Annealing

Dr. H. K. KhairaProfessor

Deptt. of Mat. Sci. and Met. Engg.

ANNEALING

Heat TreatmentHeat treatment is defined as heating a

metal to a specified temperature, keeping it at that temperature for some time followed by cooling at a specified rate.

It is a tool to get required microstructure and properties in the metal.

Handouts 23

Heat treatment

Heat treatment - controlled heating and cooling basically

The basic steps of heat treatment are:

Heating → Soaking → Cooling

Handouts 24

Heat treatment

Heating -> Soaking -> Cooling

Temperature Time of soaking Rate of cooling

Medium of cooling

- Different combinations of the above parameters

- Different compositions of materials and initial phases of materials

Give rise to different heat treatments

Heat TreatmentsThere are different types of heat

treatments.Annealing is one of the heat treatments

given to metals.Main aim of annealing is to increase the

ductility of the metal.

Types of Heat Treatments1. Annealing2. Normalizing3. Hardening4. Tempering5. Precipitation Hardening

AnnealingAnnealing is a heat treatment in which the metal is

heated to a temperature above its recrystallisation temperature, kept at that temperature for some time for homogenization of temperature followed by very slow cooling to develop equilibrium structure in the metal or alloy.

The steel is heated 30 to 50oC above Ae3 temperature in case of hypo-eutectoid steels and 30 to 50oC above A1 temperature in case of hyper-eutectoid temperature

The cooling is done in the furnace itself.

Fe-Fe3C phase diagram indicating heat treating temperature ranges for plain

carbon steel

Aims of Annealing- 1. Increase ductility

- 2. Reduce hardness

- 3. Improving formability

- 4. Recrystallize cold worked (strain hardened) metals

- 5. Remove internal stresses

- 6. Increase toughness

- 7. Decrease brittleness

- 8. Increase machinability

- 9. Decrease electrical resistance

- 10. Improve magnetic properties

Types of Annealing1. Full annealing2. Stress relief annealing3. Process annealing4. Spheroidizing annealing

Heat Treatment Temperature

The temperature ranges to which the steel has to be heated for different heat treatments

←Acm

A3→

1. Full annealingIt is heating the steel 30 to 50oC above Ae3 temperature in case of hypo-eutectoid steels and 30 to 50oC above A1 temperature in case of hyper-eutectoid temperature, keeping it at that temperature for some time for homogenization of temperature followed by cooling at a very slow rate (furnace cooling).

The cooling rate may be about 10oC per hour.

1. Full annealingIt is to get all the changes in the properties of the

metals like1. Producing equilibrium microstructure,2. Increase in ductility, 3. Reduction in hardness, strength, brittleness and 4. Removal of internal stresses.

The microstructure after annealing contains coarse ferrite and pearlite.

Annealing on TTT Diagram

The cooling rate during annealing is

very slow, about 100C per hour.

2. Stress Relief AnnealingIn stress relief annealing, the metal is heated to

a lower temperature and is kept at that temperature for some time to remove the internal stresses followed by slow cooling.

The aim of the stress relief annealing is to remove the internal stresses produced in the metal due toPlastic deformationNon-uniform coolingPhase transformation

No phase transformation takes place during stress relief annealing.

3. Spheroidizing AnnealingIn spheroidizing annealing, the steel is

heated to a temperature below A1 temperature, kept at that temperature for some time followed by slow cooling.

The aim of spheroidizing annealing is to improve the machinability of steel.

In this process the cementite is converted into spheroidal form.

The holding time varies from 15 – 25 hours.

Figure 12.6 The microstructure of spheroidite, with Fe3C particles dispersed in a ferrite matrix ( 850). (From ASM Handbook, Vol. 7, (1972), ASM International, Materials Park, OH 44073.)

4. Process AnnealingIn process annealing, the cold worked

metal is heated above its recrystallisation temperature, kept for some time followed by slow cooling.

4. Process AnnealingThe aim of process annealing is to restore

ductility of the cold worked metal.

During process annealing, recovery and recrystallization takes place.

During process annealing, new equiaxed, strain-free grains nucleate at high-stress regions in the cold-worked microstructure, and hence hardness and strength decrease whereas ductility increases

recrystallization annealing

deformed crystal undeformed crystal

Process Annealing Cold work : mechanical deformation of a metal at relatively

low temperatures. Thus, cold working of a metal increases significantly dislocation density from 108 (annealed state) to 1012 cm/cm3, which causes hardness and the strength of the metal. Example --- rolling, forging, and drawing etc.

