When dumped from the molds the castings are extremely brittle. The sprues are knocked off and the castings go to the “tumbling” mills where they are tumbled, either with the sprues, with hard iron (white iron) shot or star-shaped pieces of iron which quickly clean the sand from them and give smooth, clean surfaces.
At the chipping and sorting benches any remaining pieces of gates and other excrescences are removed while the castings are being handpicked and sorted. White iron, because of its brittleness, breaks easily and small protruding parts can more readily and cheaply be removed before annealing than after the castings have been thereby toughened.
Annealing Furnaces, Showing Sets of Pots or “Saggers” in Which the Castings Are Annealed
Having the cleaned castings of white iron of proper composition, malleability is given to them by the heat treatment known as “annealing.” Through the influence of a cherry-red heat continued over a sufficiently long period the iron and carbon, which in the white iron are chemically combined, gradually become divorced, and, after complete annealing, the casting will be found to consist of free iron in which are imbedded throughout very small particles of coke-like carbon. Castings that before this heat treatment were so brittle that they broke into many pieces under a blow and so hard that they might scratch glass, are now found to be capable of withstanding considerable distortion without fracture and so softened that a needle may scratch them.
Not only is a proper temperature necessary for best annealing, but, as stated, a sufficient time must be allowed for the separation of the carbon and iron. The separation requires many hours and the cooling from the annealing temperature must be slow in order that the carbon and iron may not again unite, as they certainly would do were the castings chilled in water or otherwise cooled too fast.
Most manufacturers produce tonnage enough that castings have to be annealed in quantity. As iron at red or higher heat wastes very rapidly on account of scaling (in fact would take fire in air if hot enough), and the quality of castings deteriorates somewhat if even a small amount of air is allowed to come in contact with them during the annealing process, they are generally protected by enclosing in iron containers on which tops are luted and cracks filled with stiff, fire-resisting mud which keeps out the air.
These iron drums are of suitable size and shape that they can be stacked one upon the other to a height of from four to six feet. The stacks or “sets” of “saggers” as they are called, are run into large brick-lined retort chambers which are heated either with coal from a grate, by powdered coal, oil, producer gas or other fuel. The larger the furnace and the greater the tonnage of iron which must be heated, the longer will be the time necessary for bringing the furnace and castings up to the cherry-red heat which is necessary for the annealing. Therefore, the larger furnaces require a somewhat longer time than those of smaller size, though the time required for the annealing of the castings themselves is no longer. In this country with ordinary-sized furnaces the usual time for the annealing operation is approximately one week. This includes the heating of the furnace and castings and the cooling to a black heat again.
No. 109. Photomicrograph of White Cast Iron