SAND CASTING EQUIPMENT.

4. Adjust the conventional chemical composition and ratio of cast iron to obtain high-strength, low-stress gray cast iron. With the carbon equivalent remaining unchanged, appropriately increasing the Si/C ratio is one of the important ways to improve the strength and rigidity of machine tool castings. By adjusting the chemical composition, especially changing the silicon/carbon ratio to make Si/C between 0.5 and 0.9, plus proper inoculation and alloying, high-strength gray iron castings with good comprehensive properties can be obtained. The law about the silicon/carbon ratio is:
4.11 Under the same carbon equivalent, the Si/C ratio is high, the tensile strength can be increased by 30~60MPa, the relative strength is high, the relative hardness is low, and the elasticity is good;
4.12 Under the same carbon equivalent, the Si/C ratio increases, the residual stress tends to decrease, and the stress tendency is also smaller;
4.13 Improve the ratio of Si/C, the tendency of white mouth is small, the sensitivity of section is small, and it has no effect on the fluidity of molten iron and linear shrinkage.
Adjust the manganese and silicon content so that the Mn content is 0.2~1.3% higher than the Si content, and another high-strength low-stress cast iron is obtained. Gray cast iron contains Mn in the range of 1.5~3.0%, increasing the Mn content, especially when the Mn content is greater than the Si content, the eutectic group can be significantly refined, and it is easy to obtain D and E type graphite and fine pearlite matrix. In addition, the difference between Mn and Si in gray cast iron and the absolute value of Mn are controlled so that the difference between Mn and Si is 0~0.5% and Mn is greater than 2%. Different types of hardening phases can also be obtained in gray cast iron. Therefore, by controlling the difference between Mn, Si, and the absolute value of Mn, high-strength gray cast iron with high mechanical properties, uniform hardness, good compression and compactness and good wear resistance can be obtained. This kind of high manganese gray castings has achieved good results in the production of xinyuanzhu Group. Mn=1.7S+0.3%
To ensure that the sulfur is completely combined with manganese. How to reduce the shrinkage tendency of high-strength gray cast iron
High-strength and shrinkage have always been a contradiction. The production of high-strength castings has a large shrinkage tendency. If the shrinkage problem cannot be solved well, a large number of shrinkage waste defects can be dealt with. To solve the problem of material shrinkage, the general principle is to have a higher carbon silicon equivalent. The high-carbon silicon equivalent plus alloying process has a smaller shrinkage tendency than the low-carbon silicon equivalent less alloying process. Therefore, under the premise of selecting high carbon silicon content, new technologies to improve performance should be developed to reduce shrinkage. Specific measures can be as follows Considerations:
4.2 Process measures to promote graphitization are the best measures to reduce the shrinkage of molten iron.
Electric furnace smelting: The application of recarburization technology is the key technology to solve the shrinkage of molten iron. Since the precipitation of graphite during the solidification of molten iron produces graphitization expansion, good graphitization will reduce the shrinkage tendency of molten iron. Therefore, the carbon increasing technology is the best process.
Since the addition of a recarburizer improves the graphitization ability of the molten iron, the shrinkage tendency of the molten iron is smaller if the process of adding a recarburizer to all scrap steel is used. This is a very important change in concept. The traditional concept is that adding more scrap will increase the tendency of molten iron to shrink. In this way, we are easy to fall into a misunderstanding that we are not willing to use more scrap and prefer to use more pig iron.
The disadvantage of multi-purpose pig iron is: there are many coarse hypereutectic graphite in pig iron. This coarse graphite is hereditary. If it is smelted at low temperature, the coarse graphite is difficult to eliminate. The coarse graphite inherits from the liquid to the solid state, which makes the solidification process inherent As the expansion of graphite precipitation is weakened, the shrinkage tendency during the solidification of molten iron is increased, and the coarse graphite will inevitably reduce the performance of the material. Therefore, compared with the use of scrap steel to increase the carbon process, the disadvantages of using a large amount of pig iron are:
4.21 Low strength performance. The same ingredients have been tested for comparison, and the performance is half a row.
4.22 The tendency to shrink is large. Under the same conditions, the shrinkage is larger than that of scrap steel.
For electric furnace smelting, the core of recarburization technology is the use of high-quality recarburizers. Using scrap steel recarburization process, the recarburizer becomes the most important link in the recarburization process. The quality of the recarburizer determines the quality of the molten iron. Whether the recarburization process can obtain a good graphitization effect and reduce the shrinkage of the molten iron mainly depends on the recarburizer:
4.23 Recarburizers must be recarburizers that have undergone high temperature graphitization treatment.
Only after high-temperature graphitization treatment, the carbon atoms can change from the original disordered arrangement to the flake arrangement, and the flake graphite can become the best core of graphite nucleation and promote graphitization.
