Cold Work  

 

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A material is considered to be cold worked if its grains are in a distorted condition after plastic deformation is completed. All the properties of a metal that are dependent on the lattice structure are affected by plastic deformation or cold working. The following properties are affected by cold work significantly:

Tensile Strength

Hardness

Yield Strength

Ductility

Tensile strength, yield strength and hardness are increased, while ductility is decreased. Although both strength and hardness increase, the rate of change is not the same. Hardness generally increases most rapidly in the first 10 percent reduction (cold work), whereas the tensile strength increases more or less linearly. The yield strength increases more rapidly than the tensile strength, so that, as the amount of plastic deformation is increased, the gap between the yield and tensile strengths decreases. This is important in certain forming operations where appreciable deformation is required. In drawing, for example, the load must be above the yield point to obtain appreciable deformation but below the tensile strength to avoid failure. If the gap is narrow, very close control of the load is required.

 

 

% Cold Work

Tensile Strength

(psi)

Elongation

(% in 2 in)

Hardness

Rockwell X

0

43,000

70

12

 

10

48,000

52

62

 

20

53,000

35

83

 

30

60,000

20

89

 

40

70,900

12

94

 

50

80,000

8

97

 

60

90,000

6

100

 

Table 1. Effect of plastic deformation on the properties of 70:30 Brass. 

 

Ductility follows a path opposite to that of hardness. A large decrease in the first 10 percent reduction and then a decrease at a slower rate is observed.(Figure 2)

Distortion of the lattice structure hinders the passage of electrons and decreases electrical conductivity in alloys.

The increase in internal energy, particularly at the grain boundaries, makes the material more susceptible to inter granular corrosion, thereby reducing its corrosion resistance. Known as stress corrosion, this is an acceleration of corrosion in certain environments due to residual stresses resulting from cold working. One of the ways to avoid stress corrosion cracking is to relieve the internal stresses by suitable heat treatment after cold working and before placing the material in service.

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.

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.

Grain Growth:

In this stage the tensile strength 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