Metallurgy

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3.0 The Peritectic Reaction
 The peritectic reaction is a 3 phase
reaction that takes place at a constant
temperature tp, when a solid solution
of constant composition reacts with a
liquid of constant composition to form
a second solid again of constant
composition.
 A peritectic reaction usually occurs
when the melting points of the 2
metals differ considerably.
3.1 Intermediate Phases
 The isomorphous, eutectic and peritectic diagrams so far
showed solid phases called terminal solid solutions which
existed over composition ranges near the concentration
extremities of the phase diagram.
 We can also find intermediate solid solutions or intermediate
phases at other than the two composition extremes
 For some systems, discrete intermediate compounds rather
than solid solutions may be found on the phase diagram.
These are called intermetallic compounds.
 This can be defined as a compound made up of 2 or more
elements, producing a new phase with its own composition,
crystal structure and properties. Intermetallic compounds are
almost always very hard and brittle
 There are 2 types of intermetallic compounds which are often
encountered
o Electron Compounds
 These compounds are of definite chemical crystal
structure and arise if the two alloying metals are
of different crystal structure, valency and if one of
these metals is electropositive with the other
being electro-negative
o Interstitial compounds
 These compounds form between metals, or metals
and non-metals, with atoms very similar to those
that form interstitial solid solution.
 The presence of an intermetallic splits the phase diagram so
that each part can be treated separately as individual
diagrams
o Stoichiometric intermetallic compounds
 Have a fixed composition and are represented by
a vertical line in the phase diagrams
o Non-stoichiometric intermetallic compounds
 Have a range of compositions and are the
intermediate solid solutions mentioned earlier
3.2 Solid State Transformations
 The Eutectoid Reaction

o Three phase reaction in which one solid phase
transforms to 2 different solid phases. (insert diagram +
equations)
 The Peritectoid Reaction
o 3 phase reaction in which 2 solids combine to form a
third solid on cooling ( insert diagram + equations)
3.3 Summary on the types of phase diagrams
 All the possibilities which exist when an alloying element is
added to the base metal
o Can be fully soluble, partly soluble or insoluble in the
liquid state (we will only be dealing with full solubility in
the liquid state)
o When fully soluble in the liquid state, it can remain fully
soluble in the solid state, become partly soluble or
completely insoluble on solidification.
o Similar changes can occur in the solid state, when a
solid solution breaks down (eutectoid and peritectoid)

3.4 The Iron Carbon Phase Diagram
 3.4.1. Polymorphism and Allotropy
o Polymorphism is a physical phenomenon where a
material may have more than one crystal structure
(BCC, FCC & HCP). A material that shows polymorphism
exists in more than one type of space lattice in the solid
state.
o If the change in structure is reversible with temperature
or pressure then the polymorphic change is known as
allotropy. The prevailing crystal structure depends on
both the temperature and the external pressure.
o Allotropes of Iron
 Iron too can exist in more than one crystal
structure and the change is reversible. The
different allotropes of iron are designated with
Greek letters.





One way of demonstrating these changes is to
heat an iron wire and to observe the changes in

length. [graph]
 Low temperature iron is BCC and this form is
called ALPHA iron (α-Fe)
 At 910 degrees Celsius, α-Fe changes to GAMMA
iron (γ-Fe) which is FCC. This change causes
contraction, since the FCC is close-packed, whilst
the BCC form is not. This change from BCC to FCC
therefor causes contraction.
 Further heating causes uniform expansion until at
1400 degrees Celsius, the FCC γ-Fe, reverts to the
BCC form with a sudden expansion. This high
temperature form is DELTA iron (δ-Fe) and it is
stable up to the melting point of iron (1537
degrees Celsius.)
 The reverse occurs upon cooling
 Iron alloys are the most important and most
widely applied metallic materials in industrial
practice.
 Iron alloys contain Iron as the fundamental or base
metal and always contain carbon which is
considered as the basic alloying element of iron
alloys. They can be called either steel or cast iron,
depending on the amount of carbon present in the
alloy.
 The iron alloys also contain several other elements
in smaller quantities, including manganese,
silicon, sulphur and phosphorous.
 These elements are found in a limited amount,
and do not influence considerably the equilibrium
diagram Fe-C. Therefor the metallographic
concepts and processes connected to iron alloys
can be studied in the two component Fe-C
equilibrium diagrams
3.4.2 Solubility of Carbon in Iron
o Interstitial solid solution of iron and carbon

