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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
Structure Of Plain Carbon Steels – cont’d
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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Structure Of Plain Carbon Steels – cont’d
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
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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.
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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
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Iron-carbon equilibrium diagram – cont’d
(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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Iron-carbon equilibrium diagram – cont’d
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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Iron-carbon equilibrium diagram – cont’d
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.
Iron-carbon equilibrium diagram – cont’d
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Iron-carbon equilibrium diagram – cont’d
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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