This paper mainly describes the types of commonly used recarburizers for casting and the factors affecting the effect of recarburization, and makes a statistical comparison of the absorption rates of several commonly used recarburizers. The results show that the absorption effect of graphitized recarburizer is better than that of calcined Petroleum coke recarburizer. The same type of recarburizer has fine particle size and good absorption effect, but its cost is relatively high. At present, the better alternative is pure Anthracite recarburizer with high coal quality, The comprehensive cost performance ratio is very high. At present, many large steel plants, special steel and casting industries are widely using pure Anthracite carburetor.
Keywords: casting; Carburizing agent; Graphitization treatment
Carburizer is a follow-up product of black or gray grain (or block) coke or Petroleum coke with high carbon content, which can reduce the oxygen content in molten iron and improve the mechanical properties of castings during metal melting. In the new synthetic cast iron smelting process, the amount of scrap steel used has greatly increased. By adding carburetors to adjust the carbon content of the molten iron, production costs have been greatly reduced while ensuring the quality of the molten iron.
1. Types of Carburizing Agents for Cast Iron
According to the crystal structure of carbon in carburetors, they can be divided into amorphous and crystalline states; According to the existence form of carbon atoms, they can be divided into graphitized carburetors and non graphitized carburetors. Graphitized carburants mainly include graphitized Petroleum coke and graphite electrode. Graphitized Petroleum coke carbon additive is the most widely used in the casting industry. Its production process is to heat the raw material Petroleum coke in a graphitizing furnace at 2200-2600 ℃, so that the amorphous disordered layer structure of Petroleum coke can be crystallized into three-dimensional ordered graphite crystals. Non graphitized carburants mainly include: calcined Petroleum coke, coal based carburants, etc. calcined Petroleum coke refers to Petroleum coke that has been heated at 1200-1500 ℃ at a low temperature and its crystal structure has not changed. Coal based carburetors are rarely used in electric arc furnace melting due to their high ash and volatile content. The main indicators for measuring the quality of carburetors include fixed carbon content, sulfur content, volatile matter, ash content, nitrogen content, and water content. After high-temperature graphitization treatment, the sulfur and nitrogen content of graphitized carburetors are significantly lower than those of non graphitized carburetors.
1.1 Fixed carbon, ash, volatile matter
The fixed carbon of a carburetor is different from its carbon content. The fixed carbon value is calculated based on the moisture, volatile matter, ash, and sulfur content of the sample, while the carbon content can be directly detected by an instrument. The higher the fixed carbon content and the lower the ash content, the better the carbon enhancement effect. The high ash content in the carburetor generates a large amount of smoke and slag during the smelting process, increasing energy consumption and increasing the labor intensity of slag removal.
1.2 Sulfur content
The sulfur content of graphitized carburetors is generally lower than 0.05%. In ordinary cast iron, sulfur element has the function of stabilizing the cementite and preventing the expansion of graphitization. When producing ductile iron and vermicular iron castings, graphitization carburetors with low sulfur content should be selected. If the sulfur content is too high, it will affect the spheroidizing effect, consume more spheroidizing agents, and increase production costs; The calcined Petroleum coke carburetor can be used in the production of some gray iron castings because of its low calcination temperature and high sulfur content, which is generally below 1%.
1.3 Nitrogen content
The nitrogen content is an important indicator for measuring the quality of carburetors. When purchasing carburetors, general foundries pay attention to the price of the carburetor; The second is the content of fixed carbon, sulfur, ash, volatile matter, and water in the carburetor, with less attention paid to nitrogen content. The common calcined Petroleum coke carbon increasing agent has low calcination temperature and high nitrogen and sulfur content. During the production of gray iron castings, because the scrap in the furnace charge will bring in part of nitrogen, if more scrap is added, the amount of carburant will increase accordingly. If the common calcined Petroleum coke carburant with higher nitrogen content is used, the nitrogen content in molten iron will increase significantly. Excessive dissolution of N in molten iron can hinder graphitization, increase the stability of carbides, promote the formation of D-type graphite, improve hardness, and deteriorate the processing performance of castings. For gray cast iron, proper amount of N element can shorten the length of graphite sheet, increase the bending degree, passivate the end, and reduce the Aspect ratio, which can stabilize the pearlite of the matrix, refine the grain structure, and improve the tensile strength and hardness of the casting. According to information, there is a possibility of nitrogen porosity defects in castings when N>0.012% in molten iron. Nitrogen holes are often found inside, on, or near the surface of castings, appearing in circular, rectangular, and irregular shapes of varying sizes, and are one of the common defects in casting production.
After high-temperature desulfurization and denitrification treatment, the content of nitrogen and sulfur elements in graphitized carburetor is very low. The nitrogen content in molten iron melted with graphitized carburetor is generally very low.
2. Factors that affect the absorption rate of carburizing agents: The main factors that affect the absorption rate of carburizing agents include the addition method of carburizing agents, the chemical composition and temperature of molten iron, and the particle size of carburizing agents.
2.1 Adding method of carburetor
The addition method has a significant impact on the carbon enhancement effect. Carburizing agent should not be added to the furnace bottom earlier than the furnace charge. It should be added in batches with the furnace charge when there is some molten iron at the furnace bottom. This is because the melting point of the carburizing agent is high and it must rely on the surrounding iron liquid to be slowly decomposed and absorbed. If directly added to the furnace bottom, it is not easy to be absorbed and reduces the service life of the furnace lining. In addition, the stirring effect of molten iron during the heating process of the electric furnace makes carbon more easily absorbed by the molten iron.
2.2 Effects of chemical composition and temperature of molten iron
According to data, the lower the initial carbon content in molten iron, the higher the absorption rate of the carburetor. When the carbon content in molten iron is above 3.6%, its absorption rate decreases; The higher the silicon content in the molten iron, the more it affects the absorption of the carburetor. This is because silicon has a carbon removal effect, reducing the solubility of carbon in the molten iron. For every 0.11% increase in silicon content, the absorption rate of the carburetor decreases by about 3% -4%. Therefore, the addition time of carburetor should be before the addition of ferrosilicon during feeding. The high content of manganese in molten iron can improve the absorption of carburizing agents. Increasing manganese by 0.1% can increase the absorption rate of carburizing agents by 2% -3%.
It is generally believed that the higher the temperature of the molten iron, the longer the action time, and the faster the absorption rate of the carburizing agent. However, according to information, when the temperature of molten iron in an induction furnace is high, the carbon content does not increase but decreases instead. This is because: 1. The main loss of graphite carbon is related to meteorological diffusion into the atmosphere outside the furnace. 2. The balance relationship between the oxidation in molten iron and C-Si-O. CO in molten iron is continuously oxidized to CO2, which is then reduced by C. The reaction produces CO, and CO2 gas floats up and overflows onto the surface of the molten iron, causing a decrease in carbon content in the molten iron. When using an induction furnace for melting, carburization should be carried out at lower temperatures, and siliconization should be carried out at higher temperatures.
2.3 Effect of Carburizer Particle Size on Absorption Rate
The smaller the carburetor, the larger the contact area with the molten iron when added, the faster it dissolves and is easy to absorb. However, if the particle size is too small, it is prone to oxidation and large loss; The particles are large, the dissolution rate is slow, and the loss is small. When selecting, choose the appropriate particle size carburetor based on the type of smelting furnace. Comparison of absorption rates of several carburizing agents
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