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The development and improvement of melting technologies have primarily focused on enhancing the quality of molten iron, increasing efficiency, conserving energy, and improving environmental performance. With the implementation of the Konda Electric Furnace Green Casting System Upgrade Project, some companies have shifted their melting processes from a combined system of cupola furnaces and induction furnaces to medium-frequency induction furnace melting.

Medium-frequency induction furnace Melting iron liquid in medium-frequency induction furnaces has several distinct advantages over cupola furnaces: namely, medium-frequency induction furnaces significantly reduce environmental pollution, offer more convenient melting operations, and enable more precise control over chemical composition and melting temperature. However, they also have certain limitations. Under the same conditions, the quality of iron liquid melted in medium-frequency induction furnaces is generally inferior to that melted in cupola furnaces—for instance, the iron liquid tends to have fewer graphite nuclei, increased supercooling, and a greater tendency toward white cast iron formation. In hypoeutectic gray cast iron, the number of A-shaped graphite flakes tends to decrease markedly, while the amounts of D- and E-shaped graphite increase. Moreover, this shift leads to an increase in the amount of ferrite accompanying D- and E-shaped graphite and a reduction in the amount of pearlite. Additionally, medium-frequency induction furnaces exhibit a greater tendency toward shrinkage, making thick-walled sections of castings prone to shrinkage cavities and looseness defects, while thin-walled sections are more likely to develop white cast iron and hard edges. If the quality of melting is not properly controlled, these issues can lead to serious consequences for product quality. In recent years, several foundries in China have successively adopted medium-frequency induction furnaces for melting iron liquid, achieving significant economic and social benefits. As a result, an increasing number of enterprises have stepped up their research efforts to enhance quality control in medium-frequency induction furnace melting processes.

2. Medium-Frequency Induction Furnace Melting Process

2.1 Raw Material Control

Raw materials entering the plant must undergo sampling and analysis; any raw materials that fail to meet the required standards shall never be put into use. First, the chemical composition of pig iron, returned scrap, and waste steel significantly affects the quality of the molten iron during smelting—primarily by exerting a substantial influence on the original molten iron’s chemical composition. This, in turn, directly impacts the target values set for the alloy composition, making the accuracy of the original molten iron’s chemical composition a critical factor. Second, the quality of pig iron itself has a significant impact on smelting quality. For one thing, the graphite flakes in pig iron tend to be relatively coarse, which makes smelting more challenging and can lead to graphite inheritance—a phenomenon that directly affects the mechanical properties of castings. Due to variations in pig iron quality, differences in gas content, non-metallic inclusions, and harmful trace elements within the pig iron can result in varying degrees of genetic inheritance in the cast iron. In particular, even trace impurities can profoundly affect the microstructure and performance of cast iron. For example, trace elements such as lead, titanium, arsenic, and aluminum can alter the shape and distribution of graphite in cast iron. In normal production, chemical analysis and control of these trace elements are seldom carried out, making it difficult to detect harmful trace elements until they cause serious consequences and only then do they attract attention. Third, certain impurity elements mixed into waste steel can pose severe hazards to castings—some of which can even be fatal. Therefore, it is essential to strictly control the entry of impurity elements, especially those that can significantly affect casting quality. This requires rigorous screening of purchased waste steel raw materials to ensure the quality of the molten iron.

In production, whenever possible, select pig iron with high carbon content and low levels of phosphorus, sulfur, and interfering elements. Use pure medium-carbon or low-carbon steel as the scrap steel for melting, and based on analytical results, decide whether to retain or discard the alloying elements and trace impurities contained in the scrap steel. Give priority to scrap steel compositions that can stably form pearlite. Both pig iron and scrap steel must be treated to remove rust before use; any oil contamination adhering to them should be removed by high-temperature baking prior to utilization. For ferroalloys and inoculants, strive for stable chemical compositions and meet the required size and particle-grade specifications; store them separately by type to prevent moisture absorption. At the same time, care should be taken to ensure that the selected raw materials avoid defects inherited from cast iron furnace charge materials.

