Normalizing


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Normalizing

 

a type of heat treatment of steel in which the metal is heated to a temperature above the upper critical point, held at this temperature, and then cooled in still air. The purpose of normalizing is to impart to the metal a uniform, fine-grained structure that could not be achieved in the preceding casting, forging, or rolling processes and, as a result, to produce improved mechanical properties (ductility and impact toughness).

In normalizing low-carbon and medium-carbon steels, decomposition of austenite takes place, with formation of a mixture of ferrite with pearlite and sorbite. Shaped steel castings are normalized to remove the coarse-grained cast structure, which results in poor mechanical properties. Rolled and forged steels are normalized to remove structural nonuniformities that are caused by the conditions of deformation and cooling. Normalization procedures are determined by the temperature to which the metal is heated (austenitization temperature), the holding time at that temperature, and the cooling rate. The heating temperature for normalization is 20°-50°C above the upper critical point. The holding time should be the minimum time that is required to achieve uniform heating throughout the cross section of the article. The cooling rate in still air is usually 150°-250°C per hr. However, for normalizing large articles the cooling rate should depend on the dimensions of the articles, the composition of the steel, and the kinetics of the austenite transformations. An increase in the rate of heating and a reduction of the holding times and temperatures to a minimum produce finer austenite grains and a more disperse mixture of pearlite or sorbite with ferrite.

R. I. ENTIN

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