Heat treatment, phases, microstructures and its properties

Malaviya National Institute of Technology Jaipur
Submitted by
Basitti Hitesh
2017PMT5094Submitted to
Dr. V.K.Sharma
Heat Treatment, Phases, Microstructures and its properties

Phase
A Phase in a material in terms of its microstructure is a region that differs in
structure and/or composition from another region.
Phase diagram
Phase diagrams are graphical representations from which it is possible to know
number and types of phases present in a material at various temperature,
pressure and composition. The following important information can be obtained
from phase diagrams:
 Number and types of phases present at different compositions and
temperatures under slow cooling (equilibrium) conditions.
Equilibrium solid solubility of one element in another.
Temperature at which an alloy starts solidifying undergoing slow cooling and
also the temperature range within which solidification take place.
Temperature at which different phases start melting and regions of stability of
phases in terms of temperature and composition.

Gibb’s phase rule:
The Gibb’s phase rule states that P+F=C+2 where
F is the degree of freedom, P is the number of phases and C is the number
of components. Gibb’s phase rule gives an idea on degree of freedom that is
permissible for a given number of components and phases in equilibrium.
Application of Gibb’s phase
rule
Fig. Phase diagram of water system

At triple point of water system, three phases are in equilibrium and water is a one
component system.
Now by applying Gibb’s phase rule F=C-P+2, we can write, for invariant
points-
F=1-3+2, i.e., F=0
Hence for this system at triple point, degree of freedom is zero which means
none of the variables viz. temperature and pressure can be changed and still
keeps three phases of coexistence. So triple point is called an invariant point.
Since most binary phase diagrams mainly deals with temperature-composition at
constant pressure at 1 atm, Gibb’s phase rule expressed as P+F=C+1.

Concept of solid solution:
An alloy is a mixture of two or more metals or a metal and nonmetal. Alloys
may have simple as well as complex structure. Solid solution is a simple type of
alloy. A solid solution is solid that consists of two or more elements atomically
dispersed in a single phase structure.
In other words, solid solution can be defined as an alloy of two or more metals,
or a metal and a nonmetal which is a single phase atomic mixture. There are
two types of solid solution –
Substitutional Solid Solution
Interstitial Solid Solution

Substitutional solid solution: In this type, solute atoms of one element can
substitute parent solvent atoms of another element. In Cu-Ni solid solution, Cu
atoms can replace Ni atoms in crystal lattice. The crystal structure of parent
solvent may remain unchanged, nut lattice may be distorted by the solute
atoms. This happens when there is significant variation in atomic diameters of
parent solvent atom and solute atoms.

Hume- Rothery, an English Metallurgist studied various alloy systems
including isomorphous Cu-Ni system and formulated the following
conditions for two elements to have complete solid solubility:
1) The crystal structure of each element of the solid solution must be the
same.
2) The size of the atoms of each of two elements must not differ by more
than 15%.
3) There should not be appreciable difference in electronegativities of the
two elements.
4) The elements should have the same valence.

Interstitial Solid Solution: This type of solid solution is formed when solute
atoms enter into the interstices of solvent atom lattice. Hydrogen, Carbon,
Nitrogen and Oxygen atoms are common which form interstitial solid
solutions.

Construction of Phase diagrams

Heat treatment, phases, microstructures and its properties

The phase composition depending on the temperature and the carbon
content can be read off this dual diagram in which the stable system iron-
graphite (dotted lines) and the meta-stable system iron-carbide (solid lines)
are shown together.
The closeness of the equilibrium lines which correspond to one another
indicates that the difference in the stability of carbide and graphite in the
alloys is not large. Therefore, the carbon may be dissolved in the iron
after solidification or, however, may precipitate in the form of graphite.
Furthermore, it can also occur in the structure in a bound form as iron
carbide (Fe3C) and is dissolved in the α and γ solid solution in both systems.
The eutectic temperature of the iron-graphite reaction is considerably higher
than that of the iron-cementite reaction. In the case of slower cooling, mainly
graphite forms, in the case of accelerated heat dissipation, on the other hand,
mainly cementite. During annealing, graphite may form, reducing
the cementite content. The tendency for graphite to form
from cementite shows that iron or iron-rich solid solutions only form a stable
equilibrium with free carbon (graphite).
Nature of Duality in Fe-Fe3C phase diagram

Purpose of the iron-carbon phase diagram?
To know what will be the crystal structure and physical and chemical
properties of iron at known carbon percentage and temperature. provided
that slow and uniform cooling rate is there and no quenching.