• % cold work = (A0 - Af)/A0 x 100%, where A0 is the original cross-sectional area and Af is the final cross-sectional area after cold working.

• With increasing % cold work, the hardness and strength of alloys are increased whereas the ductility of the alloys are decreased.

• For further deformation, the ductility has to be restored by process annealing.

Cold-rolling

Cold-drawing

Process Annealing Annealed crystal (grain) deformed or strained

crystal

When a metal is cold worked, most of energy goes into plastic deformation to change the shape and heat generation. However, a small portion of the energy, up to ~5 %, remains stored in the material. The stored energy is mainly in the form of elastic energy in the strain fields surrounding dislocations and point defects generated during the cold work.

Process Annealing : Cold worked grains are quite unstable due to the strain energy. By heating the cold worked material to high temperatures where sufficient atomic mobility is available, the material can be softened and a new microstructure can emerge. This heat treatment is called process annealing where recovery and recrystallization take place.

Cold work (high energy state)

Process Annealing

Recrystallization : occurs at 1/3 to 1/2 Tm.

during recrystallization process, new equiaxed, strain-free grains nucleate at high-stress regions in the cold-worked microstructure, and hence hardness and strength decrease whereas ductility increases.

Recrystallization temp. is that at which recrystallization just reaches completion in 1 hour.



Variation of recrystallization temperature with percent cold work for iron

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Figure 12.4 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels.

Example 12.2 SOLUTION

From Figure 12.2, we find the critical A1, A3, or Acm, temperatures for each steel. We can then specify the heat treatment based on these temperatures.

Stages of AnnealingThere are three stages of annealing

1.Recovery2.Recrystallization3.Grain Growth

Recoverythe relief of some of the internal strain

energy of a previously cold-worked material.

Recovery

Relieves the stresses from cold working Recovery involves annihilation of point defects. Driving force for recovery is decrease in stored energy

from cold work. During recovery, physical properties of the cold worked

material are restored without any observable change in microstructure.

Recovery is first stage of annealing which takes place at low temperatures of annealing.

There is some reduction, though not substantial, in dislocation density as well apart from formation of dislocation configurations with low strain energies.

RecoveryThe concentration of point defects is

decreased and dislocation is allowed to move to lower energy positions without gross microstructural change.

RecoveryLet us now examine the changes that occur

when a sample is heated from room temperature.

At first, recovery occurs in which there is a change in the stored energy without any obvious change in the optical microstructure.

Excess vacancies and interstitials anneal out giving a drop in the electrical resistivity.

Dislocations become mobile at a higher temperature, eliminate and rearrange to give polygonisation.

Modest effects on mechanical behavior

Changes in Mechanical Properties during Annealing

Annealing temperature and Mechanical Properties for a Brass

PolygonisationPolygonisation occurs during recovery.Dislocations become mobile at a higher

temperature, eliminate and rearrange to give polygonisation.

The misorientation θ between grains can be described in terms of dislocations

Inserting an edge dislocation of Burgers vector b is like forcing a wedge into the lattice, so that each dislocation is associated with a small change in the orientation of the lattice on either side of the extra half plane.

If the spacing of dislocations is d, then θ= b/d

Polygonisation

(a) Random arrangement of excess parallel edge dislocations and

(b) alignment into dislocation walls

Changes in Microstructure during different stages of

Annealing

Recrystallizationthe formation of a new set of strain-free

grains within a previously cold-worked material.

Recrystallization

This follows recovery during annealing of cold worked material. Driving force is stored energy during cold work.

It involves replacement of cold-worked structure by a new set of strain-free, approximately equi-axed grains to replace all the deformed crystals.

This process ocurs above recrystallisation temperature which is defined as the temperature at which 50% of material recrystallises in one hour time.

The recrystallization temperature is strongly dependent on the purity of a material.

Pure materials may recrystallize around 0.3Tm, while impure materials may recrystallise around 0.4Tm, where Tm is absolute melting temperature of the material.

