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Department of Technical Education Andhra Pradesh

Name :B.Tulasi RaoDesignation : LecturerBranch :Mechanical Engineering Institute :Govt. polytechnic, VisakhapatnamYear/semester :IV semesterSubject :Engineering Materials.Subject code :M 404Major Topic :Structure of materialsDuration :100 minSub Topic :Recrystallisation Teaching aids :PPT &DiagramsRevised by : K.Vijaya Bhaskar

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Recrystallisation

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In the previous class we have discussed about

• Formation of grains in the metal

• Dendrite growth

• The effect of cooling rate on grain formation.

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Recap

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Objectives

On the completion of this period, you would be

able to understand

• Recovery

• Recrystallisation

• Grain growth

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Recrystallisation

• The temperature at which the new grains are formed is called as Recrystallisation temperature

• Recrystallisation has a marked effect on properties and

microstructure

• Recrystallisation results in the formation new crystal and

grain growth

• Recrystallisation follows recovery

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• Recovery is the initial stage in which internal stresses

are eliminated

• Recovery will not effect the structure .

• There is no appreciable reduction in strength and

hardness .

• It establishes a pattern of stress free equi-axed grains.

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Recrystallisation

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Recrystallisation

• Recrystallisation is the temperature level at which the

first tiny new grain appears.

• It is lowest for pure metals and generally raises/lowers

by the presence of other elements (depends on type of

impurity/Alloy)

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• Recrystallisation temperature of a metal mainly depends on ,

(1) Type of metal

(2) Purity of metal

(3) Degree of previous deformation

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Recrystallisation

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(4) Temperature at which deformation is carried out.

(5) Grain size

(6) Annealing time

(7) alloying elements etc

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Recrystallisation

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Hot Working Vs Cold Working

Working of material above its recrystallisation temperature is known as Hot working .

Working of material below its recrystallisation temperature is known as Cold working.

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• Large grains have lower free energy than small grains .

• This is associated with the reduction of the amount of

grain boundary .

• Under ideal condition the lowest energy state for a metal

would be as a single crystal.

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Hot Working Vs Cold Working

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• As the temperature increases the rigidity of the lattice

decreases and the rate of growth is more rapid.

• It is possible to grow very large grains by holding a

specimen for a long time high in the grain growth region .

• The specimen should be held at a temperature just

below the melting point of the alloy.

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Hot Working Vs Cold Working

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• Large grain size metals are highly ductile but low

strength and hardness .

• Therefore coarse grains are not desirable in the final

structure .

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Hot Working Vs Cold Working

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Recrystallisation

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Fig.1

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Grain Growth

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Fig.2

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Recrystallisation temperature of metals

Metal Min Recrystallisation temperature Melting Point Lead Below00c 3200c Tin Below room temperature 2320c Zinc 200c To 1200c 41900c

Copper 1200c To 2400c 10830c Aliminium 1600c To2800c 6600c Silver 2800c 9600c Gold 2800c 10600c Iron 4500c 15400c

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• Recovery is the initial stage in which internal stresses

are eliminated without effecting the structure

• The process of creating new grains in a cold worked

material is known as recrystallisation.

Summary

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• Hot working is usually described as working of a

material above its recrystallisation temperature.

• Cold working is usually described as working of a

material below its recrystallisation temperature

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Summary

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1.Residual stresses of a cold worked body are treated by

a)Recrystallisation

b)Dendrites

c)Nuclei creation

d)Under cooling

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Quiz

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2.Recrystallisation temperature of an alloy will be --of its melting temperature.

a)0.2b)0.5c)0.3d)0.4

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Quiz

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Questions

1. What is recrystallisation?

2. What is recovery?

3. Explain the mechanism of recrystallisation with the help of illustration?

4. Discuss the various factors which influence the grain growth.

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