Heating the steel above the upper critical point and cooling in still air accomplish normalizing. The more rapid quenching obtained by air-cooling, as compared to furnace cooling, results in a harder and stronger material than that obtained by annealing. Recommended normalizing temperatures for the various types of aircraft steels are listed in Figure 7-6.

Case hardening produces a hard, wear-resistant surface or case over a strong, tough core. Case hardening is ideal for parts that require a wear-resistant surface and, at the same time, must be tough enough internally to withstand the applied loads. The steels best suited to case hardening are the low carbon and low-alloy steels. If high-carbon steel is case hardened, the hardness penetrates the core and causes brittleness. In case hardening, the surface of the metal is changed chemically by introducing a high carbide or nitride content. The core is unaffected chemically.

When heat-treated, the surface responds to hardening while the core toughens. The common forms of case hardening are carburizing, cyaniding, and nitriding. Since cyaniding is not used in aircraft work, only carburizing and nitriding are discussed in this section.

Carburizing is a case hardening process in which carbon is added to the surface of low-carbon steel. Thus, carburized steel has a high-carbon surface and a low-carbon interior. When the carburized steel is heat-treated, the case is hardened while the core remains soft and tough.

A common method of carburizing is called “pack carburizing.” When carburizing is to be done by this method, the steel parts are packed in a container with charcoal or some other material rich in carbon. The container is then sealed with fire clay, placed in a furnace, heated to approximately 1,700 °F, and soaked at that temperature for several hours. As the temperature increases, carbon monoxide gas forms inside the container and, being unable to escape, combines with the gamma iron in the surface of the steel. The depth to which the carbon penetrates depends on the length of the soaking period. For example, when carbon steel is soaked for 8 hours, the carbon penetrates to a depth of about 0.062 inch.

In another method of carburizing, called “gas carburizing,” a material rich in carbon is introduced into the furnace atmosphere. The carburizing atmosphere is produced by using various gases or by the burning of oil, wood, or other materials. When the steel parts are heated in this atmosphere, carbon monoxide combines with the gamma iron to produce practically the same results as those described under the pack carburizing process.

A third method of carburizing is that of “liquid carburizing.” In this method, the steel is placed in a molten salt bath that contains the chemicals required to produce a case comparable with one resulting from pack or gas carburizing.

Alloy steels with low-carbon content, as well as low-carbon steels, may be carburized by any of the three processes. However, some alloys, such as nickel, tend to retard the absorption of carbon. Thus, the time required to produce a given thickness of case varies with the composition of the metal.

Nitriding is unlike other case hardening processes in that, before nitriding, the part is heat-treated to produce definite physical properties. Thus, parts are hardened and tempered before being nitrided. Most steels can be nitrided, but special alloys are required for best results. These special alloys contain aluminum as one of the alloying elements and are called “nitralloys.”

In nitriding, the part is placed in a special nitriding furnace and heated to a temperature of approximately 1,000 °F. With the part at this temperature, ammonia gas is circulated within the specially constructed furnace chamber. The high temperature cracks the ammonia gas into nitrogen and hydrogen. The ammonia, which does not break down, is caught in a water trap below the regions of the other two gases. The nitrogen reacts with the iron to form nitride. The iron nitride is dispersed in minute particles at the surface and works inward. The depth of penetration depends on the length of the treatment. Soaking periods (as long as 72 hours) are frequently required to produce the desired thickness during nitriding.

Nitriding can be accomplished with a minimum of distortion, because of the low temperature at which parts are case hardened and because no quenching is required after exposure to the ammonia gas.

In the wrought form, commercially-pure aluminum is known as 1100. It has a high degree of resistance to corrosion and is easily formed into intricate shapes. It is relatively low in strength and does not have the properties required for structural aircraft parts. The process of alloying generally obtains high strengths. The resulting alloys are less easily formed and, with some exceptions, have lower resistance to corrosion than 1100 aluminum.

Alloying is not the only method of increasing the strength of aluminum. Like other materials, aluminum becomes stronger and harder as it is rolled, formed, or otherwise cold-worked.

Since the hardness depends on the amount of cold-working done, 1100 and some wrought aluminum alloys are available in several strain-hardened tempers. The soft or annealed condition is designated O. If the material is strain hardened, it is said to be in the H condition.

The most widely used alloys in aircraft construction are hardened by heat-treatment rather than by cold-work. These alloys are designated by a somewhat different set of symbols: T4 and W indicate solution heat-treated and quenched but not aged, and T6 indicates an alloy in the heat-treated, hardened condition.