MATERIAL SCIENCE
INTRODUCTION (COOLING)



Observation of a pure metal cooling from the
liquid state to its solid state show that it does it
in a particular well defined way.
As soon as the freezing point is reached, nucleii
begin to form at random throughout the cooling
liquid and crystal begin to form in a very special
way.
As soon as the nucleii are initiated the formation
of crystals begins with the nucleii spreading in
three directions, this process is called....
Crystal formation
FORMATION
HARDENING. (COOLING CURVE)
Gas
Evaporation/fusio
n
Liquid
Temp
Freezing Point
Solid
0ºC
Time
HARDENING. (COOLING CURVE)
Gas
Liquid
910ºC
Porridge/mixture
Temp
720ºC
Freezing Point
450ºC
Crystallisation
420ºC
0ºC
Time
Phase Diagrams
CRYSTAL STRUCTURE
EQUILIBRIUM (PHASE) DIAGRAM
Copper and Nickel
Phase diagram
Question: A liquid contain
equal amounts of the alloy will
begin to solidify at what
temperature?
1312 ºC
Question: What temperature
does it become completely
solid?
1248 ºC
EQUILIBRIUM (PHASE) DIAGRAM
Plain carbon
steel
equilibrium
phase diagram
EQUILIBRIUM (PHASE) DIAGRAM
Ferrite/Iron: This is a solid
solution containing no more than
0.04% carbon dissolved in a Body
Centred Cubic formation lattice.
Ferrite can be regarded as
almost pure iron and is very
soft, ductile and easily
worked.
= Crystal Structure
720ºC
Pearlite
+
Ferrite
Pearlite
+
Cementite
Pearlite: This structure
exist at the eutectoid of
0.83% carbon and consist
of alternate layers of
ferrite and Cementite.
The formation of Pearlite
takes place by the
breakdown of Austenite
below a temperature of
720ºC
0.83%
Pearlite
= Pearlite Crystal structure
Eutectoid
point
Cementite: This
structure exist above
0.83% carbon and is very
hard and brittle and is
usually found on the
crystal boundary
720ºC
Pearlite
+
Ferrite
Pearlite
+
Cementite
0.83%
Pearlite
= Cementite Crystal
Structure
1130ºC
Austenite
Pearlite
+
Ferrite
Pearlite
+
Cementite
Austenite: This is a
solid solution of
carbon, in face centre
cubic and iron. The
maximum carbon
content is 1.7% at
1130ºC. Austenite
only exist in plain
carbon steels above
the UCP and is a soft
non-magnetic
compound
1.7%
910ºC
Austenite
Eutectoid
point
Austenite + Ferrite
Austenite + Cementite
720ºC Lower Critical Point (LCP)
Pearlite
+
Ferrite
Pearlite
+
Cementite
0.83% carbon 100% Pearlite
Pearlite changes to Austenite above 720ºC
TWO THINGS THAT ARE NOT ON THE PHASE
DIAGRAM
Martensite, most commonly refers to a very
hard form of steel crystalline structure, but it
can also refer to any crystal structure that is
formed by displacive transformation. It includes a
class of hard minerals occurring as lath- or plateshaped crystal grains. When viewed in cross
section, the lenticular (lens-shaped) crystal
grains are sometimes incorrectly described as
Acicular, needle shape.
 Martensite is formed by rapid cooling
(quenching) of austenite which traps carbon
atoms that do not have time to diffuse out of the
crystal structure.

TWO THINGS THAT ARE NOT ON THE PHASE
DIAGRAM


Bainite: A fine non-lamellar structure, bainite
commonly consists of cemenite and dislocation-rich
ferrite. The high concentration of dislocations in the
ferrite that are present in bainite makes this ferrite
harder than it normally would be.
The temperature range for transformation to bainite
(250–550 °C) is between those for
pearlite and martensite. When formed during
continuous cooling, the cooling rate to form bainite
is more rapid than that required to form pearlite,
but less rapid than is required to form martensite in
steels of the same composition
Cooling diagram
for Hardening
plain carbon steel
Cooling
This cooling process forms crystal which in turn form grains
INTRODUCTION TO GRAIN STRUCTURES
Metals have a crystalline structure - this is not usually visible but can be seen
on galvanized lamp posts for example.
When a metal solidifies from the molten state, millions of tiny crystals start
to grow through the Dendritic Growth Process.
The longer the metal takes to cool the larger the crystals grow in the process.
These crystals form the grains in the solid metal.
Each grain is a distinct crystal with its own orientation. A crystal on a
crystal.
GRAIN STRUCTURES
Grain structures are altered by the working of the
material in its solid metallic state. In particular: -
1. Hot working and cold working
2. Heat treatment
3. Over stressing due to continued working
All of these processes have an effect on grain size,
grain growth and orientation of the crystal
structure
HOT WORKING
In metalworking, rolling is a metal
forming process in which metal stock is passed
through a pair of rolls.
 Rolling and other forms of metal forming is
classified according to the temperature of the
metal rolled.
 If the temperature of the metal is above
its recrystallization temperature, then the
process is termed as hot rolling.
 If the temperature of the metal is below its
recrystallization temperature, the process is
termed as cold rolling.
 In terms of usage, hot rolling processes more
tonnage than any other manufacturing process,
and cold rolling processes the most tonnage out of
all cold working processes.