2.2 Charge Composition

For gray cast iron parts, under the same raw material composition, the mechanical properties of castings produced using molten iron melted in a medium-frequency induction furnace are significantly inferior to those of castings made from molten iron melted in a cupola furnace. This situation can sometimes lead to extensive shrinkage cavities, looseness, and cracks in the castings, and in severe cases, even result in the rejection of the entire casting. For this very reason, in the casting industry, when using induction furnaces for melting, it has become increasingly common to employ a process that involves adding scrap steel along with carbon-increasing techniques to produce both gray cast iron and ductile iron. Such a practice not only enhances the toughness and strength—key mechanical properties—of the cast iron, but also reduces casting shrinkage by leveraging the scrap steel carbon-increasing process, thereby markedly decreasing defects related to shrinkage. In this way, the induction furnace melting process effectively combines the strengths of both methods while mitigating their respective weaknesses. Li Chuanshi proposed: “When using a medium-frequency induction furnace for melting, the proportion of pig iron added should not exceed 20%.”

2.3 Control of the Melting Process

Based on the metallurgical characteristics of medium-frequency induction furnaces, a rational melting process is formulated. Strict control is exercised over each operational step—from charging materials and temperature management to adjusting chemical composition and adding alloys and carbon additives at different temperatures, as well as controlling the tapping temperature—aiming to achieve the following objectives within the shortest possible time: minimizing alloy burn-off and oxidation, precisely controlling the chemical composition of the molten iron, stabilizing the microstructure of castings, and ultimately improving casting quality.

(1) Temperature control during the melting process

In terms of temperature control during the melting process, the melting process is generally divided into three stages: temperature control for molten iron melting, temperature control for slag removal, and temperature control for holding and tapping the molten iron.

Control of molten iron melting temperature: The temperature at which the molten iron is fully melted prior to sampling determines the absorption of alloying elements and the balance of chemical composition. Therefore, it’s crucial to avoid high-temperature melting and charging, as well as to prevent the formation of “crusts” or “bubbles.” Otherwise, the molten iron will remain in a boiling or high-temperature state, leading to increased carbon burn-off, continuous reduction of silicon, oxidation of the molten iron, and accelerated accumulation of impurities. The melting temperature should be controlled below 1400℃, and the sampling temperature should be maintained at 1430℃.

Right. If the sampling temperature is too low, the ferroalloy may not have fully melted, resulting in a sample whose chemical composition is not representative. On the other hand, if the sampling temperature is too high, some of the alloy may burn off or undergo reduction, which could also affect the adjustment of composition during the refining stage. After sampling, the power of the medium-frequency induction furnace should be carefully controlled so that, once the chemical composition results are available, the furnace temperature will just reach the slag-scraping temperature.

Slag Removal Temperature Control: The temperature at which slag is removed is a critical process that determines the quality of the molten iron. This is because it is closely related to the stability of the chemical composition and the effectiveness of inoculation treatment, and it directly affects the temperature control during the holding and tapping of the molten iron. If the temperature is too high during slag removal, it will exacerbate the burnout of graphite nuclei and the reduction of silicon in the molten iron—especially in furnaces with acidic linings. Theoretically, once the silicon content in the molten iron rises excessively, carbon segregation may occur, disrupting the crystallization process according to the stable phase diagram and increasing the tendency toward white cast iron formation. On the other hand, if the temperature is too low, the molten iron will remain exposed for an extended period, leading to severe burnout of carbon and silicon. When the chemical composition is adjusted again, not only will the melting time be prolonged, causing the molten iron to overheat, but it will also easily result in loss of control over the chemical composition, increase the supercooling degree of the molten iron, disrupt normal crystallization, and ultimately degrade the quality of the molten iron.

Hot-melt iron temperature control for heat preservation: For gray cast iron, to ensure the optimal temperatures for pouring and inoculation, we maintain the temperature within the range of 1500–1530℃.

