Material Science and Metallurgy Unit Four

UNIT-IV
Iron-carbon system
Fe-Fe3C Phase diagram: Iron Iron-carbide phase diagram and description
of microstructural aspects of phases (Ferrite, Austenite, Cementite,
Pearlite, ledeburite, hypo and hyper eutectoid steels; hypo and hyper
eutectic cast irons).
Steels: low carbon, medium carbon, high carbon, stainless, Hadfield, high
speed steels, their compositions, microstructures and applications.
Cast irons: Types of cast irons; compositions, microstructures and
applications of (Grey, white, Spheroidal graphite, Malleable) cast irons.

Allotropic forms of Pure Iron
• In the solid state, pure iron exist in three separate crystalline
forms, which are designated as Alpha-iron, Gamma-iron, and
Delta-iron.
• Alpha and delta irons consist of BCC, where as gamma iron consists
of FCC structure
• At 1539oC liquid iron transforms to delta iron which has BCC
structure and this structure is stable up to 1400oC
• As cooling is continued, delta iron is transformed to gamma iron at
1400oC
• It has FCC structure and is non-magnetic in character
• At 910oC gamma iron transforms to alpha iron, it has BCC structure
and is non magnetic up to 768oC
• Below 768oC alpha is strongly magnetic
• Alpha iron between 768oC and 910oC is usually called as Beta iron
• These changes are reversible, and same forms are observed during
heating
3

Solubility of carbon in Iron
• Austenite (FCC) has dense packing of atoms than ferrite (BCC) and has less
unfilled space 26% than ferrite (32%)
• Solubility of carbon in austenite is much greater than in ferrite
• The limited solubility of carbon in BCC (ferrite) is due to the shape and size
of the BCC interstitial positions.
• In BCC interstitial hole positions are at the mid points between the cube
center and the face centers, in FCC they are at midpoints of the cube edge
• The FCC interstitial positions are larger and therefore the strains imposed
on surrounding iron are much lower
• The solubility of carbon in austenite (max solubility 2%) is greater than the
solubility of carbon in ferrite (max solubility 0.02%)
4

• Metals and their alloys are widely used in industry
• Steel is the most important engineering metal
• Steel is an alloy of iron and carbon (up to 2%)
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• Properties can be improved by heat treatment
• Iron-carbon diagram forms the basic for study the heat treatment of steel
• For this reason, the study of iron-carbon equilibrium diagram is most important
-----------------------------------------------------------------------------------------------------------
• At room temperature, iron-carbide (Fe3C) is stable and therefore iron-carbon
diagram is considered as equilibrium diagram under the conditions of slow
heating and cooling
• The part of iron-carbon (Fe3-C) equilibrium diagram is described and represents
the portion between pure and iron-carbide containing 6.67% of carbon by
weight. 5

IRON-CARBON EQUILIBRIUM DIAGRAM
• A map of the temperature at which different phase changes occur on
very slow heating and cooling in relation to Carbon, is called Iron-
Carbon (or) Iron- Iron Carbide (or) Fe- Fe3C diagram
• Iron- Carbon diagram shows
• Type of alloys formed under very slow cooling,
• Proper heat-treatment temperature and
• How the properties of steels and cast irons can be radically
changed by heat-treatment.
6

Solid phases in Iron – Iron carbide (Fe-Fe3C) diagram
• Ferrite (alpha)
• Austenite
• Pearlite
• Cementite
• Martensite
• Ledeburite
7

• Ferrite is known as α solid solution.
• It is an interstitial solid solution of a small amount of carbon dissolved in α
iron (BCC)
• Stable form of iron below 912C The maximum solubility is 0.025 %C at 723C
(eutectoid temp.) and it dissolves only 0.008 % C at room temperature.
• It is the soft and ductile material
• It is magnetic up to 723C and non-magnetic above 768C
• Average properties are:
• Tensile strength = 40,000 psi (about 300 MPa)
• Elongation = 40 %
• Hardness: Rockwell C 0 (or) Rockwell B 90
8

