Chapter 5 - Plain Carbon Steels

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Chapter 5 – Plain Carbon Steels

ENGINEERINGMATERIALS

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Chapter Objectives

List the composition of plain carbon steelsOutline the various phases of crystallinestructures of metals

Recognize the various types of phases shown

in the iron-carbon equilibrium diagramIdentify the temperature ranges and structuresfor the slow cooling of various types of iron-carbon steels

Effect of carbon on properties of carbon steelsClassification and uses of plain carbon steels

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Composition of steelPlain carbon steels contain mainly iron(Fe) and carbon (C) up to 2.0% They contain the following elements:(a) Manganese (up to 1.0%):

- helps to reduce the sulphur

content- increases the strength and

hardness of steel

(b) Phosphorous (not to exceed 0.05%):

- impurity carried over from iron ore- causes the steel to be brittle

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Composition of steel – cont’d

(c) Sulphur (not to exceed 0.05%):- impurity from iron ore

- combines with Fe to form ironsulphide

- causes steel to be brittle

For high quality steels, max. allowable Sand P contents to be less than 0.04%

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Structure Of Plain Carbon Steels

When metals solidify from molten to solidstate, atoms align themselves in an orderlypattern or space lattice that forms crystals(grain structure)

3 main types of crystal lattice structure:(i) body centered cubic (BCC),(ii) face centred cubic (FCC) and(iii) hexagonal close-packed (HCP)

Iron is an allotropic materialIt can exist in more than one crystallattice structure

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Structure Of Plain Carbon Steels – cont’d

BODY CENTERED CUBIC

•VANADIUM

•MOLYBDENUM

•TUNGSTEN

•IRON ()

•CHROMIUM ()

FACE-CENTERED CUBIC•COPPER

•SILVER

•GOLD

• ALUMINIUM

•LEAD

•IRON ()

•CHROMIUM ()

•NICKEL•PLATINUM, COBALT ()

HEXAGONAL CLOSE-PACKED

•BERYLLIUM

•MAGNESIUM

•ZINC

•CADMIUM

•COBALT ()

Crystal Lattice Structures

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Below 910oC ironforms body-centredcubic (BCC) crystals,refer to as alpha ()iron


From 910oC to 1400oCit forms face-centredcubic (FCC) crystals,refer to as gamma ()iron

Above 14000C to 1538oCit reverts to BCC crystals,refers as delta () iron

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iron will dissolve up to 2.0%C at atemperature of 1147oC

iron will dissolve up to 0.02%C at atemperature of 723oC and 0.006%C at room

temperature

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Iron-Carbon Equilibrium Diagram Allotropic changes

that take place in

Fe is influenced by

carbon

As solubility of

carbon in ironalters, changes in

steel structure will

occur on heating &

cooling throughtransformation

temp. Fe-C equilibrium diagram will

show transformations in very slow

cooling equilibrium conditions 9



Features Of Iron-Carbon

Equilibrium Diagram

Upper critical temperature (line AEB)

- Called A3 line for hypoeutectoid steels.

- Called Acm line for hypereutectoid steels

Lower critical temperature (723oC line)

- Called the A1 line and is constant at 723oC

Eutectoid point (Point E)

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Upper Critical Temperature (UCT)

For hypoeutectoid steels – Line AE on the iron-

carbon diagram.For hypereutectoid steels – Line EB on the iron-

equilibrium diagram.

If temperature is above this line, the steel will be

wholly austenitic.

For hypoeutectoid steels, if temperature is between

the A3 line and 723oC line – phases are austenite

and ferrite.

For hypereutectoid steels, if temperature is between

the Acm line and 723oC line – the phases will be

austenite and cementite.



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Lower Critical Temperature (LCT)

This line is also called the A1 line and it is

constant at 723oC.

This is the temperature at which austenite

will transform into pearlite upon coolingunder equilibrium condition (extremely

slow cooling).



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Eutectoid Point

This is the point at which the carbon composition is 0.8%.

When the temperature falls below 723oC ,

the austenite transforms almost immediately to pearlite.

