Analysis of Carbon–iron (Fe–Fe3C) Phase Diagram
1. Experimental Objects
1) Observe the microstructures in carbon- iron alloys；2) Analyse the forming process of
iron-carbon alloys
2. Introduction 1) About the microscopy
magnified and inverted
Fig 1 Principle of Optical Microscopy
With optical microscopy, the light microscope is used to study the microstructure; optical
and illumination systems are its basic elements. For materials that are opaque to visible light,
only the surface is subject to observation, and the light microscope must be used in a reflecting
mode. Contrasts in the image produced result from differences in reflectivity of the various
regions of the microstructure. Investigations of this type are often termed metallographic, since
metals were first examined using this technique.
Normally, careful and meticulous surface preparations are necessary to reveal the
important details of the microstructure. The specimen surface must first be ground and
polished to a smooth and mirrorlike finish. This is accomplished by using successively finer
abrasive papers and powders. The microstructure is revealed by a surfacetreatment using an
appropriate chemical reagent in a procedure termed etching. The chemical reactivity of the
grains of some single-phase materials depends on crystallographic orientation. Consequently, in
a polycrystalline specimen, etching characteristics vary from grain to grain.
Also, small grooves form along grain boundaries as a consequence of etching. Since atoms
along grain boundary regions are more chemically active, they dissolve at a greater rate than
those within the grains. These grooves become discernible when
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viewed under a microscope because they reflect light at an angle different from that of the
grains themselves.When the microstructure of a two-phase alloy is to be examined, an etchant is
often chosen that produces a different texture for each phase so that the different phases may be
distinguished from each other.
Fig2 Fig 3
(a) Section of a grain boundary (a) Polished and etched grains as they and its surface groove produced by
might appear when viewed with an etching; the light reflection optical microscope. (b) Section taken
characteristics in the vicinity of from these grains showing how the the groove are also shown. (b) etching
characteristics and resulting Photomicrograph of the surface of surface texture vary from grain to grain a
polycrystalline specimen of an
because of differencescrystallographic
in
iron-chromium alloy in which the grain
orientation
boundaries appear dark.
100×
2)
Iron-Carbon alloys
Iron-carbon alloys are common engineering materials. The microstructure depends on both
the carbon content and heat treatment. The equilibrium microstructures of iron-carbon alloys
play an important roles on materials’ properties and phase transforming mechanisim. This
discussion is confined to very slow cooling of steel alloys, in which equilibrium is continuously
maintained.
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Classification and structure of iron-carbon alloys
Carbon
steels
Classification
CARBON WEIGHT（%）
Hypoeutectic steels
0.02—0.77
Eutectic steels
Hypereutectic steels
MICROSTRUCTURE OF
EQUILIBRIUM
Ferrite+pearlite
0.77
pearlite
0.77—2.11
Pearlite+cementite
Iron-carbon alloys only have two phases at room temperature,they are ferrite and
cementite. As the carbon content is quite different, the realitive weight percentage of the two
basic phases as well as their shape and distribution are different, which make them have
different microstructure. Various microscope structrue characters are discussed as follow:
a) Ferrite：
It is the solid solution which carbon inα-Fe, In the BCC ferrite, only small concentrations
of carbon are soluble, the maximum solubility is 0.022 wt% at 727℃and at room temperature,
the solubility is only 0.008wt%.Ferrite has low hardness and good ductility, it will show light
white in 4% nitric acid alcohol liquor. When the carbon weight increases, the microstructure
consists net shape ferrite around the pearlite.
b)Cementite:
The chemistrical form is Fe3C, which has a high carbon content of about 6.69wt%.It’s a
hard and strong phase. The color is light white in 4% nitric acid alcohol liquor.In hyperenutectic
steels, it is net shape around pearlite.
Both cementite and ferrite are light white in 4% nitric acid alcohol liquor.
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c) Pearlite
It is a mixture of cementite and ferrite.This microstructure is called pearlite because it has
the appearance of mother of pearl when viewed under the microscope at low magnifications.
The pearlite exists as grains, often termed “colonies”; within each colony the layers are oriented
in essentially the same direction, which varies from one colony to another. The thick light layers
are the ferrite phase, and the cementite phase appears as thin lamellae most of which appear
dark. Many cementite layers are so thin that adjacent phase boundaries are so close together that
they are indistinguishable at this magnification, and, therefore, appear dark.
In 4% nitric acid alcohol liquor, as different magnifications they have different characters:
at about 600X, both thick ferrite and thin cementite are light white colour and the boundary is
black;
At about 400X, the thin light white colour cementite will be covered by the black boundary
and the cementite will be black, at this time, pearlite is the thick light white colour ferrite and
black thin cementite;
At about 200X, the thick light white colour ferrite can not be observed as well and the
pearlite will be dark black colour.As the carbon weight in plain steel is low and the distance
between ferrite and cementite is small and even in large magnifications, the pearlite will be dark
black colony
3) Mechanical properties of various microstructure
Mechanically, pearlite has properties intermediate
between the soft, ductile ferrite and the hard, brittle
cementite. Table 2 Mechanical properties of ferrite,cementite
and pearlite
ferrite
cementite
peartite
Hardness（HB）
50—90
750—880
190—230
Tensilestrength（MPa）
190—250
30
Ductility（EL%）
40—50
0
860—900
9—12
4) Calculation the carbon weight in carbon steels
We can estimate the carbon weight by microscope. Firstly, estimate the different
microstructure’s areas, definely: n：Ferrite
area percentage K：Pearlite area
percentage B：Cementite area
percentage
According to Fe―Fe3C phase diagram, at room temperature ferrite has 0.008%wt of
carbon, pearlite has 0.77%wt of carbon and cementite has 6.69%wt of carbon, then using level
rule, we can know:
n × 0.008
To hypo-eutectoid steel : %C=
K × 0.77
+
100 100
B ×6.69 K ×0.77
To hyper-eutectoid teel: %C =+
100 100
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3. Experimental equipment and materials
1) Carbon steels specimen
2) Microscope
4. The experiment report requested
1) Observe the microstructure characters of samples in turn and draw the microstructures you
observed through the microscope.
