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1. Double-furnace melting The quality requirements for molten iron—the core of duplex melting—hinge on cupola furnace melting. In the production process, the primary roles of induction furnaces are to store molten iron, homogenize its chemical composition, and maintain and raise its temperature. The key to producing high-quality molten iron in a cupola furnace lies in the melting temperature of the iron; this temperature directly affects the stability of the microstructure and mechanical properties of the castings themselves. When the tapping temperature of the cupola furnace is too low, it directly leads to coarse graphite structures and excessively long graphite flakes in the casting matrix. Maintaining a stable molten-iron temperature is also an essential prerequisite for ensuring the stability of the cupola furnace’s molten-iron composition.

2. Quality Control of Charge Materials and Coke

The choice of coke directly affects the melting temperature in a cupola furnace. Production practice has shown that when using improved coke, the molten iron temperature at the furnace outlet typically ranges from 1420 to 1460℃. For example, in a certain casting, the graphite morphology at the specified sampling location (40 mm thickness) showed that nearly 80% of the graphite lengths were classified as Grade 3 (while the customer’s requirement was Grades 4 to 6). However, after switching to high-quality foundry coke, the molten iron temperature at the furnace outlet rose to between 1480 and 1510℃, completely resolving the issue of unqualified graphite length in this casting. Therefore, the prerequisite for producing high-quality castings through duplex melting is the use of high-quality foundry coke, which ensures that the molten iron temperature in the cupola furnace remains within the optimal range.

As-cast iron itself contains coarse graphite flakes (see Figure 1). Controlling the proportion of as-cast iron added can help improve the stability of molten iron quality. Another consequence of raising the melting temperature in cupola furnaces is an increased carbon gain rate in the molten iron, which allows for a higher proportion of scrap steel to be incorporated into the charge mix. Trace elements such as Pb and As present in as-cast iron can significantly affect casting quality. According to relevant data, when the Pb content in castings is excessively high, it can lead to abnormal graphite structures and an unusually high incidence of leakage in cylinder blocks and cylinder head castings. Therefore, when selecting as-cast iron, it is crucial to pay close attention to the content of trace elements. Using severely corroded scrap steel or thin, lightweight domestic scrap steel not only affects the iron yield during furnace melting but also has adverse effects on the quality of the molten iron.

3. Chemical Composition Control

In terms of chemical composition control, parts with different structures and characteristics require different process specifications. The double-furnace melting process employs a continuous iron-delivery melting method for production; however, in actual production, various types of parts need to be cast, and the required iron-delivery temperatures for these parts differ accordingly.

In the electric furnace, the molten iron has a carbon content of 3.20% to 3.35% and a silicon content of 1.5%.

At approximately 1.7%, the material properties of the casting are ensured by adding alloying elements.

For thick-walled parts, adding a certain amount of small scrap steel into the pre-furnace ladle can not only reduce the carbon content but also lower the temperature of the molten iron, thereby meeting the requirement for a lower pouring temperature of molten iron in thick-walled castings.

Chromium is a powerful pearlite-forming element that significantly enhances the hardness and strength of castings. Compared to other alloying elements, ferrochrome has the lowest price; thus, chromium is the most commonly used alloying element in the production of high-strength gray cast iron parts. However, during the solidification of cast iron, chromium strongly promotes the formation of cementite, thereby increasing the tendency for the cast iron to develop a white cast structure. Excessively high chromium content can easily lead to excessive hardness in the casting. Typically, the chromium content is controlled within the range of 0.15% to 0.35%.

Copper, tin, and molybdenum are primarily used in the production of engine cylinder blocks and cylinder head castings. When copper is combined with chromium, it can help reduce chromium's tendency toward white cast iron formation. Tin has a strong ability to promote the formation of pearlite; even small amounts of Sn (with a mass fraction of 0.04% to 0.06%) can increase the hardness of castings by 10–15 HBW while ensuring that the pearlite content in the castings reaches over 95%. 4. Inoculation Process

Casting treatment is the primary process approach for producing high-quality cast iron.

Proper inoculation can reduce white cast iron and promote the formation of Type A graphite. The type of inoculant should be selected according to specific requirements. In actual production, the most commonly used approach is to combine a composite inoculant with conventional silicon-iron inoculants—specifically, using a silicon-barium inoculant (or a rare-earth calcium-barium inoculant, a silicon-silver inoculant, etc.) in conjunction with a standard silicon-iron inoculant at roughly equal proportions of 50% each. Since composite inoculants are relatively expensive, adding excessive amounts not only drives up costs but also has adverse effects on the microstructure of the castings. While the choice of inoculant is one aspect, even more critical is ensuring uniform inoculation of the molten iron. Production practice has shown that directly adding the inoculant to the bottom of the ladle and employing the pouring-in method results in an uneven microstructure of the castings—especially when residual molten iron remains in the ladle, leading to poorer graphite morphology. After switching to pre-furnace flow inoculation (with a funnel added at the tapping spout, as shown in Figure 2), the issue of unstable graphite morphology in the castings has been effectively resolved.