As environmental protection requirements continue to tighten and increasing attention is paid to resource consumption, the prices of pig iron and coke have been steadily rising, driving up casting costs. Consequently, more and more foundries are starting to use electric furnaces for melting, replacing the traditional cupola furnaces. 2011 At the beginning of the year, our factory’s small- and medium-sized parts workshop also adopted electric furnace melting technology to replace the traditional cupola furnace melting process. Electric furnace melting, which makes extensive use of scrap steel, can both reduce costs and improve the mechanical properties of castings. However, the type of carbon-increasing agent used and the carbon-increasing process play a crucial role.

1 ．Main types of carbon additives

　　Many materials can be used as carbon additives for cast iron; commonly employed ones include synthetic graphite, calcined petroleum coke, natural graphite, coke, anthracite coal, and mixtures composed of these materials.
　　( 1 Artificial graphite Among the various carbon-increasing agents mentioned above, synthetic graphite is of the highest quality. The primary raw material for manufacturing synthetic graphite is high-quality calcined petroleum coke in powder form. Asphalt is added as a binder, along with small amounts of other auxiliary materials. After the various raw materials are properly blended, they are pressed into shape and then... 2500 ~ 3000 It is treated in a non-oxidizing atmosphere at a temperature of ℃ to induce graphitization. After high-temperature treatment, the ash content, sulfur content, and gas content are all significantly reduced. Petroleum coke that has not undergone high-temperature calcination or that has been calcined at an insufficient temperature will severely compromise the quality of the carbon additive. Therefore, the quality of a carbon additive largely depends on its degree of graphitization; a high-quality carbon additive contains graphite carbon (by mass fraction) at a level of... 95% ~ 98% , sulfur content is in 0.02% ~ 0.05% , nitrogen content is ( 100 ~ 200 ) ×10 - 6。
　　( 2 Petroleum coke It is a widely used carbon-increasing agent. Petrol coke is a byproduct obtained from the refining of crude oil. The residual oils and petroleum bitumen produced through atmospheric or vacuum distillation of crude oil can both serve as raw materials for manufacturing petrol coke. After coking, raw petrol coke is obtained. However, raw petrol coke contains high levels of impurities and cannot be directly used as a carbon-increasing agent; it must first undergo calcination treatment.
　　( 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 carbon-increasing agent for cast iron. Flake graphite comes in many varieties: high-carbon flake graphite needs to be extracted by chemical methods or heated to high temperatures to decompose and volatilize its oxides. Such flake graphite is produced in limited quantities and commands a high price; hence, it is seldom used as a carbon-increasing agent. Low-carbon flake graphite contains a high ash content and is therefore unsuitable for use as a carbon-increasing agent. The primary type of flake graphite used as a carbon-increasing agent is medium-carbon graphite, though even this is used only in relatively small amounts.
　　( 4 Coke and anthracite During induction furnace melting, coke or anthracite coal can be added during charging as a carbon-increasing agent. Due to their relatively high ash and volatile matter content, these materials are seldom used as carbon-increasing agents in induction furnace melting of cast iron; the carbon content in... 80% ~ 90% , sulfur content is in 0.5% Above, the nitrogen content is ( 500 ~ 4000 ) ×10 - 6 This carbon-increasing agent is relatively inexpensive and belongs to the lower-grade category of carbon-increasing agents.