Both excessively high and excessively low tapping temperatures can affect the crystallization and inoculation effects of cast iron. If the temperature is too high, even if the rapid analysis results at the furnace show that C and Si levels are moderate, the white-mouth depth of the triangular test specimens will be excessive, or a mottled structure may appear at the center. In such cases, even if measures are taken to add extra carbon into the furnace or increase the inoculant dosage, the effect will still not be satisfactory. On the other hand, if the tapping temperature is controlled too low, the molten iron will have a lower temperature during pouring, which is detrimental to desulfurization and degassing of the molten iron and will negatively impact the effectiveness of inoculation treatment. As the temperature decreases, defects such as cold shuts, insufficient filling, unclear casting outlines, and shrinkage cavities will also become more frequent.

(2) Control of molten iron composition

Medium-frequency induction furnaces have advantages in adjusting and controlling the composition of molten iron. By fine-tuning the charge mix and adding appropriate alloying elements, it is possible to ensure that the molten iron composition meets the process requirements. Therefore, the control of molten iron composition in medium-frequency induction furnaces mainly focuses on two aspects: First, the sulfur content must be maintained within the range of 0.06% to 0.10% to guarantee a good inoculation effect; second, the loss or gain of elements during the holding process must be carefully considered. Generally, below 1450℃, the composition changes in molten iron are characterized by carbon, silicon, and manganese losses. However, above 1480℃, the composition changes involve carbon and manganese losses as well as increased silicon content. The higher the molten iron temperature, the greater the tendency for carbon content to be lost—losses can reach as high as 0.02% per hour. During idle periods or production shutdowns, the holding temperature of the molten iron should be maintained between 1350℃ and 1380℃. If the molten iron is held at too high a temperature for an extended period, the number of graphite nuclei in the molten iron will decrease, thereby affecting the inoculation effect. To address this issue, it is necessary to add a certain amount of carbon-increasing agents before resuming production to restore the desired composition.

Carbon-increasing agents are essential raw materials for melting in medium-frequency induction furnaces. The quality of the carbon-increasing agent and its method of use directly affect the quality of the molten iron. The main types of carbon-increasing agents include crushed waste graphite electrode material and petroleum coke. The former has relatively poor carbon-increasing efficiency; even when used, it is added to the furnace at an early stage during the charge-melting phase. In the later stages of melting, the carbon content is primarily adjusted using high-temperature calcined petroleum-coke-based carbon-increasing agents, which can achieve a carbon-increasing rate of 80% to 90% and offer rapid carbon-increasing speed. Some foundry enterprises also use silicon carbide as a carbon-increasing material, but this is likewise applied only during the melting stage.

The selection of carbon-increasing agents significantly affects the quality of carbon enrichment. Some manufacturers use inexpensive coke powder for molten iron carbonization, but excessive usage can lead to several problems: First, it generates large amounts of smoke and dust, causing environmental pollution; second, it increases sulfur content in the molten iron, adversely affecting casting quality; third, it results in substantial burn-off of the carbon-increasing material. Our plant has chosen Nanjing Ningban carbon-increasing agents as the primary material for increasing carbon content when melting cast iron in medium-frequency induction furnaces. During the melting process, after the carbon-increasing agent is added to the molten iron, sufficient time must be allowed for its complete dissolution. The duration required for dissolution is closely related to the type and quality of the carbon-increasing agent, the production process, the temperature of the molten iron, and the melting operation procedures. If the carbon-increasing agent fails to dissolve completely, it may cause abnormal graphite morphology in the castings, low carbon content in the molten iron, and poor overall casting quality. Under normal circumstances, if a relatively large amount of carbon-increasing agent is added, the melting temperature should reach or exceed 1500°C, followed by holding the melt at this temperature for 5 to 10 minutes to ensure thorough dissolution and absorption of the carbon-increasing agent. The absorption rate of the carbon-increasing agent is primarily influenced by its carbon content: the higher the carbon content, the lower the ash content, and the higher the absorption rate. Secondly, the method of application plays a crucial role. Most companies adopt the “add-to-furnace” method, which involves first adding a certain amount of light, thin charge material to the furnace bottom and then adding the entire required quantity of carbon-increasing agent according to the recipe. If the carbon-increasing agent is added too early, it may adhere to the furnace bottom. On the other hand, adding the carbon-increasing agent late in the melting process can have two adverse effects: First, it leads to significant burn-off of the carbon-increasing agent. If the slag in the furnace is not thoroughly removed, even small amounts of slag can severely impair the absorption of the carbon-increasing agent, causing it to float on the surface of the molten iron. Second, it reduces production efficiency, as the carbon-increasing agent added later requires additional time to be fully absorbed. Some melting technicians believe that carbon-increasing agents that have undergone high-temperature graphitization treatment should be added as early as possible during the melting process, allowing them to come into direct contact with the molten iron and providing ample time for complete melting.

