Melting High Strength Cast Iron in Medium Frequency Induction Furnace




Weiye New Materials

It has become a general trend to replace power frequency furnace and cupola with intermediate frequency furnace, and has created a new way for automobile manufacturing industry to produce high-quality castings. The transmission housing is one of the basic parts of the automobile. It is not only the skeleton of the multi-stage gear, but also can withstand the local large pressure stress caused by the tightening of many high-strength bolts.

It has become a general trend to replace power frequency furnace and cupola with intermediate frequency furnace, and has created a new way for automobile manufacturing industry to produce high-quality castings. The transmission housing is one of the basic parts of the automobile. It is not only the skeleton of the multi-stage gear, but also can withstand the local large pressure stress caused by the tightening of many high-strength bolts. The casting itself must have high pressure resistance and corrosion resistance, so the casting must not have defects such as looseness and coarse grains, so as to avoid the leakage of oil that plays the role of lubrication and cooling. Traditionally, HT150 or HT200 castings are used as shell materials, and their casting quality cannot meet the requirements of the automobile industry to continuously improve the overall quality. This requires the addition of trace Cr, Mo, Cu and other alloy elements to obtain the high strength of pearlite matrix, which is suitable for the service performance of the shell. In order to produce high strength and high quality cast iron automobile basic parts, it is necessary to use medium frequency induction furnace in casting. In this paper, in the practice of using medium frequency electric furnace to produce high strength cast iron for automobile gearbox housing castings, the quality control in front of the furnace is explored.
1. Composition design of high-strength alloy gray cast iron
The transmission housing is made of HT250, with a hardness of<200HBW. Free cutting is required. No leakage is found during the oil pressure test. A small amount of multi-element alloy composition is added to the cast iron, and reasonable process parameters are selected to make the casting have relative chemical composition and cooling speed, and obtain ideal metallographic structure and mechanical properties. In order to guarantee the mechanical properties, it is necessary to control the matrix structure and graphite morphology
In the composition design of high-strength low alloyed inoculation cast iron, the influence of carbon equivalent of molten iron and cooling rate should be considered first. The carbon equivalent is too high, the cooling speed at the thick wall of the casting is slow, the thick wall of the casting is easy to produce coarse grains, loose structure, and the oil pressure test is easy to produce leakage; If the carbon equivalent is too low, it is easy to form hard spots or local hard areas at the thin wall of the casting, resulting in poor cutting performance. When the carbon equivalent is controlled at 3.95%~4.05%, the mechanical properties of the material can be guaranteed, and it is close to the eutectic point. The solidification temperature range of the molten iron is narrow, creating conditions for the realization of "low temperature" pouring of molten iron; It is also beneficial to remove the porosity and shrinkage defects of castings.
Secondly, the role of alloy elements should be considered. In the eutectic transformation of chromium and copper, chromium hinders graphitization, promotes carbide and promotes white mouth; Copper, on the other hand, promotes graphitization and reduces the white mouth of cross section. The interaction between the two elements is neutralized to a relative extent, so as to avoid the formation of cementite during eutectic transformation, which may lead to the formation of white mouth or hardness increase at the thin wall of the casting; In the eutectoid transformation, both chromium and copper can play a composite role in stabilizing and refining pearlite, but their respective roles are different. In proper proportion, they can play their respective roles. When wCu<2.0% is added to gray cast iron containing wCr=0.2%, copper not only promotes pearlite transformation, increases and stabilizes pearlite content and refines pearlite, but also promotes the formation and homogenization of A-type graphite; Copper can also slightly increase the content, and the fluidity of gray cast iron with wCr > 0.2% is especially beneficial to the thin wall cumulative casting of shell. Composite addition of chromium and copper can further improve the compactness of castings. Therefore, for castings requiring leakage resistance. Adding proper amount of chromium and copper is conducive to improving the density of the material itself and enhancing its anti leakage ability.
Pearlite is basically the organization expected in the production of high-strength gray cast iron, because only pearlite based cast iron has high strength and good wear resistance. Tin can effectively increase the pearlite content in the matrix and promote and stabilize the pearlite formation. Our production practice concluded that the tin content should be controlled within 0.7%~0.09%
2. Strictly control the quality of raw and auxiliary materials
