In modern cast iron production, cupola furnaces are being phased out due to environmental concerns, and most foundries have switched to medium-frequency induction furnaces for melting. Compared to cupolas, the medium-frequency melting process is relatively simple; the chemical composition and temperature of the molten iron are easily controlled, and the absence of carbon or sulfur pickup facilitates the production of low-sulfur iron. Environmental pollution is minimal, and the working environment and labor intensity at the furnace are significantly improved. By utilizing off-peak electricity rates for melting, production costs can be kept roughly comparable to those of cupolas. Furthermore, for castings of identical chemical composition and mold design, gray iron melted in medium-frequency furnaces exhibits higher strength and hardness than that melted in cupolas. However, medium-frequency furnace iron reaches higher superheat temperatures and has poorer fluidity; it also possesses undesirable characteristics such as a low number of crystallization nuclei and a high tendency for undercooling, chill formation, and shrinkage. Consequently, defects such as shrinkage cavities and porosity in thick-walled sections, as well as chill and hard edges in thin-walled sections, are prone to occur. In hypoeutectic gray cast iron, the amount of Type A graphite tends to decrease significantly, while the quantities of Type D and E graphite—along with associated ferrite—increase, resulting in a reduced pearlite content. These factors, combined with operational irregularities, lead to fluctuations in casting quality and disrupt normal production.

To address the new challenges associated with melting gray iron in medium-frequency furnaces, the author has overcome numerous obstacles—including a scarcity of technical data and the difficulties inherent in practical experimentation—to gradually accumulate valuable production experience and insights. It is hoped that this information will provide some assistance to small and medium-sized foundries currently struggling with operational difficulties and the growing pains of industrial upgrading.

1. Selection of raw materials and charge proportioning

The quality of charge materials directly affects the quality of molten iron; melting gray iron in a medium-frequency furnace places high demands on the cleanliness and dryness of the charge. Impure or contaminated charge materials (containing harmful elements) or poor melting control can lead to oxidation and reduced purity of the molten iron, severely degrading its metallurgical quality. This adversely affects the cast iron’s matrix microstructure and graphite morphology, leading to issues such as poor inoculation, chill formation, increased tendency for shrinkage porosity, and gas porosity. Therefore, the management of raw and auxiliary materials must be strengthened, and the use of heavily rusted or oil-contaminated charge materials strictly prohibited. To enhance purity and stabilize chemical composition, carbon steel scrap should constitute more than 50% of the charge mix. Regarding returns (remelt stock), gating systems (sprues and risers) from castings of the same material grade should be selected; these must be cleaned of adhering molding sand and coating materials before use, with an optimal usage rate of around 40%. Iron chips used should also originate from the machining of castings of the same material grade. As for pig iron—where impurities, trace elements, and microstructural defects tend to persist in the final casting—it is essential to select foundry pig iron with a stable source, minimal rust, low harmful element content, and preferably a grade of Z18 or higher; such pig iron ensures consistent, high internal casting quality. Sources of pig iron should not be changed arbitrarily, as this makes it difficult to prevent quality issues arising from substandard charge materials. Pig iron is best added during the initial melting stage (at a ratio of approximately 15%) to help improve graphite morphology. Commercial graphite recarburizers or high-temperature graphitized recarburizers should be used and added as early as possible during melting to ensure direct contact with the molten iron and allow sufficient time for dissolution and absorption. Ferroalloys and inoculants must meet chemical composition standards and have appropriate particle sizes. During charge preparation, the content of elements such as C, Si, and Mn should be calculated based on the charge ratio and material composition, with any deficiencies adjusted using recarburizers and ferroalloys. During the final stage of melting, when fine-tuning the chemical composition, pig iron can be added to increase the carbon content if it is too low, whereas scrap steel can be added to lower the carbon content if it is too high.