(3) Overheating and Pre-treatment Processes

For iron melt produced by medium-frequency induction furnaces, if the melting temperature is too low, it is difficult to eliminate the hereditary nature of coarse graphite in pig iron. The reaction can only proceed when the melting temperature is between 1480 and 1500°C.

SiO2 + 2C → Si + 2CO

This process promotes the removal of non-metallic inclusions from the molten iron, thereby purifying the molten iron and reducing its oxygen content. As a result, the active elements—such as calcium and aluminum—in the inoculant can fully exert their nucleation effects. Raising the temperature of the molten iron can improve its purity, eliminate coarse graphite structures present in the charge materials, and refine the grain size. However, if the molten iron temperature is too high or the high-temperature holding period is excessively long, it can lead to severe carbon burn-off, significantly reduce the number of graphite nuclei, and increase the likelihood of E-type graphite formation in castings, with longer graphite flakes. In practice, the overheating temperature of the molten iron should be controlled within the range of 1520–1540°C, and the molten iron should be held at this elevated temperature for 5–10 minutes before pouring. For each batch of molten iron produced during furnace lining and baking, it is generally advisable not to use it for casting cylinder heads or cylinder blocks, in order to prevent leakage defects in the castings. Additionally, prior to tapping, a certain amount of metallurgical silicon carbide should be added into the medium-frequency induction furnace for pretreatment, providing the molten iron with an adequate number of crystal nuclei to meet the metallographic requirements of the castings.

(4) Nurturing Treatment Process

To ensure high-quality castings and improve their machinability—especially to prevent undesirable phenomena such as porosity, segregation, and hard spots in thin-walled sections in certain special castings—our factory employs different inoculation processes tailored to the varying performance requirements of the parts we produce. For cylinder blocks and cylinder heads, we use a composite inoculant consisting of silicon-barium and 75% ferrosilicon. The addition of an appropriate inoculant can refine the matrix microstructure, improve the morphology of graphite, reduce the tendency toward white cast iron formation, lower section sensitivity, and enhance machinability. Currently, within the casting industry, there is no universally accepted understanding of the mechanism by which inoculants exert their effects. Some studies suggest that when inoculants are added to molten iron, they act as deoxidizers, forming numerous refractory, highly dispersed nuclei such as silicon dioxide. These nuclei have crystal structures similar to graphite and exhibit lattice constants that are remarkably close, which facilitates graphite precipitation and promotes the formation of gray cast iron while refining the pearlitic matrix and improving the mechanical properties of gray cast iron. Others argue that when inoculants are introduced into molten iron, they introduce rapid-cooling particles with non-uniform compositions, thereby creating intense fluctuations in both concentration and temperature within the melt. These fluctuations promote the nucleation of a large number of graphite crystals.

3 Conclusion

(1) Compared to cupola-furnace-melted iron, the molten iron produced by medium-frequency induction furnaces requires heightened attention to material selection, acceptance criteria, and mix design—especially in terms of controlling the quality of scrap steel.

(2) The fundamental elements for producing high-quality castings using medium-frequency induction furnaces include stable raw material quality, a rational charge mix, high-temperature melting or high-temperature superheating treatment, stable chemical composition, and effective inoculation treatment.