Application of Carbon Raiser in Casting Production

I. How to classify recarburizers

Carburizers can be roughly divided into four types according to their raw materials.

1. Artificial graphite

The main raw material for the manufacture of artificial graphite is powdered high-quality calcined petroleum coke, in which asphalt is added as a binder, and a small amount of other auxiliary materials are added. After the various raw materials are mixed together, they are pressed and formed, and then treated in a non-oxidizing atmosphere at 2500-3000 ° C to make them graphitized. After high temperature treatment, the ash, sulfur and gas content are greatly reduced.

Due to the high price of artificial graphite products, most of the artificial graphite recarburizers commonly used in foundries are recycled materials such as chips, waste electrodes and graphite blocks when manufacturing graphite electrodes to reduce production costs.

When smelting ductile iron, in order to make the metallurgical quality of the cast iron high, artificial graphite should be the first choice for the recarburizer.

2. Petroleum coke

Petroleum coke is a widely used recarburizer.

Petroleum coke is a by-product obtained by refining crude oil. Residues and petroleum pitches obtained by distillation under normal pressure or under reduced pressure of crude oil can be used as raw materials for the manufacture of petroleum coke, and then green petroleum coke can be obtained after coking. The production of green petroleum coke is approximately less than 5% of the amount of crude oil used. The annual production of raw petroleum coke in the United States is about 30 million tons. The impurity content in green petroleum coke is high, so it cannot be directly used as a recarburizer, and must be calcined first.

Raw petroleum coke is available in sponge-like, needle-like, granular and fluid forms.

Sponge petroleum coke is prepared by delayed coking method. Due to its high sulfur and metal content, it is usually used as a fuel during calcination, and can also be used as a raw material for calcined petroleum coke. The calcined sponge coke is mainly used in the aluminum industry and as a recarburizer.

Needle petroleum coke is prepared by delayed coking method with raw materials with high content of aromatic hydrocarbons and low content of impurities. This coke has an easily fractured needle-like structure, sometimes called graphite coke, and is mainly used to make graphite electrodes after calcination.

Granular petroleum coke is in the form of hard granules and is made from raw materials with high content of sulfur and asphaltene by delayed coking method, and is mainly used as fuel.

Fluidized petroleum coke is obtained by continuous coking in a fluidized bed.

The calcination of petroleum coke is to remove sulfur, moisture, and volatiles. Calcination of green petroleum coke at 1200-1350°C can make it substantially pure carbon.

The largest user of calcined petroleum coke is the aluminum industry, 70% of which is used to make anodes that reduce bauxite. About 6% of the calcined petroleum coke produced in the United States is used for cast iron recarburizers.

3. Natural graphite

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 recarburizer 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 the oxides in it. The ash content in graphite is high, so it is not suitable to be used as a recarburizer; medium carbon graphite is mainly used as a recarburizer, but the amount is not much.

4. Carbon Coke and Anthracite

In the process of electric arc furnace steelmaking, coke or anthracite can be added as a recarburizer when charging. Due to its high ash and volatile content, induction furnace smelting cast iron is rarely used as a recarburizer.

With the continuous improvement of environmental protection requirements, more and more attention is paid to resource consumption, and the prices of pig iron and coke continue to rise, resulting in an increase in the cost of castings. More and more foundries are beginning to use electric furnaces to replace traditional cupola melting. At the beginning of 2011, the small and medium parts workshop of our factory also adopted the electric furnace melting process to replace the traditional cupola melting process. The use of a large amount of scrap steel in electric furnace smelting can not only reduce costs, but also improve the mechanical properties of castings, but the type of recarburizer used and the carburizing process play a key role.

II.How to use recarburizer in induction furnace smelting

1. The main types of recarburizers

There are many materials used as cast iron recarburizers, commonly used are artificial graphite, calcined petroleum coke, natural graphite, coke, anthracite, and mixtures made of such materials.

(1) Artificial graphite Among the various recarburizers mentioned above, the best quality is artificial graphite. The main raw material for the manufacture of artificial graphite is powdered high-quality calcined petroleum coke, in which asphalt is added as a binder, and a small amount of other auxiliary materials are added. After the various raw materials are mixed together, they are pressed and formed, and then treated in a non-oxidizing atmosphere at 2500-3000 °C to make them graphitized. After high temperature treatment, the ash, sulfur and gas content are greatly reduced. If there is no petroleum coke calcined at high temperature or with insufficient calcination temperature, the quality of the recarburizer will be seriously affected. Therefore, the quality of the recarburizer mainly depends on the degree of graphitization. A good recarburizer contains graphitic carbon (mass fraction) At 95% to 98%, the sulfur content is 0.02% to 0.05%, and the nitrogen content is (100 to 200) × 10-6.

