April 07, 2020

Induction furnace casting technology for automotive transmission housing


MC Manufacturing MC Shanxi Yangcheng Huawang General Centrifugal Casting Pipe Factory / Yuan Dongzhou Sun Jiaju Gao Xiuqi Wang Zhang Long Automotive Transmission Housing Induction Furnace Casting Technology Medium frequency induction furnace is a widely used and technologically advanced melting equipment developed over the past 10 years. Because the medium frequency induction furnace has the recognized advantages in metallurgical quality, composition and temperature regulation of smelting, improvement of working conditions, convenience of melting, rapid melting, cost reduction and economic benefit, the application range is expanding. Moreover, the capacity is developing in the direction of large tonnage and high power and frequency. From the 1990s, in the developed countries such as Europe and the United States, the manufacture of power frequency induction induction furnaces has almost ceased. It has become an irresistible trend to replace the power frequency induction furnace with the medium frequency induction furnace and replace the cupola, and has created a new path for the automotive industry to produce high quality castings.
The transmission housing is one of the basic components of the automobile. It is the skeleton of the multi-stage gear. It not only bears the load, but also can withstand the local large compressive stress that many high-strength bolts should tighten when tightening. The casting itself must have a relatively high compressive stress. High pressure resistance and corrosion resistance, so strict requirements for castings should not be loose, coarse grain, so as not to cause leakage of lubrication and cooling oil. In the material selection, the cast iron of HT150 or HT200 is traditionally used as the shell material, and the quality of the casting is obviously unable to meet the requirements of the rapidly developing automotive industry to continuously improve the overall quality.
This requires the addition of trace amounts of alloying elements such as Cr, Mo, and Cu, and the high toughness of the dense pearlite matrix is ​​suitable for the performance of the shell. In order to produce high-strength, high-quality cast iron automotive basic parts, it is inevitable to use medium-frequency induction furnaces during casting.
In the practice of casting a high-strength cast iron automotive transmission housing using a medium frequency induction furnace, the pre-furnace quality control practices are as follows.
The high-strength alloy cast iron is designed to meet the requirements of the transmission housing material HT250, hardness less than HB200, and easy cutting. The oil pressure test does not leak and the economic performance of casting production. Adding a small amount of multi-alloy in cast iron The composition, the selection of reasonable process parameters, so that cast iron has a certain chemical composition and cooling rate, to obtain the ideal metallographic structure and mechanical properties. To ensure mechanical properties, it is necessary to have a good matrix structure and graphite state. The controlled matrix structure is derived from a reasonable composition of ingredients and a suitable incubation process. The inoculation treatment can change the crystallization conditions of the molten iron to obtain a fine-grained graphite structure and a fine-grained pearlite matrix.
The design of the composition of high-strength and low-alloying cast iron for modern parts must first fully consider the influence of molten iron carbon equivalent on the cooling rate. The molten iron carbon equivalent is too high, the casting cooling rate is slow, the thick wall of the casting is prone to coarse grain, and the serious structure is loose. The oil pressure test is easy to produce leakage; if the carbon equivalent is too low, the casting wall tends to cause hard spots or partial Hard zone, resulting in poor cutting performance. When the carbon equivalent is controlled at 3.85% and 4.15%, the mechanical properties of the material can be guaranteed, and the eutectic point is close to the eutectic point. The solidification temperature range of the molten iron is narrow, which creates a more objective condition for the "low temperature" casting of molten iron; Eliminate the porosity and shrinkage defects of the casting.
Second, we must consider the role of alloying elements.
With only one alloying element, it is difficult to ensure that there is no tendency to form a hard zone during eutectic transformation, and that the alloying element plays a role in stabilizing and refining the pearlite in the eutectoid transformation. If the composition contains two or more alloying elements, and the appropriate combination of the various elements, the desired effect can be achieved. For example, in the eutectic transformation of chromium and copper, chromium hinders graphitization, promotes carbide formation, and promotes white mouth; while copper promotes graphitization and reduces cross-section white mouth. The interaction of the two elements is neutralized to a certain extent, avoiding the formation of cementite in the eutectic transformation, resulting in the formation of white mouth or hardness at the casting wall; in the eutectoid transformation, both chromium and copper can be stabilized. And refine the composite effect of pearlite, but their respective roles are not the same. With the right proportions, we can better play their respective roles. Adding Cu 2.0% to Cr 0.2% gray cast iron, copper not only promotes pearlite transformation, but also increases and stabilizes pearlite volume and refines pearlite, promotes A-type graphite production and homogenizes graphite morphology; copper can also be slightly improved Cr content > 2% flowability of gray cast iron, which is especially advantageous for shell thin-walled castings. The combined addition of chromium and copper further enhances the compactness of the casting, and is therefore required for castings that are resistant to leakage. Adding an appropriate amount of alloying elements such as chromium and copper helps to improve the compactness of the material itself and improve its resistance to leakage.
