The forging of a steel component to arrive at a rough shape prior to machining is a procedure that has been adopted almost from the time that man discovered iron. Forging is a not only an ancient craft, it is a science in the manipulation of steel into a predetermined shape that is roughly the final shape of the component to be manufactured.
The forging of steel is generally accomplished at a temperature of approximately 1250-1350°C (2250-2450°F). At this temperature range, the steel is very malleable and can be manipulated with a reasonable degree of ease into the predetermined shape that is required of the component.
One of the primary reasons for forging is to acquire the geometric form of the particular component that is being manufactured. At room temperature (or even warm), it is very difficult to manipulate the steel. In addition, a great deal of induced stress will be transmitted into the steel. This induced stress will manifest itself further into the manufacturing operation after forging and, in particular, at the final heat-treatment procedure (e.g., austenitizing). This manifestation will be seen in the form of distortion.
While forging at an elevated temperature, the steel will undergo grain growth due to the time at temperature. Therefore, it can be said that “time and temperature to steel is like rain and fertilizer to grass. It makes things grow.” The steel grain size will grow in proportion to the forge temperature and to that time at the forge temperature.
Now we have a component that is in the geometric form necessary for machining into the final shape, but it has an enlarged (and deformed) grain size. Therefore, it is necessary to bring the steel grain back to a normal grain shape and size. This is the process of normalizing.
It can be said (as a simple definition) that the process of normalizing to break up non-uniform structures (created by forging and the forging temperature) and to relieve residual stress will ensure a more-uniform grain size in the steel. The steel is simply heated into the austenitizing range followed by a controlled cool-down after soaking at that particular selected process temperature.