• Austenite is an interstitial solid solution of Carbon dissolved in  - iron (F.C.C.)
• Maximum solubility is 2.0 % C at 1130°C.
• High formability, most of heat treatments begin with this single phase.
• It is normally not stable at room temperature. But, under certain conditions it
is possible to obtain austenite at room temperature.
• Tensile strength = 150,000 psi (1000 MPa)
• Hardness = Rockwell C 40 (approx.)
• Toughness = high
9

• Cementite or iron carbide, is very hard, brittle intermetallic compound of
iron & carbon.
• Cementite chemical formula is Fe3C contains 6.67% by weight.
• It has a metastable phase
• It is the hardest structure that appears on the diagram, exact melting point
unknown.
• Its crystal structure is orthorhombic
• It has
• low tensile strength (approx. 5,000 psi), but
• high compressive strength.
10

• Pearlite is the eutectoid mixture
containing 0.80 %C and is formed at
723°C on very slow cooling
• It is a very fine plate-like or lamellar
mixture of ferrite and cementite
• The white ferritic background (or) matrix
contains thin plates of cementite (dark)
11
• Tensile strength = 120,000 psi (730 MPa)
• Hardness = Rockwell C 20,
Rockwell B 95-100, and BHN 250-300

• In order to understand the transformation processes,
consider a steel of the eutectoid composition. 0.8%
carbon, being slow cooled along line x-x‘.
• At the upper temperatures, only austenite is present,
with the 0.8% carbon being dissolved in solid solution
within the FCC. When the steel cools through 723°C,
several changes occur simultaneously.
• The iron wants to change crystal structure from the
FCC austenite to the BCC ferrite, but the ferrite can
only contain 0.02% carbon in solid solution.
• The excess carbon is rejected and forms the carbon-
rich intermetallic known as cementite
12

• The net reaction at the
eutectoid is the formation of
pearlitic structure
Ferrite
Cementite
13

14
•Ledeburite is the eutectic mixture of austenite and
cementite
• It contains 4.3 % C and is formed at 1130°C

15
ABCD- liquidus line
AHECF- solidus line

• Peritectic reaction at 1492 °C with low wt% (0.18% C) alloys. The liquid
combines with d –iron to produce austenite ()
• Eutectic reaction occurs at 1130 °C, with composition of 4.3% C, alloys called
cast irons. The liquid transforms into eutectic mixture of austenite and
cementite
• Eutectoid is a solid state reaction which occurs at 723 °C with composition of
0.8% C, the austenite decomposes into ferrite (0.025%C) & cementite (6.67% C).
They are steels and this mixture is called pearlite
Liquid + d ↔ Austenite ()
Liquid ↔ austenite () + Cementite (Fe3C)
Austenite () ↔ Ferrite (a) + Cementite (Fe3C)
The diagram shows three horizontal lines which indicate isothermal reactions (on cooling / heating):
• First horizontal line is at 1492°C, where Peritectic reaction takes place:
Second horizontal line is at 1130°C, where eutectic reaction takes place:
Third horizontal line is at 723°C, where eutectoid reaction takes place:
16

• consider cooling curve of eutectoid steel (0.8% carbon)
liquid phase
• Above temperature t1 steel exist in liquid phase, just
below t1, austenite grains nucleates and grew in size as
cooling progresses.
• Below t2, the steel exist in single phase (austenite)
• Thus eutectoid steel solidifies completely in the
temperature interval from t1 and t2
• The austenite formed below t2 is stable up to A1 point
(lower critical temperature 723 deg C)
• At A1 point austenite transforms to a lamellar structure of
ferrite plus cementite
• This structure is called pearlite and this transformations is
called eutectoid reaction, this structure is stable up to
room temperature
Eutectoid steels
17

• Steels having less than 0.8% carbon are called hypo-eutectoid
steels (hypo means "less than").
• Consider the cooling of a typical hypo-eutectoid alloy along
line y-y‘
• At high temperatures the material is entirely austenite.
• Upon cooling it enters a region where the stable phases are
ferrite and austenite.
• The low-carbon ferrite nucleates and grows, leaving the
remaining austenite richer in carbon
• At 723°C, the remaining austenite will have assumed the
eutectoid composition (0.8% carbon), and further cooling
transforms it to pearlite
• The resulting structure, is a mixture of primary or pro-
eutectoid ferrite (ferrite that forms before the eutectoid
reaction) and regions of pearlite.
Hypo-eutectoid steels
19

