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To understand the complete medium-frequency induction furnace melting technology, let’s take iron melt smelting as an example and take a closer look. Medium-frequency induction furnace Technology.

1. Basic requirements for molten iron melting

To smelt high-quality molten iron using an intermediate-frequency induction furnace, it is crucial to focus on two key aspects: First, control the proportion of newly added pig iron and make greater use of scrap steel; second, ensure that the molten iron undergoes superheating at a high temperature (1500–1550℃).

2. Charge Composition

To minimize the adverse effects of coarse graphite flakes in pig iron on the microstructure of castings, the proportion of pig iron added should be controlled, and more scrap steel should be used instead. According to the experience shared by Dongfeng Motor Corporation: when producing engine cylinder block castings, the amount of scrap steel added ranges from 60% to 80%, compared to a lower addition rate (10%) for castings with the same composition.

Compared to the original composition (~20%), the strength is 19.8 MPa higher, the hardness decreases by 9 HBW, and the cylinder block leakage rate is reduced by 2%. Therefore, using more scrap steel in place of pig iron can help lower raw material costs. In actual production, the proportion of returned materials typically stands at around 30%, while the proportion of scrap steel added is controlled between 40% and 50%, with pig iron accounting for 20% to 30%.

3. Control of molten iron composition

Medium-frequency induction furnaces have advantages in adjusting and controlling the composition of molten iron. By modifying the charge mix ratio and adding supplementary elements, the composition of the molten iron can be precisely maintained to meet process requirements. Therefore, control of the molten iron composition in medium-frequency induction furnaces primarily focuses on two aspects: first, maintaining the sulfur content (ws) within the range of 0.06%~.

0.10% to ensure good casting performance; second, the gain or loss of elements during the holding process of the molten iron must be carefully considered. Typically, this is done at 1450℃.

The following changes in molten iron composition are characterized by carbon, silicon, and manganese burn-off. At temperatures above 1480℃, the composition changes in molten iron are marked by carbon and manganese burn-off as well as increased silicon content. The higher the molten iron temperature, the greater the tendency for carbon content to burn off—this can reach a rate of 0.1% per hour. During idle periods or production shutdowns, the molten iron should be kept at a holding temperature between 1350 and 1380℃. It is important to note that if the molten iron is held for an excessively long time (e.g., across shifts), the number of crystal nuclei in the molten iron will decrease, thereby affecting the inoculation effect. To address this, it is necessary to add a certain amount of cold charge before resuming production to restore the proper conditions.

Carbon additives are important raw materials for melting in medium-frequency induction furnaces.

The quality and usage method of carbon-increasing agents directly affect the melting quality of molten iron. The main types of carbon-increasing agents include crushed waste graphite electrode materials 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 process. In the later stages of melting, 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%. Some foundries use silicon carbide as a carbon-increasing material, but this is also applied exclusively during the melting stage. High-quality carbon-increasing agents must have low nitrogen content. According to available data, once the nitrogen content in molten iron reaches a certain critical value, the castings will develop dispersed, crack-like nitrogen porosity.

4. Propagation Process

Due to the tendency of molten iron melted in medium-frequency induction furnaces to form a highly chilled microstructure and the reduction in graphite nuclei within the molten iron, when selecting inoculants and inoculation processes, it is preferable to use a composite inoculant consisting of silicon-barium and silicon-iron inoculants. Some manufacturers employ silicon-zirconium inoculants during flow inoculation; these inoculants also have deoxidizing effects, which help improve the fluidity of the molten iron, reduce the tendency toward chill formation in cast iron, promote the formation of uniform, fine-type A graphite, and even slow down the degradation of inoculation effectiveness. Silicon-strontium inoculants are primarily used for producing leak-prone components such as cylinder heads; they enhance the uniformity of casting microstructure and reduce the tendency toward chill formation without increasing the number of eutectic clusters. As for the inoculation method, the inoculant is added directly into the molten iron stream as it flows out of the electric furnace.

5. Overheating treatment

Raising the molten iron temperature can improve its purity, eliminate coarse graphite structures in the charge materials, and refine the grain size. However, if the molten iron temperature is too high or the high-temperature exposure lasts for an excessively long time, it can lead to severe carbon burn-off, a significant reduction in graphite nucleation sites, and an increased likelihood of E-type graphite formation in castings, with longer graphite flakes. Typically, the superheat temperature is controlled within the range of 1500~.

1550℃, with a superheat duration of 7 to 10 minutes. For each furnace charge used for melting and sintering iron, it is generally advisable not to cast cylinder-head type castings, in order to prevent leakage defects in the castings.

 

Whether using the twin-furnace melting method or the electric furnace melting method, the fundamental elements for producing high-quality castings include stable raw material quality, a rational charge mix, high-temperature melting or high-temperature superheating treatment, consistent chemical composition, and effective inoculation treatment.