Iron forms interstitial solid solutions with carbon.
The amount of iron that iron can dissolve is
different for the different allotropes of iron.
 The alpha iron is almost pure iron since it is only
capable of dissolving 0.022% carbon in the
equilibrium state. This solid solution is called
FERRITE.
 Gamma iron can dissolve 2.14% carbon at a
temperature of 1147 degrees. FCC can dissolve
more carbon because of its empty space in the
middle of the crystal structure. This solid solution
is called AUSTENITE.
Interstitial metallic compounds between iron and
carbon.
 The interstitial compound of Fe & C is Fe3C. This is
called iron carbide or cementite. This is very hard
and rigid, it is practically non-deformable.
 By controlling the amount, size and shape of Fe 3C
we control the degree of dispersion strengthening
and the properties of steel
Phases in the Fe-Fe3C
There are 5 phases in the iron carbon phase diagram:
liquid, ferrite, austenite, delta-ferrite and the fifth phase
in which the insoluble carbon exists.
This may be the metastable interstitial compound Fe3C
or the stable form graphite. This diagram is in fact a
dual phase diagram.
Since complete stability is difficult to achieve, Fe 3C is
the more likely phase to form. However under certain
favourable conditions, the higher carbon alloys can form
graphite, giving the useful grey cast irons.
In practice, iron carbon alloys are used with a limited
carbon content, so the iron carbon phase diagram
extends only to the value of C=6.67%. This carbon
content corresponds to the Fe3C in the metastable
system. Therefor the right hand side of the boundary
line of the metastable phase diagram represents 100%
Fe3C
This equilibrium diagram is therefor often referred to as
the Fe- Fe3C system.
Cementite is a metastable phase, which on prolonged
heating breaks up into iron + graphite. Thus the
diagram cannot be called an equilibrium diagram but a
phase diagram.
Steel and Cast Iron Classifications
On the basis of the Fe- Fe 3C phase diagram, iron-carbon
alloys can be classified as:
 Up to 0.8%C – Hypo-eutectoid steels


o



3.4.3
o
o
o

o

o
o



3.4.4
o



 0.8%-2.0% - Hyper-eutectoid steels
 2.0% - 4.3% - Hypo-eutectic cast irons
 4.3% or more Hyper-eutectic Cast irons
o In practice plain carbon steels are classified as:
 Low Carbon or Mild Steels (up to 0.3%C)
 Medium carbon or midcarbon steels (0.3-0.7%C)
 High carbon or tool steels (0.7-1.4%C)
3.4.5 Reactions occurring in the Fe- Fe3C system
o Eutectic
 Occurs at T=1147 degrees between 2.06-6.67%C
 The two phase product of the eutectic reaction:
[austenite(γ)
+
cementite(Fe3C)]
is
called
ledeburite
o Eutectoid
 Occurs at T=723 degrees between 0.025-6.67%
 [Ferrite(α) + cementite(Fe3C)] is called pearlite
We can see that most of the pearlite is composed
of ferrite (88.7%). The Fe3C lamellae are in fact
surrounded by α. The pearlite structure therefore
provides
dispersion
strengthening

the
continuous ferrite phase is relatively soft and
ductile and the hard brittle cementite phase is
dispersed.
3.4.6 Mechanical Properties of Carbon Steels
o The mechanical properties of the crystalline
3.4.1 Polymorphism & Allotropy
3.4.2 Solubility of carbon in iron
3.4.3 Phases in the Iron Carbon Phase Diagram
3.4.4 Steel and Cast Iron Classifications
3.4.5 Reactions occurring in the Fe-Fe3C system
3.4.6 Development of microstructures in Fe-C alloys
3.4.7 Mechanical properties of carbon steels




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