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In recent years, induction furnaces have been increasingly used for melting cast iron. Typically, simply adding metallic charge materials into an induction furnace is insufficient to ensure the required carbon content in the molten iron; therefore, it is necessary to supplement with carbon-increasing agents. For this reason, particularly in induction furnaces... Medium-frequency induction In electric furnaces, adding a carbon-increasing agent is an important step in the melting process. This article will introduce some useful tips for using carbon-increasing agents in induction electric furnaces.

The graphitization of undissolved particles in the carbon-increasing agent occurs in the molten iron. In addition to the carbon already dissolved in the molten iron, the carbon-increasing agent also contains residual, undissolved carbon in the form of graphite, which is entrained in the stirred liquid flow in the form of discrete particles. The larger, undissolved graphite particles, when subjected to electric current, mostly remain suspended near the liquid surface close to the furnace wall, while a portion adheres to the middle section of the furnace wall—areas that correspond to dead zones where stirring is minimal. Once the electric current is turned off, these coarse graphite particles, buoyed by their own upward force, gradually begin to rise and float to the surface. As for the extremely tiny particles, too small to be observed under an optical microscope, they remain suspended in the molten iron not only during the application of electric current but also after the current has been switched off, throughout the entire process of graphite dissolution.

According to the report, the closer a substance is to forming an eutectic crystal nucleus—even if the added graphite differs slightly in crystallinity from eutectic graphite—its coupling degree will inevitably be greater compared to other substances that can be inferred to form graphite nuclei. From this perspective, it can be concluded that suspended fine graphite particles are conducive to the formation of graphite nuclei and can help prevent supercooling and white cast iron formation in cast iron.

2. The Influence of Carbon Additive Particle Size on the Carbon-Boosting Effect

 

2.1 The Influence of Carbon Additive Particle Size on Carburizing Time The particle size of the carbon additive is a key factor affecting its dissolution into the molten iron. We conducted carburizing experiments using carbon additives A, B, and C—whose compositions were roughly identical but differed in particle size—as shown in Table 1. The results are illustrated in Figure 1. Although the carburizing rates after 15 minutes were identical, the time required to reach a 90% carburizing rate varied significantly. It took 13 minutes when using carbon additive C without any particle-size treatment; only 8 minutes when using carbon additive A after removing fine particles; and just 6 minutes when using carbon additive B after removing both fine particles and coarse grains. This demonstrates that the particle size of the carbon additive has a substantial impact on the carburizing time. The presence of either fine particles or coarse grains is detrimental, especially when the content of fine particles is high.

2.2 The Influence of Carbon Additive Particle Size on Carbon Addition Amount Two researchers from Japan, Nakae and Tsukimori, conducted experiments on carbon additives with a carbon mass fraction of 99.8% and a sulfur mass fraction of 0.023%, whose particle size distribution is shown in Table 2. The experimental results are illustrated in Figure 2. As can be seen from the figure, the carbon additive E, whose particle size leans toward fine powder, exhibits extremely poor carbon-adding performance; conversely, the carbon additive G, with a particle size leaning toward coarser particles, demonstrates better carbon-adding performance. Among them, the carbon additive A, which has been appropriately freed from both fine powder and coarse particles, shows the best carbon-adding effect.

The above facts confirm that, in order to enhance the carburizing effect, the carburizing agent should undergo particle-size treatment to remove fine powders and coarse particles.

 

3. The Influence of the Chemical Composition of Molten Iron on the Carbon-Increasing Effect of Carbon Additives

3.1 The Influence of Silicon on the Carbon-Increasing Effect of Carbon-Increasing Agents The silicon content in molten iron significantly affects the carbon-increasing efficiency of carbon-increasing agents. Molten iron with high silicon content exhibits poor carbon-increasing performance. Some researchers varied the mass fraction of Si in the molten iron within the range of 0.6% to 2.1% and added two types of carbon-increasing agents—A and B—as shown in Table 1. They then observed the differences in carbon-increasing time after adding these agents. The results are illustrated in Figure 3: when the mass fraction of Si in the molten iron is high, the carbon-increasing rate slows down.

 

3.2 The Influence of Sulfur on the Carbon-Increasing Effect of Carbon-Increasing Agents—Just as the mass fraction of silicon in molten iron affects the carbon-increasing effect, the sulfur content also has a certain impact on carbon increase. Using carbon-increasing agent A from Table 2, we first added ferrous sulfide—a reagent—to the molten iron before adding the carbon-increasing agent itself, and then observed how the mass fraction of S affected the carbon-increasing process. When ferrous sulfide was added and the mass fraction of S in the molten iron reached 0.045%, the carbon-increasing rate was significantly slower compared to that of low-sulfur molten iron without added ferrous sulfide, where the mass fraction of S was only 0.0014%.

4. Selection and Addition Method of Carbon Additives

4.1 Carbon additives with low chlorine content should be selected. Typically, the mass fraction of nitrogen in cast iron molten metal is below 100 ppm. If the nitrogen content exceeds this level (150–200 ppm or higher), casting defects such as cracking, shrinkage porosity, and looseness are likely to occur—especially in thick-walled castings. This phenomenon arises from the increased need to add more carbon additives when the proportion of scrap steel is raised. Carbon additives based on coke, particularly pitch coke, contain substantial amounts of nitrogen. The mass fraction of nitrogen in electrode graphite is less than 0.1% or extremely low, whereas the mass fraction of nitrogen in pitch coke is around 0.6%. If a carbon additive containing 0.6% nitrogen is added at a rate of 2%, this alone will increase the nitrogen content by 120 ppm. Excessive nitrogen not only readily causes casting defects but also promotes the densification of pearlite and the hardening of ferrite, thereby significantly enhancing the strength of the material.

4.2 Method for Adding Carbon Additives

Stirring the molten iron can promote carbon addition. Therefore, compared with medium-frequency induction furnaces that have weak stirring power, it is significantly more difficult to achieve adequate carbon addition in medium-frequency induction furnaces with strong stirring power. As a result, medium-frequency induction furnaces may face the risk that carbon addition cannot keep pace with the melting rate of the metallic charge.

Even in industrial-frequency induction furnaces with strong stirring capabilities, the carbon-increasing operation cannot be neglected. This is because, as shown in the schematic diagram of induction furnace melting, there are separate upper and lower circulating iron flows within the induction furnace, and dead zones exist near the furnace walls at their boundaries. Graphite agglomerates that remain and adhere to the furnace walls will not melt away unless the molten iron is heated excessively and kept at a high temperature for an extended period.

It cannot be melted into the molten iron. Overheating and prolonged holding at high temperatures of the molten iron can increase its supercooling degree, thereby intensifying the tendency toward white cast iron formation in the casting. Moreover, in medium-frequency induction furnaces where strong eddy currents are generated near the furnace walls, if molten iron penetrates between graphite agglomerates adhering to the furnace walls, the penetrated metal will melt during the next smelting cycle, leading to erosion and damage of the furnace walls. Therefore, when the proportion of scrap steel is high and a large amount of carbon-increasing agents is added, extra care must be taken when adding these carbon-increasing agents.

The timing of adding the carbon-increasing agent cannot be overlooked. If the carbon-increasing agent is added too early, it tends to adhere near the bottom of the furnace, and carbon adhering to the furnace walls is not easily melted into the molten iron. On the other hand, if added too late, the optimal window for carbon enrichment will have been missed, leading to delays in melting and temperature rise. This not only postpones the time available for chemical composition analysis and adjustment but may also pose risks due to excessive heating. Therefore, it is best to add the carbon-increasing agent gradually, little by little, as the metallic charge is being introduced into the furnace.