PRODUCTION OF METAL INGOTS FREE FROM DEFECTS CAUSED BY SHRINKAGE

D. D. SARATOVKIN

A considerable part of all metal products passes through the so-called ingot phase of production. Molten metal is poured into metal molds, in which it solidifies. The structure of the solidified ingot depends on the properties of the metal, on its composition—if we are dealing with an alloy—on the method of pouring, on the pouring temperature, the pouring rate, the shape of the mold, and many other factors. However, despite the great differences among the structures of finished ingots, certain defects are always inherent in all of them, explained by the basic conditions of solidification—cooling on all sides in a metal mold. Such defects include above all the shrinkage cavity and the zone of shrinkage porosity. It is known that the zone of shrinkage porosity is very often located along the axis of the ingot throughout its entire length. While insulation of the hot top helps to a considerable extent in combating the shrinkage cavity, shrinkage porosity even in ingots obtained with an insulated hot top often extends to half their height.

Proceeding from a detailed analysis of the process of solidification of a quiet (non-boiling) metal in a metal mold, one may conclude that there exists a method capable of eliminating the heterogeneity of the ingot caused by shrinkage phenomena. This method consists in the complete elimination of cooling from all sides and in removing the heat of the solidifying ingot only through the bottom. The proposed conditions must be such that the metal solidifies in a mold whose walls at that time have a temperature not lower than the temperature of the metal itself. These conditions can be obtained, for example, when the metal is poured into cells arranged like a honeycomb. If the walls of the cells are sufficiently thin and the entire assembly has a common bottom that conducts heat well, and if from above, immediately after pouring, the metal is covered with a good heat insulator, then the conditions for solidification of each ingot located in a cell and surrounded by neighbors like itself correspond exactly to those required.

We have carried out experiments in this direction, with the pouring of zinc and tin bronze, while simultaneously carrying out control pouring of the same metals into metal molds. The control zinc specimens produced shrinkage cavities reaching, in some cases, to the middle of the ingot. In the control bronze ingot, a zone of shrinkage porosity was revealed extending to the very bottom. The ingots obtained by the proposed method, however, proved to be completely free both from shrinkage cavities and from shrinkage porosity. Zinc, possessing a great capacity for transcrystallization, produced ingots with long crystals elongated along the axis. Bronze in both cases produced a fine-crystalline structure. Chemical analysis showed that there was no noticeable segregation over the height of the bronze ingot. For both zinc and bronze, uniformity of structure was observed throughout the entire length of the longitudinal section of the ingot.

In the accompanying photograph two zinc ingots are shown: on the right—the one obtained by the indicated method, and on the left—the control ingot. For ease of judging their dimensions, they have been placed on millimeter paper.

Since the peripheral ingots in the cellular unit are not under the required conditions, they may be regarded as that percentage of metal which lowers the yield of sound product.

However, experience has shown that the peripheral cells can be made in the form of narrow wedge-shaped pockets not reaching the bottom. With a large number of cells, less than 10% of all the metal being poured enters these pockets.

Fig. 1

Pouring was carried out by us with the aid of a pouring basin with holes in its bottom corresponding to the number of cells. In order that the level of the poured metal in all cells and pockets should be the same, the walls had small openings through which metal entered underfilled cells from adjacent ones. For making the cellular unit we used ordinary molding materials. From above, after pouring, the metal was covered with ash.