yvonne@nydcasting.com 
1. The influence of melting molten iron in electric furnace on material properties
1.1 Influence on “spontaneous nucleation”
The melting point of scrap steel is higher than that of cast iron, and the melting point of carburizer is even higher. When scrap steel is melted and melted, the carburizer is heated and slowly dissolved and diffused, and the carbon in the carburizer can be eroded and absorbed by the molten steel. The molten steel gradually turns into molten iron, which is often called “synthetic cast iron”. Due to the high melting temperature of scrap steel, the overheating temperature of molten steel after it turns into molten iron is often high. At high temperatures, the carbon in molten iron is easily oxidized to CO, so some people believe that carbon in molten iron is also a “gas forming element”. CO has very little solubility in molten iron, and it is released into the atmosphere near the liquid surface after it is formed. In production practice, we will find that when high-temperature molten steel is poured into the ladle, radial sparks fly out of the ladle, which may be the carbon release phenomenon of high-temperature oxidation.
In the process of melting molten iron, the electric furnace has the characteristics of electromagnetic stirring friction. The overheating temperature of the molten iron is high, the overheating time is long, and there is stirring friction of the induced current. The fine crystalline graphite in the molten iron, namely the spontaneous crystal nucleus and the foreign crystal core, will gradually dissolve in the molten iron and disappear; or float through the liquid surface and stick to the slag collector and be picked out of the furnace. In this way, the substances in the molten iron that can serve as foreign crystal nuclei of graphite during eutectic crystallization are greatly reduced.
Sulfur is a harmful element in cast iron, especially in ductile iron. However, some information shows that when the sulfur content is less than 0.06%, some beneficial effects of sulfur cannot be exerted. There are fine and dispersed sulfide inclusions in cast iron, which can play an active and beneficial role in the nucleation and growth of graphite. The final sulfur content of synthetic cast iron made by melting scrap steel and adding carburizer in an induction furnace generally does not exceed 0.03%. If the sulfur content of the original molten iron is too low, the magnesium in the spheroidizer cannot combine with sulfur. Excessive residual magnesium not only hinders graphitization, but also causes casting defects such as shrinkage cavities and pores in the castings. If the amount of spheroidizer added is reduced, the spheroidization rate may be affected by comprehensive considerations.
In the induction furnace, the graphitized cores in the synthetic cast iron are greatly reduced due to factors such as low sulfur content, high overheating temperature, and stirring friction of the current. This kind of molten iron lacking graphitized crystal cores has a large degree of supercooling and extremely poor response to inoculation treatment. It is difficult to make the cast iron have a microstructure that meets the requirements through conventional inoculation treatment measures. Therefore, even if the chemical composition content fully meets the requirements, the cast castings are often cast with high hardness and are not easy to machine. According to some information: if sulfur increases from 0.02% to 0.06%, the tensile strength increases by more than 50MPa, which can increase one grade or more, and the hardness value can increase by HB20. Further increasing sulfur to 0.1%, the strength and hardness values do not change much. It can be seen that in gray cast iron, sulfur should be controlled at 0.06-0.1% (the automobile brake drum produced by our factory is made of HT250, and sulfur is controlled at 0.07-0.09%).
1.2 The influence of electric furnace smelting on improving material quality
1.2.1 In induction furnace smelting, the temperature of molten iron can be raised to above 1570℃, and it can be kept warm for a long time at high temperature. At this temperature, the inclusions brought in by the raw materials, as well as the slag and inclusions formed during the smelting process, can float to the surface of the molten iron. For scrap steel + carburizer, especially particle steel + scrap steel + carburizer + recycled materials, these furnace materials, whether scrap steel, particle steel or particle iron, are mostly white cast iron structures. White cast iron structures have strong heritability. To eliminate heritability, it is necessary to appropriately increase the melting temperature and increase the insulation time to better purify the molten iron and reduce casting defects.
1.2.2 The alloy element burnout is low, and the burnout of manganese and silicon in the molten iron is lower than that of cupola smelting. It is convenient to regulate each element and can stabilize the chemical composition content.
1.2.3 When producing ductile iron, excessive sulfur content will directly affect the quality of ductile iron. Such as low spheroidization level, poor material toughness, slag inclusions and other casting defects. There is no sulfur increase reaction when smelting cast iron in an electric furnace.
1.2.4 Use scrap steel + carburizer to produce synthetic cast iron. Since the inclusion content of scrap steel is low and the composition is stable, after adding carburizer and smelting at high temperature, the heredity of the charge is eliminated, and the purity of the molten iron is improved. At the same time, the carburizer has a nurturing effect, which makes the graphitization effect more stable and prominent, and the matrix structure grains of the castings will be more uniform and refined, so the toughness and strength of the castings produced are improved.
2. Make the best use of strengths and overcome weaknesses, and optimize the operation procedures
2.1 Pay attention to the cultivation of “spontaneous crystal nuclei” in the later stage of smelting. Add an appropriate amount of scrap steel to chill the molten iron, and at the same time add an appropriate amount of ferrosilicon and fine-grained carburizer, cover it with a heat preservation agent, reduce the power or shut down the power for a period of time to promote the precipitation of fine crystalline graphite.
2.2 During the process of furnace discharge or pouring, carry out sufficient multiple inoculation treatments to supplement “external crystal nuclei”. Small-grained carburizers, crushed ferrosilicon powder particles and composite inoculation agents can be added. Although the amount added is small, the effect of promoting nucleation is very good.
2.3 If the sulfur content is too low (especially when producing HT), some ferrosulphur can be added in an appropriate amount, but it must be controlled within the required range. In short, the optimization of the operating procedure refers to: the order of furnace charge entry, the control of smelting temperature and furnace discharge temperature, the selection and control of chemical composition, and enhanced inoculation and composite inoculation.