2) Rank the sequence of four samples from high to low according to the carbon content.
3) Select one specimen and estimate the areas of the different phases.Caculate the carbon
content according to the lever-rule.
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Oberservation the microstucture of carbon steels
with heat treatment
1. Experimental Objects 1) Oberserve the microstucture of carbon steels with different heat
treatment 2) Understand the heat treatment effect on the microstructures of steels 3) Distinct
states and characters with common heat treatment
2. Experimental principles After annealing or normalizing, carbon steels
can get equilibrium or close to equilibrium microstructure.But
quenching is a nonequilibrium process, we should use not only
iron-carbon phase diagram but also C-curve to analyse the
microstructure after heat treatment,
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Carbon steels will show different microstructure as the composition and heat treatments are
different. The basic characters of the microstructure will discuss as follow:
a) pearlite
Pearlite is a mechanical mixture of ferrite and layers cementite.The thickness ratio of the
ferrite and cementite layers in pearlite is approximately 8 to 1. However, the absolute layer
thickness depends on the temperature at which the isothermal transformation is allowed to occur.
At temperatures just below the eutectoid, relatively thick layers of both the -ferrite and Fe3C
phases are produced; this microstructure is called coarse pearlite, At these temperatures,
diffusion rates are relatively high, such that carbon atoms can diffuse relatively long distances,
which results in the formation of thick lamellae.With decreasing temperature, the carbon
diffusion rate decreases, and the layers become progressively thinner. The thin-layered structure
produced in the vicinity of is termed fine pearlite.
b) Bainite
The microstructure of bainite consists of ferrite and cementite phases, Bainite forms as
needles or plates, depending on the temperature of the transformation; the microstructural
details of bainite are so fine that their resolution is possible only using electron microscopy.
c)Spheroidite
If a steel alloy having either pearlitic or bainitic microstructures is heated to, and left at, a
temperature below the eutectoid for a sufficiently long period of time—for example, at about
o
(700 C ) for between 18 and 24 h—yet another microstructure will form. It is called spheroidite.
the Fe3C phase appears as sphere-like particles embedded in a continuous phase matrix.
d)Martensite
Martensite is a nonequilibrium single-phase structure.Martensite grains take on a lath-like or
flake-like appearance.Plain steels (low carbon alloy steels) after quenching will be lath
martensite and high carbon steels (high carbon alloy steels) after quenching will be flake
martensite.And the middle carbon steels (middle carbon alloy steels) can gain the mixture of
two kinds of martensite.
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e)Retained austenite
When the carbon weight is larger than 0.5%, martensite can not transformed
completely and in the room temperature there will be retained austenite.Retained austenite
is difficult to be corrosive, in the microscope it distribute between martensite grains with light
white colour. Retained austensite and martensite can not be distinguished clearly because they
are both ligh white colour before tempering, only after tempering, retained austeniste can be
distinguished clearly
e) Tempered martensite
Tempering is accomplished by heating a martensitic steel to a temperature below the
eutectoid for a specified time period. Normally, tempering is carried out at temperatures between
o
o
250 C and 650 C. The microstructure of tempered martensite consists of extremely small and
uniformly dispersed cementite particles embedded within a continuous ferrite matrix.
Tempered at low temperature, the microstructure is black needle shape.
Tempered at middle temperature, ferrite will be almost keep the shape of fomer matensite
and cementite will show quite thin grains shape, it’s not easy to be distinguished using the
optical microscope.
Tempered at high temperature, there are particle cementite grains with equiaxed shape
assembled on the ferrite-base.
Table 3 different steels samples’microstructure after various heat treatment
Heat treatment processing
No.
Steel
1
2
3
800
4
5
750
6
750
7
8
9
1100
W1-1.0C
W1-0.8C
microstructure
Heat
temperature
（℃）
1045
W1-1.2C
Corrosive
solution
750
750
Air cooling
Oil quench
Water
quench
Water
quench
Water
quench
Water
quench
Water
quench
Spherodite
annealing
300 oC
isothermal
Water
quench
F+P
M+F+B
M
600
4% nitric
acid
200
200
alcohol
liquor
Tempered M
M+F
Tempered
M+Fe3C
Tempered
M+Retained A
Spherodite
Bainite
+M+Retained
A
Cooling
methods
Tempered
temperatur
e（℃）
3. Experimental equipments and materials
Samples and microscope
4. Experimental report requested 1) Oberserve the
microstructure of the specimen and draw the
microstructure of the specimen 2) Compare the
microstructure and properties difference between
martensite and tempered martensite.
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