• (hyper means "greater than") are those that contain more than
the eutectoid amount of Carbon.
• When such a steel cools, as along line z-z' , the process is similar
to the hypo-eutectoid steel, except that the primary or pro-
eutectoid phase is now cementite instead of ferrite.
• As the carbon-rich phase nucleates and grows, the remaining
austenite decreases in carbon content, again reaching the
eutectoid composition at 723°C
• This austenite transforms to pearlite upon slow cooling through
the eutectoid temperature
• The resulting structure consists of primary cementite and pearlite.
• The continuous network of primary cementite will cause the
material to be extremely brittle.
Hyper-eutectoid steels
21

The Austenite to ferrite / cementite transformation in relation to
Fe-C diagram (hypo eutectoid and hyper eutectoid)
23

ASSIGNMENT QUESTIONS
1. Write about eutectic cast iron
2. Write about Hypo eutectic cast iron
3. Write about Hyper eutectic cast iron
24

STEELS
• Iron is soft material and not widely used
in industrial application
• Steel is most extensively used in
industry
Advantages:
It is cheap and can provide wide range of
properties
• Its properties can be improved by
alloying or heat treatment
• It posses good machinability and
weldability
• Steels are classified as plain carbon
steels and alloy steels 25

Plain carbon steels
• Iron-carbon alloys containing up to 2% carbon are called plain carbon
steels
• In addition there are small amounts of sulphur, phosphorous, silicon and
manganese
Alloy steel
• Alloys of iron and carbon containing other intentionally added elements
are known as alloy steels
• The alloys elements are nickel, chromium, vanadium etc.
26
Sulphur : increases the hardness
Phosphorus : increases metal fluidity
Silicon : changes the cementite into graphite
Manganese : remove sulphur into slag

Classification of plain carbon steels
1. low carbon steel (0.008-0.35%C)
2. Medium carbon steel (0.35-0.6%C)
3. High carbon steel (0.6-1.5%C)
27

Low carbon steels
• The carbon content varies from 0.08 to 0.35%, these steels further divided in
to two groups
A) Dead mild steel (0.08 to 0.15%)
B) Mild steel (0.15 to 0.35%)
• Dead mild steels are soft and ductile and can be cold deformed, but they can
not be hardened by heat treatment
• Applications are: boiler plates, rivets and wires
• Mild steel is a general purpose steel,
• Used where tensile strength and hardness are not the most important
requirements.
• It is tough and ductile and can not be hardened by heat treatment
• Applications: structural works, car bodies, screws, nails, axle, shafts etc.
Tough, ductile and malleable
Easily joined and welded
Poor resistance to corrosion 28

Medium carbon steels
• Steel containing between 0.35 to 0.6% carbon is called
medium carbon steel
• These are strong and can be heat treated to produce
wide range of properties, used where strength and
toughness are requisite conditions
• Applications are agricultural tools, fasteners, motor
shafts, crank shafts, connecting rods and gears
• Offer more strength and
hardness but less ductile and
malleable
• Structural steel, rails and
garden tools
29

High carbon steels
• High carbon steel containing more than 0.6% carbon
• These are strong and having very low ductility
• These are generally used for parts requiring
strength, hardness and wear resistance
• HCS with 0.6 to 0.9% carbon are mainly used for
hammers, anvil faces, cold chisels, gears, wrenches
etc.
• High carbon steels with 0.9 to 1.5% carbon are
called tool steels
• These are attain good hardness, wear resistance and
toughness with proper heat treatment
• They are widely used for cutting tools, ball bearings
and machine parts where resistance to wear is
important
• Very hard but offers Higher Strength Less ductile
and less malleable
• Hand tools (chisels, punches) Saw blades 30

Advantages and Limitations
• 1. low cost
• 2. applications are from massive I-beams to tool bits
Limitations:
1. high strength can not be obtained with good ductility and toughness
2. Rapid rate of cooling (water quenching) on hardening often leads to
distortion and cracking of the steel
3. Large sections cannot be hardened uniformly
4. Poor resistance to corrosion and oxidation at high temperature
5. Poor strength and hardness at high temperature
31