Effect of alloying elements on Recrystallisation Temperature

Increase in the recrystallisation temperature of pure copper by the addition of 0.01 atomic percent of the indicated element

Added element Increase in recrystallisation Temp. (C)

Ni 0Co 15Fe 15Ag 80Sn 180Te 240

Recrystallization laws A minimum amount of deformation is needed to cause

recrystallisation (Rx). Smaller the degree of deformation, higher will be the Rx

temperature. The finer is the initial grain size; lower will be the Rx

temperature. The larger the initial grain size, the greater degree of

deformation is required to produce an equivalent Rx temperature.

Greater the degree of deformation and lower the annealing temperature, the smaller will be the recrystallized grain size.

The higher is the temperature of cold working, the less is the strain energy stored and thus Rx temperature is correspondingly higher.

The recrystallisation rate increases exponentially with temperature.

Rate of RecrystallisationRate of Recrystallisation = Ae-Q/RT

Taking log of both sides Log (Rate) = -Q/RT

Or Log(Rate of recrystallisation) is proportional to 1/T

Recrystallization

Recrystallisation : occurs at 1/3 to 1/2 Tm.

during recrystallisation process, new equiaxed, strain-free grains nucleate at high-stress regions in the cold-worked microstructure, and hence hardness and strength decrease whereas ductility increases.

Recrystallization temperature is that temperature at which recrystallization just reaches completion in 1 hour.



RecrystallizationThe dislocation density decreases only a

little during recovery and the deformed grain structure is largely unaffected by recovery.

It takes the nucleation and growth of new grains to initiate a much larger change, i.e. recrystallisation.

RecrystallizationThe nucleation of new grains happens in

regions of high dislocation density. Nucleation begins in a jumble of dislocations.

The recrystallised grain will essentially be free from dislocations.

A greater nucleation rate leads to a finer ultimate grain size.

There is a critical level of deformation below which there will be no recrystallisation at all.

A critical strain anneal can lead to a single crystal on recrystallisation.

Recrystallization The processed of recovery and recrystallization of a cold

worked represent a structural transformation, not true phase transformations. The driving force for recovery and recrystallization is associated with the strain energy stored in the crystal as a result of cold work.

↑ the amount of cold work

↑ grain size before cold work ↑ number of strain-free nuclei

↑ annealing temp.


Annealing

Changes in Mechanical Properties during Annealing

Annealing temperature and Mechanical Properties for a Brass Alloy

Grain Growththe increase in average grain size of a

polycrystalline material.

D - Do = kt1/2

Where D is the grain diameter after time t and

And Do is initial grain diameter

Grain growth Grain growth follows complete crystallization if the

material is left at elevated temperatures. Grain growth does not need to be preceded by recovery

and recrystallization; it may occur in all polycrystalline materials.

In contrary to recovery and recrystallization, driving force for this process is reduction in grain boundary energy.

Tendency for larger grains to grow at the expense of smaller grains is based on physics.

In practical applications, grain growth is not desirable. Incorporation of impurity atoms and insoluble second

phase particles are effective in retarding grain growth. Grain growth is very strongly dependent on temperature

Grain GrowthA grain of radius r has a volume 4/3(πr3) and surface

area 4πr2. The grain boundary energy associated with this grain is 2πr2γ where γ is the boundary energy per unit area. and we have taken into account that the grain boundary is shared between two grains. If follows that:

energy per unit volume = 3γ/2r≡ 3γ/Dwhere D is the grain diameterIt is this which drives the growth of grains with an

equivalent pressure of about 0.1 MPa for typical values of γ = 0.3Jm−2 and D = 10μm. This is not very large so the grains can readily be pinned by particles.

Grain Growth

Grain Growth dn - d0n = Kt1/2


Annealing

Grain growth Grain growth :

A large concentration of grain boundaries (fine grain structure) is reduced by grain growth that occurs by high temp. annealing. The driving force for the grain growth is the reduction in the grain boundary surface energy.

Stages of the recrystallization and grain growth of brass

Recrystallization and grain growth of brass

(a) Cold-worked

(b) Initial stage of

recrystallization

c a b

(3 s at 580°C)

(c) Partial replacement of cold-worked grains by recrystallized ones (4 s at 580°C)

(d) Complete recrystallization (8 s at 580°C)

(e) Grain growth after 15 min at 580°C.

(f) Grain growth after 10 min at 700°C

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Figure 12.5 The effect of carbon and heat treatment on the properties of plain-carbon steels.

Effects of microstructure

Hardness Ductility

Annealing

Technology

Transcript of Annealing