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ADI Ductile Iron Ideal Metallographic Structure Design ADI Ductile Iron essentially adds a certain amount of graphite balls to the steel matrix. Therefore, the performance of ADI nodular cast iron is determined by the graphite sphere and matrix structure. The matrix structure properties are obtained by austempering treatment, and the graphite balls are refined to improve the roundness of the graphite spheres and to refine the grains of the ferrite, which is advantageous for improving the performance of the ductile iron.
The increase in the amount of cementite, especially in the form of a network, will seriously affect the ductile toughness. When the cementite exceeds 2, the brittle transition temperature will be increased. Therefore, the amount of free cementite should not exceed 2. The shape and size of graphite have great influence on the elongation and impact value under the condition that the matrix metallographic structure is qualified. Both spheroidal graphite and agglomerated graphite ensure good ductility of the ductile iron. The better the spheroidization, the better the toughness, and the higher the strength, elongation and impact value. The graphite is fine, the number is increased, the eutectic group is refined, and the toughness is also obviously improved.
The ideal metallographic structure design The ferrite in the ductile iron matrix should exceed 85, the pearlite should be less than 10. The phosphorus eutectic is less than 1. The chemical composition is reasonable to ensure the ductile metallographic structure and mechanical properties are qualified. Necessary conditions. Therefore, in actual production, the requirements of castings should be comprehensively considered and chemical components should be selected in combination with raw materials and production conditions. Carbon: As the carbon equivalent (CE = C 1/3Si) increases, the amount of ferrite in the as-cast nodular cast iron increases, so the impact value increases. Generally, the carbon equivalent of cast ADI nodular cast iron is selected from 4.3 to 4.6. Silicon: silicon is an element for promoting graphitization. Silicon is added by furnace charge and inoculant. Silicon added to molten iron in the form of inoculant increases the graphite crystal nucleus sharply and strongly promotes graphite. The eutectic is refined and the toughness is improved. When the spheroidizing process is used, the upper limit of silicon is determined to be 2.7 (the optimum silicon content is 2.4). Excessively high silicon content, although ensuring ferrite, significantly increases the brittle transition temperature and increases the brittleness of ductile iron at room temperature.
Manganese: The increase in manganese is unfavorable for as-cast high-toughness ductile iron. Manganese increases and refines pearlite, ferrite decreases, and elongation decreases. Foreign high-purity pig iron is used to make as-cast high-toughness ductile iron. The manganese is less than 0.03. According to China's pig iron resources, the manganese content should be less than 0.2. Phosphorus: Phosphorus has a significant effect on toughness, and the phosphorus content is controlled below 0.03. Sulfur: Sulphur has a high effect on spheroidization, which also seriously affects toughness. The original iron and water sulphur content is required to be as low as possible, and the final content is controlled below 0.03.
Magnesium and rare earth: ADI nodular cast iron should reduce the content of magnesium and rare earth under the condition of ensuring spheroidization. The content of magnesium is 0.03~0.06, and the rare earth is 0.02~0.04. In order to improve the hardenability and comprehensive mechanical properties of the new type of cast iron loop gears, some necessary alloying elements have been added. Minimize the interference with spheroidal elements. Smelting and processing technology According to the production practice experience, in order to obtain qualified as-cast ADI nodular cast iron, high-temperature low-sulfur molten iron must first be obtained. Therefore, in order to meet the chemical composition requirements of ductile ductile iron under cupola conditions, certain technical measures must be taken to ensure.
Selection of raw materials for new iron: The carbon content of pig iron can generally meet the needs. The main choice is the pig iron grade below 2.0. We choose Benxi pig iron Z15 or Q18 to strictly control the Mn, P and S content of the new iron. Scrap: Scrap is used to adjust the carbon and silicon content of molten iron and alter the genetics of pig iron. Use low-carbon steel thick steel scrap scrap without rust. The ratio is below 10. Back to the furnace material: the same grade of recycled material is selected, the surface is cleaned and sorted and stacked, the block size does not exceed 1/3 of the furnace diameter, and the proportion accounts for 20.
