Chapter Three Week 8

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Diploma in Mechanical Engineering (Material)

JF 302 MATERIAL TECHNOLOGY 1

www.pis.edu.my

peneraju ilmu sejagat

Week 8


CLO 2 : Synthesis course knowledge into an effective

course assignment and present technical findings

both orally and in writing.


Understand cold work

Understand hot work

Describe the casting process

Describe the heat treatment of steels


cold work hot work casting process Heat treatment of steels

HEAT

TREATMENT

Principle of Operation:

Heat treatment may be defined as an operation involving heating and cooling of metal/alloy in solid

state to obtain desirable conditions and properties.



HEAT

TREATMENT

Principle of Operation:

Heat treatment may be defined as an operation involving heating and cooling of metal/alloy in solid

state to obtain desirable conditions and properties.

Stress relieving Better machinability

Improve ductility



CLASSIFICATION

OF HEAT

TREATMENT

PROCESS Classification of heat treatment: Various heat treatment processes can be classified

as:

1. Annealing

2. Normalizing

3. Quenching

4. Tempering

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PURPOSES

OF HEAT

TREATMENT Purposes of heat treatment: 1. Relief of internal stresses which is developed

during cold working, welding, casting or forging

2. Harden and strengthen metal/alloy

3. Improve machinability

4. Change of grain size

5. Soften metal/alloy for further mechanical working

6. Improve ductility and toughness

7. Increase heat/wear and corrosion resistance

8. Improve magnetic and electrical properties



STAGES OF

HEAT

TREATMENT Stages of heat treatment:

1 2 3

1. Heating

2. Holding (Soaking)

3. Cooling



Think critically 3.5

Briefly describe the three basic

stages of heat treatment process.




Briefly describe the three basic

stages of heat treatment process.

1. Heating the steel to define temperature

2. Soaking at that temperature for sufficient period to allow

necessary changes to occur

3. Cooling at a rate to obtain desired properties associated with

changes in the form, size and distribution of micro-constituents

(such as ferrite, pearlite and martensite)



Fig 3.1 Temperature range for heat treatment of steel

A1 - Eutectoid line

727C or

Lower critical

tempt

A1

A3

Acm A3 Upper critical tempt

Acm austenite + cementite line



ANNEALING

Definition:

Annealing is the process of heating a steel to a

temperature about 727C, which will remove the

instability or distortion and then cooling at a slow

rate (furnace cooled).

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ANNEALING

Purposes:

1. Induce a completely stable structure (full

annealing)

2. Refine the structure

3. Reduce hardness

4. Improve machinability

5. Remove residual stresses

6. Improve mechanical, physical, electrical and

magnetic properties




Give the classes of annealing of

steels.




Give the classes of annealing of

steels.




Briefly explain the annealing

temperatures.




Briefly explain the annealing

temperatures.




Explain the annealing for

hypoeutectoid steels .

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Explain the annealing for

hypoeutectoid steels .



NORMALIZING

Definition:

Normalizing is the process of heating a steel to a

temperature about 40-50C above its upper critical

temperature, holding it at that temperature for a

short time and then cooling in air at room

temperature.



NORMALIZING

Purposes:

1. Refine the grain size of steel

2. Reduce internal stresses

3. Produce a harder and stronger steel than

annealing

4. Improve machinability, ductility and toughness




Why normalizing produces harder

and stronger steel than annealing?




Why normalizing produces harder

and stronger steel than annealing?

Normalizing produces harder and stronger steel than annealing due

to a faster cooling rate. Thus, normalizing is often used as a

final heat treatment.



QUENCHING

Definition:

Quenching is the process of heating a steel with

sufficient carbon (0.35-0.7%) to a temperature

about 30-50C above A3 line (Fig 3.1), holding it at

that temperature from 15 to 30 minutes and then

cooling rapidly or quenched in a suitable medium

(brine, water or oil).

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QUENCHING

Purpose:

1. Improve hardness

2. Improve wear resistance




How does the cooling rate affect the

formation of martensite?




How does the cooling rate affect the

formation of martensite?

During fast cooling rate, austenite cannot transform into ferrite due

to insufficient time for carbon atoms to diffuse out of austenite.

Most of the carbon atoms remain trapped in austenite to distort

the structure and become elongated. The structure is called

martensite. It is hard and brittle.



TEMPERING

Definition:

Tempering is the process of heating hardened steel

below the lower critical temperature, holding it at

that temperature from 3 to 5 minutes and then

cooling rapidly or quenched in a suitable medium

(brine, water or oil).



TEMPERING

Purposes:

1. Relieve residual stresses

2. Improve ductility

3. Improve toughness

4. Reduce hardness




What happens during tempering?

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What happens during tempering?

During tempering, the excess carbon atoms, trapped in martensite,

gradually come out as extremely fine cementite particles and

the metastable martensite transforms into stable ferrite. The

resulting microstructure is called tempered martensite or bainite.


List FOUR (4) types of surface hardening

processes?



List FOUR (4) types of surface hardening

processes?








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Chapter Three Week 8

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