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We’ve mentioned before. Melting The quality and purity of the ingredients are extremely critical, and the proportioning of the charge materials is actually another equally crucial factor.
I. Proportions of Raw Materials for Electric-Furnace Cast Iron and Synthesis of Synthetic Cast Iron
In the casting industry, it is often said that the composition of casting materials determines the microstructure, and the microstructure, in turn, influences the material’s performance. However, this statement is actually not entirely comprehensive. In our production practice, we have found that many types of cast irons, even when their compositions are identical, can exhibit significant differences in mechanical properties. The quality of molten iron is not only related to its chemical composition but also closely linked to factors such as the charge mix (the proportions of pig iron, scrap steel, return materials, and alloy additions), the melting and tapping temperatures, and the inoculation process. So-called synthetic cast irons refer to cast iron materials made by synthesizing with more than 50% scrap steel through a carburization process. Due to their higher melting temperatures, these synthetic cast irons are best melted in electric furnaces. Currently, the main types of synthetic cast irons include synthetic gray iron and ductile iron.

Through extensive practice, it has been found that for high-strength gray cast irons such as HT250 and HT300, scrap steel significantly influences strength, while pig iron affects the microstructure.
1. Ingredient Contraindications
(1) When using a high proportion of scrap steel (especially ship plates) combined with a high proportion of remelted materials (risers, waste castings, and iron chips), the amount of scrap steel added to produce gray iron should not exceed 50%.
(2) A high proportion of scrap steel (especially ship plates) combined with pig iron containing high levels of sulfur and phosphorus;
(3) The proportion of recycled materials exceeds 40% (including risers, scrap castings, and iron chips).
2. Optimized ingredient combination (%)
Composition of pig iron and scrap steel returned to the furnace: Ratio A 403030, Ratio B 304030, Ratio C 204040, Ratio D 205030.
3. Manganese and sulfur content
When it is necessary to increase hardness, the manganese content can reach 1.0–1.2%, but there is no need to correspondingly increase the sulfur content (the sulfur content in gray iron will be analyzed separately).
To cut costs, a certain company has been increasingly relying on scrap steel. Within two months, they conducted trial production of high-grade gray cast iron using scrap steel exclusively, with the proportion of scrap steel reaching as high as 60% at one point. For a while, in addition to adding scrap steel, they also incorporated return furnace charge and a small amount of iron chips. Initially, the quality was quite good; however, after a certain period, they began to notice that batches of castings were plagued by shrinkage cavities, looseness, and white, hard spots—and these defects continued to worsen over time.
Cause of this defect: Preliminary analysis indicates that the excessive MnS content in the molten iron is responsible for microscopic shrinkage cavities and looseness in the castings, with MnS accumulating to form white, hard spots. This phenomenon arises because high-grade gray iron HT300 has relatively high Mn requirements (around 1%), coupled with the inherently high Mn content in scrap steel (for instance, the 16Mn steel used in ship plates contains about 1.6% Mn). Furthermore, as S in the scrap steel and S from returned iron (including iron chips) react with manganese to form MnS, the accumulation of MnS in the charge material eventually reaches a critical level, leading to an excess and, consequently, the defects described above.
To reduce the MnS content in molten iron, it is common practice to add a certain amount of high-quality pig iron (low in S and Mn) for adjustment. Additionally, improving the inoculation effect can refine the MnS particles and mitigate their adverse effects.
When the amount of scrap steel added is excessive, since the melting point of scrap steel is around 1530°C, whereas the melting points of pig iron and returned materials are only about 1230°C, using more scrap steel increases electricity consumption, intensifies the tendency for the molten iron to become supercooled, and also causes it to absorb large amounts of nitrogen. Generally speaking, the resulting synthesis... Cast iron craftsmanship It is not suitable for gray cast iron but is more suitable for ductile iron.