How to improve the quality of electric furnace cast iron?

Given the unique characteristics of electric furnace cast iron smelting, foundry professionals must update their thinking in many areas—including cast iron composition selection, charge composition, scrap steel usage, inoculation processes, carbon enrichment and de-carbonization, sulfur enrichment and de-sulfurization, spheroidization processes, creep hardening processes, temperature control, and pouring techniques—and adopt practical measures to ensure and improve product quality.

Drawing on thirty years of experience in cupola furnace production and over a decade of electric furnace production, technical experts have summarized lessons learned regarding the ten major issues mentioned above. We share these insights with our colleagues in the foundry industry, hoping they will prove helpful to professionals in the field.

1.Proportions of Charges for Electric Furnace Cast Iron and the Production of Cast Iron

In the foundry industry, it is often said that the composition of casting materials determines the microstructure, and the microstructure dictates the properties; however, this statement is not entirely accurate. In our production practice, we have found that many cast irons exhibit significant differences in mechanical properties even when their compositions are identical. The quality of molten iron is not only related to its composition but is also closely linked to the charge composition (proportions of pig iron, scrap steel, return material, and alloy additions), melting and tapping temperatures, and inoculation processes. So-called synthetic cast iron refers to cast iron produced using a method of carbon enrichment, in which scrap steel constitutes more than 50% of the charge. Because this process requires a high melting temperature, it is suitable only for smelting in electric furnaces. Currently, the main types of synthetic cast iron are synthetic gray iron and ductile iron.

Through extensive practical experience, it has been observed that for high-strength gray cast irons such as HT250 and HT300, scrap steel primarily influences strength, while pig iron affects the microstructure.

Material Batching Taboos

(1) High proportion of scrap steel (especially ship plates) should not be combined with high proportion of recycled materials (gating risers, scrap castings, iron filings). The amount of scrap steel added to the synthetic gray iron should not exceed 50%.

(2) High proportion of scrap steel (especially ship plates) should not be combined with pig iron with high sulfur and phosphorus content.

(3) Recycled materials should not exceed 40% (gating risers, scrap castings, iron filings).

Optimized Material Batching Combinations (%)
Composition of Pig Iron, Scrap Steel, and Recycled Materials

Ratio A 40 30 30, Ratio B 30 40 30, Ratio C 20 40 40, Ratio D 20 50 30

Manganese and Sulfur Content

When increased hardness is required, the manganese content can reach 1.0-1.2%, but a corresponding increase in sulfur content is not necessary (the sulfur content in gray iron will be analyzed separately).

A company, in an effort to save costs, used excessive amounts of scrap steel to trial-produce high-grade gray cast iron within two months. Scrap steel usage once reached 60%. For a period, in addition to scrap steel, recycled materials and a small amount of iron filings were added. Initially, the quality was good, but after a while, batches of castings were found to have shrinkage cavities, porosity, and white hard spots, which continued to worsen.

Cause of this defect: Preliminary judgment is that the excessive MnS content in the molten iron caused micro-shrinkage cavities and porosity in the castings, with MnS enrichment forming white hard spots. This is because high-grade gray iron HT300 requires a relatively high Mn content (around 1%), and scrap steel itself is also high in manganese (16Mn steel in ship plates contains about 1.6% Mn). The accumulation of sulfur (S) in the scrap steel and the MnS produced by the reaction of S and manganese in recycled iron (including iron filings) in the furnace charge reaches a certain level, resulting in excessive MnS and causing the aforementioned defects.

To reduce the MnS content in molten iron, a certain amount of high-quality pig iron (low S, low Mn) is generally added to adjust the content. Additionally, improving the inoculation effect can refine the MnS and reduce its adverse effects.

When too much scrap steel is added, since the melting point of scrap steel is around 1530 degrees Celsius, while the melting point of pig iron and recycled materials is only around 1230 degrees Celsius, using more scrap steel increases power consumption, increases the tendency for supercooling of molten iron, and also adsorbs a large amount of nitrogen. Generally speaking, synthetic cast iron processes are not suitable for gray cast iron, but are more suitable for ductile iron.