(2) Petroleum coke is a widely used recarburizer. Petroleum coke is a by-product obtained from refining crude oil. Residues and petroleum pitches obtained from regular pressure distillation or vacuum distillation of crude oil can be used as raw materials for the manufacture of petroleum coke. After coking, raw petroleum coke can be obtained. The content is high and cannot be used directly as a recarburizer, and 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 recarburizer 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 the oxides in it. The ash content in graphite is high and should not be used as a recarburizer. Medium carbon graphite is mainly used as a recarburizer, but the amount is not much.

(4) Carbon Coke and anthracite In the process of induction furnace smelting, coke or anthracite can be added as a recarburizer when charging. Due to its high ash and volatile content, induction furnace smelting cast iron is rarely used as a recarburizer. , The price of this recarburizer is low, and it belongs to the low-grade recarburizer.

2. The principle of carburization of molten iron

In the smelting process of synthetic cast iron, due to the large amount of scrap added and the low C content in the molten iron, a carburizer must be used to increase the carbon. The carbon that exists in the form of element in the recarburizer has a melting temperature of 3727°C and cannot be melted at the temperature of the molten iron. Therefore, the carbon in the recarburizer is mainly dissolved in the molten iron by two ways of dissolution and diffusion. When the content of graphite recarburizer in molten iron is 2.1%, graphite can be directly dissolved in molten iron. The direct solution phenomenon of non-graphite carbonization basically does not exist, but with the passage of time, carbon gradually diffuses and dissolves in the molten iron. For the recarburization of cast iron smelted by induction furnace, the recarburization rate of crystalline graphite recarburization is significantly higher than that of non-graphite recarburizers.

Experiments show that the dissolution of carbon in molten iron is controlled by the carbon mass transfer in the liquid boundary layer on the surface of the solid particles. Comparing the results obtained with coke and coal particles with the results obtained with graphite, it is found that the diffusion and dissolution rate of graphite recarburizers in molten iron is significantly faster than that of coke and coal particles. The partially dissolved coke and coal particle samples were observed by electron microscope, and it was found that a thin sticky ash layer was formed on the surface of the samples, which was the main factor affecting their diffusion and dissolution performance in molten iron.

3. Factors Affecting the Effect of Carbon Increase

(1) Influence of the particle size of the recarburizer The absorption rate of the recarburizer depends on the combined effect of the dissolution and diffusion rate of the recarburizer and the rate of oxidation loss. In general, the particles of the recarburizer are small, the dissolution speed is fast, and the loss speed is large; the carburizer particles are large, the dissolution speed is slow, and the loss speed is small. The choice of the particle size of the recarburizer is related to the diameter and capacity of the furnace. In general, when the diameter and capacity of the furnace are large, the particle size of the recarburizer should be larger; on the contrary, the particle size of the recarburizer should be smaller.

(2) Influence of the amount of recarburizer added Under the conditions of a certain temperature and the same chemical composition, the saturated concentration of carbon in the molten iron is certain. Under a certain degree of saturation, the more recarburizer 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 the recarburizer In principle, the higher the temperature of the molten iron, the more conducive to the absorption and dissolution of the recarburizer. On the contrary, the recarburizer is difficult to dissolve, and the recarburizer absorption rate decreases. However, when the temperature of the molten iron is too high, although the recarburizer is more likely to be fully dissolved, the burning loss rate of carbon will increase, which will eventually lead to a decrease in the carbon content and a decrease in the overall absorption rate of the recarburizer. Generally, when the molten iron temperature is between 1460 and 1550 °C, the absorption efficiency of the recarburizer is the best.

(4) Influence of molten iron stirring on the absorption rate of recarburizer Stirring is beneficial to the dissolution and diffusion of carbon, and avoids the recarburizer floating on the surface of molten iron and being burned. Before the recarburizer is completely dissolved, the stirring time is long and the absorption rate is high. Stirring can also reduce the carbonization holding time, shorten the production cycle, and avoid burning of alloying elements in the molten iron. However, if the stirring time is too long, it not only has a great influence on the service life of the furnace, but also aggravates the loss of carbon in the molten iron after the recarburizer is dissolved. Therefore, the appropriate stirring time of molten iron should be suitable to ensure that the recarburizer is completely dissolved.

(5) Influence of the chemical composition of molten iron on the absorption rate of the recarburizer When the initial carbon content in the molten iron is high, under a certain solubility limit, the absorption rate of the recarburizer is slow, the absorption amount is small, and the burning loss is relatively large. The recarburizer absorption rate is low. The opposite is true when the initial carbon content of the molten iron is low. In addition, silicon and sulfur in molten iron hinder the absorption of carbon and reduce the absorption rate of recarburizers; while manganese helps to absorb carbon and improve the absorption rate of recarburizers. In terms of the degree of influence, silicon is the largest, followed by manganese, and carbon and sulfur have less influence. Therefore, in the actual production process, manganese should be added first, then carbon, and then silicon.