1 Graphitization of unmelted particles in carburizer
In the molten molten iron, in addition to the carbon that has been dissolved in the molten iron, there is residual, undissolved, carbon in the form of graphite, which is entangled into the stirred liquid stream in a granular form. Most of the unmelted, coarse graphite particles are suspended in the molten iron surface near the furnace wall when the current is applied, and a part of the graphite particles adheres to the middle of the furnace wall, which is equivalent to the dead angle of stirring. At this time, once the energization is stopped, these coarse graphite particles will be gradually suspended due to buoyancy. During the melting process of graphite, extremely small particles that are beyond the observation range of the optical microscope can be suspended in the molten iron not only when the current is energized, but even when the energization is stopped.
According to reports, the closer it is to the material that forms the eutectic nucleus, even if the added crystals have a slightly different crystallinity from the eutectic graphite, compared with other materials that can be inferred to form a graphite core, the degree of convergence is bound to be greater. From this point of view, it can be considered that the suspended fine graphite particles are conducive to the formation of graphite cores, and can prevent the cast iron from being undercooled and whitened.
2 Effect of particle size of carburizer on carburization effect
2.1 Effect of particle size of recarburizer on recarburization time
The particle size of the recarburizer is the main factor affecting the melting of the recarburizer into the molten iron. A, B, and C recarburizers with approximately the same composition and different particle sizes in Table 1 were used for the recarburization effect test. The results are shown in FIG. 1. Although the carbon replenishment rate is the same after 15 minutes, the carbon replenishment time to reach 90% carbon replenishment rate is quite different. It takes 13 minutes to use the C recarburizer without particle size treatment, 8 minutes to remove the fine powder A recarburizer, and only 6 minutes to remove the fine powder and coarse B recarburizer. This shows that the particle size of the recarburizer has a greater effect on the recarburization time, and it is not good to mix fine and coarse particles, especially when the content of fine powder is high. Effect of particle size of recarburizer on recarburization time Table 1 Composition and particle size (mm) distribution of recarburizer for test (1)
2.2 Effect of particle size of recarburizer on recarburizer
Japan's Nakae and Mochizuki have tested the carbon content of the carbon recarburizers with the particle size distribution shown in Table 2 for C with a mass fraction of 99.8% and S with a mass fraction of 0.023%. The test results are shown in Figure 2. It can be seen from the figure that the carburizing effect of the recarburizer E having a particle size that is finer than the fine powder is extremely poor, and the carburizing effect of the carburizing agent G having a particle size that is coarser is better; and the carburizing agent that appropriately removes the fine powder and coarse particles A has the best carburizing effect.
The above facts confirm that in order to improve the effect of carburizing, the carburizing agent should be treated with particle size to remove fine powder and coarse particles. Figure 2 The influence of the particle size of the recarburizer on the amount of recarburization Table 2 Composition and particle size (mm) distribution of the recarburizer for the test (2)
3 Effect of chemical composition of molten iron on the effect of carburizing agent
3.1 Effect of silicon on the recarburization effect of carburant
The silicon in the molten iron has a great influence on the effect of carburizing. Iron with high silicon content has poor carburizing properties. Someone changed the mass fraction of Si in the molten iron in the range of 0.6% ~ 2.1%, and added two types of A and B recarburizers as shown in Table 1. The difference between the recarburization time after adding the recarburizer was observed. The results are shown in Fig. 3. When the mass fraction of Si in the molten iron is high, the rate of carbon addition is slow. Figure 3 Effect of silicon content in molten iron on carburization
3.2 Effect of sulfur on the recarburization effect of the recarburizer
Just as the mass fraction of silicon in the molten iron affects the carburization effect, the sulfur content also has a certain effect on the carburization. Use the A carburizing agent in Table 2 and add iron sulfide for reagents before adding. Observe the effect of S mass fraction on carburization. When the mass fraction of S in the iron sulfide and molten iron is 0.045%, compared with the low sulfur iron liquid phase with 0.0014% of the mass fraction of S in the iron sulfide and molten iron, the rate of carbon increase is much slower. .
4 carburant selection and adding method
4.1 Carbogens with low nitrogen content should be selected
The mass fraction of nitrogen in cast iron molten iron is usually below 100 ppm. If the nitrogen content exceeds this concentration (150-200 ppm or higher), it is easy to cause cracks, shrinkage or loose defects in castings, and thick-walled castings are more likely to occur. This is due to the increase in the amount of carburant added when the proportion of scrap steel is increased. Coke-based recarburizers, especially pitch coke, contain a large amount of nitrogen. The mass fraction of nitrogen in the graphite of the electrode is less than 0.1% or a very small amount, and the mass fraction of pitch coke nitrogen is about 0.6%. If you add 2% of a carbon recarburizer with a mass fraction of 0.6% nitrogen, this adds 120 ppm mass nitrogen alone. A large amount of nitrogen is not only easy to cause casting defects, but nitrogen can promote pearlite compaction, ferrite hardening, and strongly increase strength.
4.2 Adding method of carburant
Stirring of molten iron can promote carbon increase. Therefore, medium frequency induction electric furnaces with weak stirring force are much more difficult to increase in power than industrial frequency induction electric furnaces with strong stirring force. Possibility. Even if it is a power frequency induction electric furnace with strong stirring force, the carbon increasing operation cannot be ignored. This is because from the principle diagram of induction furnace melting, it can be seen that there is a stirred iron flow in the induction furnace that is separated from the top and bottom, and there is a dead angle near the boundary of the furnace wall. The graphite clusters that stay on the furnace wall and adhere to it cannot be melted into the molten iron without excessive temperature rise and long-term heat preservation of the molten iron. Excessive heating of the molten iron and long-term heat preservation will increase the degree of molten iron subcooling, and tend to increase cast iron whitening. In addition, for medium-frequency induction electric furnaces that generate a strong induced current near the furnace wall, if molten iron is drilled between the graphite clusters attached to the furnace wall, the molten metal is melted during the next furnace melting, causing erosion And damage the furnace wall. Therefore, when the proportion of scrap steel is high and the amount of recarburizer is added, more attention should be paid to the addition of recarburizer.
The time of adding the recarburizer cannot be ignored. If the time of adding the recarburizer is too early, it is easy to make it adhere to the furnace bottom, and the recarburizer attached to the furnace wall is not easily melted into the molten iron. In contrast, if the addition time is too late, the opportunity for carbon addition will be lost, resulting in a delay in melting and heating time. This not only delays the analysis and adjustment of chemical composition, but also may cause harm due to excessive temperature rise. Therefore, it is better to add the recarburizer little by little during the process of adding the metal charge.