At present, the majority of machine tool castings are still gray cast iron. The practice at home and abroad has proved that high carbon equivalent and high strength are the development direction of gray cast iron. It is an important way for gray cast iron to obtain low casting stress, good processability and castability under high strength. It is an important measure to achieve comprehensive balance in these three aspects. However, obtaining high strength under high carbon equivalent is not a simple adjustment of composition, but a good control of smelting and inoculation, especially to improve the metallurgical quality of gray cast iron, which is the basis of producing high carbon equivalent and high strength cast iron.
(1) Control of carbon equivalent. Comparing the carbon equivalent of machine tool castings at home and abroad, we can see that the carbon equivalent of machine tool castings at home is lower than that of foreign countries.
(2) Control of silicon carbon ratio. The comparison of silicon carbon ratio at home and abroad shows that the silicon carbon ratio of domestic machine tool castings is also lower than that of foreign ones. It is suggested that the silicon carbon ratio should be controlled between 0.55 and 0.62.
(3) Control of alloying. The addition of alloy elements to the molten iron can increase the number of austenite dendrites, increase and refine the pearlite, strengthen the ferrite, refine the graphite, refine the eutectic group, and improve the cross-section sensitivity. The main foreign machine tools have been standardized by adding alloy, generally 0.4% ~ 0.6% for w (Cu) or 0.2% ~ 0.4% for w (CR). The specification of alloy cast iron for machine tools was issued in 1980s in China, but it was not continued after a period of implementation. At present, the alloying of machine tool castings in different factories is not included in the standard, and the role of adding alloy elements in production is also insufficient. Some stable pearlite alloy elements must be added under high carbon equivalent to increase its strength and hardness and reduce the sensitivity of cross section. Low alloying is an indispensable measure for high carbon equivalent and high strength gray cast iron. The commonly added alloy elements are Cu, Cr, Sb, Sn, which are often combined with Cr, Cu and Sb, Cu and Sn. The mass fraction of added alloy elements is recommended as follows: 0.4% ~ 0.6% Cu and 0.2% ~ 0.4% Cr, 0.4% ~ 0.6% Cu and 0.02% ~ 0.04% Sb, 0.4% ~ 0.6% Cu and 0.02% ~ 0.04% Sn.
Under the condition of cupola smelting, the scrap ratio of machine tool casting at home and abroad is higher than that at home.
Under the condition of electric furnace smelting, the amount of scrap steel added in China is less, the amount of pig iron added is more, and the quality of graphite is poor. The amount of pig iron added abroad is generally less than 10%. The technology of high scrap ratio and carburizing is an important measure of high carbon equivalent and high strength. A large number of practices have proved that under the same carbon equivalent, the cast iron with high scrap ratio has high tensile strength, low hardness and high quality coefficient. Performance comparison of different pig iron ratio under the same carbon equivalent.
3 Temperature of molten iron
Comparison of temperature of cupola molten iron at home and abroad. FeO in slag is less than 3% in foreign countries and 5% ~ 8% in China. Comparison of coke consumption at home and abroad.
High molten iron temperature is the basis of high carbon equivalent and high strength gray iron. High molten iron temperature can refine graphite and improve strength. When the temperature of the molten iron is 1500 ~ 1550 ℃, the oxidation of the molten iron is reduced, and the SiO2 ash is significantly reduced.
(1) Problems. At present, most of the domestic enterprises adopt the inoculation in the tap hole. The oxidation and sticking caused by the decline of inoculation or the premature addition of inoculant in the ladle often affect the inoculation effect. Inoculation decay is an important factor for the unstable quality of machine tool castings. The main reason is the lack of understanding or implementation of the instantaneous incubation. The low temperature of molten iron, the insufficient amount of scrap, the low undercooling of molten iron before inoculation, the s < 0.05% in the original molten iron melted by electric furnace, and the serious oxidation of cupola molten iron all affect the inoculation effect to varying degrees.
(2) Solutions. The selection of inoculation method is more important than the selection of high-efficiency inoculant. According to the actual situation of the enterprise, we should strive to adopt the following flow inoculation, sprue cup inoculation, floating silicon inoculation, in mold inoculation and other instantaneous inoculation methods to ensure the inoculation quality. Triangle sample, thermal analysis curve and cold ratio before and after inoculation should be used to test the inoculation effect, that is to prevent undercooled graphite caused by insufficient inoculation and shrinkage caused by excessive inoculation. When the best results are not obtained by instantaneous inoculation, the choice of high-efficiency inoculant should be considered. The melting temperature of molten iron should be 1500 ~ 1550 ℃.
5 Performance test
The existing problems in performance testing are as follows: single cast test bar rather than attached cast test bar is used for performance testing; only testing performance, not metallographic structure; no testing for elastic modulus, casting stress, casting deformation, etc.; no effective testing for section sensitivity and processing performance, etc. Therefore, it is necessary to strengthen the inspection of the properties and metallographic structure of the attached casting test bar, especially the inspection of supercooled graphite, carbide and pearlite in the metallographic phase, so as to improve the metallurgical quality of the machine tool casting materials, so that its maturity is more than 1, its hardenability is less than 1, and its quality coefficient is more than 1. It has low stress and good processing performance under high carbon equivalent, The experiments of elastic modulus, casting stress, section sensitivity and machining performance are mainly carried out to provide high-quality castings for the production of high-quality CNC machine tools.
The above measures are mainly aimed at the gray cast iron machine tool castings. In recent years, large-scalecastings are used in heavy-duty machine tools. We can learn from the production experience of Beijing No.1 Machine Tool Plant. Every year, the factory needs more than 70 tons of large-scale machine tool castings of 5000 tons. In the past, the single length of machine tool castings was not more than 10 m, and the weight is 20-50 T, but now the length is 20 m, and the weight is 70-145 t, The factory has successfully produced more than 3000 tons of large-scale ductile iron parts with over 80 tons and 135 tons of cast ductile iron crossbeam (see Figure 10). The material is QT600-3, which actually reaches qt700-5. The guide rail is 19.5 m long and 250 mm thick. The grade of spheroidization is 2.5, the hardness of the guide rail is hbw200, the hardness difference is less than hbw10, and the guide rail surface is free of casting defects as a whole. The complete set of technology for their production of large-scale ductile iron machine tool castings is worth learning and promoting.