chapter two week 5

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Diploma in Mechanical Engineering (Material)

JF 302 MATERIAL TECHNOLOGY 1

www.pis.edu.my

peneraju ilmu sejagat

Week 5


CLO 1 : Explain various types of materials used in

manufacturing industry.


Understand the iron production

Understand steel production

Describe the plain carbon steel

Describe the alloy steel

Describe the cast iron


Iron production steel production carbon steel alloy steel cast iron

Fig 2.5 The iron-carbon phase diagram

The composition axis in Fig 2.5 extends to 6.7 wt% C. But we will only observe the phase transformation up to 1.7 wt% C.

Fe3C

Fe3C



At room tempt. the stable form, called BCC ferrite or iron. Ferrite experiences

transformation to FCC austenite, or iron

at 912C. This austenite persists to 1395C,

at which tempt. the austenite reverts back to

BCC phase known as ferrite, finally melts

at 1538C.

Fe3C

Fe3C



Cementite (Fe3C) forms when the solubility limit of carbon in ferrite is exceeded below

727C. Fe3C will also coexist with the

phase between 727C to 1147C.

Mechanically, it is very hard and brittle. Fe3C

Fe3C

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Phase changes that occur upon passing from the phase into the + Fe3C phase as shown

in Fig. 2.6.

An alloy of eutectoid composition (0.76 wt% C) as it is cooled from phase, says 800C, will

occur no changes until the eutectoid

tempt.(727C) is reached.

The microstructure for this eutectoid steel consists of alternating layers or lamellae of two

phases ( + Fe3C). This phase is called

pearlite.

Pearlite has properties intermediate between the soft, ductile ferrite and the hard, brittle

cementite.

Fig 2.6 Schematic representations of the

microstructures for an iron-carbon alloy



Think critically 2.3

Match the following microstructures

with their descriptions.

1. Austenite

2. Ferrite

3. Cementite

4. Martensite

: an iron carbide contains 6.67%C by weight. It is hard and

brittle interstitial compound of low tensile strength but high

compressive strength.

: a metastable phase of supersaturated solid solution of

carbon in -iron. It is a needle like structure and hard, brittle

in properties.

: a BCC iron phase with very limited solubility for carbon. It is

the softest structure.

: a solid solution of carbon and other alloying elements in

iron. It is non-magnetic and soft.



Types Characteristic

Low carbon steel Contains less than 0.3% carbon (0.8% C)

Low in strength, high in hardness and wear resistance after heat treatment

Applications : cold chisels, cold working dies, hammers, hand taps and reamers, shear blades

Also known as tool steel.

Tempering process can accelerate martensite formation and maintain the low strength

properties.

Fig 2.7 Types of plain carbon steel



Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties.

The alloying elements are added to accomplish the following purposes: a. To impart a fine grain size to steel

b. To improve casehardening properties

c. To improve elasticity

d. To improve corrosion and fatigue resistance

e. To improve hardness, toughness and tensile strength

f. To improve machinability



Alloying elements Properties

1. Nickel Increase toughness, electrical and

magnetic properties

Improve forming property in steel

2. Chromium Provide stainless property in steel

3. Manganese Improve properties to heat

treatment

Provide control of structure

4. Tungsten Retention of hardness and

toughness at high temperature

5. Silicon Provide high electrical resistance

and magnetic permeability

6. Copper Improve atmospheric corrosion

resistance

Fig 2.8 The effects of alloying elements



Fig 2.9 Main classes of alloy steels

Stainless steel Contains at least 12% chromium.

Chromium or nickel forms an oxide layer which protects the underlying steel alloy from

corroding.

Tool steel Contains 0.6-1.5% carbon

High hardenability, Wear and corrosion resistance, Cannot be reshaping, Require high

toughness and resistance to shock.

Structural steel Contents : nickel, manganese, chromium, molybdenum.

High strength ,toughness, resistance to softening at elevated temperatures, resistance

to corrosion, good weldability, workability , high hardenability.

Magnetic steel Form in 2 methods :

Hard magnet used to produce permanent magnet

Soft magnet use to produce impermanent magnet

Heat resistance steel Contains chromium, nickel, silicon and manganese, others are carbon and bismuth

(30% carbon, 3.5% bismuth).

High resistance to corrosion and oxidation.

High in hardness and used for high temperature cutting.

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The product of the blast furnace, pig iron is unsuitable for casting as it contains high impurities. To render it suitable for desired purpose it is refined

in the furnace known as cupola. The refined product is called cast iron.

Cast iron contains more than 2% carbon.



Fig 2.10 Classifications of cast iron






What is the factors that affect the

structures of cast iron ?

1.The rate of solidification

Slow rate of solidification allow for

graphite formation. Rapid solidification

tends to give white irons structures.

2. Carbon content

The higher the carbon content of the

iron, the greater will be the tendency

for it to solidify grey.






3. The presence of other elements

Some elements promote the formation

of graphite in an iron structure.

4. The effect of heat treatment

The prolonged heating of a white iron

will cause graphitization to occur.






5. The cooling rate

Rapid cooling rate during solidification

tends to prevent the decomposition of

cementite in an iron which, when on

slow cooling would become graphitic.

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Answer Yes or No 1. Cast iron is an alloy.

2. Manganese reduces hardness and brittleness.

3. Grey cast iron contains carbon in free form (graphite

flakes).

4. Nodular cast iron is also called ductile cast iron.

5. Chromium provides stainless property in steel.

chapter two week 5

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