STRUCTURE- PROPERTY AND

PERFORMANCE
OF METLAS AND NON METALS

1
Crystal Structure of metals

2
Slip in metals

3
Diffusion in metals

4
5
6
7
8
9
10
11
12
 PHASE DIAGRAMS
&
HEAT TREATMENT OF
CARBON STEEL
 Phase Diagram
Phase
Transformation
Heat Treatment

14
 Phase
Phase is a homogeneous portion of a system
having the same composition and the same state of
aggregation throughout its volume, and separated from the
other portions of the system by interfaces.

For instance, a homogeneous pure metal or

alloy is a single phase system. A state in which a liquid
alloy (or metal) coexists with its crystals is a two-phase
system.

15
 Gibbs Phase Rule

F = C- P + n

F = degrees of freedom of the system (or independently

variable factors)

C = number of components forming the system

P = number of phases in equilibrium in the system

n = number of external factors(e.g. temp., pressure etc.)

The number of degrees of freedom i.e. the variance of the
system is the number of factors, such as temperature,
pressure and concentration that can be independently
varied without changing the number of phases in
equilibrium.

Now three types of equilibrium are possible:

 Invariant equilibrium (F=0)
 Univariant equilibrium (F=1)
 Bivariant equilibrium (F=2)
 Phase Diagram

A “map” that will guide us in answering the

general question:

“What microstructure should exist

at a given temperature for a given
metal composition?”
• So phase diagram, the graphical representation of the
state variables associated with microstructures, can be
explained with three types based on Gibbs Phase
Rule:

 Unary Diagrams (put C = 1 in Gibbs phase rule)

 Binary Diagrams (put C = 2 in Gibbs phase rule)
 Ternary Diagrams (put C = 3 in Gibbs phase rule)

Binary Phase diagrams can be classified on the basis of

liquid solubility and solid solubility. So before going into
details, let’s first discuss about solid solution.
Unary Phase diagram

Melting point Boiling point

Triple point

20
 Solid Solution

Solid solution is a solution within a solid

where solute and solvent both are solid.
 Substitutional Solid solution

In the substitutional solid solution alloy the involved

solute and solvent atoms are randomly mixed on
lattice sites. The single-phase solid alloys that
extend across the entire phase diagram in the Cu-
Ni and Ge-Si systems are good examples of
random substitutional solid solutions. Both atoms
are randomly distributed on FCC lattice sites in Cu-
Ni and on diamond cubic sites in Ge-Si.
Contd.
A necessary condition for single phase solid solution formation across the
entire phase diagram is that both components have the same crystal
structure. It is not a sufficient condition, however, because there are
combinations like Ag-Cu (both FCC) and Fe-Mo (both BCC) that do not
form an extensive range of solid solutions. Instead terminal solid
solutions, so named because they appear at the ends of the phase
diagram, form. The terminal α and β phases in the Pb-Sn diagram are
examples. They are both substitutional solid solutions and display the
limited solubility often exhibited by such phases. Introduction of foreign
atoms into the lattice, whether by design (as dopants or solutes) or
accident (as impurities), will always create dilute solid solutions which are
often substitutional in nature.

The lattice parameter of substitutional solid solutions is usually an average

of the interatomic spacing in the pure components weighted according to
the atomic fractions present. This observation is known as Vegard's law. It
predicts, for example, that the lattice parameter of a 25% Cu-75% Ni alloy
will be approximately 0.25 x 0.3615 nm + 0.75 x 0.3524 nm = 0.355 nm.
 Interstitial Solid Solution

When undersized alloying elements dissolve in

the lattice they sometimes form interstitial solid
solutions. Important examples include carbon
and nitrogen in BCC iron, the former being the a
phase on the Fe-Fe3C phase diagram.
 Intermediate Solid Solution

Unlike terminal solid solutions that extend inward from the

outer pure components, intermediate phase fields are
found within the phase diagram. Two phase regions
border either side of an intermediate phase. In the case of
Cu-Zn, α and η are terminal phases, and β, γ, δ and ε are
intermediate phases. These single phases are generally
stable over a relatively wide composition range.
 Ordered Solid Solution

Atoms within certain solid solutions can, surprisingly, order

and give every appearance of being a compound. In the
alloy 50% Cu-50% Zn β'-brass we can imagine a CsCl-like
structure populated by atoms of Cu and Zn. Above 460°C
the ordering is destroyed and a random solid solution
forms. The order-disorder transformation, interestingly,
bears a close resemblance to the loss of magnetism
exhibited by magnets that are heated above the Curie
temperature.
 Binary Phase Diagram

