SAND CASTING EQUIPMENT.

The amount of gray iron scrap added should be determined by the production workshop''s control of the purity of the scrap and the cost. Xinyuanzhu Group is affected by the high cost of pig iron. In the casting, all scrap steel plus a part of recycled material are used. The amount of scrap added is controlled at about 40%.
1. Casting production from scrap steel into gray iron, carbon increase is the most important process in the process. Therefore, it is particularly important to choose a recarburizer that matches the type of scrap steel. The quality of recarburizers in the market is now uneven. A good recarburizer can stabilize recarburization and promote absorption. But the proportion of carbon increase depends on the composition of the scrap.
Matters needing attention for recarburizer in gray iron casting
1.1 The composition of the recarburizer should be distinguished by the amount of nitrogen content. Taking the induction furnace to add more than 50-60% of scrap steel to smelt synthetic cast iron, the greater the amount of scrap added, the greater the nitrogen content of the molten iron. Since the synthetic cast iron molten iron contains low harmful elements such as titanium, lead, and antimony, low-nitrogen recarburizers should be used. If the recarburizer has a high nitrogen content, it is easy to cause nitrogen holes in the casting. Generally speaking, low-end recarburizers have relatively high nitrogen content.
Since the content of titanium in the molten iron is very low, it is impossible to consume a large amount of nitrogen, so that the castings are prone to nitrogen holes, crack-like nitrogen holes or related defects due to the high nitrogen content. Calcined petroleum coke with low nitrogen content. Many foundry friends believe that the high sulfur content of the recarburizer will not affect the quality of ordinary gray iron (inoculated cast iron), but the high sulfur content of the recarburizer will be accompanied by high nitrogen, which will cause quality problems.
1.2 Internationally, there are only three types of recarburizers: high nitrogen, medium nitrogen and low nitrogen. The nitrogen content of recarburizers is a very important indicator! Ordinary coal, uncalcined petroleum coke has a high nitrogen content, generally exceeding 500-4000PPM (0.05-0.40%). The nitrogen content of petroleum coke calcined at high temperature is generally less than 300PPM, but when the supplier produces the recarburizer, the control measures such as the calcination temperature are different, and the nitrogen content is also different, especially the nitrogen content detection, which is not available in many factories, resulting in factory acceptance More blindly, just look at the degree of graphitization of the carbon, and use the recarburizer particles to write on white paper, feel comfortable, and judge whether the strokes are clear or not. The best petroleum coke recarburizer nitrogen content is less than 100PPM.
1.3 Of course, various recarburizers are gradually adapting in China, especially high-nitrogen recarburizers, which involve the cost of castings, and are also used in large quantities, such as poorly graphitized petroleum coke, clean coal, etc. What are these low-end products for? Cast iron smelting use? Is it used for high titanium cast iron? Or is the molten iron gas discharged during the smelting and heat preservation? It also needs to gradually gain experience in the future. For a recarburizer with poor graphitization, adding molten iron will have a low absorption rate (only up to 40-60%), a slow absorption rate, and a large amount of slag in the furnace. It is easy to cause the nitrogen content of the molten iron to exceed the standard, resulting in nitrogen holes. Sometimes a large number of stomatal defects do not appear. The reason is estimated to be related to elements such as titanium and pickaxe in the molten iron. These elements have a strong affinity for nitrogen. I hope everyone will pay attention and gradually summarize experience.
1.4 How to add recarburizer, when making synthetic cast iron, the amount of recarburizer is very large, and it can be added in the early or mid-stage of electric furnace smelting and feeding, and added at the same time as scrap, and at the same time adding silicon carbide. It involves the addition amount in the later stage of smelting, which acts as a pretreatment and increases the core role of graphite. At this time, a recarburizer with low nitrogen content must be added. It is best not to add too much, not more than 0.2%, to avoid coarse graphite. Finally, add recarburizer. The ideal molten iron temperature is about 1500 degrees. Add it to the clean liquid surface of the slag. After a few minutes, the temperature is right and the furnace can be released. Do not pay attention to the residual recarburizer that is not absorbed by the liquid surface because it is being released. The impact of molten iron can also play a role in incubation.
1.5 Recarburizers with high nitrogen content are easy to produce nitrogen pore defects when smelting gray iron. For ductile iron, nitrogen pore defects also appear, and the probability is lower than that of gray iron. It is estimated that the nodulizer contains rare earths. Waiting for the degassing element to work.

2. Regarding the nitrogen content in gray cast iron
2.1 After the gray iron smelting is switched from the cupola to the electric furnace, the non-synthetic cast iron ingredients, the same raw material, the same carbon equivalent, the strength of the electric furnace hot metal is always inferior to the cupola furnace, the reason, from the composition point of view, it can be found that the content of trace elements is different , Especially the titanium content is different. The titanium content of cupola hot metal is sometimes lower, generally less than 0.025%, while the titanium content of electric furnace hot metal is generally above 0.04-0.05%. Nitrogen can strengthen the matrix and is one of the factors that affect the strength of gray iron. However, the metallurgical quality is different and there is no reliable explanation.
2.2 In gray iron, nitrogen should be treated as an alloying element, so that everyone gradually realizes that the alloying elements that affect nitrogen, titanium, and even zirconium, must be controlled. In cast iron, as the nitrogen content increases, the strength of the cast iron increases, until the content exceeds 150PPM or more, and the porosity appears, the strength of the cast iron increases a lot. The composition of cast iron is: W (C) 3.12%, W (Si) 1.35%, W (Mn) 0.71%, W (S) 0.09%, W (P) 0.13%. As the nitrogen content increases, the strength of cast iron is also Gradually increase.
