by adding more charge and scrap steel, and the final casting has strict requirements for the silicon content of the molten iron. Under the premise that the inoculation amount can not be further reduced, low-silicon nodulizing agent can be used for treatment, which can increase the regrind charge by 8% to 15%, which can reduce the production cost of the foundry. Sulfur content of raw iron liquid: If the raw iron liquid has a high sulfur content, if no desulfurization treatment is required, nodulizers with high magnesium and high rare earth are required, and the addition amount will be higher, if the raw iron liquid has a lower sulfur content, then Spheroidizing agent with low magnesium and low rare earth can be used, and the addition amount will be lower, and the cost of spheroidizing agent with low magnesium and low rare earth will also be cheaper. Different spheroidization methods The currently commonly used spheroidization methods are as follows: in-bag treatment (including straight-through method, sandwich method and lid bag method), in-mold spheroidization method, flow method, pure magnesium treatment process (including transfer Package method and core wire method). The advantages and disadvantages of these spheroidization methods are briefly introduced below. In-bag treatment method: This is the most common spheroidization process. It has a wide range of applications, ranging from a few kilograms of automobile parts to tens of tons of wind power parts. The magnesium absorption rate by the cover method is the highest, followed by the sandwich method. The disadvantage is that the current degree of automation is not high, and there are already some domestic equipment factories that are developing automatic feeding systems. Inner spheroidization method: There are not many foundries using this process at present, because the shortcomings of this process are more obvious, and the slag generated by the spheroidization process sometimes enters the cavity, causing defects caused by slag inclusions and producing waste products. In addition, this spheroidization process has high requirements on the temperature of the molten iron and the flow rate of the molten iron, otherwise the spheroidization will be uneven. Flow method: As the name implies, the flow method is that molten iron flows through a spheroidizing chamber filled with a spheroidizing agent for spheroidization. At present, this process is not used too much. The advantage is that the degree of automation is relatively high; the disadvantage is that the requirements for molten iron temperature and molten iron flow rate are stricter. Pure magnesium spheroidization process: Sometimes called high magnesium spheroidization process, there are currently two main forms, subcontracting method and core wire method. The advantage of this method is that it has a high degree of automation and is also environmentally friendly; the disadvantage is that the magnesium absorption rate is low, and more smoke and slag are generated. The drawing compares various spheroidizing processes from the aspects of smoke, slag and magnesium absorption. Precautions for the production of ductile iron Now briefly summarize the precautions for the production of ductile iron. (1) The sulfur content and other trace elements of the original molten iron should not be too high.
