The Microstructure of Cast Iron and
Copper-Silver Eutectic Alloys
Abstract
The microstructures of five samples of different forms
of cast iron were studied and drawn using an optical microscope. The
differences in microstructure were accounted for by the different heat
treatment processes, and additives present and the process by which the steel
was cooled.
Four samples of copper silver eutectic alloys of
differing compositions were also studied using an optical microscope and their
structures drawn.
Introduction
Cast irons are a class of ferrous alloys with a carbon
content of between 2.0 – 4.5% [1]. They are the most economical of
cast materials with respect to foundry costs which makes them desirable for
many engineering applications even though they tend to be quite brittle. There
is a high demand for cast irons due to their versatility. They vary in strength
but have good resistance to wear, abrasion and corrosion and are easily
machined. They have a high damping capacity and are also easily melted and
cast, which means they take good casting impressions[2].
The carbon within cast irons exists in two forms,
either in the combined form as cementite which is an unstable iron carbide, or
in the free form as graphite. Cementite is extremely hard and brittle which
makes the iron hard and difficult to machine. Graphite however is soft making
the iron softer and easier to machine. Graphite normally occurs in flakes which
weaken the metal by breaking up its continuity. The flakes also have sharp
edges which act as stress raisers in the metal causing localised concentrations
of stress. Due to the properties of these two forms of carbon, the shape,
distribution and relative amounts in the cast iron produce a variety of
different cast irons with differing properties [2].
There are a number of factors which affect the
structure of the cast iron. The rate of solidification determines the type of
iron to be formed, for example slow cooling will produce grey iron whereas more
rapid cooling will result in white iron structures. The carbon content of the
melt and the presence of other elements have a large effect on whether graphite
or cementite is formed and in what amounts. For example nickel and silicon
promote the formation of graphite in the iron structure. The type of heat
treatment will also affect the structure, as if cementite is heated and
decomposes to ferrite and graphite it produces a completely different structure
[3].
Copper and silver form a binary alloy which shows
three single-phase regions on the phase diagram. These are 
, 
and liquid. The
phase is a
solid solution rich in copper which has silver as the solute and an FCC
structure and is considered to include pure copper. The phase solid
solution has copper as the solute, has an FCC structure and is considered to
contain pure silver.
As silver is added to copper the temperature at which
the alloys become totally liquid decreases. This can also be said for the
addition of copper to silver. The composition where these two temperatures meet
is known as the eutectic. In the case of copper silver this is at 72% Ag 28% Cu
[1].
Experimental
Five prepared micro specimens were provided and the
microstructure of each studied and drawn using a microscope. A magnification of
400 x was used for each specimen to give the best representation of the
microstructure.
The specimens provided were:-
·
Blackheart malleable cast
iron
·
Ferritic spheroidal
graphite iron
·
Pearlitic spheroidal
graphite iron
·
White cast iron
·
Phosphoric grey cast iron
Four polished and etched micro sections of copper silver alloys were
provided. These were inspected under a microscope at 400x magnification and
drawn.
The samples provided had the following compositions:-
·
90%Ag 10% Cu
·
72%Ag 28% Cu
·
50% Ag 50% Cu
·
30% Ag 70% Cu
The phase diagram for the copper-silver alloy system was constructed
from information provided.
Results
Copper Silver Phase diagram
The first set of drawings show the microstructures of the different cast
irons at 400 x magnification. The second set of drawings show the
microstructures of the copper silver alloys of differing compositions also at
400 x magnification.
Discussion
Different structures of cast iron can be produced by the way the metal
is cooled. Two of these structures are white and grey cast iron. On cooling the
liquid austenite begins to form and as the metal is cooled further the amount
of austenite in the matrix increases. At the eutectic temperature of 1130°C the
remaining liquid solidifies producing austenite in a eutectic matrix. At 723°C
this structure begins to decompose to form a structure consisting of cementite,
Fe3C and eutectoid iron. The eutectoid is a mixture of cementite and
ferrite, which contains areas of pearlite and cementite. The pearlite is formed
as a result of the decomposition of the austenite on cooling. The structure
described above is that of white cast iron which is formed when the rate of
cooling is rapid [3].
Grey cast iron is produced when the cementite in the structure
dissociates to form ferrite and precipitates of graphite. This leaves no pure
cementite in the structure and reduces the amount of pearlite, as the cementite
lamellae within the pearlite will also dissociate forming ferrite and graphite.
Incomplete dissociation means that there may still be traces of pearlite within
the grey iron structure. The cooling rate for this process is much slower than
that used for the white cast iron, as the cementite needs time to dissociate [2].
