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Hypoeutectoid Carbon Steels
Another example: Amount of carbon?
1035 Steel: whiteregions are pro-eutectoid ferrite
grains
By the end of this lecture you should be able to predict the amount of carbon in a
plain-carbon hypoeutectoid steel by just looking at a micrograph
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Hypereutectoid Carbon SteelsThe proeutectoid phase now is
cementite
Photomicrograph of a 1095 (plaincarbon) steel. Notice the network shape
Proeutectoid cementite tends to form in the parent austenite grain boundaries. This worsensthe brittleness of these steels even more. High carbon steels have limited applications.
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Example ProblemHomework: a) Determine the value x that allows you to obtain92 o total errite. b Determine the value x that allows you toobtain 30% proeutectoid ferrite
2.0The variation of proeutectoid ferrite andproeutectoid cementite according to
0.022 0. 806.67
e p ase agram s near rom eeutectoid composition
100% ProeutectoidFe3C
0%
ear e
steel and you need 10% proeuctectoid Fe3C
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So, why do I care about, ,
cementite?
• Because those proportions (or percent) regulate the mechanicalproperties of the steel.
• You can ick a chemicalcomposition that fits your steelneeds (according to your design):
–
– Hardness
– Toughness, etc.
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This is a brief (and very limited) classification of solid-solid phase transformations in crystalline engineeringmaterials:
-change of number of phases and their composition:
– Recr stallization
• Diffusion-controlled phase transformations withchange of number of phases and composition
– Isothermal transformations (eutectic, etc.)
• Difussionless or dis lacive transformations.
– Martensitic transformations
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We need to know the kinetics of diffusion-controlledp ase rans orma ons:Remember recrystallization. Thefraction of transformed hasefollows the Johnson-Mehl- Avrami (JMA) equation:
= 1 - ex -k·tn
The JMA model only describesthe phenomenon at one
tem erature.The inverse is the transformationtime to achieve 50% (or 0.5 infraction) of the transformation is“the rate of the transformation:”
r = t -10.5 This is the inverse of the maximum slope
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also depends on temperatureaccording to an Arrheniusequation:
r = r o exp (-Q/RT)
again Q is the activation energy
for the transformation.
emem er: e recrys a za on
rate is an example of the
application of an Arrhenius
equation.
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Plotting the same data as a function of the amount of phaserans orme we o a n one curve a eac empera ure:
Each curve follows
the JMA equation:
y = 1 - exp (k· tn)
Note that there is a “nucleation time” too: each transformation
doesn’t start from t = 0. It takes some time for the transformation
to start.
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Microstructure and Property Changes in Fe-C Alloys
’e s app y ose ne c mo e s o rans orma on n s ee s.
Remember the definition of heat treatment :
con ro e ea ng an coo ng cyc e or cyc es n en e o a us e
microstructure and mechanical properties of a material for a specificpurpose
Examples: annealings, normalizing, quenching and tempering, etc.
First we’ll perform an isothermal annealing in a eutectoid plain carbonsteel. Let’s assume we austenitize a eutectoid steel and drop thetemperature just below the eutectoid temp: Te (this is the equilibriumtemperature for the eutectoid transformation)
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First, we will study a eutectoid steel annealed just below the eutectoid.
What if we choose
temperature?
Austenite will transform isothermallyinto earlite followin the JMA model
Remember lower temperatures→ lessdiffusivit → smaller lamellar s acin
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Now let’s trace the isothermal decomposition of austenite at lower temperatures
• Degree of instability
• Diffusivity
emem er a n a us on- r ven rans orma ons, e rac on otransformed phase follows the Avrami equation: y = 1 - e -k·tn
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Diagram for a
When the temperature of low, carbon diffusion isheavily compromised andano er ype otransformation takes place.
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A new metastable phase shows up: martensite.
It is the result of fast
cooling a steel starting
from an austenitic
microstructure.
Bain
Transformation As a result of the high cooling rate
carbon atoms cannot diffuse faster
.
Then they supersaturate the BCC
structure and promote the
formation of a BCT structure
supersaturated with carbon atoms
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Martensitic TransformationsThey are examples of displacive (diffusionless) transformations. They are not assistedby diffusion!
Steel martensite starts to form at a given temperature Ms and finish forming at another temperature Mf .
Types of
martensite inplain carbon
steels:
• Lath martensite
• Plate martensite
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INGE 4001INGE 4001 -- Engineering MaterialsEngineering MaterialsThe Role of Carbon in the Shape of Martensite BCT
Crystal Structure
FCC BCC BCT
Note the effect of carbon’
lattice parameters. That
means the unit cell volume is.
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Hardness and Strength of Fe-C MartensiteMartensite mechanicalproperties stronglydepend on the carbonlevel in the steel.
Strengthening mechanisms:
• High dislocation densities
in lath martensite• High dislocation densities
plus solid solution
twinning deformations in
plate martensite
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Properties of Individual
•
Microconstituents in Steel
– Yield strength 200 - 800 MPa – -
• Bainite
– Yield strength 800 - 1300 MPa
– Tensile strength 1300 - 1400 MPa
• Martensite
– Yield strength 500 - 1800 MPa
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Let’s go back to the TTT curves but now for a hypereutectoidp a n car on s ee w .
So, if it’s
hypereutectic what
is the proeutectoid
p ase
Can you see thedifference with
the eutectoid
curve c ose
to the eutectoidEutectoid steel
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Now let’s see a TTT curve for a hypoeutectoid plaincar on s ee w .
What is the
proeutectoid
hase?
Is it easy to
Eutectoid steel
martensite in
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ContinuousCooling
Transformation
Eutectoid Steel
The diagram is
produced without
but by tracking the
transformation
continuously in the
cooling media
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CCT Curves:
Different
Cooling Media
Please, define
the critical cooling
rate CCR.
Sometimes the
hardenability of a
steel is measured
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Quenching and TemperingHigh cooling rate during quenching
Tempering
temperature regulates
the final hardness andtensile strength
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In this plot, look at the effect of carbonin the final hardness of the tem ered
This image shows the effect of tempering temperatures and times in
steelsthe final hardness of a eutectoid steel
steels
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Let’s summarize what we’ve learned about hasetransformations in plain carbon steels:
B controllin the hase selectionprocess you can control the finalmechanical properties of a steel.
many uses of steel: cheap and
versatileNow, think that you can addmany elements to diversify
.