Unit 3.ppt

Unit 1II:
Ferrous Metals and
Designation

In this unit we are going to study:
Allotropy of Iron
 Iron-iron carbide diagram
 Plain carbon steels
 Limitations of plain carbon steel
Unit 3: Ferrous metals and Designation

In this unit we are going to study:
Alloy steels
Advantages of alloy steels
Effect of alloying elements on
mechanical
properties of steel
 Tool steels
 Stainless steels
 Cast irons
 Designation of steels and cast iron

What is steel?
Steel is a interstitial solid solution of
iron and carbon containing 0.008 to
2% carbon by weight

local atomic fluctuation
formation of many small
nuclei
growth of nuclei with
critical size or greater
• Homogeneous nucleation : occurs within a homogeneous medium.
• Heterogeneous nucleation : nucleation occurs at some structural
imperfection such as foreign surface, and hence with reduced surface
energy

910 0C
1400 0C
1539 0C
Temp
Time

Phases in Steel
α-ferrite
Interstitial solid solution of carbon dissolve in
α-iron having BCC structure.
Maximum solubility of carbon in α-iron is
0.02% (at 7270C)
At room temperature solubility is 0.008%

Phases in Steel
Properties of α-ferrite
Soft and ductile phase
Ferromagnetic upto curie temperature(7680C)
Tensile Strength 40,000psi
Elongation 40% (2in GL)
Hardness 80 BHN
Toughness Low

Phases in Steel
Microstructure of α-ferrite

Phases in Steel
Austenite (γ)
γ-iron having FCC structure.
Maximum solubility of carbon in γ-iron is 2.%
(at 11470C)
Stable only above 7270C

Phases in Steel
Properties of Austenite
Soft and ductile phase
Non magnetic
It can be extensively worked at the temperature
of its existence.
Tensile Strength 1,50,000psi
Hardness Rc 40
Toughness High

Phases in Steel
Microstructure of Austenite

Phases in Steel
δ-ferrite
δ-iron having BCC structure.
Maximum solubility of carbon in δ-iron is
0.1% (at 14920C)
Stable only above 14000C

Phases in Steel
Iron Carbide (Cementite)
Intemetallic compound of iron and carbon
with fixed carbon content of 6.67% and having
orthorhombic structure.
Chemical formula Fe3C
Metastable phase

Phases in Steel
Properties of Iron Carbide
(Cementite)
Extremely hard and brittle phase
Ferromagnetic upto 2100C
Tensile Strength 5000psi
Elongation 1%
Hardness 900-1200 VHN
Toughness Very Low
Compressive Strength Very High

Transformations
 Peritectic reaction:
S1 + L S2
 Eutectic reaction:
L S1 + S2
 Eutectoid reaction:
S1 S2 + S3
9-5

Transformations
Peritectic reaction:
General Reaction:
S1 + L S2
Reaction in steel:
Liquid + δ γ
0.55%C 0.1% C 0.18% C
BCC FCC
9-5
14920C
Cooling

Transformations
Eutectic reaction:
General Reaction:
L S1 + S2
Reaction in steel:
Liquid γ + Fe3C
4.3%C 2% C 6.67% C
FCC Orthorhom
9-5
11470C
Cooling

Transformations
Eutectoid reaction:
General Reaction:
S1 S2 + S3
Reaction in steel:
γ α + Fe3C
0.8%C 0.02% C 6.67% C
FCC BCC Orthorhombic
9-5
7270C
Cooling

Eutectoid reaction:
γ α + Fe3C
0.8%C 0.02% C 6.67% C
FCC BCC Orthorhombic
This eutectoid mixture is called Pearlite due to its pearly
appearance under microscope.
Pearlite: It is a eutectoid mixture of alpha ferrite
and cementite formed from austenite containing
0.8%C while cooling at 7270C
9-5
7270C
Cooling

Phases in Steel
Properties of Pearlite
Good Hardness and T.S.
magnetic
Tensile Strength 1,20,000psi
Hardness Rc 20 (250 BHN)
Toughness High

Phases in Steel
Microstructure of Pearlite

Phases in Steel

Phases in Steel
Laminar (Platelike) or Fingerprint Microstructure
Ferrite (white)
Cementite
(dark)