The pearlite matrix is ​​the tissue that is sought to be obtained in the production, because only the pearlite-based cast iron has high strength and good wear resistance. Tin can effectively increase the pearlite content in the matrix structure and promote and stabilize the pearlite formation. When the tin content is added to 0.1%, the matrix structure of the cast iron consists essentially of pearlite.
Tin is the best element to promote the distribution of graphite in type A. In order to ensure that the matrix is ​​pearlite, especially in the case of a high proportion of recycled iron, a small amount of alloying element tin can be added. In production practice, the sample matrix structure containing no tin was found to be approximately half of the microstructure of ferrite and pearlite.
As long as a small amount of tin (0.01%) is added, the ferrite in the cast iron can be reduced by nearly 20%. As the tin content increases, the tensile strength and flexural strength of the cast iron are significantly improved. According to the data, when the tin content exceeds 0.26%, the strength and hardness tend to increase, but the carbides in the cast iron matrix structure appear, and the brittleness increases at the same time, which brings the problem of difficult cutting of the casting. Therefore, the conclusion of our production practice is that it is better to control the tin content to 0.07%0.09%.
The quality of raw and auxiliary materials is the basis of high-quality cast iron. We start from the raw and auxiliary materials, and the raw materials and auxiliary materials must be sampled and analyzed. Unqualified raw and auxiliary materials will never be put into use. To ensure high quality raw iron water, it is necessary to use high carbon, low, low sulfur, low interference (the pig iron supplier must have trace element analysis report) element of pig iron; use pure medium carbon steel, its composition of Cr, Trace elements such as Mo, Sn, V, Ti, Ni, and Cu are determined by the test results, and are preferred for scrap steel capable of stabilizing pearlite. Pig iron and scrap steel can only be used after derusting treatment, and the oil stain should be baked at 250*C. For ferroalloys and inoculants, fixed-point procurement is also used, and the ingredients are stable, the blockiness (grain size) is qualified, and the piles are stacked to avoid moisture. This requirement avoids the drawbacks caused by the "hereditary" cast iron charge. Accurate measurement before use is the quality assurance of smelting qualified molten iron. In particular, it is forbidden to mix sealed vessels and explosives in the charge, especially for induction furnaces.
The principle of ingredients should be adhered to by combining theoretical ingredients (ingredient calculation) and practical experience. No matter the trial algorithm or graphic method, the theoretical calculation of the ingredient data can not be determined as the final ratio, but also the elements in the melting process of the iron furnace. Change the law. In the case of the intermediate frequency furnace, if the furnace lining is an acidic material, the temperature of the molten iron is higher than 1500 C, and the lower limit can be taken on the amount of Si added, and C must take the upper limit. This is determined by the characteristics of the metallurgical process of the induction furnace and must obey the physicochemical laws of its metallurgical process.
Modern parts master the chemical composition of various metal materials in the furnace and the burning and reduction laws of various elements. This classifies the classification and stacking of the reclaimed iron, pouring risers and scrapped castings, and records the strict requirements of the components. The elements reduced in the furnace are subtracted from the ingredients, and the elements that are burned in the furnace are replenished.
The alloying elements are based on the principle of one-time dispensing. The alloys (Mo, Cr, Cu, Sn, etc.) can be added after the melting of the slag, and the burning is less in the acid furnace. C and Si can also be added during slag and gestation. In the case of induction furnace melting of cast iron, the principle of first adding carbon and then adding silicon must be followed.
The control of P and S content is mainly from new iron. The amount of P and S can be controlled within the range by selecting the charge. Therefore, the Wp of the new iron must be less than 0.06%; the Ws content is less than 0.04%. In this way, the amount of P and S can be ignored in the calculation of the ingredients (all the metal materials that are included in the furnace are strictly required to be accurately measured.
Induction Furnace Smelting Technology The purpose of smelting high-strength cast iron is to achieve high-strength molten iron composition and ideal casting properties. Therefore, according to the metallurgical characteristics of the medium frequency induction furnace, a reasonable smelting process must be prepared, which must be strictly controlled from the charging, temperature control and alloying, recarburizing agent, slag forming agent and tapping temperature at different temperatures. With the shortest smelting time, the smallest alloy burning and oxidation, the purpose of controlling and stabilizing the metallographic structure and improving the quality of the casting is achieved.
MC manufacturing MC In the production practice, we divide the whole process of melting into three stages of temperature control. The so-called three-stage temperature here refers to the melting temperature, the slag temperature and the tapping temperature.
The melting temperature, which is the melting period before the sampling temperature, determines the balance between the absorption of the alloying elements and the chemical composition. Therefore, it is necessary to avoid high-temperature melting and feeding, and avoid the scuffing of the crust. Otherwise, the molten iron is in a boiling or high temperature state, the burning of carbon element is intensified, the silicon element is continuously reduced, and the oxidation of molten iron increases the impurity. According to the process requirements, the melting temperature is controlled below 1365*C, and the sampling temperature is controlled at 1420*C*10*C. The sampling temperature is low, and the ferroalloy is not melted. The chemical composition of the sample taken is unrepresentative; the temperature is too high. Burning or reduction of the alloy also affects the composition adjustment during the refining period. The intermediate frequency power should be controlled after sampling, and the quality management instrument will enter the slag temperature just after the chemical composition shows the result.
