Making a casting involves three basic steps: (1) heating metal until it melts, (2) pouring the liquid metal into a mold cavity, and (3) allowing the metal to cool and solidify in the shape of the mold cavity. Much of the art and science of making castings is concerned with control of the things that happen to metal as it solidifies. An understanding of how metals solidify, therefore, is necessary to the work of the foundry-man. The control of the solidification of metal to produce better castings is described in later chapters on casting design, gating, risering, and pouring.

The change from hot molten metal to cool solid casting takes place in three main steps. The first step is the cooling of the metal from the pouring temperature to the solidification temperature. The difference between the pouring temperature and the solidification temperature is called the amount of superheat. The amount of superheat determines the amount of time the foundryman

has available to work with the molten metal before it starts to solidify.

The second step is the cooling of the metal through the range of temperature at which it solidifies. During this step, the quality of the final casting is established. Shrink holes, blow holes, hot cracks, and many other defects form in a casting while it solidifies.

The third step is the cooling of the solid metal to room temperature. It is during this stage of cooling that warpage and casting stresses occur

THE START OF SOLIDIFICATION

Solidification of a casting is brought about by the cooling effect of the mold. Within a few seconds after pouring, a thin layer of metal next to the mold wall is cool enough for solidification to begin. At this time, a thin skin or shell of solid metal forms. The shell gradually thickens as more and more metal is cooled, until all the metal has solidified. Solidification always starts at the surface and finishes in the center of a section. In other words, solidification follows the direction that the metal is cooled.

The way in which metal solidifies from mold walls is illustrated by the series of steel castings shown in figure 1. The metal that was still molten after various intervals of time was dumped out to show the progress of solidification. All metals behave in a similar manner. However, the time required to reach a given thickness of skin varies among the different metals.

The speed of solidification depends on how fast the necessary heat can be removed by the mold. The rate of heat removal depends on the relation between the volume and the surface area of the metal. Other things being equal, the thin sections will solidify before the thick ones. Outside corners of a casting solidify faster than other sections because more mold surface is available to conduct heat away from the casting. Inside corners are the slowest sections of the casting to solidify. The sand, in this case, is exposed to metal on two sides and becomes heated to high temperatures. Therefore, it cannot carry heat away so fast.

Changes in design to control solidification rate sometimes can be made by the designer. If, however, a change in solidification rate is required for the production of a good casting, the foundryman is usually limited to methods that result in little or no change in the shape of the casting. The rate of solidification can be influenced in three other ways: (1) by changing the rate of heat removal from some parts of the mold with chills; (2) by proper gating and risering, mold manipulation, and control of pouring speed, and (3) by padding the section with extra metal that can be machined off later