Cooling of Eutectoid Steel (0.8% C)

Off Eutectoid Steels
Steels with compositions between
0.008 to 0.8 wt% Carbon are
hypoeutectoid steels.
Steels with compositions between
0.8 to 2 wt% Carbon are
hypereutectoid steels

33
Fe
3
C
(cementite)
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.7
L
g
(austenite)
g+L
g +Fe3C
a+Fe3C
L+Fe3C
d
(Fe) C, wt% C
1148°C
T(°C)
a
727°C
C0
0.76
a
pearlite
g
g g
g
a
a
a
g
g
g g
g g
g
g
Cooling of Hypoeutectoid Steel (0.8% )
Hypoeutectoid Steels

34
Proeutectoid α-ferrite
 Formed before the eutectoid
 Ferrite that is present in the pearlite is called eutectoid ferrite.
 The ferrite that is formed above the Teutectoid (727°C) is proeutectoid.
Cooling of Hypoeutectoid Steel (0.8% )

Fe
3
C
(cementite)
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.7
L
g
(austenite)
g+L
g +Fe3C
a +Fe3C
L+Fe3C
d
(Fe) C, wt%C
1148°C
T(°C)
a
0.8
Fe3C
g
g
g g
g
g
g g
g
g
g g
pearlite
Cooling of Hypereutectoid Steel (0.8% )
Hypereutectoid Steels

Proeutectoid Cementite
Cooling of Hypereutectoid Steel (0.8% )

Hypereutectoid Steel (0.8% )
Microstructure of Hypereutectoid Steel

The Lever rule
Amount of phase 1 = (C2 - C) / (C2 - C1)
Amount of phase 2 = (C - C1) / (C2 - C1).
If an alloy consists of more than one phase, the amount of each phase presen
can be found by applying the lever rule to the phase diagram.
The composition of the alloy is represented by the fulcrum,
and the compositions of the two phases by the ends of a bar.

Ex.1:For eutectoid steel determine amount of
alpha ferrite and cementite just below the
eutectoid temperature
Numerical Examples

Solution:
Eutectoid steel means 0.8%C steel
Co=0.8
Cα=0.02
CFe3C=6.67
(a)Amount of alpha
Ferrite
=(6.67- 0.8)/(6.67- 0.02)
=88.27%
(b) Amount of Cementite
=(6.67- 0.8)/(6.67- 0.02)
=11.72%

Numerical Examples
Ex.2:For peritectic steel determine amount of
delta ferrite and austenite just below the
peritectic temperature
Ex.3:For eutectic cast iron determine amount of
austenite and cementite just below the
eutectic temperature
Ex.4: Find the maximum amount of
proeutectoid cementite in any steel.

Ex.5:For 0.40 % C steel at a temperature just
below the eutectoid, determine the
following
a) Amount of pearlite and proeutectiod ferrite
(a)
b) the amount of cementite
Numerical Examples

Solution:
(a)The amount of pearlite
=(0.4 – 0.02) /( 0.8 -0.02) x 100
=48.71%
(b)Amount of
proeutectoid ferrite
=(0.8-0.4 )/ (0.8 -0.02) x 100
=51.28%
(c)Amount of Cementite
=(0.4-0.02)/(6.67-0.02) x 100
=5.7%
CO = 0.40 wt% C
Ca = 0.02 wt% C
Cp = 0.8 wt% C
Fe
C
(cementite)
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.7
L
g
(austenite)
g+L
g + Fe3C
a+ Fe3C
L+Fe3C
d
Co, wt% C
1148°C
T(°C)
727°C
CO
R S
CFe C
3
Ca

Ex.5:For 1.1 % C steel under equilibrium
conditions determine the following:
a) Amount of austenite and proeutectiod
cementite just above eutectoid temp.
b) Amount of pearlite and proeutectiod
cementite and total cementite at room
temperature.
Numerical Examples

Ex.6:A slowly cooled steel contains 10% proeutectoid
ferrite at room temperature. Determine the
amount total ferrite and cementite present in the
alloy.
Ex.7:A slowly cooled steel contains 60% ferrite and
40% pearlite at room temperature. Determine the
amount of pearlite, total ferrite and cementite
present in the alloy at the temperature just below
eutectoid temperature.
Numerical Examples

