Heat-treated, and hardened steels are sometimes required in building operations. The most familiar heat treatment is annealing, a reheating operation in which the metal 4.62 SECTION FOUR is usually heated to the austenitic range (Fig. 4.2) and cooled slowly to obtain the softest, most ductile state. Cold working is often preceded by annealing. Annealing may be only partial, just sufficient to relieve internal stresses that might cause deformation or cracking, but not enough to reduce markedly the increased strength and yield point brought about by the cold working, for example.
Another type of heat treatment that may be used is normalizing. It requires heating steel to 100 to 150F above the A3 temperature line in Fig. 4.2. Then, the steel is allowed to cool in still air. (The rate of cooling is much more rapid than that used in annealing.) Normalizing may be used to refine steel grain size, which depends on the finishing temperature during hot rolling, or to obtain greater notch toughness.
Thick plates have a coarser grain structure than thin plates and thus can benefit more from normalizing. This grain structure results from the fewer rolling passes required for production of thick plates, consequent higher finishing temperature, and slower cooling.
Sometimes, a hard surface is required on a soft, tough core. Two principal casehardening methods are employed. For case carburizing, a low- to medium-carbon steel is packed in carbonaceous materials and heated to the austenite range. Carbon diffuses into the surface, providing a hard, high-carbon case when the part is cooled. For nitriding, the part is exposed to ammonia gas or a cyanide at moderately elevated temperatures. Extremely hard nitrides are formed in the case and provide a hard surface.