Coke: Select the machine coke with a sulfur content of less than 1.0 and dry to obtain high temperature low sulfur hot metal. Spheroidizing agent: XtMg5-8 rare earth ferrosilicon magnesium alloy is selected. Inoculant: Fe-Si75 ferrosilicon is used as inoculant and sputum compound inoculant.
Cupola smelting We use 1.5t/h two rows of large-pitch hot-air acid cupola smelting. The temperature of the tapping iron is high and stable. When the total coke ratio is 1:6, the temperature of the molten iron reaches 1500 °C, and the calcium carbide is desulfurized in the furnace. The dosage is 2 to 4, and 5-8 limes are added into the furnace to remove the total sulfur. Up to 40 ~ 50. In order to avoid serious erosion of the furnace lining, increase the amount of quicklime added, increase the amount of slag and operate the slag opening.
The spheroidizing treatment process is spheroidized by the pit punching method, and the spheroidizing agent having a blockiness of 15-20 mm is stacked in a groove preheated to dark red, and the covering inoculant 0.7 is added, and a layer of coke is covered thereon. The powder and grass ash are firm and generally no longer covered with iron filings or steel. At this time, the coke powder is filled in the gap between the inoculant and the spheroidizing agent to temporarily isolate the molten iron and the spheroidizing agent, thereby delaying the reaction time of the spheroidizing agent, substantially no magnesium light and soot, which improves the front of the furnace during the spheroidizing treatment. Visibility improves the environment of the workshop. At this time, the coke powder burns in the high-temperature molten iron to release heat to compensate for the cooling caused by spheroidization. The absorption rate is also greatly improved, the tapping temperature is >1500 °C, the secondary unpacking treatment temperature is 1450 °C ~ 1470 °C, the spheroidizing agent is added in 1.5, the reaction speed is controlled at 1 min, and the slag is stirred after the treatment to cover the perlite. Casting, pouring temperature is controlled at 1360 °C.
The inoculation treatment process is to make full use of the good effect of the inoculation treatment, using three incubations and compounding, the covering agent is inoculated with 0.7, the second inverting treatment is added to the floating silicon 0.4, and the 0.2é“‹ compound inoculant is added, and the inoculation is carried out three times. When adding 0.2 ferrosilicon powder (particle size 3~5mm) with the iron flow, the molten iron is always in a good gestation state during the pouring process and avoids the decline. After the sample is made into the gear blank, the quality of the casting is first checked for defects such as pores and cold partitions. Then, the sample is cut from the gear blank for composition, organization and performance analysis to adjust and determine the pressing process parameters.
The preheating temperature of the mold preheating temperature mold has a great influence on the mechanical properties of the workpiece. Therefore, the preheating temperature of the mold must be considered in terms of the quality of the workpiece. The temperature of the mold mainly affects the solidification and cooling rate of the part.
The working temperature of the mold part of the tooth blank is preferably selected from 200 ° C to 300 ° C. In order to keep the mold temperature substantially constant, in the actual production, the heating device of the mold should be provided and the cooling measures should be taken. Pouring temperature In order to increase the life of the metal type, a lower pouring temperature should be used, but for smaller parts, in order to maintain a longer solidification time for pressurization, a higher pouring temperature should be taken. The casting temperature of ductile iron is low, although pores are easily generated, but the metal solidifies and crystallizes under pressure, which helps to eliminate pores. Therefore, the pouring temperature is between 1320 and 1380 °C.
Compression temperature compression temperature is a key factor. The temperature is too high, and the molten iron is splashed when pressed, and there are defects such as gas. If the temperature is too low and the flowability of the molten iron is not good, there is a phenomenon that the upper type pressure is not lowered. The liquidus temperature of the experimental molten iron was about 1130 °C, and the compression temperature was controlled at a liquidus temperature of 20 °C.
Discussion on the experiment of pressing and casting of new type cast iron loop gear
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