Inoculation treatment has an important role in improving the performance of castings. In summary, the main effect of inoculation on gray cast iron castings is to promote graphitization, reduce the white tendency, increase the number of eutectic aggregates, and promote the formation of A-type graphite and fine flake pearlite. For nodular cast iron, the role of inoculation treatment is mainly reflected in reducing the white tendency, promoting graphite precipitation, improving the inhibition of eutectic cementite, improving the roundness of graphite balls, increasing the number of graphite balls per unit area, refining eutectic aggregates, making the organization of nodular cast iron more uniform, improving the plasticity and toughness of nodular cast iron.
The operation process of inoculation treatment is a very important process, and various details should be paid attention to, otherwise the ideal inoculation effect can not be achieved. In general, the effect of on-stream incubation is the best, so if the conditions allow, it is best to use on-stream incubation. When incubating in front of the furnace, the artificial incubating is generally carried out in the cupola hot metal outlet tank, which can achieve better incubating effect. Some also carry out on-flow inoculation under automatic control conditions, and its advantage is that the control of inoculation amount is more accurate.
There are three aspects of the incubation treatment operation process, the first is the selection of inoculant to ensure uniform particle size and moderate size, so as not to affect the inoculation effect, and the impregnated hot metal should be fully stirred and then slag treatment. The second is to strictly control the time of inoculation treatment, especially the use of Si-Fe inoculant, the anti-recession performance is not good, and the inoculation time must be controlled at more than 70% of the iron time. The third is to determine an optimal inoculation amount, for gray iron parts, generally controlled at 0.3%-0.6%, do not blindly increase the inoculation amount, too high inoculation amount will increase the viscosity of liquid iron and slag inclusion, shrinkage hole, shrinkage tendency.
For the inoculation of gray cast iron, the reasonable inoculation amount should be determined according to the ratio of charge and the experience value of alloy burning loss for the raw hot metal melted by cupola. The raw molten iron smelted by electric furnace is different from the raw molten iron smelted by cupola, and the sulfur content of the raw molten iron smelted by cupola is relatively low, generally within the range of 0.015%-0.035%, while the sulfur content of the raw molten iron smelted by cupola is about 0.1%, although sulfur is a harmful element, too high sulfur content makes the casting prone to “hot cracking”. However, too low sulfur content also has an adverse effect on the inoculation treatment of high grade gray cast iron, so the hot metal smelted in the electric furnace should be sulfurized before inoculation, generally increasing the sulfur content to about 0.06%-0.10%, which can achieve the ideal inoculation effect.
In addition, for the inoculation treatment of large thick wall gray cast iron, considering the long cooling and solidification time of hot metal, in order to prevent inoculation decay, long-term inoculants such as Si-Ba and Si-Ca are generally used. For nodular cast iron, the method of multiple inoculation is generally used, that is, pre-furnace inoculation combined with lagging inoculation. For nodular cast iron with thick and large section, the delayed inoculation method of using long-acting inoculant should be considered to extend the incubation decay time and increase the number of graphite spheres. For ductile ductile cast iron pipe fitting with high plastic requirements, the usual practice is to use in-bag inoculation in front of the furnace combined with late flow inoculation, the amount of addition is controlled at about 0.5%, the inoculant can be placed on the gate tray during secondary inoculation, of course, when conditions are available, the inoculant effect will be better, the secondary inoculation generally uses long-acting powder inoculant, the dose is controlled at about 0.1%. Too large dose or too large particle size will affect the inoculation effect and easily cause slag leakage.
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