In order to produce grey cast iron instead of white cast iron a number
of variables must be controlled. The rate of cooling must be controlled and
made as slow as possible to form grey cast iron [3]. A high silicon
content will also promote the formation of grey cast iron as silicon has strong
graphitising tendencies [2]. Phosphoric grey iron is stronger, has a
lower melting point and better fluidity than normal grey cast iron [4].
The Blackheart malleablising process involves packing
white iron castings into pots with a neutral packing, such as sand or crushed
slag, and heating them to 900°C for three days. They are then cooled very
slowly, at about 3°C per hour. This results in the cementite in the white iron
being decomposed into ferrite and graphite being precipitated in a finely
dispersed form[1]. The structure is composed entirely of ferrite and
graphite. The graphite present in the structure is shown as ‘rosettes’ of
carbon in the ferrite. There is usually a small amount of pearlite left after
the process which does not affect the properties of the casting [4].
The steel produced in this process has good wear resistance and strength and
reasonable toughness [2].
To produce spheroidal graphite iron rather than flake
graphite iron a number of additives are needed. Magnesium amounting to 1-2% of
the weight of the iron is added in the form of a nickel magnesium alloy of
10-20% magnesium. The alloy is used to prevent an extremely violent reaction
from occurring. Magnesium can be replaced by Cerium, which has a similar
effect. The presence of silicon also assists the formation of the nodules
therefore ferro-silicon is added [2].
In order to produce a ferritic spheroidal graphite
iron from a pearlitic spheroidal graphite iron the steel must be heated to just
below the lower critical temperature. This has the effect of making the
cementite lamellae present in pearlite or free, to coalesce and become
spheroidal. This is possible due to the surface tension, because at this
temperature there is sufficient mobility to enable the particles to contract
their surfaces and become globular. The spheroids embedded in the ferrite
matrix reduce the hardness of the steel considerably [2].
The cooling of copper silver alloys of different compositions:-
·
90%Ag 10%Cu
The liquid cools until it reaches approximately 870°C where the first
solid starts to form.
The crystals
continue to grow until the melt reaches the eutectic temperature of 780°C. At
this temperature the liquid solidifies forming a eutectic intergrowth of and. The composition of and stays the same
as the solid cools to ambient temperature.
·
72%Ag 28%Cu
The liquid cools until it reaches the eutectic temperature of 780°C at
which point all of the liquid solidifies forming a pure eutectic phase.
·
50%Ag 50%Cu
The liquid cools until it reaches approximately 870°C where the first
solid forms. The crystals grow
until the eutectic temperature is reached when the remaining liquid solidifies.
This forms a eutectic intergrowth with the presence of crystals.
·
30%Ag 70%Cu
The liquid cools until it reaches approximately 980°C where the first
solid forms. The
crystals continue to grow as in the 50-50% alloy, but due to the larger
temperature difference between the formation of the first crystals and
the eutectic, the amount of formed is
greater. At the eutectic temperature the remaining liquid solidifies to form
the eutectic intergrowth with large amounts of present.
Non equilibrium cooling produces the effect of coring. In a cored
crystal the composition is not the same at all points. The crystal lattice will
be continuous but there will be a gradual change in composition across each
crystal [3]. The dendrites within the structure represent the first
solid formed which will be rich in copper as it has the higher melting point.
The regions between the dendrites are rich in silver as it has the lower
melting point and is the last to form a solid. The composition and properties
of the + phase differs
from one region to the next, which results in the casting having poorer
properties. Also if heated, melting of the regions between the dendrites may
occur at a temperature below the equilibrium solidus of the alloy.
Conclusion
·
Cast irons have many
different structures each one caused by a different cooling rate, additives and
different heat treatments.
·
Slow cooling rates and
high silicon content promotes the formation of grey cast iron over white cast
iron
·
Magnesium and silicon help
to produce spheroidal graphite iron rather than flake graphite iron
·
To produce a ferritic
spheroidal graphite iron from a pearlitic spheroidal graphite iron the steel
must be heated to just below the lower critical temperature.
·
The composition of the
copper silver eutectic alloys has a very large effect on the microstructure of
the alloy, with different amounts of the phases being produced on cooling.
References
[1] William D. Callister
Jr. ‘Fundamentals of Materials Science and Engineering an integrated approach’
– Second edition, 2005, John Wiley and Sons Inc. p372-373, 539-540
[2] W. Dennis ‘Metallurgy
of the Ferrous Metals’ , 1963, Sir Isaac Pitman and Sons Ltd, Woking and London
p107-109, 234,309
[3] V. John ‘Introduction
to Engineering Materials’ - 4th Edition, 2003, Palgrave Macmillan,
[4] Richard Henry Greaves
and Harold Wrighton ‘Practical Microscopical Metallography’ 1967, Chapman and
Hall,
[5] D.R. Askeland ‘The
Science and Engineering of Materials’ - 5th Edition, 2006, Chapman
and Hall,