2.Regarding the issue of sulfur increase in gray cast iron from electric furnaces

As mentioned earlier, compared to cupola melting, medium-frequency induction furnace casting of cast iron has several disadvantages besides its higher melting temperature, mainly in three aspects:

1. A greater tendency for undercooling of the molten iron, easily producing D and E type graphite that affects the mechanical properties of the material;
2. The purity of the molten iron results in fewer heterogeneous crystal nuclei, leading to poor inoculation effects. Under the same composition conditions, the castings have lower strength and are harder;
3. A greater tendency for shrinkage. In high-grade gray cast iron with high manganese content, micro-shrinkage cavities and porosity are easily produced.

The solutions to the above problems are as follows:

1. Increase the high-temperature holding time in the later stages of melting to ensure the uniformity of ferrocrystalline grains in the various furnace charges, especially refining graphite;
2. Appropriately increase the amount of exogenous heterogeneous nuclei (such as sulfides) to enhance the inoculation effect and promote the formation of type A graphite;
3. Control the sulfur and manganese content and their ratio in high-grade gray cast iron, and control the proportion of recycled materials to achieve suitable composition.

These measures vary depending on the structure of the casting and need to be mastered in practice.

Case Study

One day, a company smelted six heats of gray iron HT300 in an electric furnace to cast hydraulic valves G03 and G02, among other products. Internal analysis revealed extensive microscopic shrinkage cavities, porosity, and cracks, rendering all 830 pieces scrap (see attached image). The Brinell hardness was HBS241, and the chemical composition was C3.27, Si1.78, Mn0.83, S0.087, P0.04. Pearlite content was 98%, E-type graphite reached 80% (A-type 20%), and the graphite length was grade 5. According to relevant personnel, the problem lay with the molten iron material.

While the chemical composition analysis results appear normal for typical thin-walled HT300 castings, the hydraulic valve castings (with thicker walls) presented a problem. The cause of this defect is preliminarily determined to be excessive MnS content in the molten iron, leading to micro-shrinkage cavities, porosity, and cracks in the casting. In other words, the S and Mn content in the molten iron exceeds the suitable range for the casting (the composition varies depending on the casting).

Due to the addition of a certain amount of S-increasing agent during smelting, the S and Mn content in the molten iron accumulates to a certain level, causing the S content to exceed the requirements for normal solidification and crystallization of the casting, thus producing this type of defect. The solution is to stop adding the S-increasing agent, adjust the Mn content to ensure the normal content of the five elements in the HT300 gray iron, and after adjustment, all defects were eliminated.

In electric arc furnace gray cast iron, adding sulfur-enhancing agents to form a certain amount of MnS serves as a heterogeneous nucleus, improving the inoculation effect. This is theoretically correct. However, most recent literature suggests that the sulfur content in high-grade gray cast iron from electric arc furnaces should be controlled between 0.05% and 0.10%. Yet, many factories have demonstrated that when the Mn content is around 1%, if the sulfur content in the casting exceeds 0.05% according to composition analysis, shrinkage cavities begin to appear. When the sulfur content exceeds 0.07%, mass shrinkage cavities occur. How can this phenomenon be explained?

S in gray cast iron exists in two forms: elemental and combined MnS. The sulfur that acts as a nucleus in gray cast iron is primarily combined MnS. Current testing methods (both chemical and spectroscopic) can only analyze elemental S in castings and molten iron; combined MnS cannot be detected. When the elemental sulfur (S) content exceeds 0.05%, the combined sulfur content is relatively high. At this point, in the molten iron: MnO + FeS = MnS + FeO, FeO + C = Fe + CO, or 2FeO + C = 2Fe + CO₂.

During the solidification process, the molten iron precipitates CO or CO₂ and simultaneously produces some brown MnS powder, forming slag reaction shrinkage pores. Under certain conditions, these shrinkage pores can occur not only in electric arc furnace molten iron but also in cupola furnace molten iron. In fact, during the electric arc furnace melting process, we have already added some sulfur, which comes from:

1. Sulfur and phosphorus content from the remelting process: The sulfur and phosphorus content in the remelting system is much higher than that in the castings.
2. Sulfur in pig iron: Generally, the sulfur content in pig iron is not high. However, ordinary pig iron we buy carries varying degrees of slag (debris), which we do not test. But this debris contains high levels of sulfur and phosphorus, which will be carried into the furnace.
3. Rust from scrap steel and pig iron: Rust has a high iron oxide content, which increases the sulfur absorption rate in the molten iron. Under these circumstances, adding iron sulfide to increase sulfur is excessive. In actual production of high-grade gray cast iron parts, the elemental sulfur in the molten iron should be controlled between 0.03% and 0.05%.

