Carbonizing agents can be roughly divided into four types based on their different raw materials.
1. The main raw material of artificial graphite is powdered high-quality calcined Petroleum coke, in which asphalt is added as binder, and a small amount of other auxiliary materials are added. After various raw materials are combined, they are pressed into shape and then treated in a non oxidizing atmosphere at 2500-3000 ℃ to graphitize. After high-temperature treatment, the ash content, sulfur content, and gas content are significantly reduced. Due to the high price of artificial graphite products, most of the commonly used artificial graphite carburetors in foundries are materials that are recycled during the manufacturing of graphite electrodes, such as chips, waste electrodes, and graphite blocks, in order to reduce production costs. When melting ductile iron, artificial graphite is preferred as the carburetor to ensure the superior metallurgical quality of the cast iron.
2. Petroleum coke Petroleum coke is a widely used carburetor at present.
Petroleum coke is a by-product obtained from refining crude oil. The residual oil and petroleum asphalt obtained from crude oil regular pressure distillation or vacuum distillation can be used as raw materials for manufacturing Petroleum coke, and then raw Petroleum coke can be obtained after coking. The output of raw Petroleum coke is less than 5% of the crude oil used. The annual output of raw Petroleum coke in the United States is about 30 million tons. The content of impurities in raw Petroleum coke is high, so it cannot be directly used as a carburetor, and must be calcined first.
Raw Petroleum coke has sponge, needle, granular and fluid types.
Sponge Petroleum coke is produced by delayed coking method. Because of its high sulfur and metal content, it is usually used as fuel during calcination, and also as raw material for Petroleum coke calcination. The calcined sponge coke is mainly used in the aluminum industry and as a carburizing agent.
Needle Petroleum coke is produced by delayed coking from raw materials with high Aromatic hydrocarbon content and low impurity content. This type of coke has a needle like structure that is prone to cracking, sometimes referred to as graphite coke, and is mainly used to manufacture graphite electrodes after calcination.
Granular Petroleum coke is hard granular, which is made from raw materials with high sulfur and asphaltene content by delayed coking method and is mainly used as fuel.
Fluidized Petroleum coke is produced by continuous coking method in a fluidized bed. It is in the form of fine particles, directionless in structure, high in sulfur content and low in volatile matter.
Petroleum coke is calcined to remove sulfur, water and volatile matter. The raw Petroleum coke can be calcined at 1200~1350 ℃ to make it basically pure carbon.
The largest user of calcined Petroleum coke is the aluminum industry, 70% of which is used to manufacture anodes for bauxite reduction. The calcined Petroleum coke produced in the United States is used as a carburizing agent for cast iron, accounting for about 6%.
3. Natural graphite can be divided into two types: flake graphite and microcrystalline graphite.
Microcrystalline graphite has a high ash content and is generally not used as a carburizing agent for cast iron.
There are many varieties of flake graphite: high carbon flake graphite needs to be extracted by chemical methods or heated to high temperature to decompose and volatilize its oxides. This type of flake graphite has low production and high price, and is generally not used as a carburetor; Low carbon flake graphite has a high ash content and is not suitable for use as a carburizing agent; The main carbon enhancer used is medium carbon graphite, but the amount used is also limited.
4. In the process of electric arc furnace steelmaking with coke and Anthracite, coke or Anthracite can be added as carburizing agent during charging. Due to its high ash and volatile content, induction furnace melting of cast iron is rarely used as a carburizing agent.
With the continuous improvement of environmental protection requirements and increasing emphasis on resource consumption, the prices of pig iron and coke continue to rise, leading to an increase in casting costs. More and more casting manufacturers are using electric furnaces for melting to replace traditional cupola melting. At the beginning of 2011, our small and medium-sized parts workshop also adopted the electric furnace melting process to replace the traditional cupola melting process. The extensive use of scrap steel in electric furnace melting can not only reduce costs but also improve the mechanical properties of castings. However, the types of carburizing agents used and the carburizing process play a crucial role.
02. Usage of Carburizer in Induction Furnace Melting
1. The main types of carburants are many materials used as cast iron carburants, such as artificial graphite, calcined Petroleum coke, natural graphite, coke, Anthracite, and mixtures made of these materials.
