1. Burden selection
The scrap steel + recarburizer process is preferred.
(1) Choose carbon steel as much as possible and use less alloy steel. If you want to use the alloy in it, you must separate it separately and control its ratio;
(2) The pig iron contains coarse stone flakes, which cannot be completely melted in the electric furnace and is brought into the casting, and it is easy to form coarse block graphite, thus reducing the strength of the casting.
(3)Pig iron contains more elements such as phosphorus, titanium and lead, which also affect the performance of brake discs.
(4)The recarburizer can provide carbon, especially the fully graphitized recarburizer, which not only has good absorption rate, but also can be used as the core of graphite nucleation, which is the preferred recarburizer. However, this kind of recarburizer is expensive, and its nitrogen content is very low, it needs to add additional elements to improve the strength.
For brake disc castings, due to the large amount of iron filings after processing, this part can also be applied after treatment, but the addition amount does not exceed 25%. Under normal circumstances, the ratio of the charge, scrap: recharge: iron filings = 45% ~ 55%: 25% ~ 30%:
In the 15% ~ 25% charge ratio, use as little or no iron as possible to reduce the genetic influence of pig iron. If pig iron is to be used, it must be verified, and the source of pig iron ore can be replaced at will. The recycled materials should be stored separately, preferably shot blasting. It is recommended to use the recycled materials in proportion to the same grade.
2. Melting process
The molten iron has an equilibrium temperature. When the molten iron is higher than the equilibrium temperature, the oxygen content in the molten iron is significantly reduced, the element burning loss is significantly reduced, and the purity of the molten iron is improved. The equilibrium temperature of molten iron is related to its chemical composition. The theoretical equilibrium temperature at 3.2% Ci and 1.6% Si is 1 415. C, the actual overheating temperature should be above the equilibrium temperature of 50 °C. However, in production practice, the actual overheating temperature often reaches 1 500 to 1 550 °C. At this temperature, both the graphite and the matrix are refined, the strength is improved, the hardness is reduced, and the quality factor is improved. Standing at a high temperature for 5 to 10 minutes is very helpful to eliminate the heredity of the charge and improve the purity of the molten iron. When using a recarburizer with a poor degree of graphitization to increase recarburization, it is necessary to pay attention to sufficient melting time after recharging to ensure sufficient absorption.
Accurately detect the content of carbon, silicon and various combined elements. It is recommended to use a thermal analyzer or a carbon-sulfur analyzer for the detection of carbon content, and spectroscopy for the detection of alloying elements. The molten iron with all qualified components and temperatures should be discharged as soon as possible. The temperature should be kept for more than 30 minutes, and carbon should be supplemented and refilled to melt.
3. Inoculation treatment
Commonly used inoculants are, 75 ferrosilicon is the most commonly used inoculant, of which calcium and aluminum play a greater role, and the content of the two should be tested when the incoming material; the silicon strontium inoculant has a strong ability to eliminate white mouth, but does not increase the total Crystal clusters are more suitable for products that are prone to shrinkage; silicon-barium inoculants have a long duration and strong ability to eliminate white mouths, but the number of eutectic clusters increases more, which is more suitable for thick and large pieces; rare earth inoculants have the effect of deoxidation and degassing , It can also offset the adverse effects of lead, bismuth and other elements.
The amount of inoculant is limited to no white mouth. Too much will cause the strength to decrease and the tendency to shrink and shrink. Flow inoculation is the best method of inoculation, but the amount should not be too much, and the particle size should be fine; there are lumps in the mold, and the lumpy inoculant cannot guarantee that the first molten iron can be melted. It usually starts to melt after 3 seconds. It plays a role of inoculation, so it is not suitable for small pieces with a short pouring time. Even large pieces can only be used as an aid, not only in-mold inoculation. No inoculant has the best effect. Each inoculant has 1-2 outstanding effects. The combination of different inoculants can often achieve the best results. You can choose the combination method according to your own needs. and amount.
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