Dense castings understand that inoculation treatment is to add inoculants to molten iron before it enters the casting cavity to change the metallurgical state of the molten iron, thereby improving the microstructure and properties of cast iron. These performance improvements cannot be explained by the changes in the chemical composition of the molten iron after the addition of inoculants. With the improvement of inoculants and inoculation methods, inoculation treatment has become an important means to improve the performance of cast iron in modern casting production.
1) Purpose of inoculation: Promote graphitization and reduce the tendency of white cast iron; Improve cross-section uniformity;
Control graphite morphology, reduce the formation of eutectic graphite and symbiotic ferrite to obtain medium-sized A-type graphite; Appropriately increase the number of eutectic clusters and promote the formation of fine lamellar pearlite; Improve the mechanical properties and other properties of cast iron.
2) Evaluation of inoculation effect:
Different inoculation purposes have different indicators for evaluating inoculation effects. However, it is often evaluated by reducing the tendency of chilling, increasing the number of eutectic clusters and reducing the degree of supercooling.
① To reduce the tendency of chilling, the depth or width of the chilling of the triangular sample is often used to evaluate the tendency of chilling before and after inoculation. Different forms of triangular samples can be used for different castings.
② The number of eutectic clusters is measured on the sample to measure the difference in the degree of nucleation before and after inoculation. It should be pointed out that the sensitive comparison of eutectic clusters must be carried out under similar conditions, because the charge, melting conditions, superheat treatment, inoculant, inoculation method, etc. will cause changes in the number of eutectic clusters; some inoculants, such as strontium-containing inoculants, do not increase the number of eutectic clusters too much, but have a strong effect of reducing the tendency of chilling.
③ Eutectic supercooling, after the molten iron is inoculated, the number of crystallization cores increases significantly, which makes the eutectic nucleation temperature start and end early, and the absolute supercooling decreases accordingly. Therefore, the change of supercooling before and after inoculation can be used to detect the inoculation effect.
Actual production cannot pursue a large amount of inoculation effect. In order to prevent defects such as looseness, many companies stipulate that a relative supercooling of less than 4°C is considered over-inoculation, and strive to obtain a relative supercooling of 6~8°C after inoculation.
The effectiveness of inoculants for machine tool castings declines over time. Therefore, when selecting inoculants, the duration of the inoculation effect is often used as an evaluation indicator.
(2) Under certain conditions, each inoculant has its optimal addition amount. Excessive use of inoculants will not bring greater inoculation effects, but will waste inoculants, reduce the temperature of molten iron, and increase defects and costs of castings. It is generally recommended that the amount of silicon brought into the molten iron by the inoculant should not exceed 0.3% and the amount of carbon should not exceed 0.1%. The degree of oxidation of molten iron in China is relatively high, so the amount of inoculant used is mostly higher than this value.
To date, most foundry workshops at home and abroad still use FeSi75 as the inoculant. The reason for this is that in addition to being cheap and easy to obtain, it has a good inoculation effect in a short period of time (about 5~6 minutes) after inoculation.
(3) Inoculation method
In-ladle flushing method: the inoculant is added into the ladle and then flushed into the molten iron; the method is simple, but the inoculant is easily oxidized and has a large burnout; it is easy to float up and mix with the slag in the ladle and has no inoculation effect; the amount of inoculant used is large; the interval from inoculation to pouring is long and the decay is serious;
Inoculation in the tapping trough: when tapping iron, the inoculant is added to the molten iron flow in the tapping trough by hand, inoculant hopper or vibrating feeder. Or when transferring, add it to the transfer iron liquid flow; the oxidation of the inoculant is reduced; the waste of the inoculant is small, but the amount is still too much; the residence time before pouring is long, and the decay is serious;
Cup inoculation: put the inoculant (granules or molded blocks) into the pouring cup, and the molten iron enters the pouring cup, so that the inoculant melts and enters the mold; increase the workload of molding; the inoculant particles are easy to float, which is wasteful; after inoculation, the molten iron enters the mold immediately, and there is basically no decay; the amount of inoculant is less than the inoculation method in the ladle;
FeSi rod inoculation: when pouring, the ferrosilicon rod at the ladle mouth is inoculated by the molten iron flow; less decay; the amount of inoculant is less than the ladle method; the manufacturing of ferrosilicon rods is troublesome; the amount of inoculant is not easy to control; the casting process is required to be high;
Large floating silicon inoculation: put large silicon inoculants at the bottom of the ladle, and pour into the molten iron to make the inoculant blocks melt and float, and there is still 1/4~1/5 of ferrosilicon block, or sprinkle a layer of ferrosilicon on the liquid surface after the ladle flushing method; the iron liquid surface is rich in silicon, the poured iron liquid is like fresh inoculation, and the decay is small; simple operation; reduce the workload of crushing; but the block size must match the temperature and ladle capacity; the inoculant consumption is large;
Inoculation wire inoculation: wrap the inoculant in a hollow metal wire, use an improved welding wire feeder, and evenly feed it into the iron liquid in the sprue or pouring cup; the amount of inoculant can be reduced to less than 0.08%; the inoculation wire can automatically and evenly enter the iron liquid; no decay; the inoculation wire supply cost is high; all are used at fixed points; reliable control system is required; iron liquid flow inoculation: add the inoculant to the iron liquid flow entering the mold by gravity or air force; the amount of inoculant can be reduced to 0.1%; the inoculant particles can evenly enter the iron liquid flow; no decay, the effect is better than the ladle inoculation method, good for fixed-point use, and the control system must be reliable;
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