2 ．Principle of Carbon Increase in Molten Iron

　　During the melting process of synthetic cast iron, due to the large amount of scrap steel added, the molten iron... C The content is low, so a carbon-increasing agent must be used to increase the carbon content. The carbon existing in elemental form within the carbon-increasing agent has a melting temperature of... 3727 ℃, it cannot melt at the temperature of molten iron. Therefore, the carbon in the carburizing agent primarily dissolves into the molten iron through two mechanisms: dissolution and diffusion. When the content of graphite carburizing agent in the molten iron is... 2.1% At this point, graphite can dissolve directly into the molten iron through a process known as direct dissolution. By contrast, direct dissolution of non-graphitic carbon additives is virtually nonexistent; rather, over time, carbon gradually diffuses and dissolves into the molten iron. In the case of carbon addition during the melting of cast iron in induction furnaces, the rate of carbon addition using crystalline graphite is significantly higher than that achieved with non-graphitic carbon additives. Experiments have shown that the dissolution of carbon in molten iron is controlled by mass transfer of carbon across the liquid boundary layer surrounding the surface of solid particles. When comparing results obtained using coke and coal particles with those obtained using graphite, it was found that graphite-based carbon additives exhibit a markedly faster diffusion and dissolution rate in molten iron than carbon additives such as coke and coal particles. Examination of partially dissolved coke and coal particle samples using an electron microscope revealed the formation of a very thin, sticky layer of ash on the sample surfaces—this layer is the primary factor influencing their diffusion and dissolution performance in molten iron.

3 ．Factors Affecting the Carburizing Effect

　　( 1 ) The effect of carbon additive particle size The absorption rate of carbon-increasing agents depends on the combined effects of their dissolution and diffusion rates as well as their oxidation loss rates. In general, smaller carbon-increasing agent particles dissolve more quickly but also oxidize and lose mass at a faster rate; larger particles dissolve more slowly but experience slower oxidation and loss. The selection of carbon-increasing agent particle size is related to the furnace diameter and capacity. Generally speaking, the larger the furnace diameter and capacity, the coarser the particle size of the carbon-increasing agent should be; conversely, the smaller the particle size should be.
　　( 2 ) The impact of the amount of carbon-increasing agent added Under conditions of constant temperature and identical chemical composition, the saturation concentration of carbon in molten iron is fixed. At a given saturation level, the greater the amount of carbon-increasing agent added, the longer it takes for the carbon to dissolve and diffuse, resulting in greater losses and a lower absorption rate.
　　( 3 ) The effect of temperature on the absorption rate of carbon-increasing agents In principle, the higher the molten iron temperature, the more favorable it is for the absorption and dissolution of the carbon-increasing agent. Conversely, at lower temperatures, the carbon-increasing agent is harder to dissolve, resulting in a reduced absorption rate. However, when the molten iron temperature is too high, although the carbon-increasing agent dissolves more readily and completely, the burn-off rate of carbon increases, ultimately leading to a decrease in carbon content and a lower overall absorption rate of the carbon-increasing agent. Typically, the molten iron temperature is... 1460 ~ 1550 At ℃, the carbon-increasing agent exhibits the best absorption efficiency.
　　( 4 ) The effect of molten iron stirring on the absorption rate of carbon additives Stirring promotes the dissolution and diffusion of carbon, preventing the carburizing agent from floating on the surface of the molten iron and being burned off. Before the carburizing agent is completely dissolved, longer stirring times lead to higher absorption rates. Stirring can also reduce the holding time required for carburization, thereby shortening the production cycle and minimizing the burn-off of alloy elements in the molten iron. However, excessively long stirring not only significantly affects the furnace’s service life but also, once the carburizing agent has dissolved, further stirs up and accelerates the loss of carbon from the molten iron. Therefore, the optimal stirring time for molten iron should be just sufficient to ensure complete dissolution of the carburizing agent.
　　( 5 ) The Influence of Molten Iron Chemical Composition on the Absorption Rate of Carbon Additives When the initial carbon content in the molten iron is high, under a given solubility limit, the absorption rate of the carbon-increasing agent is slow, the amount of carbon absorbed is small, and the burn-off is relatively high, resulting in a low absorption efficiency of the carbon-increasing agent. Conversely, when the initial carbon content in the molten iron is low, the situation is reversed. In addition, silicon and sulfur present in the molten iron hinder carbon absorption and reduce the absorption efficiency of the carbon-increasing agent; whereas manganese promotes carbon absorption and enhances the absorption efficiency of the carbon-increasing agent. In terms of the degree of influence, silicon has the greatest effect, followed by manganese, while carbon and sulfur have relatively minor effects. Therefore, in actual production, it is advisable to increase manganese first, then carbon, and finally silicon.