(1) Among the various carburizing agents mentioned above, the best quality is artificial graphite. The main raw material for manufacturing artificial graphite is powdered high-quality calcined Petroleum coke, in which asphalt is added as binder, and a small amount of other auxiliary materials are added. After various raw materials are combined, they are pressed into shape, and then treated in a non oxidizing atmosphere at 2500-3000 ℃ to graphitize. After high-temperature treatment, the ash content, sulfur content, and gas content are significantly reduced. If Petroleum coke is not calcined at high temperature or the calcination temperature is insufficient, it will seriously affect the quality of the carburetor. Therefore, the quality of the carburetor depends mainly on its graphitization degree. A good carburetor contains 95%~98% of graphite carbon (mass fraction), 0.02%~0.05% of sulfur and (100~200)% of nitrogen × 10-6.
(2) Petroleum coke is a widely used carburetor at present. Petroleum coke is a by-product obtained from refining crude oil. The residual oil and petroleum asphalt obtained from the regular pressure distillation or vacuum distillation of crude oil can be used as raw materials for manufacturing Petroleum coke. After coking, raw Petroleum coke can be obtained. Raw Petroleum coke has high impurity content and cannot be directly used as a carburetor. It must be calcined first.
(3) Natural graphite can be divided into two types: flake graphite and microcrystalline graphite. Microcrystalline graphite has a high ash content and is generally not used as a carburizing agent for cast iron. There are many varieties of flake graphite: high carbon flake graphite needs to be extracted by chemical methods, or heated to high temperature to decompose and volatilize its oxides. This type of flake graphite has a low yield and high price, and is generally not used as a carburetor; Low carbon flake graphite has a high ash content and is not suitable for use as a carburizing agent. The main carbon enhancer used is medium carbon graphite, but the amount used is also limited.
(4) During the smelting process of coke and Anthracite induction furnace, coke or Anthracite can be added as the carburizer when charging. Due to its high ash and volatile content, induction furnace melting of cast iron is rarely used as a carburizing agent. The carbon content ranges from 80% to 90%, the sulfur content exceeds 0.5%, and the nitrogen content ranges from 500 to 4000 × 10-6, this type of carburetor has a lower price and belongs to a lower grade of carburetor.
2. During the melting process of synthetic cast iron based on the principle of increasing carbon content in molten iron, due to the large amount of scrap added and the low C content in molten iron, carburizing agents must be used to increase carbon content. The carbon present in the form of a single substance in the carburetor melts at a temperature of 3727 ℃ and cannot melt at the temperature of molten iron. Therefore, the carbon in the carburetor is mainly dissolved in molten iron through dissolution and diffusion. When the content of graphite carburetor in molten iron is 2.1%, graphite can be directly dissolved and dissolved in the molten iron. The direct dissolution phenomenon of non graphite carburization basically does not exist, but over time, carbon gradually diffuses and dissolves in the molten iron. For the carburization of induction furnace melting of cast iron, the carburization rate using crystal graphite is significantly higher than that using non graphite carburizing agents.
The experiment shows that the dissolution of carbon in molten iron is controlled by the carbon mass transfer in the liquid Boundary layer on the surface of solid particles. The results obtained from using coke and coal particles were compared with those obtained from graphite, and it was found that the diffusion and dissolution rate of graphite carburetors in molten iron was significantly faster than that of carburetors such as coke and coal particles. Observing partially dissolved coke and coal particle samples using an electron microscope, it was found that a thin viscous gray layer formed on the surface of the sample, which is the main factor affecting its diffusion and dissolution performance in molten iron.
3. The factors that affect the decarburization effect (1) The influence of the particle size of the carburetor on the absorption rate of the carburetor depends on the comprehensive effect of the dissolution diffusion rate and oxidation loss rate of the carburetor. In general, carburetors have small particles, fast dissolution rate, and high loss rate; The carburetor has large particles, slow dissolution rate, and low loss rate. The selection of the particle size of the carburizing agent is related to the diameter and capacity of the furnace. In general, the diameter and capacity of the furnace are large, and the particle size of the carburetor is larger; On the contrary, the particle size of the carburetor should be smaller.
(2) The effect of the amount of carburetor addition on the saturation concentration of carbon in molten iron is constant under certain conditions of temperature and chemical composition. At a certain saturation, the more carburetor is added, the longer the time required for dissolution and diffusion, the greater the corresponding loss, and the lower the absorption rate.