Stainless steel
• Addition of small percentage of chromium to a plain carbon steel can increase
strength and hardness and cause a reduction in the critical cooling rate
• If large amount of chromium is used, the steel is provided with exceptionally good
corrosion resistance, such steels are called stainless steels
• Steel alloyed with chromium (18%), nickel (8%), magnesium (8%)
• Hard and tough
• Corrosion resistance
• Comes in different grades
• Sinks, cooking utensils, surgical instruments
32

1. Ferritic stainless steel
• Ferritic stainless steel contains 12 to 25%
chromium and less than 0.1% carbon
• Structure of these steels consist ferrite phase
which can not be hardened by heat treatment
• They are magnetic and can be hardened by cold
working
• It possess high corrosion and scaling resistance
• used where good corrosion resistance is required
without the need of high strength
• Applications are gas and electric stoves, domestic
applications such as utensils, spoons and forks, car
silencers, surgical instruments
33
Classified into three groups
1. Ferrite stainless steels
2. Martensitic stainless steels
3. Austenitic stainless steels

2. Martensitic stainless steel
• Steel contain 12 to 18% chromium and about 0.1 to 0.2% carbon
• These steels with suitable combination of chromium and carbon, can be
quenched to give Martensitic structure
• These steels can be hardened by heat treatment
• These steels are magnetic and can be cold worked and machined
satisfactorily
• These steels possess high strength and hardness than ferritic stainless
steel
• Applications are cutlery, springs, ball bearings, gas turbine parts and
instruments subjected to high temperature and corrosion conditions
34

3. Austenitic stainless steel
• Steels contains of 18% chromium, 8% nickel and less than 0.1% carbon
• Steels with this composition are referred to as 18/8 stainless steels.
• They possess austenite structure at room temperature
• They are usually quenched to minimize the formation of chromium
carbide which reduce the corrosion resistance of steels
• 18/8 steels possess high strength as well as corrosion resistance and
non magnetic
• Used for aircraft manifolds, food, and chemical processing equipment,
utensils and sanitary fittings
35

Hadfield steel
• High manganese steel is called Hadfield manganese
steel
• It has retained austenite which work hardens very
rapidly and has initially high strength due to presence
of manganese and carbon in solid solution
• It contains about 12 -13% manganese and 1.2% of
carbon
• It is extremely tough, wear-resistance and non-
magnetic
• This steel is widely used in mining (excavator
buckets), railway track work, jaw crushing machinery
(stone crusher jaws)
• This steels are available as castings, forgings or hot
rolled sections, but it difficult to machine as it tends
to harden when machining is attempted
36

37
High Speed Steel (HSS)
Medium Carbon steel alloyed with 18% Tungsten, 4% chromium, 1% vanadium
Tungsten (W) – increases hardness particularly at elevated temperatures due to stable
carbides
Chromium (Cr) – improves hardenability, strength and wear resistance, sharply increases
corrosion resistance at high concentrations
Vanadium (V) – increases strength, hardness, creep resistance and impact resistance due to
formation of hard vanadium carbides
Very hard, Resistant to frictional heat even at high temperature
Machine cutting tools (lathe and milling), Drills

CAST IRON
• Cast iron exhibits low melting point, good
fluidity, low shrinkage etc.,
• As compared to steel, CI possesses high
compressive strength, damping capacity, wear
resistance.
• The iron-carbon alloy with more than 2% carbon
and considerable amount of silicon is called cast
iron
• This iron is known as cast iron because it is used
in the cast form
• Very hard and brittle
• Strong under compression
• Suitable for casting [can be pour at a
relatively low temperature]
• Engine block, engineer vices, machine
parts
38

CAST IRON-composition
• Cast iron consists of 2 to 6.67% carbon and significant amount of silicon
• Carbon: 2 to 6.67%, present as cementite
• Silicon : 1 to 3%, it helps to change cementite to graphite and also helps to
form sound castings
• Manganese : 0.5 to 1%, it helps to remove sulphur into slag
• Phosphorus : 0.1 to 0.9%, increases metal fluidity, but tends to weaken the
castings
• Sulphur : 0.07 to 0.1%, it tends decrease fluidity and increases the hardness
39

