annealing

Dr. H. K. KhairaHead

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.

AnnealingAnnealing is a heat treatment in which the metal is

heated to a temperature above its recrystallisation temperature, kept at that temperature 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 Acm temperature in case of hyper-eutectoid temperature

The cooling is done in the furnace itself.

Aims of Annealing- Increase ductility

- Reduce hardness and brittleness

- Alter microstructure to soften the metal prior to shaping by improving formability

- Recrystallize cold worked (strain hardened) metals

- Remove internal stresses

- Increase toughness

- Increase machinability

- Decrease electrical resistance

- Improve magnetic properties

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

Full annealingIt is heating the steel 30 to 50oC above Ae3 temperature in case of hypo-eutectoid steels and 30 to 50oC above Acm 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.

The cooling rate may be about 10oC per hour.

It is to get all the changes in the properties of the metals like producing equilibrium microstructure, increase in ductility, reduction in hardness, strength, brittleness and removal of internal stresses.

The microstructure contains coarse ferrite and pearlite.

Annealing on TTT Diagram

The cooling rate during annealing is

very slow, about 100C per hour.

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.

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.

Process AnnealingIn process annealing, the cold worked

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

The aim of process annealing is to restore ductility of the cold worked metal.

During process annealing, recovery and recrystallization takes place.

Heat Treatment Temperature

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

←Acm

A3→

Fe-Fe3C phase diagram in the vicinity of eutectoid, indicating heat treating temperature ranges for plain carbon steel

Recrystallization and Melting Temperatures

Recrystallization proceeds more rapid in pure metals that in alloys. For pure metals, the recrystallization temperature is about 0.3Tm (Tm is absolute melting temp.) For some alloys, the recrystallization temperature can be as high as 0.7Tm.

ProcessHeat the metal to a temperatureHold at that temperatureSlowly cool

PurposeReduce hardness and brittlenessAlter the microstructure for a special propertySoften the metal for better machinabilityRecrystallize cold worked (strain hardened) metalsRelieve induced residual stresses

AnnealingIn case of annealing of steels, the steel is

heated to different temperatures depending upon the aim of annealing followed by furnace cooling.

AnnealingAnnealing is a heat treatment designed to

eliminate the effects of cold working. The properties of a metal may revert back to the precold-worked states by Annealing, through recovery, recrystallization and grain growth.

Spheroidizing: held at A1 or 700 °C for 15-25 hrs.

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.

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 but little change in hardness .

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

Changes in Mechanical Properties during Annealing

Annealing temperature and Mechanical Properties for a Brass Alloy

PolygonisationThe misorientation θ between grains can be

described in terms of dislocationsInserting 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

Recrystallizationthe formation of a new set of strain-free

grains within a previously cold-worked material.

RecrystallizationThe dislocation density decreases only a

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

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 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 (Zener drag).

Changes in Microstructure during different stages of

Annealing

Annealing

Annealing is a heat treatment designed to eliminate the effects of cold working. The properties of a metal may revert back to the precold-worked states by Annealing, through recovery, recrystallization and grain growth.

Recovery: the relief of some of the internal strain energy of a previously cold-worked material. Recrystallization: the formation of a new set of strain-free grains within a previously cold-worked material. Grain Growth: the increase in average grain size of a polycrystalline material. An

elevated temperature heat treatment (annealing) is needed for these 3-processes.

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

f d e

Grain Growth

Grain Growth dn - d0n = Kt

Annealing Cold work : mechanical deformation of a metal at relatively

low temperatures. Thus, cold work of a metal increases significantly dislocation density from 108 (annealed state) to 1012 cm/cm3, which causes the metal to be hardened. ex) 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.

Cold-rolling

Cold-drawing

Annealed crystal (grain) deformed or strained crystal

When a metal is cold worked, most of energy goes into plastic deformation to change the shaped 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.

Annealing : a 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 annealing where recovery and recrystallization take place.

Cold work (high energy state)

Recovery and recrystallization Recovery :

A low temperature annealing. The concentration of point defects is decreased and dislocation is

allowed to move to lower energy positions without gross microstructural change.

Modest effects on mechanical behavior while electrical conductivity increases significantly.

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.

recrystallization annealing

deformed crystal undeformed crystal

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.

Influence of annealing temperature on the tensile strength and ductility of a brass alloy

Variation of recrystallization temperature with percent cold work for iron

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

annealing

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