The incoming raw and auxiliary materials must be sampled and analyzed to be sure. Unqualified raw and auxiliary materials will not be put into use. To ensure the workload of raw iron, pig iron with high carbon, low phosphorus, low sulfur and lower interference (the pig iron supplier shall have a trace element analysis report) must be selected; The pure medium carbon steel shall be selected. The composition of Cr, mo, Sn, v, Ti, Ni, Cu and other trace elements shall be determined according to the test results. The composition of scrap steel that can stabilize pearlite shall be selected preferentially. Pig iron and scrap steel can only be used after rust removal, and those with oil stain shall be baked at 250 °.
Ferroalloys and inoculants shall also be purchased at fixed points to ensure stable composition and qualified lump size (particle size). Stack them by category to avoid moisture. This requirement avoids the defects caused by the "heredity" of cast iron charge.
The accurate measurement before use is the quality assurance for smelting qualified molten iron. It is pointed out that for induction furnace smelting, the rigorous charge is mixed with sealed vessels and explosives.
(1) Adhere to the combination of theoretical ingredients (ingredient calculation) and practical experience. No matter the trial calculation method or graphical method is adopted, the burden data calculated theoretically cannot be determined as the proportion, and the change law of elements in the melting process of intermediate frequency furnace must be mastered. If the furnace lining is made of acid material, and the temperature of molten iron is more than 1500 ℃, the lower limit can only be taken for the addition of Si, while the upper limit must be taken for carbon.
(2) Grasp the chemical composition of various metal materials and the burning loss and reduction law of each element. Strict requirements for clear composition are put forward for classified stacking and numbering records of recycled iron (gating and riser, scrap castings). The reduced elements in the furnace shall be subtracted during batching, and the burned elements in the furnace shall be added during batching.
(3) The alloy elements shall be added in one time, and other ingredients except Si shall be subject to the medium limit. Alloys (Mo, Cr, Cu, Sn, etc.) can be added after slag removal and melting, so that they are less burnt in the acid furnace. C. Si can also be added during slag raking and inoculation. For induction furnace melting castings, the principle of adding carbon first and then silicon is followed.
(4) For the control of P and S content, the P and S content mainly comes from the new pig iron. The P and S content can be controlled within the required range by selecting the furnace charge. Therefore, it is necessary to make the wP of new pig iron<0.06% and wS<0.04%, so that the P and S content can not be taken into account in the batching calculation. (Due to the technical requirements of castings: wP ≤ 0.06%, wS ≤ 0.04%).
(5) All metal materials into the furnace shall be accurately measured in strict accordance with the requirements.
3. Control of medium frequency electric furnace smelting
According to the metallurgical characteristics of the medium frequency electric furnace, a reasonable smelting process should be prepared. The temperature control and the addition of alloy, carburizing agent, slag making agent and tapping temperature at different temperatures shall be strictly controlled, so as to control and stabilize the metallographic structure and improve the casting quality with short melting time, small alloy burning and oxidation.
In production practice, we divide the whole smelting process into three phases for temperature control. The so-called third stage temperature here refers to: melting temperature, slagging temperature and tapping temperature.
Melting temperature: refers to 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 shall be avoided to avoid "crusting". Otherwise, the molten iron is in boiling or high temperature state, the burning loss of carbon element is aggravated, the silicon element is continuously reducing, and the oxidation of molten iron aggravates the increase of impurities. According to the process requirements, the melting temperature is controlled below 1365 ℃, and the sampling temperature is controlled at (1420 ± 10) ℃. If the sampling temperature is low, the ferroalloy is not completely melted, and the chemical composition of the sample taken is inevitably unrepresentative; If the temperature is too high, the alloy will be burnt or reduced, which will also affect the composition adjustment during the scouring period. The power of intermediate frequency furnace shall be controlled after sampling. After the quality control instrument in front of the furnace displays the results of chemical composition, it just enters the slagging temperature.
Slag raking temperature: Slag raking temperature is an important link to determine the quality of molten iron, because it is closely related to the stability of composition and the effect of inoculation, and directly affects the control of tapping temperature. Too high slagging temperature will aggravate the burning loss of graphite crystal nucleus in molten iron and the reduction of silicon. For acid furnace lining, theoretically, carbon removal will occur after the silicon content in molten iron is too high, which will affect the crystallization according to temperature system, and there is a tendency to produce anti white mouth; If the temperature is too low, the molten iron will be exposed for a long time, and carbon and silicon will be seriously burned. When the composition is adjusted again, not only the melting time is prolonged to overheat the molten iron, but also the composition is easily out of control, the undercooling of molten iron is increased, and the normal crystallization is destroyed.