On the basis of Liquid and Solid Solubility, Binary

Solid Solution can be classified as

100 % liquid solubility & 100 % solid solubility

100 % liquid solubility & 0 % solid solubility
100 % liquid solubility & partial solid solubility
0 % liquid solubility & 0 % solid solubility
 100 % liquid solubility & 100 % solid solubility

0% A
20% A
Liquidus
40% A

60% A

80% A

100% A
Solidus

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29
Pure copper
Pure Nickel

35% Cu & 65% Ni

30
 100 % liquid solubility & 0 % solid solubility

0%A
20%A

40%A
80%A
60%A

100%A
Liquidus

Solidus

31
32
 100 % liquid solubility & partial solid solubility

0% A

80% A

100% A 60% A 40% A

Liquidus

Solidus

Solvus

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34
 0 % liquid solubility & 0 % solid solubility

100% B

100% A

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 Lever rule
Why
We want know the
phase composition at a
particular temperature
and alloy composition

Draw a horizontal
line (tie line) passing
through the point

Find the overall alloy

composition on the
tie line.α
Cα C0 CL

Cα – C0 C0– CL
Now calculate, WL = Wα =
Cα – CL Cα – C L
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 Fe-Fe3C Phase Diagram

 Alloys with a carbon content upto

2.0% are called STEEL.

 Alloys with a carbon content

exceeding 2.0% are called CAST
IRON.

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Cooling Curve for pure Iron
1538
a=2.93 A

1401

a=3.63 A

1130

910 a=2.90 A

768

723 a=2.86 A

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39
BCC

FCC

BCC

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41
Hypoeutectoid and Hypereutectoid steel

42
CHT & CCT with reference to Fe-Fe3C metastable binary diagram

43
 Phase Transformation

Simple diffusion- Diffusion-dependent Diffusionlesss

dependent transformation transformation
transformation

Metastable phase is
No change in either the Some alteration in
produced.
number or composition phase compositions
of the phases present. and often in number
of phases present.
Martensitic
Solidification of pure Transformation
metal, allotropic Eutectoid reaction
transformations,
recrystallization and grain
growth.
44
Kinetics of Phase Transformation

Nucleation Grain Growth

45
 Nucleation

Homogeneous Nucleation Heterogeneous Nucleation

46
Mechanical Behavior of Iron-carbon Alloys

Fine Pearlite

Coarse Pearlite

Bainitte

Martensite

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48
49
50
51
 Phase Transformation Diagram

Isothermal Transformation Continuous Cooling

Diagram (TTT) Transformation Diagram
(Time –Temperature – (CCT)
Transformation)

52
 % of
Transformation

450 C 250 C
400 C 700 C

53
TTT diagram for Eutectoid Steel

54
TTT diagram for Hypoeutectoid and Hypereutectoid Steel

Hypoeutectoid Steel Hypereutectoid Steel

55
 Austenite to pearlite

400 sec

1150 sec

1320 sec

1450 sec

4000 sec 56
 Austenite to Bainite

400 sec

500 sec

850 sec

900 sec

2500 sec
57
Continuous Cooling Transformation Diagram

58
59
Martensitic Transformation of Steel

BCT unit cell

FCC unit cell

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Heat Treatment
 WHY?
Primary concern is to
increase the strength of Theoritical Strength

the material
Whiskers

WHIISKERING (costly)
Heat treated

HEAT TREATMENT Pure

metal
(cheaper)

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 Heat Treatment

Annealing Tempering

Normalizing Hardening

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 Annealing
 To obtain softness Full Annealing
 Improve machinability
 Increase / restore Incomplete
ductility / toughness Annealing
 Relieve internal stress
Isothermal
 Reduce / eliminate Annealing
structural homogeneity
Spherodising
 Refine grain size
Diffusion Annealing
(Homogenising)
64
 Full Annealing
Heating a hypoeutectoid steel 30-50o C above the critical
point A3, holding at this temperature and slowly cooling (@
30-200o C /hour)

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 Incomplete Annealing
 Heating steel to a temperature somewhat above the
critical point A1, holding it at this temperature and
slowly cooling

 Incomplete annealing associated with only partial

recrystallisation; excess ferrite of hypoeutectoid steel or
excess cementite of hypereutectoid steel does not
pass over into solid solution and is not recrystalised.

 Incomplete annealing is applied chiefly to eutectoid and

hypereutectoid steel.

66
 Isothermal Annealing

Steel is heated as for ordinary annealing and then cooled

comparatively rapidly (in air or by a blast in a furnace) to a
temperature 50o to 100o C Eutectoid temperature.