4.24 Good recarburizers have very low sulfur content, and w(S) less than 0.03% is an important indicator.
For cupola smelting: high-temperature smelting is the most critical technical indicator, and high-temperature smelting can effectively eliminate the heredity of coarse graphite in pig iron. High temperature smelting can increase the carburizing rate and reduce the amount of pig iron added in the ingredients. The carbon obtained by carburizing has good activity and has a better graphitization effect than the carbon brought by pig iron. It is reflected in the casting that the graphite has a better shape and more uniform distribution. The good shape of graphite will improve the performance of the material, including cutting performance, and the good graphitization effect will reduce the shrinkage tendency of molten iron.
4.3 Increase the amount of silicon in the original molten iron and control the inoculation.
Part of the silicon in gray cast iron is the silicon in the original molten iron, and part is the silicon that is inoculated.
Many people like the low silicon in the raw iron and then use a large amount of inoculation. This approach is not scientific: a large amount of inoculation is not advisable, which will increase the tendency of contraction. Inoculation is to increase the number of crystalline cores and promote graphitization. A small amount of inoculation (0.2% to 0.4%) can achieve this goal. From the perspective of process control, the inoculation volume should be correspondingly stable without excessive changes. This requires the amount of silicon in the original molten iron to be correspondingly stable. Increasing the amount of silicon in the original molten iron can not only reduce the white mouth and shrinkage tendency, but also play the role of silicon solid solution strengthening the matrix, but the performance will not decrease. The current scientific approach is to increase the silicon content of the original molten iron of gray cast iron, and the inoculation amount is controlled at about 0.3%, which can exert the solid solution strengthening effect of silicon, which is beneficial to increase the strength and reduce the shrinkage of the casting.
4.4 The method of alloying has a great influence on the shrinkage of molten iron.
Alloying can effectively improve the properties of cast iron. Our commonly used alloying elements are chromium, molybdenum, copper, tin, and nickel.
Chromium: Chromium can effectively improve the performance of gray cast iron. As the amount of addition increases, the performance will continue to improve. The white mouth tendency of chromium is relatively large, this is the problem that everyone is most concerned about. If the amount is too large, carbides will appear. As for how to control the upper limit of the amount of chromium, different chromium addition processes have different upper limits. If chromium is added to the original molten iron, the upper limit should not exceed 0.35%. Increasing the amount of chromium in the original molten iron will make the molten iron white. and the tendency to shrink increases, which is very harmful.
Another method of adding chromium is not to increase the chromium of the original molten iron, but to add the chromium to the molten iron ladle and use the flushing method to flush in. This process will greatly reduce the white mouth and shrinkage tendency of the molten iron, the same as the previous one. Compared with this process, with the same amount of chromium, the whiteness and shrinkage tendency will be reduced by more than half. With this chromium addition method, the upper limit of chromium can be controlled to 0.45%.
Molybdenum: The properties of molybdenum are very similar to those of chromium, so no detailed description will be given. Due to the high price of molybdenum, adding molybdenum will greatly increase the cost. Therefore, molybdenum should be added as little as possible and more chromium should be added. Adding chromium and molybdenum by the impulse method is an effective measure to reduce alloying shrinkage.
4.5 The influence of molten iron pouring temperature on shrinkage.
High temperature iron liquid tends to shrink, which is the experience that everyone has. It is very important to control the pouring temperature within a reasonable range. If the pouring temperature is higher than the reasonable temperature of 20-30 ℃ specified by the process, the shrinkage tendency will increase significantly. Pay attention to this phenomenon in production. Electric furnaces without automatic heat preservation function may increase the temperature of molten iron. The pouring temperature of the first ladle of molten iron will be lower, and then the temperature will become higher and higher. If it is not controlled, It is possible to produce shrinkage waste. In the production, the first ladle of molten iron should be ironed, and the hot ladle should be used again, and the pouring temperature of the first ladle of molten iron should be controlled at the lower limit, not the upper limit, to prevent the temperature from rising continuously. Electric furnace smelting to control the pouring temperature is a key measure to prevent shrinkage and waste of castings.
4.6 The oxidation tendency of molten iron cannot be ignored: large oxidation and large shrinkage.
The high tendency of molten iron to oxidize is very harmful and will increase the tendency of shrinkage. In order to reduce the oxidation of molten iron, the cupola smelting must achieve rapid smelting. At present, foreign advanced electric furnace smelting technology can quickly melt the added iron material within a few minutes, greatly shortening the time of the iron material in the high-temperature oxidation stage, and greatly reducing the oxidation tendency. At the same time, due to the application of electric furnace recarburization technology, the molten iron The oxidation of iron is further reduced, so electric furnace smelting can also produce molten iron with low oxidation and low shrinkage. As long as the pouring temperature is strictly controlled, electric furnace smelting to produce complex cylinder blocks and cylinder head castings is also very advantageous.

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