Critical Temperatures in
Iron-Iron Carbide Equilibrium Diagram
S.
N.
Critical
Points
Temp Significance During Heating
1 A0 210 0C Cementite becomes
paramagnetic
2 A1 727 0C Perlite Austenite
3 A2 768 Ferrite becomes
paramagnetic
4 A3 727-
9100C
Completion of
Ferrite Austenite
5 Acm 727-
11470C
Completion of
Cementite Austenite

Critical Temperatures in
Iron-Iron Carbide Equilibrium Diagram
S.
N.
Critical
Points
Temp Name of Critical Temperature
1 A0 210 0C Curie Temperature of Fe3C
2 A1 210 0C Lower Critical Temperature
3 A2 768 Curie Temperature of Ferrite
4 A3 727-9100C Upper Critical Temperature for
Hypoeutectoid Steel
5 Acm 727-
11470C
Upper Critical Temperature for
Hypereutectoid Steel

Property Variation With Microstructure
Mechanical properties are structure sensitive
Mech Properties= f (Types of phases, amount of
phases and morphology of
structure)
Morphology means distribution of phases
For two phase alloy with α-β structure, the property on an
average can be expressed as(when morphology is
insignificant):
Average property=
Amount of α x property of α
+ Amount of β x property of β

Since hardness is less sensitive to morphology.
For hypoeutectoid steels:
Hardness (BHN) =
Amount of α-ferrite x Hardness of of α-ferrite
+ Amount of pearlite x Hardness of pearlite
= Amount of α-ferrite x 80 BHN
+ Amount of pearlite x 230 BHN

Since hardness is less sensitive to morphology.
For hypereutectoid steels:
Hardness (BHN) =
Amount of Fe3C x Hardness of of Fe3C
+ Amount of pearlite x Hardness of pearlite
= Amount of Fe3C x 900 +
Amount of pearlite x 240

Many of the mechanical properties like T.S., ductility,
toughness are very sensitive to morphology. Hence,
unless the distribution of phases are not known
correctly, property prediction can not be done
accuratly.
Since phases are well distributed in
hypoeutectoid steels T.S. is less sensitive to
morphology.
T.S. (psi)
= Amount of α-ferrite x 40000 psi
+ Amount of pearlite x 1,20,000 psi

T.S. (kg/mm2)
= Amount of α-ferrite x 28 kg/mm2
+ Amount of pearlite x 84 kg/mm2
= (1- %C/0.8) x 28 kg/mm2
+ ( %C/0.8) x 84 kg/mm2
T.S. of hypoeutectoid steel increases by
7kg/mm2 for every 0.1%C rise.
T.S. (kg/mm2)=0.36 X BHN

%C
Property
Hardness
T.S.
Ductility
0.8 2.0
350 BHN
84 kg/mm2
230 BHN
10%
80 BHN
28 kg/mm2
40%
0.4 1.2

Non Equilibrium Cooling of
Steels

Non Equilibrium Cooling of
Steels
Non-equilibrium cooling means cooling faster
than equilibrium cooling.
Faster cooling lowers all transformation
temperatures
Eutectoid point shift towards left side for
hypoeutectoid steel and towards right for
hypereutectoid steel
Amount of proeutectoid phase decreases and
amount of pearlite increases
Pearlite becomes finer.

Why Solubility of Carbon in Austenite is Very
High?
Austenite, being FCC have four atoms per unit cell
Ferrite, being BCC have two atoms per unit cell
Empty space in FCC Austenite=25%
Empty space in BCC Ferrite=32%
This means Austenite have much denser packing of
atoms than ferrite.
Still Solubility of Carbon in Austenite (2%) is very
high compare to Ferrite (0.02).

