Transcript Casts Irons The cast irons are made and used by many industries

M e c h 4 7 3 L e c tu re s
P ro fe s s o r R o d n e y H e rrin g
Casts Irons
The cast irons are made and used by many industries
including the automotive industry, farming industry (e.g.,
tractors), construction industry, etc to make the housings
of crank shafts, piston rods, heads, intake and exhaust
manifolds, transmission housing, starter motor housing,
gear shafts and assembles, wheels, etc.
A hard, good wear-resistant material with a reasonable
amount of toughness is required.
Cast iron is cheaply made using a blast furnace from Pig Iron
since it contains a high carbon and Si content.
Thus, the cast irons are not an iron-carbon alloy but an ironcarbon-silicon alloy of high C content between 3-3.75% C
and high Si content of 1.5-3% Si.
Casts Irons
The main factors determining the type of cast iron are:
• 1) The solidification rate
• 2) The composition
Solidification Rate
• The stable Fe-C system is iron-graphite – rather than ironcementite
• So slow solidification from the melt favours the irongraphite system while rapid solidification will favour the
Fe-Fe3C system.
• In order to obtain some areas of Fe-Fe3C within a Fegraphite casting, to make surface hard areas in a casting,
nails with large heads (called chills) are inserted at the
mould face to form local spots of cementite – instead of
graphite.
Casts Irons
Composition
Silicon and carbon in solution in the melt – both enhance
the formation of graphite eutectic transformation at
1154oC.
These elements are both naturally present in relatively
high concentrations in pig iron, which is the major raw
material – mixed with iron and steel scrap – for making
cast iron.
Typical Composition of Pig Iron
C
Si
Mn
3.00 – 3.75
1.5 – 3.00
0.1 – 1.0
S
P
0.05-0.06
0.3-1.5
Fe-Cementite Phase Diagram
Iron-Graphite Phase Diagram
Note:
Changes in
eutectic and
eutectoid
compositions
and
temperatures
The Need for Silicon
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The eutectic of graphite occurs at 1154 oC,
This eutectic reaction is given by
L4.26%C  g2.08%C + Cgraphite
The eutectic graphite reaction is competing with the
eutectic reaction of Fe3C at 1148 oC.
In the cast irons, Si is used to control the formation of
the graphite phase.
The eutectic reaction does not actually occur at 1154
oC nor 1148 oC nor 4.3 %C because of the addition of
Si.
The Need for Silicon
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Cast irons with a carbon equivalent, C.E., less than 4.3
wt% C are hypoeutectic.
Cast irons with C.E. more than 4.3 wt% C are
hypereutectic.
C.E. close to 4.3 wt% induces the graphite reaction
producing gray irons.
Hypoeutectic cast irons will tend to have Fe3C and
form white cast irons.
The Need for Silicon
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In order to induce the eutectic graphite reaction to go
to completion, a certain amount of Si is used to
complete the formation of graphite from the available
carbon.
The addition of Si retards the Fe3C formation, which
allows more time to form graphite.
Si stabilizes the formation of graphite structures.
It does this by increasing the undercooling of the
eutectoid reaction.
This encourages the formation of the ferrite and
graphite phases from austenite.
This is seen in the (Fe-2%Si)-C phase diagram.
Do you recall what undercooling is?
(Fe-2%Si)-C Phase Diagram
1.5%C
0.05%C
3.66%C
undercooling
(Fe-2%Si)-C Phase Diagram
Cast irons contain 1.15 – 2.85% Si – and are thus
conveniently described in terms of a pseudo-binary
(Fe+2%Si)-C phase diagram.
• The g-field is severely constricted – compared to the Fe-C
diagram.
• The maximum solubility of C in g-Fe is 1.5%C – instead of
2.11% C.
• The eutectic is 3.66%C – instead of 4.26%C
• Thus, the eutectic is given by
L3.66%C  g1.5%C + Cgraphite
The eutectic temperature is increased to 1154 – from 1148 oC.
(Fe-2%Si)-C Phase Diagram
The eutectoid
• The eutectoid is 0.60%C – instead of 0.77%C.
• The eutectoid temperature is increased to 765 – from 727
oC.
• The maximum solubility of C in a-Fe is increased to 0.05% from 0.02%C.
• The eutectoid reaction is given by:
g0.60%C  a0.05%C + Cgraphite
The Need for Silicon
The Si is converted to an equivalent carbon and the
desired total carbon content is about 4.3 wt% C .
This is expressed as:
Carbon Equivalent
(C.E.)  %C 
%Si
 4.3
3
This equation is taken from the ranges of carbon and Si in
ferrous alloys for different types of cast irons.
