Chilled Castings is a name applied to a class of casting wherein the metal has been suddenly cooled or chilled immediately upon pouring. The means usually adopted for chilling castings is to pour the metal against a piece of iron that forms a part of the mould. The effect of this sudden chilling is to produce great hardness on the face of the casting. The most common example of chilled castings is the cast iron car wheels that are used on all American railways. In this case the tread and flange are chilled. The chill is that portion of the mould which is made of iron against which the hot metal strikes. Chills for car wheels are made in several ways. The original form was a plain smooth ring that was turned to the contour of the tread and face of the flange. This ring was about 3 inches thick, in order that it might not itself become over heated. Of recent years the contracting chills are used. These are of various forms. One is shown in Fig. 70. In this the chilling face a is divided into n large number of segments which are attached to the main ring b by narrow necks. The object of the contracting chill is to keep the metal of the chill and the casting in contact until the latter has cooled.
The molten metal heats and expands the common chill and at the same time the cooling lasting contracts or becomes smaller. The result is that the time of actual contact is very brief and the easting is likely to become imperfect in shape when cold. With the contracting chill the face a with its neck in heated and expanded by the molten metal. It is fastened at its outer end to the heavy solid ring b, Which is too far away from the molten metal to become heated. As a result the face and neck are pushed toward the center by their expansion and the diameter or the chill is lessened. This type of chill, therefore, follows the shrinking casting and remains in contact with it until it is cold. Other types of contracting chills are simply plain rings of the original form, which have a stream of cold water circulated through them after the metal has been poured. Some of these are plain rings, in others the contracting necks are as in Fig. 70, with the outer ring b, through which the water is circulated.
CAR-WHEEL BORER. Baker Brothers.
The moulding of castings that are to be chilled is done in the same manner as ordinary castings. The chill is either set in position like a core or forms a part of the flask as in the case of a car wheel chill.
Chilled castings should be annealed before using. The sudden cooling of one part sets up internal stresses that will result in fracture unless relieved by annealing. The usual method of annealing car wheels is to remove the casting from the mould while it is still red hot. It is placed in a pit 7 or 8 feet deep and having a diameter a little larger than its own, with enough others to fill it. They are covered and then buried in ashes. Thus protected the wheels cool very slowly. The internal stresses are relieved because of the shape which the metal takes. This annealing usually takes about one week.
The depth of the chill, or the thickness of the exceedingly hard metal varies from 1/16 to 5/8 inch. It depends upon the character of the metal used, the heat at which it is poured, and the rapidity with which it is cooled. In car wheels the depth of the chill is about 3/8 inch.
All irons are not suitable for the making of chilled castings. No. 1 and No. 2 iron that are rich, or comparatively so, in uncom-bined or graphitic carbon do not chill readily. The best chilled car wheels are made from the irons containing combined carbon. Salsbury and Lake Superior pig has, until recent years, been most extensively used for car wheel making. As these irons, especially when made in a charcoal furnace, are expensive, they are being supplanted by cheaper mixtures.