chapter three week 6

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Diploma in Mechanical Engineering (Material)

JF 302 MATERIAL TECHNOLOGY 1

www.pis.edu.my

peneraju ilmu sejagat

Week 6


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

Mechanical Working Mechanical working of a metal is simply plastic deformation performed to

change dimensions, properties and surface condition.

It is worked for three purposes: a. To produce a particular shape

b. To break down the cast structure and improve the properties of the metal

c. To impart a desirable dimension and surface finish

Mechanical working of metals may be classified into: i. Cold working

ii. Hot working



Cold working Cold working of a metal is carried out

below its re-crystallization

temperature.

Hot working Hot working of a metal is carried out

above its re-crystallization

temperature but below its melting

point.

Metal working performs above room temperature that the metal does not

work harden.

Plastic deformation of a metal which results in strain hardening. It usually

involves working at room

temperature.



Think critically 3.0

What is re-crystallization

temperature?

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What is re-crystallization

temperature?

The approximate minimum temperature

at which complete re-crystallization of a

cold worked metal occurs within a

specified time.



Cold working Cold working distorts grain structure

of the metals.

Hot working Hot working refines grain structure of

the metals. The coarse columnar

dendrites of cast metal are refined to

smaller grains and improve the

mechanical properties of the

component.

Good surface finish is obtained.

Surface finish of hot worked metal is not as good as with cold working,

because of oxidation and scaling.



Cold working Excessive cold working gives rise to

the formation and propagation of

cracks in the metal.

Hot working Defects in the metal, such as holes,

internal porosity, cracks; when they

get welded up during hot working.

The loss of ductility during cold working has a useful side effect in

machining. With less ductility, the

chips break more readily and

facilitate the cutting operation.

During hot working, self annealing occurs and re-crystallization takes

place immediately following plastic

deformation. This will cause to loss of

ductility.



Purposes of Cold working 1. To obtain thinner work piece

2. To obtain good surface finish

3. To obtain greater dimensional

accuracy

4. To improve mechanical properties

Purposes of Hot working 1. To improve properties of the

metal, as compared to those in

cast condition

2. To produce raw material to be

used for subsequent cold working

operations



Cold working

Advantages: 1. Increase:

a. Ultimate tensile strength

b. Yield strength

c. Hardness

d. Fatigue strength

e. Residual stresses

2. No heating is required.

3. It is more economical than hot

working for certain ductile metals.

Hot working

Advantages: 1. It does not affect tensile strength,

hardness, corrosion resistance for

work piece.

2. It promotes uniformly of work

piece by facilitating diffusion of

alloy constituents and breaks up

the brittle films or impurities in

metals.



Cold working

Limitations : 1. Possess less ductility

2. Strain hardening occurs

3. Metal surface must be clean and

scale free before cold working

4. Higher forces are required for

deformation than hot working

Hot working

Limitations : 1. Oxide forms rapidly on metal

surface

2. Surface scaling of the product

3. Surface decarburization

4. The volumetric expansion of the

work piece during heating and its

contraction during cooling

preclude the accurate final

dimension

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Think critically 3.0(a)

What is the effect of cold working on

the mechanical properties of copper?



Think critically 3.0(a)

What is the effect of cold working on

the mechanical properties of copper?

As the percent cold work increases,

both the yield strength and the tensile

strength increase. However, the

ductility decreases and approaches

zero.

The metal will break if more cold work

is attempted.



Cold working processes 1. Rolling

2. Wire and tube drawing

Hot working process 1. Hot Rolling

2. Forging

3. Extrusion

4. Welding



ROLLING

Fig 3.0 Rolling process

Principle of Operation:

A pair of cylindrical rollers made of iron or steel rotate in opposite direction with a gap in between.

The worked piece is entered into the gap and passes between the rollers. It is squeezed and its

cross section is reduced.



ROLLING

Cold rolling is applied to sheet or plate to make it thinner, better surface finish, maintain an accuracy

dimension and increase the strength and hardness.

Hot rolling is used to convert ingots into smaller sections known as billets. It is used to produce

plate, sheet and also rods, bars and various

structural shapes through rollers at certain

temperature.

Fig 3.1 Cold and hot rolling



ROLLING

Rolling temperature:

Hot rolling is carried out at tempt. above 0.7Tm (where Tm is the melting point of the metal in C)

1000C for mild steel and 450C for aluminium.

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ROLLING

Advantages :

i. High speed of rolling

ii. Economy of the process

iii. Refine the cast coarse crystalline structure and

eliminate the gas pockets and shrinkage cavities.

Limitations :

i. Non-uniform surface finish of the hot rolled metals

ii. The degree of size or dimensional control

attainable in hot rolling is not narrowly precise.



WIRE AND

TUBE

DRAWING

Fig 3.2 Wire and tube drawing


Wire drawing involves reducing the diameter of a rod or wire by passing through a series of drawing

dies or plates.

Tube drawing involves reducing the cross section and wall thickness through a draw die.



WIRE AND

TUBE

DRAWING Wire drawing:

The finished wire is coiled onto a motor driven block. The dies are manufactured from hard

material, for example tungsten carbide. Diamonds

dies are used for small diameter wire. Soap is used

for lubrication. If large diameter reductions are

required, the rod is progressively drawn through

several stages of dies and blocks.

Wire can be extruded at speeds of up to

2000m/min.



WIRE AND

TUBE

DRAWING

Tube drawing:

A draw bench is used to pull the tube through a drawing die. One end of the tube to be drawn is

collapsed and fed through the die. This end is then

clamped to a travelling carriage. The carriage is moved away from the die either mechanical or

hydraulically traction. In practice several tubes may

be drawn in parallel to increase productivity.



FORGING

Fig 3.3 Forging process


Forging is the shaping of metal either by impact or steady compression between a hammer and an

anvil. Usually the metal is heated to increase its

plasticity.

Working: a. Metal to be forged is first formed into bars or

blocks, called billets.

b. The billets are heated to permit an easy flow of

the metal for shaping purposes before forging.

c. Hammer rests on the anvil and cause the metal

to change shape.




When do the cold forging process is

used?

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When do the cold forging process is

used?

It is necessary to:

a. develop strength and hardness in a

component

b. have bright and clean surface

c. eliminate forging scale

d. eliminate decarburizing



EXTRUCTION

Fig 3.4 Extrusion process


Extrusion is a metal working process that produces long lengths of uniform cross sectional area from a

metal billet by causing the latter to flow under

hydrostatic pressure through a restricted die.

Working: a. A heated cylindrical billet is placed in the

container and forced out through a steel die by a

ram.

b. The metal takes in cross section the shape of the

die.




What are the two types of hot

extrusion process? Discuss briefly.




What are the two types of hot

extrusion process? Discuss briefly.

a. Direct extrusion

b. Indirect extrusion




a. Direct extrusion

i. A hot billet is placed within the container

ii. A ram forces hot billet through the die opening,

producing the extruded product.




b. Indirect extrusion

i. The billet remains stationary while the die is

pushed into the billet by the hallow ram through

which the extrusion takes place

ii. It does not require as much force as direct

extrusion because no force is required to move

the hot billet inside the container.

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HOT

EXTRUCTION




List four typical types of cold

extrusion processes.




List four typical types of cold

extrusion processes.

a. Hooker extrusion

b. Hydrostatic extrusion

c. Impact extrusion

d. Cold extrusion forging


Name the following

metal working

processes:


chapter three week 6

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