Because of the considerable change in form by contraction during cooling, steel castings of large bulk and particularly varying thickness, are apt to be under stress due to the contraction of a heavier and hotter part after a thinner part has cooled. Especially is this the case with castings naturally brittle from a high per cent of carbon. The stresses are removed, or at least are reduced within safe limits, by annealing.

This may be done in any kind of a brick furnace which can be evenly heated to the temperature required. After the heat has remained for a while at its maximum, the furnace openings are stopped with bricks or clay to insure slow and even cooling as the fire dies out. Castings may be covered with sand to assist in the gradual and even heating and cooling of light and heavy parts. Castings are heated red, and annealing requires from 60 to 180 hours.

The elastic and tensile strength of a casting is controlled to such a degree by annealing that many investigators are seeking to find the factors controlling this process. Method of heating, time and temperature, all enter into the problem. Annealing furnaces need pyrometers to insure best results.

By exercising care in allowing small and medium-sized castings to cool in the moulds, the necessity of annealing may be avoided.