3.Inoculation and Modification Treatment of High-Grade Gray Iron in Electric Furnaces

Regarding the inoculation process for high-grade gray iron (taking HT300 as an example), the traditional inoculation amount is 0.3-0.4% of the molten iron (mainly for cupola furnace production). In recent years, with the popularization of electric furnaces, the inoculation amount has gradually increased, with the latest data recommending 0.5-0.6%. Through long-term practice, I have selected an inoculation amount of around 0.8%, achieving a comprehensive improvement in strength, hardness, and machinability, and significantly reducing internal defects in the finished castings.

Case Study

A company produces high-grade solenoid valves. The technical requirements are a casting hardness greater than HB200 and a strength greater than 300 N/mm². The main wall thickness of this product exceeds 50 mm. Through multiple experiments, while increasing the primary inoculation amount, a secondary in-flow inoculation was adopted. This eliminated the defects of coarse microstructure caused by thick walls, improved the density of the castings, and ensured product quality.

Regarding secondary in-flow inoculation of molten iron, adding a uniform inoculant with a particle size of 0.2-0.7 mm before pouring is more suitable for thick parts, but when used for small parts, it increases the shrinkage of the molten iron.

For a period of time, some products from a certain company exhibited white, bright spots on their surface after processing, indicating high hardness and causing tool slippage. Analysis revealed that the inoculant was too large and incompatible with the volume of the molten iron ladle. This resulted in the inoculant not completely dissolving during pouring, leading to localized silicon enrichment and the formation of a hardened phase in the casting. The same defect also occurred when secondary in-flow inoculation was performed at a lower molten iron temperature.

A factory specializing in the production of HT300 gray iron hydraulic components was casting a KP pump body with a wall thickness of approximately 30mm. Following the experienced composition guidelines for HT300, the molten iron composition was: C 3.0-3.1%, Si 1.7-1.8%, Mn 0.95-1.05%, P 0.05%, S 0.04%. The tensile strength of the casting body reached 300 N/mm². However, multiple batches of products experienced shrinkage and cracking near the ingate. No matter how the gating system was adjusted, there was no improvement. As a last resort, the carbon equivalent was increased to reduce strength. When adjusted to C 3.2-3.3% and Si 1.8-2.0%, the defects disappeared. However, after processing and pressure testing, most of the products showed expansion and leakage, and the tensile strength of the casting body also failed the test, resulting in a batch of returns from the OEM. Recalling a previous batch of similar pump bodies, which, following advice, had sulfur added using ferrous sulfate, resulting in large-scale shrinkage cavities in the castings when the sulfur content in the molten iron exceeded 0.07%, leading to a large accumulation of scrap, a method was employed to address this issue. Based on the principle of rare earth desulfurization, when such scrap was added, a small amount of rare earth magnesium ferrosilicon (approximately 0.2%) was added during the inoculation process. This effectively reduced the sulfur content and resolved the shrinkage cavity problem.

Regarding the shrinkage depressions and cracks present in the KP pump at that time, although the original molten iron did not have a high sulfur content, a similar experiment was conducted by adding a small amount of rare earth magnesium ferrosilicon (approximately 0.2%) during the inoculation process, achieving satisfactory results and completely resolving the shrinkage cavity problem. Analyzing the mechanism, the shrinkage of cast iron is mainly caused by gases (including oxygen, nitrogen, hydrogen, etc.) in the molten iron. When these gases precipitate in the later stage of solidification, the molten iron cannot replenish them, resulting in defects. Rare earth magnesium silicon iron, as a gray iron modifier (and also a kind of inoculant), is very good at removing gases. The gas content of the molten iron is greatly reduced, and the defects are eliminated.