Application of carburizer in induction furnace smelting

Application of carburizer in induction furnace smelting

 

With the continuous improvement of environmental protection requirements, more and more attention is paid to resource consumption. The prices of pig iron and coke continue to rise, resulting in an increase in casting costs. More and more foundries have begun to use electric furnace smelting to replace traditional cupola smelting. In early 2011, many small and medium-sized workshops of factories also adopted electric furnace smelting technology to replace traditional cupola smelting technology. The large-scale use of scrap steel in electric furnace smelting can not only reduce costs but also improve the mechanical properties of castings, but the type of carburizer used and the carburizing process play a key role.

 

1. Main types of carburizers

 

There are many materials used as cast iron carburizers, and the commonly used ones are artificial graphite, calcined petroleum coke, natural graphite, coke, anthracite, and mixtures made of these materials.

 

(1) Artificial graphite: Among the various carburizers mentioned above, artificial graphite has the best quality. The main raw material for making 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, they are pressed into shape and then treated in a non-oxidizing atmosphere at 2500~3000℃ to make them graphitized. After high-temperature treatment, the ash, sulfur and gas content are greatly reduced. If the petroleum coke is not calcined at high temperature or the calcination temperature is insufficient, it will seriously affect the quality of the carburizer. Therefore, the quality of the carburizer mainly depends on its degree of graphitization. A good carburizer contains graphite carbon (mass fraction) of 95%~98%, sulfur content of 0.02%~0.05%, and nitrogen content of (100~200)x10-6.

 

(2) Petroleum coke is currently a widely used carburizer. Petroleum coke is a by-product obtained from refining crude oil. The residual oil and petroleum asphalt obtained by normal pressure distillation or vacuum distillation of crude oil can be used as raw materials for making petroleum coke. After coking, raw petroleum coke is obtained. The impurity content in raw petroleum coke is high and cannot be used directly as a carburizer. It must be calcined first.

 

(3) Natural graphite: It can be divided into two categories: 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 temperatures to decompose and volatilize the oxides in it. This type of flake graphite has a small output and a high price, and is generally not used as a recarburizer; low-carbon flake graphite has a high ash content and is not suitable for use as a recarburizer. The main recarburizer is medium-carbon graphite, but the amount used is not large.

 

(4) Coke and anthracite: During the induction furnace smelting process; coke or anthracite can be added as a recarburizer when loading. Due to its high ash and volatile content, it is rarely used as a recarburizer in induction furnace smelting cast iron. The carbon content is 80%~90%, the sulfur content is above 0.5%, and the nitrogen content is (500~4000)x10-6. This recarburizer is relatively low in price and belongs to a low-grade recarburizer.

 

2. Principle of carbonization of molten iron

 

During the smelting process of synthetic cast iron, carbonization must be performed by using a carbonizer because of the large amount of scrap steel added and the low C content of molten iron. The carbon in the carbonizer in the form of a single substance has a melting temperature of 3727℃ and cannot melt at the temperature of molten iron. Therefore, the carbon in the carbonizer is mainly dissolved in the molten iron by dissolution and diffusion. When the content of graphite carbonizer in molten iron is 2.1%, graphite can be directly dissolved in the molten iron. The direct dissolution phenomenon of non-graphite carbonization is basically non-existent, but with the passage of time, carbon gradually diffuses and dissolves in the molten iron. For the carbonization of cast iron smelted in an induction furnace, the carbonization rate of crystalline graphite carbonization is significantly higher than that of non-graphite carbonizer.

 

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 with coke and coal particles are compared with those obtained with graphite. It is found that the diffusion and dissolution rate of graphite carbonizer in molten iron is significantly faster than that of carbonizers such as coke and coal particles. Using an electron microscope to observe the partially dissolved coke and coal particle samples, it was found that a very thin sticky ash layer was formed on the surface of the sample, which is the main factor affecting its diffusion and dissolution performance in molten iron.

 

3. Factors affecting the recarburization effect

 

(1) The effect of recarburizer particle size: The absorption rate of the recarburizer depends on the combined effect of the dissolution and diffusion rate of the recarburizer and the oxidation loss rate. In general, the smaller the recarburizer particles, the faster the dissolution rate and the greater the loss rate; the larger the recarburizer particles, the slower the dissolution rate and the smaller the loss rate. The selection of recarburizer particle size is related to the furnace diameter and capacity. In general, the larger the furnace diameter and capacity, the larger the recarburizer particle size; conversely, the smaller the recarburizer particle size.

 

(2) The effect of the amount of recarburizer added: Under certain temperature and chemical composition conditions, the saturation concentration of carbon in the molten iron is certain. At a certain saturation, the more recarburizer 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 carburizer: In principle, the higher the temperature of molten iron, the more conducive it is to the absorption and dissolution of carburizer. On the contrary, the carburizer is difficult to dissolve and the absorption rate of carburizer is reduced. However, when the temperature of molten iron is too high, although the carburizer is easier to dissolve fully, the carbon burn-off rate will increase, which will eventually lead to a decrease in carbon content and a decrease in the overall absorption rate of carburizer.

 

(4) The effect of molten iron stirring on the absorption rate of carburizer: Stirring is conducive to the dissolution and diffusion of carbon, and prevents the carburizer from floating on the surface of molten iron and being burned. Before the carburizer is completely dissolved, the longer the stirring time, the higher the absorption rate. Stirring can also reduce the carburizer 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 will not only have a great impact on the service life of the furnace, but also after the carburizer is dissolved, stirring will aggravate the loss of carbon in the molten iron. Therefore, the appropriate stirring time of molten iron should be appropriate to ensure that the carburizer is completely dissolved.

 

(5) The influence of the chemical composition of molten iron on the absorption rate of carburizer: When the initial carbon content in the molten iron is high, under a certain solubility limit, the absorption rate of the carburizer is slow, the absorption amount is small, the burn loss is relatively large, and the carburizer absorption rate is low. When the initial carbon content of the molten iron is low, the situation is the opposite. In addition, silicon and sulfur in the molten iron hinder the absorption of carbon and reduce the absorption rate of the carburizer; while manganese helps the absorption of carbon and increases the absorption rate of the carburizer. In terms of the degree of influence, silicon has the greatest influence, followed by manganese, and carbon and sulfur have less influence. Therefore, in the actual production process, manganese should be added first, then carbon, and finally silicon.

 

Conclusion:

 

(1) Factors affecting the absorption rate of the carburizer include carburizer particle size, carburizer addition amount, carburizing temperature, molten iron stirring time and molten iron chemical composition.

 

(2) High-quality graphite-type carburizer can not only improve the mechanical properties and metallographic structure of castings, but also improve the processing performance of castings. Therefore, when using the induction furnace smelting process to produce key products such as cylinder blocks and cylinder heads, it is recommended to use high-quality graphite-type recarburizers.