Annealing

As a result of cold working, the hardness, tensile strength, and electrical resistance increase, while ductility decreases. There is also a large increase in the number of dislocations, and certain planes in the crystal structure are severely distorted. Most of the energy used to cold work the metal is dissipated in heat, and a finite amount of energy is stored in the crystal structure as internal energy associated with the lattice defects created by the deformation.

Full annealing is the process by which the distorted cold worked lattice structure is changed back to one which is strain free through the application of heat. This process is carried out entirely in the solid state and is usually followed by slow cooling in the furnace from the desired temperature. The annealing process may be divided into three stages:

 Recovery

 Recrystallization

 Grain growth

Recovery:

This is primarily a low temperature process, and the property changes produced do not cause appreciable change in microstructure or the properties, such as tensile strength, yield strength, hardness and ductility. The principal effect of recovery is the relief of internal stresses due to cold working.

When the load which causes plastic deformation is released, all the elastic deformation does not disappear. This is due to the different orientation of crystals, which will not allow some of them to move back when the load is released. As the temperature is increased, there is some spring back of these elastically displaced atoms which relieve most of the internal stresses. Electrical conductivity is also increased appreciably during the recovery stage.

Since the mechanical properties of the metal are essentially unchanged, the main purpose of heating in the recovery range is stress relieving cold worked alloys to prevent stress corrosion cracking or to minimize the distortion produced by residual stresses. Commercially, this low temperature treatment in the recovery range is known as stress relief annealing or process annealing.

Recrystallization:

As the temperature of the recovery range is reached, minute new crystals appear in the microstructure. These new crystals have the same composition and lattice structure as the original undeformed grains and are not elongated but are uniform in dimensions. The new crystals generally appear at the most drastically deformed portions of the grain, usually the grain boundaries and slip planes. The cluster of atoms from which the new grains are formed is called a nucleus. Recrystallization takes place by a combination of nucleation of strain free grains and the growth of these nuclei to absorb the entire cold worked material.

The term recrystallization temperature does not refer to a definite temperature below which recrystallization will not occur, but refers to the approximate temperature at which a highly cold worked material completely recrystallizes in one hour. The recrystallization temperatures of several metals and alloys are listed in Table 1.

Material

Recrystallization Temperature (oF)

Copper ( 99.99 %)

250

Copper ( 5 % Zinc)

600

Aluminum (99.99 %)

175

Aluminum alloys

600

Low carbon steel

1000

Zinc

50

Tin

25

Lead

25

Table 1. Recrystallization temperatures for various metals and alloys.

Pure metals have low recrystallization temperatures as compared with alloys. Zinc, tin and lead have recrystallization temperatures below room temperature. This means that these metals cannot be cold worked at room temperature since they crystallize spontaneously, reforming a strain free structure.

The greater the prior deformation, the lower the temperature for the start of recrystallization.

Increasing the annealing time decreases the recrystallization temperature for the start of recrystallization.

During recrystallization stage, there is a significant drop in tensile strength, hardness and a large increase in the ductility of the material (See Figure 1).

 

Grain Growth:

In this stage the tensilestrength and hardness continue to decrease but at a much less rate than the recrystallization stage. The major change observed during this stage is the growth of the grain boundaries and reaching the original grain size (See Figure 1).

Figure 1. Effect of annealing on tensile strength, hardness, ductility and grain size.

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Last Updated : September 17, 1999

By: Serdar Z. Elgun