Heat treatment of Metals (5)

November 4, 2009

Heat treatment of Metals (5)


The primary purpose of annealing is to soften hard steel so that it maybe machined or cold worked. This is usually accomplished by heating the steel to slightly above the critical temperature, holding it htere until the remperature of the piece is uniform throughout, and then cooling at a slowly controlled rate so that the temperature of the surface and that of the center of the piece are approximately the same. This process is known as full annealing because it wipes out all trace of previous structure, refines the crystalline structure, and softens the metal. Annealing also relieves internal stresses previously set up in the metal.

The temperature to which a given steel should be heated in annealing depends on its composition; for carbon steels it can be obtained readily from the partial iron-iron carbide equilibrium diaagram. The heating rate should be consistent with the size and uniformity of sections, so that the entire part is brought up to temperature as uniformly as possible. When the annealing temperature about 45 min for each inch (25mm) of thichness  of the largest section. For maximum softness and ductility the cooling rate should be very slow, such as allowing the parts to cool down with the furnace. The highest the carbon content, the slower this rate must be.


The process of normalizing consists of heating the steel about 50′ to 100’F (10′ – 40’C) above the upper critical range and cooling in still air to room temperature. This process is principally used with low end medium carbon steels as well as alloy steels to make the grain structure more uniform, to relieve internal stresses, or to achieve desired results in physical properties. Most commercial steels are normalized after being rolled or cast.

Spheroidizing is the process of producing a structure in which the cementite is in a spheroidal distribution. If a steel is heated slowly to a temperature just below the cirtical range and held tere for a prolonged period of time, this structure will be obtained. The globular structure obtained gives improved machinability to the steel. This treatment is particularly useful for hypereutectoid steels that must be machined.

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