Different phases present in Iron-Iron carbide system
‘γ’ phase or Austenite:
Interstitial solid solution of carbon in iron of FCC crystal structure having
solubility limit of 2.11 wt.% at 1147⁰C with respect to cementite. The stability of
the phase ranges between 727-1495⁰C and solubility ranges 0-0.77 wt.%C with
respect to alpha ferrite and 0.77-2.11 wt.% C with respect to cementite, at 0
wt.%C the stability ranges from 910-1394⁰C.
α - ferrite:
Interstitial solid solution of carbon in iron of BCC crystal structure (α-iron)
having solubility limit of 0.0218 wt % C at 727⁰C with respect to austenite.
The stability of the phase ranges between low temperatures to 910⁰C, and
solubility ranges 0.008 wt. % C at room temperature to 0.0218 wt%C at 727⁰C
with respect to cementite.
Pearlite:
This phase is a product of eutectoid decomposition of austenite into mixture of
ferrite and cementite (Fe3C).

Delta ferrite:
The high-temperature ferrite is labeled delta-iron, even though its crystal
structure is identical to that of alpha-ferrite. The delta-ferrite remains stable
until it melts at 1538 °C. Maximum solubility is 0.1 wt% C at 1495 °C.
Fe3C or Cementite:
Interstitial intermetallic compound of C & Fe with a carbon content of 6.67
wt.% and orthorhombic structure consisting of 12 iron atoms and 4 carbon
atoms in the unit cell. Stability of the phase ranges from low temperatures to
1227°C. This is a chemical compound of high hardness. In steel, it can be
associated with carbides of other elements, such as Mn.
Ledeburite :
Eutectic mixture of austenite and cementite is known as ledeburite.

Actually there is no difference in beta phase and alpha phase when we talk
about crystal structure of iron. Beta phase has the same structure as the
alpha phase. The only difference is the magnetic properties which are
absent in beta phase due to the expanded lattice parameter.
Why beta phase is not there in iron carbon phase
diagram?

Why is solubility of ‘C’ higher in FCC than in
BCC?
Austenite is having FCC (Face Centred Cubic) structure and Ferrite is
having BCC (Body Centred Cubic) structure.
There are 2 types of interstitial sites octahedral & tetrahedral. In
FCC, octahedral void is significantly larger than tetrahedral void. Whereas in
BCC these are nearly same.
The total open space is shared by more number of sites. Therefore
interstitial gap in BCC is much smaller than that of FCC. This is why carbon
which occupies interstitial site has higher solubility in austenite (FCC).

Peritectic reaction:
Liquid+Solid1↔Solid2
L (0.53wt%C) +δ (0.09wt%C) ↔γ (0.17wt%C) at 1495⁰C
Liquid(18.18wt%) +δ-ferrite (81.82 wt. %)→100 wt. % γ
Invariant reactions in Iron-Iron carbide phase diagram

Eutectic reaction:
Liquid↔Solid1+Solid2
Liquid (4.3wt%C) ↔ γ (2.11wt%C) + Fe3C (6.67 wt.%C) at 1147˚C
Liquid-100 wt.% →51.97wt% γ+Fe3C (48.11 wt.%)
The phase mixture of austenite and cementite formed at eutectic temperature
is called ledeburite.

Eutectoid reaction:
Solid1↔Solid2+Solid3
γ (0.77wt%C) ↔ α(0.0218 wt.%C) + Fe3C(6.67 wt.%C) at 727 ⁰ C
γ (100 wt.%) →α(89 wt.% ) +Fe3C(11 wt.%)
Typical density: α-ferrite=7.87 g/cm3
Fe3C=7.7 g/cm3
Volume ratio of α-ferrite: Fe3C=7.9:1

Sometimes the letters c, e, or r are included:
• Accm — In hypereutectoid steel, the temperature at which the solution of
cementite in austenite is completed during heating.
• Ac1 — The temperature at which austenite begins to form during heating, with
the c being derived from the French chauffant.
• Ac3 — The temperature at which transformation of ferrite to austenite is
completed during heating.
• Aecm, Ae1, Ae3 — The temperatures of phase changes at equilibrium.
• Arcm — In hypereutectoid steel, the temperature at which precipitation of
cementite starts during cooling, with the r being derived from the
French refroidissant.
• Ar1 — The temperature at which transformation of austenite to ferrite or to
ferrite plus cementite is completed during cooling.
• Ar3 — The temperature at which austenite begins to transform to ferrite during
cooling.
• Ar4 — The temperature at which delta-ferrite transforms to austenite during
cooling.

 A1 = Temperature at which austenite begins to form during heating
 A2 = Temperature at which alpha iron becomes non-magnetic
 A3 = Temperature at which transformation of alpha iron to austenite is completed
during heating
 A4 = Temperature at which austenite transforms to delta ferrite
 Am = Temperature at which solutionizing of cementite in austenite is
complete
Critical Temperatures

Phase transformations may be classified according to whether or not
there is any change in composition for the phases involved. Those for
which there are no compositional alterations are said to be congruent
transformations. Conversely, for incongruent
transformations, at least one of the phases will experience a change
in composition. Examples of congruent transformations include
allotropic transformations and melting of pure materials. Eutectic and
eutectoid reactions, as well as the melting of an alloy that belongs to an
isomorphous system, all represent incongruent transformations.
Congruent Phase Transformation

