Medium frequency furnace manufacturers share the characteristics of molten iron and defect prevention strategies of medium frequency induction heating furnaces

A factory has completed the technical transformation of the smelting process of medium frequency induction heating furnaces in five foundry workshops. Each foundry workshop is equipped with two sets of medium frequency coreless induction furnaces. The two furnaces operate at the same time. One melts and the other keeps hot metal for continuous production. It not only improves production efficiency, improves the quality and yield of castings, but also improves the working environment in the factory, reduces the labor intensity of workers, and creates good economic and social benefits. However, when using electric furnaces for smelting production, it was found that electric furnaces There are many differences in characteristics between molten iron and cupola molten iron. They have excellent characteristics, such as easy control of temperature and composition, higher relative purity, etc., but they also have undesirable characteristics, mainly manifested in fluctuations in casting quality. Conduct in-depth and detailed research on the problems that arise in production after adopting the medium frequency induction furnace smelting process, and explore some experiences and countermeasures.

(1) Undesirable characteristics of electromelted iron. Under the conditions of the same raw materials, there are certain differences in the casting matrix structure and graphite morphology of the molten iron smelted in the electric furnace and the molten iron smelted in the cupola furnace. Moreover, there are few technical data related to the electric furnace direct melting process, making it difficult to practice and research.

1. Compared with cupola furnace molten iron, the number of crystal nuclei in electric molten iron is smaller, the degree of supercooling is increased, and the tendency of whitening is greater.

2. In hypoeutectic gray cast iron, the amount of type A graphite is easily reduced, and the amount of type D and E graphite increases. This increases the amount of ferrite associated with type D and E graphite, and decreases the amount of pearlite.

3. It has a greater tendency to shrink. Shrinkage cavities and shrinkage porosity are prone to occur in thick-walled parts of castings, and casting defects such as white holes and hard edges are prone to occur in thin-walled parts.

(2) Impact and analysis of undesirable characteristics

① Casting defects. After the electric furnaces in each workshop were put into production, the following typical quality problems occurred during the production process:

a. Crack defects appear in engine body and tractor box castings, and the scrap rate reaches 15%.

b. The engine cylinder head casting leaks, and the leakage waste reaches 50% on some working days.

c. The engine gear chamber cover casting has serious white spots, and the scrap rate once reached more than 40%.

Most castings have had defects such as poor graphite morphology and low pearlite content to varying degrees.

②Defect analysis. After adopting the electric furnace smelting process, the raw and auxiliary materials used for smelting have basically remained unchanged, and strict inspection procedures are required for materials entering the factory. Therefore, such differences caused by raw material factors are basically eliminated.

After the casting quality problems occurred, the technicians followed the work, carefully scrutinizing every ingredient and every detail during the operation, recording and analyzing the composition and metallography of the defective castings, and found a phenomenon: all castings with crack defects , the graphite form is mostly E-type graphite or E-type graphite content is relatively high; the graphite form of castings with white defects is mainly D-type. For this phenomenon, the reasons are analyzed as follows.

a. Liquid iron has low sulfur content. The "sulfide core theory" believes that when the sulfur content of the molten iron is low, the white hole depth is larger. As the sulfur content increases, the white hole depth gradually decreases and reaches a minimum value. At this time, the sulfur content is 0.05%. -0.06%. Then, as the sulfur content increases, the depth of the white hole increases again.

Some data point out: "At low sulfur, the number of eutectic clusters is small, that is, the degree of nucleation is very small. As the amount of sulfur increases, the number of eutectic clusters increases sharply. When the sulfur content reaches about 0.05%, the number of eutectic clusters tends to increase. Slow down”. The sulfur content of our electromelting HT molten iron is generally only about 0.03%. Practice has proved that when the S content of molten iron in the electric furnace is below 0.05%, often 0.03%, the tendency of castings to become white will increase. Analysis shows that due to low sulfur and sulfide content, the number of crystal nuclei decreases, the nucleation ability is low, the white mouth increases, A-type graphite decreases, and D and E-type graphite increases.

b. The main factor for the low sulfur content is that the molten iron is kept at high temperature for a long time. During the cupola furnace smelting process, due to the influence of sulfur content in the coke, sulfur increases in the molten iron. However, during the electric furnace smelting process, there is no source of sulfur increase, and there is no sulfur increase. Instead, S and other elements easily synthesize sulfides to form molten slag. It floats on the surface of the molten iron and is removed together with the slag. Not only does S not increase, but it decreases accordingly. Due to its own smelting characteristics, electric molten iron requires a long high-temperature holding time. As nucleation grains, sulfur compounds melt in large quantities during the holding period, resulting in a reduction in sulfide crystal nuclei and a reduction in graphite nucleation ability, and as the molten iron As the holding time continues to extend, the degree of subcooling continues to increase. The higher the grade of cast iron, the more significant the impact of holding temperature and time on the degree of subcooling. Regardless of whether it is incubated or not, as the temperature of the molten iron increases and the holding period increases, the degree of subcooling and the depth of the white spot increase.