Entering the new century, science and technology are constantly innovating and developing, key projects are constantly being carried out, and local investment and construction are constantly expanding, which has led to an increasing demand for machine tool products. It is necessary to continuously upgrade and transform products to make them enter a golden period of vigorous development. It is the only means to solve the problems of large components such as impellers, blades, and propellers of traditional large ships. At the same time, it will have a profound impact on the development of high-density instruments such as aerospace and aviation.
01
Characteristics and applications of high-quality ductile iron for machine tools
It mainly refers to cast iron in which the carbon in the liquid iron is mainly precipitated as spherical graphite during the solidification process. Compared with gray cast iron, it changes its structure to the maximum extent, avoids the excessive appearance of other impurities, greatly reduces the effect of graphite on metal cutting, and basically eliminates the phenomenon of excessive concentration of graphite stress. It will greatly increase the strength of the machine body, so that the performance of all parties can be maximized. It can show the same characteristics as steel in future use. In addition, it can be heat treated or other measures can be taken to greatly improve its toughness or change other characteristics.
02
What defects will appear in the casting process of ductile iron
1. Poor spheroidization or certain degradation
The fracture is silver-gray, too many black spots appear on the surface, and the metallographic structure is also distributed in a large number of thick graphite flakes. The main reason for this phenomenon is that the sulfur content in the molten iron is too high. The sulfur in the molten iron will react chemically with the oxygen in the air. During the reaction, excessive anti-spheroidizing elements will appear. Therefore, it is recommended to use low-sulfur carbon or treat the sulfur in the carbon to a certain extent. If necessary, even a certain amount of sulfide rare earth agent can be added. Secondly, the blasting device should be reasonably installed and the blasting force can be controlled.
2. Certain shrinkage and loose phenomena
Shrinkage often appears in the first shrinkage stage, causing a certain depression on the surface. At the same time, there will be certain dark shrinkage holes in the depressed part, making the surface rougher. Looseness will occur in the second shrinkage stage, and more loose holes will be produced. The main reason for this is that the carbon content is too low and the phosphorus content is too high, which will increase the shrinkage of the holes. Therefore, it is necessary to increase the rigidity of the casting and increase the proportion of resin added.
3. There is a certain amount of graphite floating phenomenon. Graphite balls will first appear during the cooling process of molten iron crystallization. They will float up and gather, and are mainly distributed in the last upper part of the casting, showing a graphite blooming state. The reason is that the original size of the charge is too large, which makes the carbon content and rare earth content too high, which are the main reasons for graphite floating. Therefore, it is necessary to control the rare earth content and pay attention to the proportion of primary iron and other charges.
4. The phenomenon of anti-white mouth appears. It will be clearly visible that there are certain white blocks on the cross section, and they will show a white and bright policy-like shape, which is mainly produced during the heat treatment process. The main reason is that a large amount of magnesium, manganese and other elements that affect white mouth appear during the heat treatment solidification process. At the same time, the reaction speed is too fast during the heat treatment solidification process. This effect will also occur. Therefore, in order to better avoid the occurrence of this result, it is necessary to minimize the appearance of elements related to white mouth, increase the intensity of the reaction in the furnace, and increase the temperature of small castings.
5. The surface has a certain amount of slag inclusions
At the cross-section or corner of the casting, a large amount of black and dull impurities are black substances, which are slag inclusions. The process of forming slag inclusions is mainly divided into two stages. The first stage is that the sulfur content in the original iron liquid is relatively high, and the sulfite produced by chemical reaction with oxygen and water vapor in the air is a kind of slag inclusion. In the second stage, the magnesium element in the residue is too high. After the sulfur element reacts, magnesium reacts with oxygen in the air to produce a layer of oxide film covering the surface, which will produce the second type of slag inclusions. Therefore, the main measures to be taken are to reduce the sulfur content in the iron liquid, and at the same time, avoid excessive air contact, and try to isolate it from the air. At the same time, a certain amount of rare earth should be added to reduce the temperature after the mold is formed. The casting system should adopt a more substantial and stable structure and be equipped with a slag inclusion outlet.
6. Effectively control the impact of ductile iron
Under low temperature and pressure, ductile iron will produce brittle fractures due to its own cracks. This fracture will be irreparable, mainly because the energy absorbed during the manufacturing process is very low, resulting in a significant reduction in toughness. Therefore, it is necessary to choose a higher grade material that can withstand low temperature impact. This can better demonstrate its performance and bring out its maximum potential.
03
Reasonable quality control during the casting process
1. Make a scientific and reasonable mold
The mold should be made of multi-layer wood board, which can greatly improve its own strength, and it is not easily affected by the external environment during the lifting and transportation process, ensuring its safety and stability. In order to make the mold surface more glossy and effectively anti-corrosion, first brush a layer of anti-rust paint on the surface, and then apply the relevant gloss brightness paint, so as to achieve better results.
2. Reasonable smelting casting and timely cleaning
The smelting electric furnace should use a more advanced electric melting furnace with a larger tonnage, which can better meet the requirements of different tonnages, and at the same time can better control the temperature of the molten iron in the furnace, and can first preheat the auxiliary catalysts such as spheroidizers, which can greatly reduce the loss of molten iron temperature caused by spheroidization. Secondly, after the specific implementation of the molten iron incubation, the waste slag in the furnace must be cleaned in time to ensure the effective utilization rate of the furnace next time.
04
Conclusion
At present, my country has gradually become a foundry power, and various foundry technologies have been widely used and developed. However, there is still a clear gap between my country's technology and that of developed countries. We need to continue to learn and enhance our own technical deficiencies, and truly make the best use of our strengths and avoid our weaknesses, so that we can also move to the forefront of world-leading technology, create greater economic benefits for enterprises, better realize social value, make effective contributions to the development of the foundry industry, and create more benefits for society.
