Quality Control of Molten Iron in Induction Electric Furnace Melting

Quality Control of Molten Iron in Induction Electric Furnace Melting

1. Current Quality Issues of Molten Iron in Induction Electric Furnace Melting

1) Without using the carbon addition process, the number of nuclei is low, the degree of undercooling is large, and the tendency for white cast iron is significant.

2) If the molten iron has a large degree of undercooling and the inoculation is ineffective, overcooled D and E type graphite and accompanying ferrite can easily form in the gray cast iron structure, leading to a decrease in mechanical properties or the formation of white cast iron at the edges and corners.

3) Molten iron from induction electric furnaces has a high tendency to shrink, making castings prone to shrinkage porosity, shrinkage cavities, leakage, or cracks at the junctions of thick and thin walls.

4) The electromagnetic stirring characteristics of electric furnaces make it difficult for slag inclusions to float, easily causing slag inclusion and resulting in casting leakage or reduced mechanical properties.

5) The "decarbonization" reaction that occurs when the molten iron temperature in an electric furnace reaches 1500-1550°C has both beneficial effects on improving molten iron quality and adverse effects such as reducing the number of nuclei and increasing lining erosion.  Prolonged high-temperature holding can exacerbate decarbonization and silicon increase, leading to severe lining damage and increased white cast iron tendency.

2. Key Control Points for Improving Molten Iron Quality in Induction Electric Furnace Melting

Based on the characteristics of molten iron from induction electric furnaces, the key points for controlling molten iron quality are as follows:

1) Use less pig iron. When producing gray cast iron above HT250, the proportion of pig iron should be less than 10% to reduce coarse graphite and genetic issues of trace elements.

2) Use the scrap steel + carbon addition process to increase the number of nuclei, improve graphite quality, reduce shrinkage tendency, and lower the content of harmful trace elements.

3) Although low-sulfur molten iron obtained through the scrap steel + carbon addition process is beneficial for spheroidizing in ductile cast iron, for gray cast iron, a sulfur addition process should be used to ensure inoculation effectiveness and reduce overcooled D and E type graphite.

4) Strengthen inoculation, especially when carbon and sulfur additions are insufficient, by appropriately increasing the inoculant dosage or using instantaneous inoculation methods.

5) Purify furnace charge materials, strictly prohibiting rusty, sand-adhered, or other contaminated materials from entering the furnace. Return scrap, risers, etc., should be shot blasted to reduce slag inclusions that are difficult to float due to electromagnetic stirring.

6) Shorten high-temperature holding time and adopt "fast melt and quick pour" operations to prevent excessive "decarbonization" reactions that can cause excessive oxygen content (<10 PPM), reduced nuclei, and increased lining erosion.

7) Pay attention to nitrogen content in the molten iron.

The nitrogen content in induction electric furnace molten iron is typically higher than in cupola furnace molten iron. When scrap steel accounts for more than 40%, molten iron nitrogen content is linearly related to scrap steel addition, as shown in Figure 3-1.  Statistics show that when nitrogen content exceeds 100-140 PPM, castings are at risk of nitrogen porosity, so the upper limit should be controlled, with thin-walled castings <130 PPM and thick-walled castings <80 PPM.

8) Control molten iron temperature and holding time to prevent excessive carbon reduction and silicon increase that can cause composition fluctuations.