 The microstructure of crystalline materials is defined by the type, structure,
number, shape and topological arrangement of phases and/or lattice
defects .
 Elements of microstructure: Point defects, point-defect clusters,
dislocations, stacking faults, grain boundaries, interphase interfaces are
important elements of the microstructure of most materials.
Definition of Microstructure

 When Fe-alloy of 0.77% of C is cooled
slowly it transforms from single phase
of austenite to pearlite structure, a
lamellar or layered structure of two
phases: ferrite and cementite.
 In the micrograph, dark regions
are cementite and bright regions
are ferrite
Microstructure of Eutectoid steel

 Layered structures are formed
because of redistribution of C
atoms between ferrite (0.022 wt
%) and cementite (6.7 wt %) by
diffusion.
 Mechanical properties of
pearlite are in between that of
ferrite (soft) and cementite
(brittle)
Formation of layered structure !

Fig. Photomicrographs of (a) α ferrite and (b) austenite (325X)
(copyright 1971 by united states steel corporation)

Fig. Photomicrograph of a
eutectoid steel showing the
pearlite microstructure consisting
of alternating layers of α ferrite
(the light phase) and Fe3C (thin
layers most of which appear
dark). 500X. (Reproduced with
permission from metals
handbook, 9th edition, Vol.9,
Metallography and
Microstructures, American
Society for Metals, Materials
park, OH, 1985.)

Fig. Schematic representations of
the microstructures for an iron-
carbon alloy of hypoeutectoid
composition C0 (containing less
than 0.76 wt% C) as it is cooled
from within the austenite phase
region to below the eutectoid
temperature.

Fig. Photomicrograph of a 0.38 wt% C steel having a microstructure consisting
of pearlite and pro-eutectoid ferrite. 635X. (Photomicrograph courtesy of
Republic Steel Corporation

Fig. Schematic
representations of the
microstructures for an iron-
carbon alloy of hyper
eutectroid composition
C1(containing between 0.76
and 2.14 wt% C), as it is
cooled from within the
austenite phase region to
below the eutctoid
temperature

Fig. Photomicrograph of a
1.4 wt% C steel having a
microstructure consisting
of a white proeutectoid
cementite network
surrounding the pearlite
colonies. 1000X.
(Copyright 1971 by United
States Steel Corporation

Fig. Microstructure of DP steel

Temperature-Time Transformation diagram

MARTENSITIC TRANSFORMATION
• Martensite: austenite quenched to roomtemperature.
• Austenite to martensite does not involve diffusion no activation: athermal
transformation.
• Each atom displaces small (sub-atomic) distance to transform FCC ƴ-Fe (austenite) to
martensite, a Body Centered Tetragonal (BCT) unit cell (like BCC, but one unit cell axis
longer than other two).
• Martensite is metastable - persists indefinitely at room T: transforms to equilibrium
phaseson at elevated temperature
• Sincemartensite is ametastable phase, it does not appear in Fe-Cphase
diagram. The amount of martensite formed is a function of the temperature to
which the sample is quenched and not of time.
• The shear changes the shape of the transforming region:
→ results in considerable amount of shear energy
→ plate-like shape of Martensite

The martensitic transformation involves the sudden reorientation of C and
Fe atoms from the FCC solid solution of γ-Fe (austenite) to a body
centered tetragonal (BCT) solid solution (martensite)
Austenite to Martensite

Fig. Microstructure of
martensite transformation
in Fe-31wt%Ni-0.02wt%C
transformed by cooling into
liquid nitrogen. Micrograph
obtained by J. R. C.
Guimarães.

Martensite is so brittle it needs to be modified for practical applications. Done by heating
to 250-650oC for some time.
Temperedmartensite,extremely fine-grained,well dispersed cementite grains in a
ferrite matrix.
Tempered martensite is more ductile.
Mechanical properties depend upon cementite particle size: fewer, larger
particles means less boundary area and softer, more ductile material - eventual
limit is spheroidite.
Particle size increases with higher tempering temperature and/or longer time
(more carbon diffusion)
Tempered Martensite

 Tempered martensite is less brittle than martensite; tempered at 594°C.
 Tempering reduces internal stresses caused by quenching.
 The small particles are cementite; the matrix is α-ferrite. US Steel Corp.
4340 steel

 Bainite is a Plate-like microstructure that forms in steels at temperatures
of 250–550 °C (depending on alloy content).
 First described by E. S. Davenport and Edgar Bain, it is one of the
products that may form when austenite (the face centered cubic crystal
structure of iron) is cooled past a critical temperature.
 This critical temperature is 1000 K (727 °C) in plain carbon steels.
 Davenport and Bain originally described the microstructure as being similar
in appearance to tempered martensite.
 A fine non-lamellar structure, bainite commonly consists of cementite
and dislocation-rich ferrite.
 The high concentration of dislocations in the ferrite present in bainite
makes this ferrite harder than it normally would be.
BAINITE

Fig. Microstructure of Bainite
(a) Upper Bainite (b) Lower Bainite (c) Retained austenite

Heat treatment, phases, microstructures and its properties

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