The composition of steel at this point is called eutectoid composition.



Iron-carbon equilibrium diagram – cont’d

Types of phases in carbon steels:(a) CEMENTITE (Iron carbide):

- excess of C combines with Fe to form Fe3C

- contains 6.67%C.

- hardest and most brittle

(b) AUSTENITE ( iron):

- contains up to a max. of 2.0%C at 11470C

- it has a face-centred cubic structure

- non-magnetic, soft and ductile

(c) FERRITE ( iron):

- contains up to a max. of 0.02%C at723oC, falling to 0.006%C at20oC

- a weak solution of carbon in BCCiron. Soft,

ductile, low strength and magnetic (up to 770oC) 14




(d) PEARLITE:

- eutectoid mixture containing 0.8%C occursat 723oC

- has lamellar structure of ferrite andcementite

- hardness between hard cementite and soft

ferrite

Note: A lamellar structure is one that

consists of alternate layers of

ferrite and cementite

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Steels can be differentiated into three types

when referring to the iron-carbon phase

diagram:(i) Hypo-eutectoid steels

(ii) Hyper-eutectoid steels

(iii) Eutectoid steels

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Hypo-eutectoid SteelsThese steels contain less than 0.8% carbon.

- At room temperature, the microstructures consist

of ferrite and pearlite.

- Above the upper critical temperature, the steel

is wholly austenitic .

- Between upper critical temperature and 723oC ,

the phases are ferrite and austenite.- Below 723oC , the structures are ferrite and

pearlite.

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Hyper-eutectoid SteelsThese steels contain more than 0.8% carbon.

- At room temperature, the microstructureconsists of pearlite and cementite.

- Above upper critical temperature, thesteel is wholly austenitic .

- Between upper critical temperature and723oC , the phases are austenite and

cementite.- Below 723oC , the structures are pearliteand cementite.




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These steels contain 0.8% carbon.

- Microstructure is mainly pearlite at room

temperature.

- At a temperature above eutectoid temperature (723oC), the steel is wholly

austenitic .

- The transformation is almost immediate

from austenite to pearlite.



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CASE 1: Steel containing 0.4%C (hypo-eutectoid steel)

On cooling from 16000C:

1) At Q1 it begins to solidify at15000C

2) At S1 solidification is completeat 14500CStructure of uniform austenite

3) At U1, Upper Critical Temp. atabout 8250C

Ferrite will start to grow at grainboundaries of austenite.Bulk ofcarbon remains in austenite

4) Just above 7230C (LowerCritical Temp.): Formation of

Austenite and Ferrite complete5) At 7230C, austenite will form

layers of ferrite and cementitecalled pearlite6) Below 7230C, final structure is

ferrite and pearlite



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CASE 2: Steel containing 0.8%C (Eutectoid Steel)

1) At Q2, it begins to solidifyas austenite at 14900C

2) At S2, solidificationcomplete at 14100C

3) Upper critical and lowercritical temperaturescoincide at E (7230C): Nochange in austenite

structure4) Below E (7230C) austenitewill transform to finalstructure of pearlite



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CASE 3: Steel containing 1.2%C(Hyper-eutectoid steel)

1) At Q3, it begins to solidify bydepositing dendrites (type ofcrystal growth) of austenite at

14800C2) At S3, solidification complete

at 13500C. Structure ofuniform austenite forms

3) At U3, upper critical

temp.(9000

C), cementiteforms in needle-like crystals,mainly at grain boundaries ofaustenite

4) At 7230C, remainingaustenite will reduce to

0.8%C5) Below 7230C, remaining

austenite changes to finalstructure of pearlite andcementite



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Effect Of Carbon On Properties Of Plain

Carbon Steels

Increase the amount

of carbon in mediumcarbon steels

promotes the

formation of cementite

This results in an

increased presence of

pearlite, making such

steels stronger,tougher and harder

but less ductile



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Uses Of Plain Carbon SteelsPlain carbon steels can be classified according to their

carbon contents along with their uses as follows:

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Chapter 5 - Plain Carbon Steels