Treatment of coarse grain in castings caused by metal melting

When the melting operation is small, it will have an impact on the structure of the remaining particles. For different cast metals, the same melting process must be adopted.

(1) The imbalance of blast volume and coke in cupola melting gray cast iron will cause excessive carbon increase. For example, too high bottom coke height and low blast volume will cause excessive carbon increase. When the lining is melted, the carburization will be more serious. Because after the cupola diameter becomes larger, in order to maintain the same carbon content, it is necessary to increase the blast capacity. Melting at too high temperature will increase the carbon content, which will be encountered if hot air smelting is used. According to experience, for every 55 ℃ increase in blast temperature, 0.10% carbon (mass fraction) will be added. If oxygen is used to raise the temperature, it does not necessarily cause the same problem.

If the interval of tapping liquid is too long, or the molten iron stays in the hearth for too long, it will also lead to carbon increase. The production of low carbon cast iron generally uses shallow hearth, and shortens the interval time of tapping liquid, so as to achieve continuous tapping liquid as far as possible.

Intermittent melting will cause excessive carburization, resulting in coarse-grained structure. In addition, due to the interruption of melting due to the stop of wind, almost all of them lead to the fluctuation of carbon and silicon content. After the wind stops, it usually takes 15 minutes to get the original chemical composition again.

(2) The deviation in weighing or proportioning of malleable cast iron will lead to the change of chemical composition; the blast volume in the furnace is not guaranteed, which will affect the control of chemical composition; the melting overheat or smoke filling in the flame will cause carbon increase.

(3) The dirty crucibles used for brass and bronze, as well as the remaining shell or metal thin layer left at the bottom and side wall of the crucible during the melting of the last furnace will cause pollution to the next melting. Therefore, the waste materials from unknown sources should be avoided in production, and the raw materials that will produce gas, such as wet, oil polluted or other dirty materials, should not be mixed into the metal furnace burden.

(4) It is a common reason for the coarseness of aluminum alloy grain that the molten aluminum is overheated due to the improper control of melting temperature. Therefore, the superheated aluminum liquid should be cooled down slowly to lower pouring temperature. In addition, carelessness or charge pollution in the batching process may also cause coarse grain defects.

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