Cold Work

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.

Figure 1. Effect of cold working on tensile and yield strength of copper.

 

 

% 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.

Figure 2. Effect of cold working on tensile strength, hardness, ductility and grain size. (The curve below ductility represents the change in grain size)

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Last Update: September 18, 1999

By: Serdar Z. Elgun