Advantages and limitations and applications
Advantages
1. low melting point (1130 Deg C), high fluidity and low shrinkage
2. Cheapness-castings can be produced quickly and inexpensively
3. Free graphite in cast iron acts as lubricant, when used for machine slides, they move
in minimum friction
4. High compression strength with damping capacity
5. Better corrosion and wear resistance
Limitations
1. it is brittle
2. it can not be forged or hardened
3. it is very weak in tension
Applications
• Cylinder heads, machine tools beds, pulleys and agriculture tools 40

GREY CAST IRON
• Grey iron is a high-carbon-silicon alloy
• The carbon is present in the form of graphite flakes in
the matrix of ferrite or pearlite
• The pearlite GCI is stronger than ferrite GCI
• The cast iron is known as GCI because of the gray
appearance of its freshly fractured surface
• By increasing the carbon content to 4.3% (eutectic
point) lowers the melting point and favors the
formation of graphite rather than cementite.
• The combined effect of carbon, silicon, and
phosphorous is presented by the value of carbon
equivalent which is computed as
• Carbon equivalent % = %C + 1/3 (Si + P)
41

GREY CAST IRON….contd
• The carbon equivalent near to eutectic value is likely to produce grey C.I
• A composition of iron as 1.5% Si, 0.3% P and 3.5% C, then the carbon
equivalent is 4.1% and thus the iron is very close to eutectic point
• Carbon: 3.2 to 3.5%
• Silicon : 1.3 to 2.3%,
• Manganese : 0.5 to 0.7%
• Phosphorus : 0.15 to 1.0%
• Sulphur : 0.1%
42

GREY CAST IRON….contd
Properties
• Excellent compressive strength and
machinability
• Good wear resistance
• Very good damping capacity
• Excellent casting properties such as
low melting point, good fluidity and
low shrinkage
• It provides lubrication is due to
presence of graphite
Applications
• Machine tools beds, slides
• Cylinder blocks for automobile engines,
brake drums and various machine parts
• Used for light and intricate castings
such as gates, pipes and agriculture
tools
43

WHITE CAST IRON
• White C.I usually contains silicon content below 1.3%
• All the carbon exist as cementite
• Name white is refers to the bright fracture produced by this brittle
constitute
• This structure is obtained by rapid cooling of iron consisting of low silicon
and high manganese
• Carbon: 2.0 to 3.0%
• Silicon : 0.9 to 1.65%,
• Phosphorus : 0.15%
• Sulphur : 0.15%
44

Properties
• Very hard and brittle
• Difficult to machine and can not be welded
Applications
- Used for rollers for crushers,
- brake shoes
45

MALLEABLE CAST IRON
• Malleable C.I are produced by the heat treatment of white C.I
• They possess good ductility combined with high strength
• Carbon: 2.0 to 2.8%
• Silicon : 0.7 to 1.4%,
• Phosphorus : 1.0% max.
• Sulphur : 0.2% max.
46

Properties:
• Possess high strength, ductility and wear resistance
• Possess good machinability and castability
• They can also withstand impact loads
Applications
• Agricultural tools
• Automobile parts
• Man hole covers
• Railroad equipment's
• Gears, pipe fittings
47

SPHEROIDAL (Ductile) CAST IRON
• Spheroidal –graphite C.I exists in the form of spheres or nodules
• It is also called nodular or ductile cast iron
• Addition of magnesium to prevent the formation of graphite flakes during the
cooling
• The structure of S.G iron possess graphite nodules in the ferrite or pearlite
matrix
• The resulting cast iron is more ductile and stronger than grey C.I
• Carbon: 3.0 to 3.5%
• Silicon : 2.0 to 2.5%,
• Phosphorus : 0.025% - 0.4% max.
• Sulphur : 0.015% - 0.4% max.
• Magnesium : 0.015 -0.1%
48

Properties
• Strength of S.G iron is nearly equal to steel
• It also possess toughness and ductility
• It has superior wear resistance, good shock resistance and strength
Applications
• Crankshaft, hydraulic cylinders, valves, cylinder heads, connecting
rods and high pressure pipes
• Because of good compression and corrosion resistance it is also used
for steam plants and marine applications
49

Material Science and Metallurgy Unit Four

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