Furnace control for melting high-strength cast iron with medium-frequency induction heating equipment

(1) Control of melting of medium frequency induction heating equipment. It is necessary to prepare a reasonable smelting process based on the metallurgical characteristics of the intermediate frequency electric furnace, and strictly control all aspects from charging, temperature control, adding alloy, carburizer, slagging agent and tapping temperature at different temperatures, and strive to use the shortest smelting time. , minimize alloy burning loss and oxidation, achieve the purpose of controlling and stabilizing the metallographic structure and improving the quality of castings. In production practice, the entire smelting process is divided into three stages for temperature control. These three temperatures refer to: melting temperature, slag removal temperature and tapping temperature.

① Melting temperature: that is, the melting period before the sampling temperature, which determines the absorption of alloy elements and the balance of chemical composition. Therefore, high-temperature melting and feeding should be avoided, and "crushing" of the shed should be avoided. Otherwise, the molten iron will be in a boiling or high-temperature state, the carbon element will be burned more severely, the silicon element will continue to be reduced, and the oxidation of the molten iron will aggravate and the impurities will increase. According to the process requirements, the melting temperature is controlled below 1365°C, and the sampling temperature is controlled at (1420 ± 1)°C. If the sampling temperature is low, the ferrous alloy has not been completely melted, and the chemical composition of the sample taken is bound to be unrepresentative; if the temperature is too high, the alloy will be burned. Or reduction will also affect the composition adjustment during the refining period. After sampling, the intermediate frequency power should be controlled, and the slag removal temperature should be reached just after the quality management instrument in front of the furnace displays the results of the chemical composition.

②Slag removal temperature: The slag removal temperature is an important link in determining the quality of molten iron, because it is closely related to the stability of the composition and the effect of the inoculation treatment, and directly affects the control of the tapping temperature. Excessive slagging temperature will aggravate the burning of the graphite crystal core of the molten iron and the reduction of silicon. Especially for acidic furnace linings, theoretically high silicon content in the molten iron will produce carbon emissions, which will affect the crystallization according to the stable system and may produce white spots. tendency; if the temperature is too low, the molten iron will be exposed for a long time, and the carbon and silicon will be severely burned. When the composition is adjusted again, not only will the smelting time be extended to overheat the molten iron, but it will also easily cause the composition to get out of control, increase the degree of subcooling of the molten iron, and destroy normal crystallization.

③Tapping temperature: In order to ensure the best temperature for pouring and incubation, it is generally controlled at 1520~1550℃. The high or low tapping temperature will have an impact on the crystallization and inoculation effects of cast iron. If the temperature is too high (more than 30°C above the process specified temperature), although the quick analysis results in front of the furnace for C and Si are also moderate, the trial pouring triangle test The depth of the white patch may be too large or the center may appear numb. When this happens, even if measures are taken to add carbon to the furnace or increase the size of the heart, the pit will appear. 6. Take out the old wpP and add carbon to the furnace or increase the size and perform cooling treatment in the furnace, that is, Add 10% to 15% of the total amount of baked new iron into the furnace, so that the pit at the center of the specimen's fractured edge will turn into gray, and the depth of the white edge at the top will become smaller. If the high temperature continues for a long time, after taking the above method, carbon supplementation measures in the furnace must still be implemented. The tapping temperature is controlled according to the pouring temperature. The suitable pouring temperature for shell iron castings is (1440 ± 20) ℃, which can achieve "high temperature tapping and appropriate temperature pouring". It is of course better to strictly control it. Because the low tapping temperature will cause the pouring temperature to be lower than 13°C, which is not conducive to desulfurization and degassing, and especially affects the inoculation treatment effect. As the temperature decreases, problems such as cold insulation and unclear outlines increase significantly.

(2) Incubation treatment of molten iron. The production of transmission casings is incubated with HT250 to improve the wear resistance of the material, significantly improve the structure and performance of the casting, significantly increase the hardness value on each section, and stabilize the pearlite on the thick section. It has the same effect in terms of quantity, and can also improve the sensitivity of its wall thickness and the good cutting performance of castings during machining. It has a special role in preventing looseness and leakage of shell castings.

① The amount of inoculant added is determined based on the wall thickness, chemical composition, pouring temperature and other factors of the shell casting. The principle is that there will be no looseness or leakage in the thick wall and no hard areas in the thin wall.

Production practice shows that Sr, Ba, Ca, Si-Fe inoculants are ideal inoculants for high-strength gray cast iron. This inoculant exerts the anti-decay ability of barium (Ba) and increases the occupancy rate of A-type graphite. Strontium (Sr) ) has a strong ability to eliminate white spots, the proportion of calcium (Ca) and silicon A-type graphite, strontium "(Sr)" and silicon is an ideal choice for the inoculation treatment of high-strength cast iron.

②The relationship between the number of incubations and the effect of incubation. As the number of incubations increases, the uniformity of the graphite distribution inside the cast iron improves, and the difference between the A-type graphite occupancy rate and the graphite length is large. Type A graphite that has been incubated more than twice has a high occupancy rate, even distribution and moderate length. What's more important is that multiple incubations increase the number of non-spontaneous crystal nuclei and strengthen the matrix, thereby improving and stabilizing the strength of cast iron.

After the flow-compound inoculation treatment and the inoculation with barium ferrosilicon + 75 ferrosilicon in a funnel-type inoculation bag, the key to controlling the inoculation effect is to prevent the flow-following inoculation from lagging behind the pouring. The molten iron after inoculation should be poured within a limited time, generally no more than 8 minutes. The second incubation in the bag for 3 to 5 minutes will give better incubation effect. Silicon barium inoculant can eliminate the white spots of HT250, improve its graphite shape and distribution, and eliminate E and D type supercooled graphite. Because the E-type graphite and ferrite structure will reduce the density of the material and seriously deteriorate the anti-leakage performance.

(3) Production effect. There is no white spot in the thinnest part of the casting soil. Its tensile strength reaches above HT250. The hardness of the test rod reaches 190~230HB. The shell body is dissected and the hardness is around 190HB. The quality coefficient of the casting is significantly improved. The metallographic structure reaches According to the casting level of foreign prototype casings, the pearlite content is 85% to 90% or more, which meets the requirements of the reducer casing, and its mechanical properties reach the material level of the transmission casings of similar foreign models.