lecture eutectoid steel 12-2007

7/27/2019 Lecture Eutectoid Steel 12-2007

http://slidepdf.com/reader/full/lecture-eutectoid-steel-12-2007 1/12

INGE 4001INGE 4001 -- Engineering MaterialsEngineering Materials

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


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.




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


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.




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


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




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.


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.




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)


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




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


Diagram for a

When the temperature of low, carbon diffusion isheavily compromised andano er ype otransformation takes place.




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


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



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.


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




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


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




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


ContinuousCooling

Transformation

Eutectoid Steel

The diagram is

produced without

but by tracking the

transformation

continuously in the

cooling media




CCT Curves:

Different

Cooling Media

Please, define

the critical cooling

rate CCR.

Sometimes the

hardenability of a

steel is measured


Quenching and TemperingHigh cooling rate during quenching

Tempering

temperature regulates

the final hardness andtensile strength




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


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

.

lecture eutectoid steel 12-2007

Documents