Structural steels may be annealed to relieve stresses induced by cold or hot working. Sometimes, also, annealing is used to soften metal to improve its formability or machinability. Annealing involves austenitizing the steel by heating it above the A3 temperature line in Fig. 1.14, then cooling it slowly, usually in a furnace. This treatment improves ductility but decreases tensile strength and yield point. As a result, further heat treatment may be necessary to improve these properties.
Structural steels may be normalized to refine grain size. As pointed out in Art. 1.21, grain size depends on the finishing temperature in hot rolling.
Normalizing consists of heating the steel above the A3 temperature line, then cooling the metal in still air. Thus the rate of cooling is more rapid than in annealing. Usual practice is to normalize from 100 to 150F above the critical temperature. Higher temperatures coarsen the grains.
Normalizing tends to improve notch toughness by lowering ductility and fracture transition temperatures. Thick plates benefit more from this treatment than thin plates. Requiring fewer roller passes, thick plates have a higher finishing temperature and cool slower than thin plates, thus have a more adverse grain structure. Hence the improvement from normalizing is greater for thick plates.