(3) The effect of temperature on the absorption rate of carburizing agent: In principle, the higher the temperature of the molten iron, the more conducive it is to the absorption and dissolution of the carburizing agent. On the contrary, the carburizing agent is difficult to dissolve and the absorption rate of the carburizing agent decreases. However, when the temperature of the molten iron is too high, although the carburetor is more easily dissolved, the burning loss rate of carbon will increase, ultimately leading to a decrease in carbon content and a decrease in the overall absorption rate of the carburetor. When the temperature of molten iron is generally between 1460 and 1550 ℃, the absorption efficiency of carburizing agents is the best.
(4) The effect of stirring on the absorption rate of carburizing agent in molten iron. Stirring is beneficial for the dissolution and diffusion of carbon, preventing the carburizing agent from floating on the surface of the molten iron and being burned. Before the carburetor is completely dissolved, the stirring time is long and the absorption rate is high. Mixing can also reduce the carbonization and insulation time, shorten the production cycle, and avoid the burning of alloy elements in the molten iron. However, if the stirring time is too long, it not only has a significant impact on the service life of the furnace, but also exacerbates the loss of carbon in the molten iron after the carburizing agent dissolves. Therefore, the appropriate stirring time for molten iron should ensure the complete dissolution of the carburetor.
(5) The effect of chemical composition of molten iron on the absorption rate of carburizing agents When the initial carbon content in molten iron is high, at a certain dissolution limit, the absorption rate of carburizing agents is slow, the absorption amount is small, the burning loss is relatively high, and the absorption rate of carburizing agents is low. When the initial carbon content of molten iron is low, the situation is opposite. In addition, silicon and sulfur in molten iron hinder the absorption of carbon, reducing the absorption rate of carburizing agents; And manganese element helps to absorb carbon, improving the absorption rate of carburetors. In terms of the degree of impact, silicon is the largest, followed by manganese, and carbon and sulfur have a smaller impact. Therefore, in the actual production process, manganese should be added first, followed by carbon, and then silicon.
4. The Effect of Different Carburizing Agents on the Properties of Cast Iron (1) Experimental Conditions: Two 5t medium frequency coreless induction furnaces were used for melting, with a maximum power of 3000kW and a frequency of 500Hz. According to the daily batching list of the workshop (50% of the body return material, 20% of pig iron, and 30% of scrap steel), a low nitrogen calcination type carburizing agent and a graphite type carburizing agent are used to melt one furnace of molten iron respectively. After adjusting the chemical composition according to the process requirements, a cylinder body main bearing cover is poured separately.
Production process: Carburizer is added to the electric furnace for melting in batches during the feeding process, and 0.4% primary inoculant (silicon barium inoculant) is added to the molten iron process for inoculation. During the pouring process, 0.1% secondary flow inoculant (silicon barium inoculant) is added. Use the DISA2013 styling line.
(2) To verify the effects of two different carburizing agents on the properties of cast iron, and to avoid the influence of iron liquid composition on the results, the composition of iron liquid melted with different carburizing agents was adjusted to be basically consistent. In order to fully verify the results, in addition to pouring a set of 30mm test bars into two batches of molten iron during the experiment, 12 castings were randomly selected from each batch of molten iron for Brinell hardness testing (6 pieces/box, two boxes).
When the composition is almost the same, the strength of the test bars produced with graphite type carburizing agents is significantly higher than that cast by melting with calcined carburizing agents, and the processing performance of the castings produced with graphite type carburizing agents is significantly better than that produced with calcined carburizing agents (when the hardness of the castings is too high, there will be knife bouncing phenomenon at the edges of the castings during processing).
(3) The graphite morphology of the samples using graphite type carburizing agents is all A-type graphite, with a larger number and smaller size of graphite.
From the above experimental results, the following conclusion can be drawn: high-quality graphite type carburizing agent can not only improve the mechanical properties and metallographic structure of castings, but also improve the processing performance of castings.
03. Finally
(1) The factors that affect the absorption rate of carburizing agents include the particle size of the carburizing agent, the amount of carburizing agent added, the carburizing temperature, the stirring time of the molten iron, and the chemical composition of the molten iron.
(2) High quality graphite type carburizing agent can not only improve the mechanical properties and metallographic structure of castings, but also improve the processing performance of castings. Therefore, it is recommended to use high-quality graphite type carburizing agents when producing key products such as cylinder blocks and cylinder heads using the induction furnace melting process.
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