Advantage:
Reduces time required for heat
treatment
Reduce hardness

Application:
Produces good results in
treating relatively small charges
of rolled stock or small forgings.

67
 Spheroidising
Spheroidising is performed by heating the steel slightly
above 730o-770o C with subsequent holding at this
temperature followed by slow cooling @ 25o to 30o /hour
to 600o C.

68
 Homogenising
 Homogenising is carried out at temperatures from 1100o to 1200o C
(optimum temperature is 1150o C) at which diffusion proceeds quite easily
and to some extent equalises the composition of steels having developed
dendritic segregations.

 Scaling is very intensive at high temperatures and this leads to

excessive losses of metal. Holding time, therefore should be minimum.

 Cooling with the furnace for 6 to 8 hours to 800o-850o C and then further
cooling in air.

 After homogenising, Castings undergo full annealing to refine their

structure.

Application

Alloy steel ingots and heavy complex castings for eliminating the chemical
inhomogeneity within the separate crystals by diffusion.
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 Normalising
Heating steel to a temperature from 40o to 50o C above A3 ,
holding at this temperature for a short time and subsequent
cooling in air.

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 Normalising
Why it is done

 To eliminate coarse grain structures obtained in

previous working(rolling, forging or stamping)

 To increase the strength of medium carbon steels

to a certain extent (in comparison with annealed
steel)

 To improve the machinability of low carbon steels

 To improve the structure in welds

 To reduce internal stress

 To eliminate the cementite network in

hypereutectoid steels
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 Hardening
Steel is heated to a temperature above the critical point, held at this
temperature and then quenched (rapidly cooled) in water, oil, or molten salt
baths.

Why it is done
To increase hardness and
wear resistance retaining
sufficient toughness at the
same time

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 Effect of hardening of HYPOEUTECTOID steel

Initial structure: pearlite + ferrite After quenching from the range

770oC: martensite + ferrite

After hardening at a normal Overheated (1000o C): coarse

temperature (840o C): martensite acicular martensite 73
 Effect of hardening of HYPEREUTECTOID steel

Initial structure: Structure after properly Hardening with

granular pearlite conducting hardening: overheating
martensite and
cementite
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Precaution
Reaction of furnace gases (combustion products and air) with the
surface of articles heated in flame and electric furnaces will lead to
oxidation and decarburization of steel.

Oxidation in the heating Decarburisation of the

process results in irretrievable surface layers of the steel
losses of metal, detoriation of reduces the hardness in the
in the condition of the ordinarily as quenched condition as
most highly stressed layers of well as the water resistance
metal and necessity for and fatigue strength.
subsequent descaling.

Oxidation and decurburization may be prevented if a protective

gaseous medioum is introduced into the furnace,called controlled or
protective atmosphere.
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 Quenching Media
Quenching medium must provide for a cooling rate above the critical
value to prevent austenitic decomposition in the pearlitic and
intermediate regions.
In the martensitic transformation temperature range, cooling should be
slower to avoid high internal stress, warping of the hardened part and
cracking.

76
 Stages of Quenching

 A thin vapor film or blanket surrounds the hot metal. Cooling

proceeds by film boiling, the cooling rate is relatively slow and is
determined by the radiation and conduction of vapor.

 Vapor film breaks up and liquid boils with bubbles on the surface of
the metal being cooled. During this period , liquid wets the metal
surface in direct contact and cooling is accomplished by vapor
generation on this surface. Since all quenching media have a high
latent heat, this is the fastest stage of cooling.

 At temperature below the boiling point, cooling is much slower as

heat is extracted mainly by convection. The cooling rate decreases
as the temperature of metal falls.

77
Effect of different Quenching media

78
 Hardening Procedure

Conventional Stepped Quenching Isothermal quenching

quenching in a (Martempering) (Austempering)
single medium

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Effect of Hardening
Quenching in
Water

Quenching in oil

Cooling in air after forging

Annealing at 900o C and

cooling in the furnace

Annealing at 730o-760oC

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 Tempering

Low-temperature Medium-temperature High-temperature

tempering tempering tempering
(150o to 250o C) (350o to 450o C) (500o to 650o C)

The purpose of this It is employed for coil Almost completely

tempering is to reduce and laminated eliminates internal
internal stress and to springs and provides stress and provides
increase the toughness the highest attainable the most favorable
without any appreciable elastic limit in ratio of strength to
loss in hardness conjunction with toughness for
ample toughness. structural steels.
After this type of
tempering, the martensite Steel has a troosite The tempered steel has
produce by quenching is structure after this a sorbite structure after
transformed into tempering procedure. this treatment
tempered martensite
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82
 Sub-zero Treatment
Why it is done
A certain amount of retained austenite may always be found in
hardened steel. Retained austenite reduces the hardness, wear-
resistance and thermal conductivity of steel and makes its
dimensions unstable.