The Need for Silicon
If Phosphorus is added to the cast iron, the C.E. is:
C.E.  %C 
%Si  %P
3
 4.3
Effect of Other Elements on the Fe-C Eutectic
• Beside Si and C – the g-Fe-Graphite eutectic is also
promoted by the following elements:
•
Ni – Mg – Al – Ti – Zr – Cu
• which are expensive so not generally added.
• Carbide forming elements – such as Cr & Mo – stabilize the
carbide phase and thus promote the (g-Fe + Fe3C) eutectic.
• Mn is also a strong carbide (Fe3C) forming solute – but its
effect on the eutectic depends on the presence – or
absence – of Sulphur.
• Sulphur is present in cast irons – from the coke used to
smelt the pig iron.
• It does not actually form a carbide – but strongly promotes
carbide formation, e.g., 0.01%S can offset the graphitizing
effect of 0.15% Si.
Effect of Other Elements on the Fe-C Eutectic
• S has a similar affinity for Mn – forming MnS, which itself is
neutral - but the first additions of S remove some of the
carbide-stabilizing Mn – and thus can cause the amount of
graphite to increase.
• Similarly – the first additions of Mn can remove the S from
solution – and again increase the amount of graphite.
• Phosphorus can act in two ways:
• 1) Physically – it forms a phosphide eutectic with a lower
melting point than FeC (graphite) – which increases the
time available for Si to promote graphite.
• 2) Chemically – it promotes carbide formation – so large
amounts increase carbide formation.
The various alloying elements must thus be carefully
balanced to obtain a desired grey or white cast iron.
The Need for Silicon
For Si contents > 2%
• The eutectic is composed of (g–Fe + C
(graphite)) when the alloy is cooled slowly.
• These Fe-C alloys are the grey cast irons.
• The eutectic is composed of (g–Fe + Fe3C)
when the alloy is chill cast.
• These alloys are the white cast irons.
Casts Irons
• When a sample of cast iron is fractured – the exposed
surface may be grey – white- or a mottled grey/white
mixture.
• A sooty grey fracture indicates that the microstructure
is composed of graphite flakes in a ferrite matrix.
• This graphite is relatively weak – as the fracture goes
from flake to flake – so the grey fracture is mostly
exposed graphite.
• Gray cast iron contains small, interconnected graphite
flakes in an alpha iron matrix.
• It has low strength and low ductility.
• Gray cast iron is the most commonly used cast iron
for engine blocks.
Casts Irons
• A white fracture means that the microstructure consists
of cementite and ferrite – with the fracture going along –
or through – the brittle white areas of cementite.
• White cast iron contains massive amounts of cementite.
• When it fractures it’s surface appears white, hence the
name.
• White cast irons are very hard and resistant to wear.
• A mottled colour means that graphite flakes are present
in some areas, while cementite is present in others.
Casts Irons
Schematic drawings of five types of cast irons a) gray iron, b) white iron,
c) malleable iron, d) ductile iron and e) compacted graphite iron.
We will discuss
these in detail.
Casts Irons
Sketch in a) and photograph in b) of the flake structure of graphite in
gray cast iron.
Time Temperature Transformation (TTT)
Diagram of Cast Iron
Later we will
discuss TTT
diagrams in
detail.
In fact we’ll
use TTT
diagrams to
make steel
alloys.
Forms of Graphite Flakes
Many forms of cast iron exist depending on its solidification.
• The iron-carbon eutectic can solidify by one or other of the
two reactions:
liquid  austenite + graphite What does superheated
liquid  austenite + cementite mean?
If a cast iron is superheated to destroy the graphite nuclei –
undercooling will result – and the lower temperature (gFe+Fe3C) will form.
However – if there is sufficient Si and/or C present – the
cementite will break down to austenite plus graphite – the
sequence is thus:
liquid  austenite + cementite – then cementite 
austenite + graphite
This secondary graphite is quite distinct from the eutectic
graphite and forms between the austenite dendrites.
Dendritic Growth Mechanism
Liquid temperature ahead of growing
dendrite is cooler than dendrite,
which promotes formation of
protuberances and nodules.
Heat of fusion, DHf heats liquid metal
ahead of dendrite, slowing its growth.
Images of dendrites
What is inoculation?
A
B
D
C
E
Eutectic Graphite
A. Uniform flakes – random orientation
B. Rosette graphite – by inoculation
C. Non-uniform flakes – random orientation
Secondary Graphite
D. Interdendritic – random orientation
E. Interdendritic – preferred orientation
Sizes of Graphite Flakes
• Rapid solidification results in finer graphite flakes.
