The most important step in annealing is to raise the temperature of the metal to the critical point, as any hardness that may have existed will then be completely removed. Strains that may have been set up through heat treatment will be eliminated when the steel is heated to the critical point, and then restored to its lowest hardness by slow cooling. In annealing, the steel must never be heated more than approximately 28 to 40C (50 to 75F) above the critical point. When large articles are annealed, sufficient time must be allowed for the heat to penetrate the metal.

Steel is usually subjected to the annealing process for the following purposes:

1. To increase its ductility by reducing hardness and brittleness.

2. To refine the crystalline structure and remove residual stresses. Steel that has been cold worked is usually annealed to increase its ductility.

Assuming that the part to be annealed is heated to the proper temperature, the required slow cooling may be accomplished in several ways, depending on the metal and the degree of softness required.

Normalizing, although involving a slightly different heat treatment, may be classed as a form of annealing. This process removes all strains due to machining, forging, bending, and welding. Normalizing can only be accomplished with a good furnace, where the temperatures and the atmosphere may be closely regulated and held constant throughout the entire operation. A reducing atmosphere will normalize the metal with a minimum amount of oxide scale, while an oxidizing atmosphere will leave the metal heavily coated with scale, thus preventing proper development of hardness in any subsequent hardening operation. The articles are put in the furnace and heated to a point above the critical temperature of the steel. After the parts have been held at this temperature for a sufficient time to allow the heat to penetrate to the center of the section, they must be removed from the furnace and cooled in still air. Drafts will result in uneven cooling, which will again set up strains in the metal.

Prolonged soaking of the metal at high temperatures must be avoided, as this practice will cause the grain structure to enlarge. The length of time required for the soaking temperature will depend upon the mass of metal being treated.

The carburizing process may be applied to plain carbon steels provided they are within the low-carbon range. Specifically, the carburizing steels are those that contain no more than 0.20 percent carbon. The lower the carbon content in the steel, the more readily it will absorb carbon during the carburizing process. The amount of carbon absorbed and the thickness of the case obtained increase with time; however, the carburization progresses more slowly as the carbon content increases during the process. The length of time required to produce the desired degree of carburization and depth of the case depend upon the composition of the metal, the kind of carburization material used, and the temperature to which the metal is subjected. It is apparent that in carburizing, carbon travels slowly from the outside toward the center; therefore, the proportion of carbon absorbed must decrease from the outside to the center.

A common method of carburizing is called "pack carburizing." When carburizing is to be done by this method, the steel parts are packed with the carburizing material in a sealed steel container to prevent the solid carburizing compound from burning and retaining the carbon monoxide and dioxide gases. The container should be placed in a position to allow the heat to circulate entirely around it. The furnace must be brought to the carburizing temperature as quickly as possible, and held at this heat from 1 to 16 hours, depending upon the depth of the case desired and the size of the work.

After carburizing, the container should be removed and allowed to cool in the air, or the parts removed from the carburizing compound and quenched in oil or water. The air coding, although slow, reduces warpage, and is advisable in many cases.

In another method of carburizing, called "gaseous carburizing," a carbonaceous material is introduced into the furnace atmosphere. When the steel parts are heated in this carburizing atmosphere, carbon monoxide combines with the iron to produce results that are practically the same as those described under the pack carburizing process.

The case obtained in this manner is due principally to the formation of carbides on the surface of the steel. The use of a closed pot is required for cyaniding, as cyanide vapors are extremely poisonous.