The slag temperature is an important part of determining the quality of molten iron, because it is closely related to the effect of stable composition and inoculation treatment, and directly affects the control of tapping temperature. Excessive temperature of the slag increases the burning of the graphite graphite nucleus and the reduction of silicon, especially for the acid lining. Theoretically, the high content of Si in the molten iron will produce a C-carrying effect, which affects the crystallization according to the stable system. If the temperature is too low, the molten iron will be exposed for a long time, and the carbon and silicon will be burned seriously. When the composition is adjusted again, not only the smelting time is extended, but the molten iron is overheated, and the composition is easily out of control. During the pouring, the excessive cooling of the molten iron is increased due to the cooling, and the normal crystallization is destroyed.
The tapping temperature, we generally control (the lost foam casting is higher than the sand casting casting temperature), which is the best temperature to ensure the pouring and gestation. The high and low iron temperature will affect the crystallization and inoculation effect of cast iron. If the temperature is too high (above the specified temperature of 30C), although the results of rapid analysis before the furnace are moderate, C and Si are moderate, but the test triangle test The depth of the white mouth will be too large or the center will appear. In this case, even if measures are taken to add carbon to the furnace or increase the amount of gestation, the author's practical experience is not effective. Then, after lowering the intermediate frequency power, the furnace is cooled. That is, 15% of the total amount of molten iron is baked into the furnace, so that the mouth of the test piece turns into a gray mouth, and the depth of the white mouth of the tip becomes small. If the continuous high temperature time is long, after adopting the above method, the furnace should still be supplemented with FeSi75 according to the "add one to one" experience method to control the white mouth width within the specified range of the process. The tapping temperature is controlled according to the pouring temperature, and the suitable pouring temperature of the shell casting casting mold is 1440C*20*C, which can realize “high temperature tapping and suitable temperature pouring”. Strict control and control is of course the best. Because the low temperature of the tapping will result in a pouring temperature lower than 1380C, it is not conducive to desulfurization and degassing, and particularly affects the inoculation treatment effect. As the temperature decreases, the cold separation and unclear contour will increase significantly.
The iron inoculation treatment technology is used to produce the HT250 for the transmission of a casing, which relies on improving the wear resistance of the material, so that the microstructure of the casting can be significantly improved, the hardness value on each end face is remarkably improved, and the thick end face is stabilized. The amount of pearlite on the same effect has the same effect, and can also improve the sensitivity of the wall thickness and the good cutting performance of the casting during machining, in particular, to prevent the looseness and leakage of the casing casting.
The amount of inoculant added is determined according to factors such as the wall thickness, composition requirements and casting temperature of the casting of the casing, so that there is no looseness or leakage at the wall thickness, and no hard zone appears at the thin wall. In production practice, it is proved that Sr, Ba, Ca, Si-Fe inoculant is the most ideal inoculant for refining high-strength gray cast iron. This kind of modern parts agent plays the role of Ba (Bar) anti-recession ability and enhances the retention of A-type graphite. The rate and sputum (Sr) eliminate the white mouth ability, and the auxiliary gestation and osmosis effected by calcium (Ca) and iron (Fe). This strong combination of inoculants is an ideal choice for the production of high-strength cast iron.
The relationship between the number of births and the effect of birth.
With the increase of the number of gestation, the uniformity of graphite distribution inside the cast iron is improved, and the difference between the A-type graphite occupancy and the graphite length is large. The high-strength A-type graphite that has been inoculated twice or more has a high occupancy rate, a uniform distribution, and a moderate length. More importantly, multiple births promote the generation of non-spontaneous crystal nuclei and strengthen the matrix, thereby increasing and stabilizing the strength of the cast iron.
After instant incubation, and using the funnel-style gestation bag, BaSiFe+FeSi75 is used as the post-inoculation treatment to avoid the long pouring time of the molten iron after the gestation, and the post-cultivation is the key to control and stable gestation. The molten iron after inoculation should be poured within a limited time, generally not more than 8min, and the second inoculation in the bag for 24 minutes is the best. Silicon germanium inoculant can eliminate the white mouth of HT250, improve its graphite shape and distribution, and eliminate E and D type supercooled graphite. Because of the E-type graphite and ferrite structure, the material compactness will be lowered, and the leakage resistance will be seriously deteriorated. The castings made of ferroniobium iron have no white mouth at the thinnest part, and the tensile strength reaches the requirement of 6,250N/mm2. The hardness of the test bar reaches 190230HB. The shell body is dissected and the hardness is about 190HB. The quality coefficient of the casting is remarkable. Improve, the metallographic organization reaches the casting level of the foreign prototype shell, the pearlite is 85% and 90%, which satisfies the strength requirements and performance of the automobile transmission housing, and its mechanical performance reaches the material level of the foreign similar type transmission housing. .
MC article query number: W1012

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