• But too rapid solidification will result in cementite
unless there is a very high Si and C concentration.
Graphite flakes are classified in sizes – like ASTM grain
sizes – according to the maximum length observed at a
magnification of 100x.
What does ASTM stand for?
No. 1 Longest flakes > 4 in (100 mm)
No. 2 Longest flakes 2 - 4 in (50 – 100 mm)
No. 3 Longest flakes 1 - 2 in (25 – 50 mm)
No. 4 Longest flakes 0.5 – 1 in (12.5 – 25 mm)
No. 5 Longest flakes (0.25 – 0.5 in (6.25 – 12.5)
No. 6 Longest flakes (0.125 – 0.25 in (3.125 – 6.25)
Etc.
Sizes of Graphite Flakes
What does BHN
And UTS stand for?
Note: C-Eq.
(carbon equivalent)
Graphitization in the “Solid State”
Both grey and white cast irons contain austenite.
• In hypereutectic irons – the austenite is only in the eutectic.
• In hypoeutectic irons – the austenite is the pro-eutectic
constituent – as well as being a constituent of the
eutectic – so the structure consist of austenite
dendrites surrounded by interdendritic eutectic.
• This helps us to understand the structures we see.
• During slow cooling from the eutectic to the eutectoid
temperature – the proeutectic austenite will precipitate
carbon.
• The graphite – or carbide – in the eutectic exerts a strong
nucleation effect – so that the precipitation of excess
carbon from the proeutectic austenite results in the
growth of the eutectic graphite – or cementite.
Graphitization in the Solid State
During slow cooling from the eutectic to the eutectoid
temperature – the proeutectic austenite will precipitate
carbon.
The Eutectoid Transformation in White Cast Irons
On cooling through the eutectoid temperature – the austenite
transforms by:
austenite  ferrite + graphite
or
austenite  ferrite + cementite (i.e., pearlite)
In a hypoeutectic white cast iron – the pearlite reaction will
normally occur – but austenite in the eutectic will
precipitate carbide on to the cementite formed in the
eutectic – instead of forming pearlite between these
carbide plates.
That is, the pre-existing cementite acts as a nucleation site
for the precipitation of cementite from austenite.
(see next slide)
The Eutectoid Transformation in White Cast Irons
pearlite
Interdendritic
carbide
Nucleation sites
The Eutectoid Transformation in Grey Cast Irons
In a grey cast iron – the type of eutectoid reaction will
depend on the Si and C – and on the rate of cooling.
For a given composition –
With slow cooling
the eutectic will be austenite + coarse graphite –
and on passing through the eutectoid – this austenite
will transform to ferrite + graphite – with the graphite
depositing on the existing flakes.
With rapid cooling
the eutectic structure will be austenite + fine graphite –
but on passing through the eutectoid – the austenite
will transform to a regular pearlite structure.
(see next slide)
The Eutectoid Transformation in Grey Cast Irons
Malleable Cast Irons
It is possible to have a composition that –
would form a grey cast iron on extremely slow cooling
but form a white cast iron on regular cooling
Reheating a regularly cooled sample – and holding at high
temperature – will cause decomposition of the carbide
– to form graphite + austenite.
The graphite formed by this process is quite different from
eutectic graphite – it grows in the form of compact
aggregates – rather than flakes.
On slow cooling to room temperature –
the austenite will decompose to ferrite + graphite –
which will deposit on the previously formed aggregates
– so the alloy will no longer exhibit brittleness.
(see next slide)
Malleable Cast Irons
Nodular or Spherulitic Cast Iron
Cast irons with tensile ductility can be formed – by adding
Mg (or Ce Ca Li or Na) to a very low S alloy (0.01%) –
just before casting.
These additives cause the graphite to form as tiny balls of
spherules rather than flakes – during the eutectic
solidification of grey irons.
Since Mg is above its boiling point at casting temperatures of
1400 – 1500 oC, it is necessary to add it in the form of a
Mg-Ni alloy.
Alternatively – the cooling of a white cast iron can be
arrested to permit the eutectic carbide to decompose
to austenite + aggregates of graphite – (like the
malleable irons) – an then rapidly cool through the
eutectoid temperature – to transform the austenite to
pearlite.
(see next slide)
Nodular or Spherulitic Cast Iron
The pearlitic nodular irons have a greater strength than the
ferritic nodular irons – but still have reasonable
pearlite
ductility.
graphite
Compositions and Properties of Ductile Cast Irons
The End
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