Tapping temperature: in order to ensure a better pouring and inoculation temperature, we generally control it at 1520~1550 ℃. Both high and low tapping temperatures will affect the crystallization and inoculation effect of cast iron. If the temperature is too high (more than 30 ℃ higher than the specified process temperature), although the rapid analysis of C and Si in front of the furnace is also moderate, the white mouth depth of the trial cast triangular test piece will be too large or the central part will show pockmarks. In this case, even if measures are taken to add carbon into the furnace to increase the inoculation amount, the practical experience of the author is that the effect is not good, and it is necessary to reduce the temperature in the furnace after the medium frequency power is reduced, that is, add 10% - 15% of the total amount of molten iron into the furnace, and the baked new iron, so that the pockmark in the fracture center of the test piece will turn to gray, and the white mouth depth will become smaller. If the high temperature lasts for a long time, carbon supplement measures in the furnace must still be carried out after the above methods are adopted. The tapping temperature is controlled according to the pouring temperature. The proper pouring temperature for shell type iron castings is (1440 ± 20) ℃, which can realize "high temperature tapping and proper temperature pouring". It is certainly good to strictly control and control it. Because the low tapping temperature will cause the pouring temperature to be lower than 1380 ℃, which is not conducive to desulfurization and degassing, but also affects the inoculation treatment effect. With the decrease of temperature, the problems of cold shut and unclear contour increase obviously.
4. Inoculation treatment of molten iron
Inoculate the HT250 used to produce the transmission housing to improve the wear resistance of the material, so that the structure and performance of the casting can be significantly improved, and the hardness value on each section can be significantly improved. In addition, it has the same effect on stabilizing the pearlite volume on the thick section, and it can also improve the sensitivity of its wall thickness and the good cutting performance of the casting during machining, especially for preventing the looseness and leakage of the housing casting.
The amount of inoculant shall be determined according to the wall thickness, chemical composition, pouring temperature and other factors of the shell castings produced. The principle is that there is no looseness and leakage at the wall thickness, and there is no hard zone at the wall thickness. The production practice shows that Sr, Ba, Ca, Si Fe inoculants are ideal inoculants for improving the strength of gray cast iron. This inoculant gives play to the anti fading ability of barium (Ba) and improves the occupancy of A-type graphite, the special white mouth ability of strontium (Sr), and the auxiliary inoculation and penetration role of calcium (Ca) and silicon (Si). The inoculant with this strength combination is an ideal choice for the inoculation treatment of high-strength cast iron.
The relationship between the number of inoculations and the effect of inoculation. As the number of inoculations increases, the distribution of graphite inside the cast iron is improved. The difference between the percentage of A-type graphite and the length of graphite is large. The percentage of A-type graphite after more than two inoculations is high, the distribution is uniform, and the length is moderate. What is more important is that multiple inoculations increase the number of non self developing nuclei, strengthen the matrix, thus improving and stabilizing the strength of cast iron.
The key to control the inoculation effect is to avoid that the stream inoculation of molten iron lags behind the pouring after the stream compound inoculation treatment and the funnel type inoculation package is inoculated with barium ferrosilicon+75 ferrosilicon. The molten iron after inoculation treatment shall be poured within a limited time, generally not more than 8 minutes. The secondary inoculation in the ladle for 3~5 minutes has a good inoculation effect. The silicon barium inoculant can improve the white mouth of HT250, its graphite shape, distribution, E, D type undercooled graphite. Because of the E-type graphite and ferrite structure, the material density will be reduced and the anti leakage performance will be seriously deteriorated.
5. Actual production effect
There is no white mouth on the thin part of the casting, its tensile strength is above HT250, the hardness of the test bar is 190~230HBW, the shell body is dissected, the hardness is about 190HBW, the quality coefficient of the casting is significantly improved, the metallographic structure reaches the casting level of the foreign prototype shell, the pearlite is 85%~90%, which meets the strength requirements of the transmission shell, Its mechanical properties have reached the material level of the transmission housing of the same type abroad.

Keyword: Recarburizer

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