2.Influence of chemical composition

Carbon and silicon are elements that strongly promote graphitization; excessively high levels of C and Si lead to graphite coarsening, an increase in ferrite content, a decrease in pearlite content, and a reduction in the strength and hardness of the cast iron. Since the strength of the cast iron matrix increases with the pearlite content, the C and Si levels in high-strength gray iron should be appropriately lowered within a specific range; this helps refine graphite, promote pearlite formation, and enhance mechanical properties while still ensuring a gray iron structure. The carbon equivalent (CE) and the Si/C ratio significantly influence the microstructure and properties of gray iron; selecting appropriate CE and Si/C ratios is beneficial for improving the microstructure and performance of the casting. CE is the primary factor affecting the internal quality of gray iron castings; increasing the CE can greatly improve casting properties, reduce defects such as chill (white iron), shrinkage cavities, porosity, and leakage, and lower the scrap rate—factors that are particularly important for thin-walled castings. However, if the CE is too high, the amount of precipitated graphite increases and the tendency toward ferritization rises, reducing the casting’s tensile strength and hardness; additionally, slow cooling rates in thick-walled sections can lead to coarse grains and a loose microstructure. Conversely, if the CE is too low, localized hard zones may form in thin-walled sections, impairing machinability. A low CE promotes the formation of eutectic ledeburite and D- or E-type undercooled graphite in the microstructure, resulting in reduced casting properties, increased section sensitivity, higher internal stress, and elevated hardness. Appropriately increasing the Si/C ratio can enhance the strength and machinability of the cast iron. Under identical conditions, varying the Si/C ratio can lead to significant differences in mechanical properties and microstructure. When the CE is held constant, the strength and hardness of gray iron peak as the Si/C ratio increases from 0.6 to 0.8; conversely, when the Si/C ratio is constant, strength and hardness decrease as the CE increases. Therefore, in production, it is essential to strictly control the CE while also selecting and maintaining an appropriate Si/C ratio. When melting gray iron in a medium-frequency induction furnace, the Carbon Equivalent (CE) should be approximately 0.3% higher than that used in a cupola, and the carbon content should be about 0.1% higher. Additionally, the Si/C ratio should be maintained between 0.6 and 0.7 to ensure the cast iron retains appropriate hardness and high tensile strength.

Manganese and sulfur are elements that stabilize pearlite and inhibit graphitization. Manganese promotes and refines pearlite; increasing manganese content enhances the cast iron’s strength and hardness as well as the pearlite content in the microstructure. It promotes carbide formation and stabilization while suppressing the generation of FeS. Manganese also combines with sulfur to form high-melting-point compounds that act as heterogeneous nuclei to refine grains; consequently, higher manganese levels are used in high-grade gray iron. However, excessive manganese interferes with nucleation during solidification, reduces the number of eutectic cells, leads to coarse graphite and the formation of undercooled graphite, and ultimately lowers the cast iron’s strength. Sulfur is considered a restricted element in gray iron; however, an appropriate amount plays a positive and beneficial role in graphite nucleation and growth, improving inoculation effects and machinability. For gray iron melted in medium-frequency furnaces, a sulfur content of w(S) ≥ 0.06% is generally required to ensure effective inoculation. Moderately increasing sulfur content improves graphite morphology and refines eutectic cells—shortening flake graphite, curving its shape, and blunting its tips—thereby reducing the disruptive effect of graphite on the matrix and enhancing the cast iron’s properties. Thus, lower sulfur content is not necessarily better for gray iron. In contrast, phosphorus is generally a harmful element in gray iron; it tends to form low-melting-point phosphide eutectics at grain boundaries, leading to cold cracking. Therefore, lower phosphorus content is usually preferred; for castings requiring high density, the phosphorus content should be kept below 0.06%.

In actual production, the chemical composition design should be optimized based on factors such as the gray iron grade, wall thickness, and structural complexity. Strictly controlling the fluctuation range of each element is crucial for ensuring the quality and performance of the gray iron castings.