2.3 A large number of synthetic cast iron ingredients are used in the electric furnace smelting gray iron, and the recarburizer ingredients are added to increase the carbon content of the molten iron, which greatly increases the nitrogen content of the molten iron, especially the bad recarburizers, which have extremely high nitrogen content and scrap steel. Adding a lot of hot metal will accumulate the nitrogen content of the molten iron (plus inoculation and resin sand core), which will cause the problem of nitrogen holes in the castings, which also affects the quality of the castings. The addition of titanium or zirconium alloys successfully overcomes the nitrogen pores of gray iron castings. However, from the perspective of nitrogen increasing the strength of castings, coupled with the difficulty of accurately analyzing the nitrogen content of molten iron by a large number of foundry enterprises, the final control of the nitrogen content of molten iron is insufficient. There are also problems.
2.4 Too high nitrogen content causes porosity in castings. Nitrogen further increased, and cracked nitrogen holes appeared. Of course, in actual casting, it is inevitable that other gases will dissolve into the molten iron during the smelting process. Once hydrogen is dissolved, the maximum nitrogen content that produces pore defects will be reduced. Generally, the nitrogen content should not exceed 120PPM. The higher the nitrogen content, the higher the strength of gray iron, until finally the strength suddenly decreases due to the appearance of pores. Although the hydrogen content acts at the same time and produces pore defects, its maximum allowable content in the molten iron is one order of magnitude lower than the nitrogen content. The main cause of pore defects in gray iron is nitrogen.
2.5 Nitrogen content has an impact on the mechanical properties of gray iron, but many practical situations are difficult to explain.
The analysis result of the 2.51 oxygen and nitrogen analyzer is the total nitrogen content, while the influence on the mechanical properties is the dissolved nitrogen, and the combined nitrogen has little effect.
2.52 Most factories do not have the means to test the nitrogen content. Sometimes the spectrometer in the cast steel factory has a nitrogen channel, which can analyze nitrogen, but the accuracy is problematic.
2.6 The mechanism of the influence of silicon, titanium, zirconium and other trace elements combined with nitridation on the properties of gray iron is not clear. Data accumulation and experimental research are insufficient in actual production.
2.61 Gray iron smelting is transformed from a cupola to an induction electric furnace. The simple understanding is that the smelting time of the induction electric furnace is relatively long, and the graphite crystalline core becomes less and less at high temperature, and it is easy to form a white mouth. The general explanation is that the metallurgical quality is not as good as the cupola. However, there are very few specific and detailed theoretical explanations, and there are many theoretical opinions on the different content of trace elements in the two smelting equipment.
2.62 Induction electric furnace smelting, the burning loss of various elements in raw materials such as pig iron is much less than that in the cupola, that is, it retains the raw materials, especially various trace elements in pig iron, among which the harmful elements are titanium, lead, tellurium, etc. In cupola smelting, the burning loss of various elements is large, and the harmful elements are also more burning. In order to reduce the influence of harmful elements in pig iron on molten iron, the foundry industry now uses synthetic cast iron technology and high-purity pig iron materials to smelt molten iron.
2.7 Then how to make the metallurgical characteristics of the hot metal in the induction electric furnace close to that of the cupola hot metal? In recent years, foundry workers have proposed the following methods in use:
2.71 The power density configuration of the induction electric furnace is relatively large to ensure that the smelting time is shortened, that is, fast smelting. (It takes 3-4 hours to smelt 10 tons in Cangzhou factory).
2.72 It is strictly forbidden to keep the molten iron at a high temperature above the equilibrium temperature for too long, so as to reduce the reduction of graphite crystal cores at high temperatures. (Silica microcrystalline problem).
2.73 During production, if the molten iron needs to be taken out of the furnace several times, and the molten iron is stored for too long in the furnace, various treatment measures to increase the graphite core of the molten iron should be used to "pre-treat" the molten iron. Generally speaking, in large-piece foundry workshops, after a furnace of molten iron temperature composition is qualified, it is released immediately, or after one package is completed, or after 2-3 packages are completed, the castings are poured immediately. In the assembly line foundry, if the furnace configuration is large, the molten iron must be tapped several times. (Pouring 20 tons of molten iron a day. The 10 tons furnace is discharged in 4 times, and the wait is more than 1 hour.
2.74 There are various methods for pretreatment operations: leaving part of the pig iron, reheating materials (removing rust and sand) or high-quality recarburizers of 0.05%-0.1% (highly affecting the coarseness of graphite), which are all added in the later stage of smelting. Is to increase the crystalline core.
2.75 The temperature of the molten iron is qualified. Before the furnace is ready to be released, add a pretreatment agent containing zirconium and barium, or add a small amount of fine-particle metallurgical silicon carbide as a pretreatment agent, and then release the furnace, the purpose is to increase the graphite core.
2.76 Gray iron cylinder block and cylinder head casting assembly line workshop, every time after the molten iron is tapped from the large electric furnace, immediately add the same brand of recharge to the furnace, reduce the temperature in the furnace to below the equilibrium temperature, and wait for the next ladle of molten iron to be prepared When incubating, the temperature is raised to the specified temperature and then released. These recycled materials of the same brand not only reduce the temperature of the furnace, but also have a pretreatment effect on molten iron.