High?
Reason:
The largest interstitial sphere that would just fit in
BCC cell has radius of 0.36 x 10-8cm
The largest interstitial sphere that would just fit in
FCC cell has radius of 0.52x10-8cm

High?
α-ferrite
BCC
Radius
=0.36x 10- 8 cm

Why Solubility of Carbon in Austenite
is Very High?
Austenite
FCC
Radius
=0.52 x 10- 8 cm

Classification of Steels
Steels are classified base on various
criterions:
Amount of carbon
Amount of alloying elements
Amount of deoxidation
Grain Coasening Characteristics
Method of Manufacturing
Depth of Hardening
Form and use

Amount of carbon
Low carbon steel
(0.008 – 0.3 %C)
Medium carbon steel
(0.3 – 0.6 %C)
High carbon steel
(0.6 – 2 %C)

Amount of carbon
Low carbon steel
Soft
ductile
malleable
tough
machinable
weldable
non hardenable by heat treatment

Applications of Low carbon steel
Good for cold working such as rolling into thin sheets
Good for fabrication work by welding, pressing or
machining
Used for wire, nails, rivets, screws, panels, welding rod,
ship plates, boiler plates, tubes for bicycles and
automobiles
Steels with o.15 to 0.3 %C are widely used as Structural
steels and used for building bars, grills, beams, angles,
channels etc.
Mild Steel is well known from this group

Medium carbon steel
Medium Soft
Medium ductile
Medium malleable
Medium tough
Depth of hardening is less
Slightly difficult to machine, weld and
harden
Difficult to cold work
They are also called as machinery
steels

Applications of medium carbon steel
Used for
Bolts
Axles
Lock washer
Forging dies
Springs
Wheel Spokes
Railway rails

High carbon steel
Hard
Wear Resistant
Brittle
Difficult to cold work
Very difficult to machine and weld
Depth of hardening is more
They are also called as Tool Steels

Applications of high carbon steel
Used for
Dies
Punches
Hammers
Chisels
Drills
Metal cutting saws
Razor blades

On the basis of alloying elements
Low alloy steels
(Total alloying elements are less than 10%)
High alloy steels
(Total alloying elements are more than 10%)

On the basis of alloying elements and
carbon content
Low carbon Low alloy steels
Low carbon High alloy steels
Medium carbon Low alloy steels
Medium carbon High alloy steels
High carbon Low alloy steels
High carbon High alloy steels

On the basis of deoxidation
Rimmed steels
Killed steels
Semi-killed steels

A molten steel contains large amount of dissolved
oxygen and other gases.
The solubility of gases is more in the liquid metal than
in the solid metal and hence the dissolved oxygen along
with other gases tries to go out as CO during
solidification and a large part of it gets entrapped into
solidified ingot.
Rimmed steels
In rimmed steels no treatment is given to dissolved
gases.
The tapped gases form blow holes which compensate
for the usual liquid to solid shrinkage.

Rimmed steels
The thin solidified layer of ingot i.e. rim (skin)
formed at surface which contains low carbon, less
impurities and no blow holes
These steels coarsen rapidly during heating in
austenitic region.
Generally low carbon steels containing less than
0.15% carbon are produced in sheet form in
rimmed condition and used for deep drawing and
forming operations.

Rimmed steels

Killed steels
The dissolved oxygen from the melt is completely
removed by addition of strong deoxidising agents
like Al, Si, Mn and V.
The deoxidisers are added to the steel in the
furnace prior to pouring into mould.
They rapidly combine with the dissolved oxygen
and form respective oxides thus reduces dissolve
oxygen.
Killed steel shows more shrinkage (called as pipe)
during solidification due to absence of blow holes.

Killed steels

Killed steels
Killed steels shows fined grain characteristics
since oxide inclusions which inhibits the grain
boundary migration
Killed steel ingot has sound , defect free, less
segregated structure throughout the cross section
Usually high carbon steels and alloy steels are
produced in the killed condition.

Semi-killed steels
In these steels part of the dissolved oxygen is
removed by addition of deoxidisers.
Blow holes formed compensate for part of the
shrinkage and hence pipe is less.
They show intermediate grain coarsening
characteristics.
Usually steels containing carbon between 0.15 to
0.25% are produced.