RECRYSTALLIZATION TEMPERATURE
The lower limit of the hot working temperature is determined
by its recrystallization temperature. As a guideline, the lower
limit of the hot working temperature of a material is 0.6 times
its melting temperature (on an absolute temperature scale).
In our case we are going to use plain
carbon steel with 0.83% carbon.
What is the re-crystallisation temperature?
720ºC x 0.6 = 432ºC
EFFECTS ON STRUCTURE
COLD WORKING
Deformed crystals
EFFECTS OF COLD WORKING

Breaks down the crystal structure

Destroys the lattice structure

Deformation occur along the crystal edges

Much more pressure is required to work the material

The elasticity limit is exceed

Work hardening occurs when not required.

Internal stress occurs known as residual stress.
To combat these effects the material has to be annealed
HEAT TREATMENT (ANNEALING)
Annealing, in metallurgy and materials science,
is a heat treatment that alters a material to
increaseare
its ductility
and to make it more
There
two "softening"
processes
workable.

commonly used when
metalworking:
normalizing
and

It involves heating material
to 30 to 50ºC
above
its upper critical temperature, maintaining a
annealing.
suitable temperature, depending on the mass of
the material,
then of
it isboth
cooled very
slowly, usually
The
objective
processes
is to
leaving it in the furnace when switch off.
soften the metal and to make it less

AnnealingThis
can induce
ductility,
soften the
brittle.
makes
further
work on
material, relieve internal stresses, refine the
the
piecebyeasier
safer. and
structure
making itand
homogeneous,
improves cold working properties.
ANNEALING
Deformed crystals
Apply heat
here
SOFTENING PROCESSES (NORMALISING)




Normalizing is the heating of steel to 30-50ºC
above its upper critical point (UCP) followed by
an air cool.
The cooling is faster than annealing and this is
the main difference between the two processes.
This limits the grain growth to a more refined
grain structure and a better quality of material.
The hardness and strength of normalised steel
are better than that of annealed steel but it
looses out where ductility is concerned.
THE IRON CARBON PHASE DIAGRAM
Copy this one now
Temperature in ºC
Annealing and Hardening 940ºC to 960ºC
1300
1200
1100
1000
900
Normalizing
800
700
600
720ºC
Upper Critical Point
910ºC
500
Lower Critical Point
400 to 450ºC
400
300
Re-crystallisation range
200
100
0.2
0.4
0.6
0.8
1.0
% of carbon in the steel
1.2
1.4
1.6
1.8
HEAT TREATMENT (HARDENING
PRESENTATION)
The Hall–Petch method
Quenching Hardening
Precipitation hardening
Work hardening
Solid solution strengthening
Martensitic transformation hardening
You must talk about the following in your presentation:1. Describe the process using photos and text.
2. The lattice formation (BCC) (FCC) etc.
3. Temperatures used in the process and why.
4 The percentage carbon in the steel
5. How the steel was cooled (speed)
6 What effect the process has on the steel (crystals)
HEAT TREATMENT (HARDENING)
Do
not
Will harden but
temperatures varies
according to carbon
content
Temperature in ºC
These steels will harden but have a
constant temperature
1300
1200
1100
1000
900
Hardening temperature
range 940ºC to 960ºC
800
700
600
720ºC
Upper Critical Point 910ºC
Lower Critical Point
500
400
300
200
0.3% carbon
0.83% carbon
100
0.2
0.4
0.6
0.8
1.0
% of carbon in the steel
1.2
1.4
1.6
1.8
PLASTICS
PLASTICS
PLASTICS
PLASTICS
 Thermo
Plastics comprise long-chain
molecules held together by weak
bonds (Figure a). When heat is
applied, the molecules "slide past"
one another and the polymer softens.
On cooling, the molecules cannot
slide past each other easily and the
polymer hardens
PLASTICS
 Thermo
Setting long chain
molecules, however, are linked
together by small molecules via
strong chemical bonds, a process
sometimes referred to as
vulcanization (Figure b). This threedimensional network is so rigid that
the molecules cannot move very
much even when the polymer is
heated. Thus, TSs do not soften