A sub-zero treatment has been devised to reduce the retained austenite in

hardened steel. It consists in cooling the metal being treated to sub-zero
temperatures. Such treatment is suitable only when the temperature, at
which the martensitic transformation is complete( Mf), is below zero. 83
 Defects due to Heat Treatment

The main types of rejects in annealing and normalising

are due to faulty regulation of heat temperature,
including overheating, burning, underheating etc.

The chief cause of quenching defects in high residual

(internal) stresses occurring in hardening articles. These
stress may cause distortion, warping and even cracking.

84
 Surface Hardening
Surface hardening is a selective heat treatment in which the surface layers
of a metal are hardened to a certain depth while a relatively soft core is
maintained.

Types of Surfece hardening

 Hardening with high frequency induction heating
 Hardening with electrical contact resistance heating
 Hardening with electrolytic heating
 Oxyacetylene flame hardening

Purpose
 To increase the hardness and wear resistance of
the structures of metal articles
 To improve the reliability in operation of a machine
component
 To increase fatigue limit 85
 Carburising of Steel
Carburisation is the process of saturating the surface layer of steel with carbon.

Purpose
To obtain a hard and wear resistant surface on machine parts by enrichment
of the surface layer with carbon to a concentration from 0.75 to 1.2 % and
subsequent quenching.

Carburised and Quenched (case-hardened) steel has a higher fatigue limit.

Mechanism of Carburisation
 Dissociation of the carbonaceous gases with the evolution of atomic carbon

 Enrichment of surface layer with carbon (degree of saturation depends on

carburizing temperature and carburizer composition)

 Diffusion of carbon, absorbed by the surface, deep into the metal.

86
Microstructure of Steel after Carburisation

87
Variation of carbon contents depends on
the temperature of the process, the
holding time, the steel composition and
the activity of the surrounding medium
which supplies carbon atoms to the
surface.

1 hr 5 hr 10 hr

This results in the formation of “soft spots” on the surface of the part after
quenching.

This may be eliminated by heating to higher hardening temperature and by

using a highly effective quenching medium.
88
Classification according to carbon source

 Pack carburising with solid carbonaceous mixtures

(carburisers)
 Gas carburizing
 Liquid carburizing

Heat treatment after carburizing

Parts of carbon steel, requiring high mechanical properties, are usually

subject to double-hardening, followed by tempering, after carburizing.

To improve the structure of the core and to impart optimum properties

to the surface layers, a single heating to one temperature will evidently
be insufficient.

89
Stages

 First hardening or
normalizing is conducted at
a temperature of 880o-900oC
to improve the core
structure of the work which
Normal structure
has been overheated in
carburization.

 The second hardening

operation is conducted at
750o-780oC to eliminate the
effects of overheating and
to impart a high hardness With an increased amount of retained austenite
to the carburized case.

 Heat treatment is
completed by tempering at
150o to 180oC. 90
 Nitriding
Process of saturating the surface of steel with nitrogen by
holding for a prolonged period at a temperature from 480o to
650oC in an atmosphere at Ammonia (NH3).

Purpose
 Increases the hardness of the surface to a very high degree.

 Increases the wear resistance.

 Improve the fatigue limit under corrosive condition.

91
Mechanism ε + γ’

 Solid solution of nitrogen in α-iron

(α-phase) at the eutectoid
temperature((591oC), nitrogen γ’
concentration in the alpha phase will
be 0.42% and reduced to 0.015% at
α + γ’
room temperature.
(eutectoid)
 γ’ phase, a solid solution on the
basis
of iron nitride Fe4N (5.5 to 5.95 % N)

 ε-phase, a solid solution on the basis

of iron nitride Fe2N (8 to 11.2% N)

α + γ’(excess)
92
 Effect of alloying elements
Al

Cr
Mo

Mn Si
W

Ni

Highly dispersed particles of these nitrides interlock the slip planes and thus
considerably increase the hardness of the nitrided layer. Al, Cr, Mo and V
increase the hardness to the greatest extent.
93
Procedures

 Hardeniung and tempering is performed to impart the required the

mechanical prpoperties to the core of the work, i.e. to increase its
strength and toughness.

 All required machining ooperations are done.

 All areas which are not to be nitride , are protected by a thin layer of
tin applied by an electrolytic method.

 Nitriding

 Finishing grinding and lapping is applied in accordance with the

specified tolerance on the work.

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 Steel
600o 550o

500o

Alloy structural Steel

Carbon steel

95
Thank You

96