annealing

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annealing

(ənēl`ĭng), process in which glass, metals, and other materials are treated to render them less brittle and more workable. Annealing consists of heating the material and then cooling it very slowly and uniformly; the time and temperatures required in the process are set according to the properties desired. Annealing increases ductility and lessens the possibility of a failure by relieving internal strains. The process, also called hot working, was known to the ancients.

Annealing

 

a type of heat treatment of metals and alloys (mainly steels and cast irons) that consists of heating to a certain temperature, holding, and subsequent cooling (usually slow cooling). Annealing is used to accomplish the processes of return (recovery) of metals, recrystallization, and homogenizing. The objectives of annealing are improvement of machinability and structure, attainment of greater homogeneity of the metal, and removal of internal stresses.

The classification of A. A. Bochvar distinguishes two types of annealing. Annealing of the first type, without phase recrystallization, is used to convert the metal into a structural state of greater equilibrium; such treatment eliminates the effects of cold working, decreases the hardness, increases the plasticity and impact strength, and relieves internal stresses (through recovery and recrystallization processes). Annealing of the second type is accompanied by phase recrystallization: the steel is heated to a temperature above the critical points, held for various periods of time, and subsequently cooled slowly.

In full annealing of steel, the metal is heated to a temperature 30°–50°C above the upper critical point to achieve complete transformation of the steel structure into austenite, and then it is cooled slowly to 500°–600°C for the purpose of formation of ferrite and pearlite. The rate of cooling is about 50°–100°C per hr for carbon steels. If the cooling is performed in air, normalizing takes place. In process annealing, the metal is heated to temperatures between the upper and lower critical points and then cooled slowly; this method is most frequently used to produce the structure of divorced pearlite, which leads to a decrease in hardness and an improvement in machinability.

Isothermal annealing is used for alloy steels. In this method the metal is heated above the upper critical point, held, cooled to a temperature below the lower critical point, held for a period sufficient for complete conversion of austenite to pearlite, and cooled to room temperature. In homogenizing, the metal is heated to temperatures significantly above the critical points and then held for a prolonged period. This method is used for equalization of inhomogeneities in the distribution of elements throughout an article. Homogenizing leads to attainment of more uniform properties throughout the article and to a pronounced improvement of the mechanical properties in the direction lateral with respect to the direction of rolling. If necessary, annealing is performed in protective atmospheres to prevent decarburization of steel.

REFERENCES

Bochvar, A. A. Metallovedenie, 5th ed. Moscow, 1956.
Guliaev, A. P. Termicheskaia obrabotka stali, 2nd ed. Moscow, 1960.

R. I. ENTIN

annealing

A process of holding a material at an elevated temperature, but below its melting point, to permit the relieving of internal stresses in the material.

annealing