On the basis of Grain Coarsening
characteristics
During heating,100 % austenite is formed at just above
the upper critical temperature and the grains are of
smallest size. As the temperature increases above this,
the grain size may increase. Depending on the grain
coarsening characteristics, steels are classified into two
types as :
Coarse grained steels
Fine grained steels

characteristics
Coarse grained steels
Coarse grained steels coarsen rapidly with temperature.
Fine grained steels
These steels maintain a relatively fine and uniform grain size
even after holding for long time at high temperature. Fine
grained steels do not coarsen up to a definite temperature.
Above this temperature, they coarsen very fast and may
reach a size greater than those of coarse grained steels.

characteristics
Temperature
Austenitic
Grain
Size
700 800 900 1000 1100 1200
Coarse Grained Steels
Fined Grained Steels

characteristics
Usually rimmed steels behave coarse grained
steels.
Killed steels and alloy steels behave fine grained
steels.
The oxide inclusions in killed steels and
undissolved alloy carbides in alloy steels inhibit
the grain boundary migration, thus reducing grain
coarsening.
In the absence of such particles, grain
coarsening is rapid.

On the basis of Method of Manufacuring
Basic open hearth
Acid open hearth
Acid Bessemer
Basic oxygen process
Electrical Furnace

On the basis of Depth of Hardening
Non-hardenable
Can’t be hardened by quenching
Shallow hardening steels
hardened by quenching up to a small
depth
Deep hardening steels
hardened by quenching deeply

On the basis of Depth of Hardening
Non-hardenable
Very low hardenablilty
Low carbon steels with no alloying elements
Shallow hardening steels
Medium hardenability
Medium carbon steels with low alloying
elements
Deep hardening steels
High hardenability
High carbon steels with high alloying
elements

On the basis of form and us
Based on form:
Cast steels
Wrought steels
Based on Use:
Boiler steels
Case hardening steels
Corrosion and heat resistant steels
Deep drawing steels
Electrical steels
Free Cutting steels
Machinery steels
Structural steels
Tool steels

Specification of Steels
Steels are specified on the basis of
criteria like:
Chemical Composition
Mechanical Properties
Method of manufacturing
Heat Treatment
Quality
Majority of specifications are based
on chemical composition.

Standard practiced for specifying steels in
India are:
IS : Indian Standard
AISI : American Iron and Steel Institute
SAE: Society for Automotive Engineers
BIS: British Institute of Standards
In addition to above every country has its own
specification system e.g.
DIN :Germany
JIS : Japan
GHOST : Russia

However, attempts are being made
to uniquely identify material by a
numbering system such as UNS (Unified
numbering System) by initiative of AISI
and SAE.

Indian Standard Designation
System:
Indian standard code for designation of steel
was adopted by Indian standard Institution (ISI)
in 1961.
Indian specifications are based on
Mechanical Properties
Chemical Composition

Indian Standard Designation System:
Code designation on the basis of Mech
properties is based upon T.S. and Y.S.
Property Symbol
T.S. (N/mm2) Fe
T.S. (Kg/mm2) St
Y.S. (N/mm2) FeE
Y.S. (Kg/mm2) StE

Indian Standard Designation System:
Designation of steels on the basis of chemical
composition consists of numerical figure
indicating 100 times the average % of carbon
content.
Alphabets are prefixed to identify type of
steels of various class
Letters C or T is followed by figure indicating
10 times the average % of Mn content
Symbols S,Se,Te,Pb or P are used to indicate
free cutting steels followed by a figure indicating
100 times the percent content of the respective
element.

Alphabets are prefixed to identify type of steels
of various class:
Type of Steels Prefix Used
For plain carbon steels C
For high alloy steels X
For low alloy steels -
For Plain carbon tool steels T
For alloy tool steels XT
For Wrought steel S
For Cast steel CS

Alloy steels are designated in the symbolic form on the
basis of their alloy content by first specifying the average
content of carbon, followed by the chemical symbols of
the significant elements in the descending order of of
percentage content
If the average alloy content is upto 1%, the index
number is expressed upto 2 decimal places underlined
by a bar
If the alloy content is between 1 and 10% the index
number is rounded to the nearest whole number
If two or more significant alloying elements have same
alloy content, the chemical symbols are grouped
together followed by alloy content

