?????Most of the materials used for the hubs and brake drums assembled in the front and rear axle assemblies of automobiles use ductile iron. A large number of studies have shown that in large-section ductile iron, bismuth and antimony are added at a certain ratio with rare earth elements. It can effectively improve the shape of graphite, increase the number of graphite spheres, reduce or eliminate the variation of graphite in the core of large-section ductile iron castings, and improve the performance of ductile iron.
?????1) Carbon equivalent High carbon equivalent can easily lead to graphite floating, but too low can easily lead to defects such as shrinkage and shrinkage. Considered from improving molten iron casting performance. Try to make the carbon equivalent of the molten iron close to the eutectic point. Due to the influence of spheroidizing elements, the eutectic point of ductile iron moves to the right from 4.6% to 4.7%. Some people think that it is more appropriate to control the carbon equivalent between 4.1% and 4.3%. The general control of w (c) is 3.4% to 3.7%. During the trial production of Φ600 × 1500mm large-section ductile iron castings, the speed selection w (c) is 3.5% -3.7%, and good results have also been achieved.
2) Si silicon has the functions of promoting graphitization and increasing the content of ferrite on the one hand, and on the other hand, promoting the spheroidization decline, leading to low temperature brittleness, and promoting the formation of fragmented graphite. Most studies believe that in thick section ductile iron, the amount of silicon should be limited to 1.8%-2.2%.
3) Mn Manganese is an element that promotes the formation of carbides and is prone to segregation. Especially in thick sections, the segregation is very serious. It is concentrated on the grain boundaries and the mechanical properties of thick sections are reduced. Generally, the amount of manganese in the thick section ductile iron should be controlled at 0.1% to 0.4%, the lower limit of ferritic ductile iron and the upper limit of pearlite ductile iron.
4) P Phosphorus is prone to segregation, forming phosphorus eutectic, causing brittleness of castings and reducing toughness, so its content should be as low as possible, preferably below 0.02%.
5) S sulfur is a recognized harmful element and should be as low as possible. It is necessary to take certain pre-desulfurization measures for the molten iron to reduce the sulfur content of the molten iron. However, some studies have shown that in order to ensure good spheroidization, the original molten iron w (s) should not be too low.
6) Mg and RE Magnesium and rare earth are all spheroidizing elements. Magnesium is the main spheroidizing element, and excessive whiteness occurs. On the premise of ensuring the spheroidizing quality, the residual balance should be controlled below 0.08%, generally 0.044% to 0.06%; At the same time, it also has the functions of deoxidation, desulfurization, degassing, neutralizing the spheroidizing interference elements lead, qin, crowing, antimony, secretion, etc., purifying molten iron and promoting graphitization. If the residual amount is too high, it will cause whitening and deteriorate the shape of graphite. It is generally controlled to be less than 0.025%.
The spheroidizing agent suitable for large-section ductile iron has no essential difference from the commonly used spheroidizing agent. Nodule and anti-decay trace elements need to be added to the nodularizer. Heavy rare earth spheroidizing ability is stronger than light rare earth and strong anti-recession ability, but the cost is higher than using light rare earth. According to China's raw material resource conditions and melting conditions, the combination of light and heavy rare earth is a reasonable choice. However, if the material requirements are not particularly high, the reasonable use of ordinary rare earth nodulizer alone can also meet the requirements. In the production experiment of thick section ductile iron parts of high-quality injection molding machine, light rare earth and antimony nodulizer have been used to achieve good results. When spheroidizing, the initiation time, reaction speed and reaction time should be controlled. The spheroidizing method adopts the punching method, which is a simple, flexible and the most economical method, which is also widely used abroad.