S.
N.
Steel Specification
1 Fe410K
2 St42
3 FeE270
4 C20
5 C40
6 25C5
7 80T11
8 15Ni13Cr1Mo12
9 35S18
10 35Mn1S18

S.
N.
Steel Specification
11 T75W18Cr4V1
12 T105Cr1Mn60
13 T85W6Mo5Cr4V2
14 T35CrMo1V30

AISI/SAE Designation System
American Iron and Steel Institute and Society of
Automotive Engineers, London have almost similar
method of designation of steel and is based on the
chemical composition of steel.
The method consists of designating the steel with four
or five numerical digits. The first digit from left indicates
the types of steels as follows:
Digit Types of Steels
1 Carbon Steels
2 Ni Steels
3 Ni-Cr Steel
4 Mo Steels
Digit Types of Steels
5 Cr Steels
6 Cr-V Steels
7 Tungsten Steels
8 Ni-Cr-Mo Steels
9 Si-Mn Steels

For simple alloys, the second digit indicates the approximate % of
the predominant alloying elements and for othera it indicates
modification of the alloy in that group.
The last two or three digits divided by 100 usually the average %
carbon in the steel.
In addition to the numerals, AISI specification may include a letter
prefix to indicate the manufacturing process of that steel as below:
Letter Manufacturing Process
A Basic open hearth alloy steel
B Acid bessemer carbon steel
C Basic open hearth carbon steel
D Acid open hearth carbon steel
E Electric furnace steel

Some Important AISI/SAE steel designation groups
S.
N.
Details of the steel AISI/SAE Group
1 Carbon Steels
(i)Plain carbon
(ii)Free Cutting Steel
(iii)Manganese Steel
1XXX
10XX
11XX , 12XX
13XX
2 Nickel Steels
(i)0.5% Ni
(ii)1.5% Ni
(iii)3.5%Ni
(iv)5%Ni
2XXX
20XX
21XX
23XX
25XX

S.
N.
3 Nickel-Chromium Steels 3XXX
4 Molybdenum steels
(i)Cr-Mo
(ii)Cr-Ni-Mo
(iii)Ni-Mo
4XXX
41XX
43XX
46XX

S.
N.
5 Chromium steels 5XXX
6 Chromium-Vanadium
steels
6XXX
7 Tungsten steels 7XXX
8 Ni-Cr-Mo steels 8XXX
9 Silicon sttels 92XX

Steel Specification
AISI1035 Steel with 0.35%C
AISI4340 Mo steel with 0.4%C
AISI52100 Cr-steel with 1%C
and
AISI2440 Steel with 4%Ni and
0.4%C
AISI9260 Steel with 2%Si and
0.6%C

British Specification
British system of designation of steels is known as En series.
The En number of a steel has no corelation with the composition
or mechanical properties of steels
The new British system, BS970, used the first three digits for the
content of alloying elements, followed by a letter significance of
that is as shown below:
The last two digits after this letter is meant for carbon content
S.
N.
Letter Significance
1 A Analysis
2 H Hardenability
3 M Mechanical properties

British Specification
British
Old
En
British New Indian Standard AISI/SAE
En6 080M41 C35Mn75 AISI1035
En24 817M40 40Ni2Cr1Mo28 AISI4340
En31 534A99 109Cr1Mn60 AISI5210
0
En42 070A72 C75 AISI1074

Questions
Q.1 State true or false and justify in brief.
a) Iron shows allotropic changes.
b) Carbon has more solubility in ferrite than in
austenite
c) Austenite is observed at room temperature in
plain carbon steel.
d) Hypoeutectoid steels shows cementite in their
structure at room temperature.

Questions
Q.1 State true or false and justify in brief.
e) Killed steels show better resistance to
grain coarsening than rimmed steels above
11000C.
f) Chemical analysis of steel can be obtained
from microscopic examination.

Eutectoid Steel
NO Proeutectoid phase!

The alternating α and Fe3C
layers in pearlite causes
the redistribution of C by
diffusion as shown
during phase
transformation:
Hypoeutectoid
alloys: Alloys with
C content between
0.022 and 0.76
wt% are
hypoeutectoid
alloys.

Free
Energy
Metastable
Stable
Unstable

The Iron-Carbon Diagram
Callister, Materials Science and Engineering An
Introduction, John Wiley & Sons

Unit 3.ppt

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Unit 3.ppt