Hari Prasad

Presentation by

Hariprasad (Asst. Professor)

Steels

L+Cementite

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0.18

0.008

Pure iron, upon heating, experiences two changes

in crystal structure before it melts.

At room temperature the stable form, called

ferrite, or iron, has a BCC crystal structure.

Ferrite experiences a polymorphic transformation

to FCC austenite, or iron, at 9120C.

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This austenite persists to 1394oC, at which

temperature the FCC austenite reverts back to

a BCC phase known as ferrite, which finally

melts at 15380C

All these changes are apparent along the left

vertical axis of the phase diagram.

The 𝜹-ferrite is virtually the same as 𝜶-ferrite,

except for the range of temperatures over

which each exists.

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(a) Ferrite (90X) (b) Austenite (325X)

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Maximum solubility of carbon in various single

phases:

𝞭 − 𝑰𝒓𝒐𝒏(0.1%)

𝞬 − 𝑰𝒓𝒐𝒏(2.1%)

𝞪−𝑰𝒓𝒐𝒏(0.008%)

𝑭𝒆𝒓𝒓𝒊𝒕𝒆(0.025%)

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Room temp solubility

Stable at only high temp

The solubility of carbon varies in different formsof iron.

In delta iron, maximum solid solubility of carbonis 0.1%

In gamma iron, the maximum solid solubility ofcarbon is 2.03%

Austenite is a solid solution of carbon in FCCiron and solute atoms occupy interstitialpositions in this lattice.

In alpha iron, carbon has a limited solidsolubility of about 0.008% at room temperature

The maximum solubility of carbon in ferrite is0.025%.

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Description of the phases in Iron-Iron Carbide system

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Steels

L+Cementite

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• Interstitial solid solution of small amounts carbondissolved in BCC-Iron

• Max. solubility of carbon is 0.025% at 727oC and0.008% at room temp

• Softest among all the phases

𝞪-Ferrite

• Interstitial solid solution of carbon in FCC iron.

• Max solubility of carbon is 2.1%

• It is not stable below 727oC

• Non-magnetic

Austenite

• Interstitial solid solution of carbon in BCC iron

• Upon heating Austenite changes to BCC and gets𝞭-Ferrite

• Max solubility of carbon is 0.1%𝞭-Ferrite

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• This is also known as Iron Carbide (Fe3C)

• It’s a chemical compound because it contains afixed percentage of carbon (6.67%)

• It has an orthorhombic crystal structure (a≠b≠c)

Cementite

• It is the eutectic lamellar mixture of austenite(light phase) and cementite (dark phase)

• It is unstable below 727oC and transforms into𝞪-ferrite and cementite.

Ledeburite

• Austenite containing 0.8%C forms pearlite uponslow cooling below 727oC

• Its lamellar structure of 𝞪-ferrite and cementite.• It is very bright in appearance (like a pearl)

• It has a fingerprint like appearance

Pearlite

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There are three reactions which occur in iron –

cementite phase diagram.

1.Peritectic

2.Eutectic

3.Eutectoid

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Steels

E

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Peritectic reaction: In the alloy containing 0.18%

carbon, the initial crystals of delta solid solution

and the whole of liquid phase is completely

transformed to from austenite on cooling at

14930C

L+𝜹 𝜸 (austenite)

Eutectic reaction: alloy with carbon content

4.33%, the liquid is transformed into austenite

and cementite on cooling at 11470C

L 𝜸 (austenite)+cementite

cooling

heating

cooling

heating

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“The eutectic of austenite and cementite is known as

ledeburite”

Eutectoid reaction: in Iron – carbon alloy with

0.8% carbon, the austenite is transformed into

ferrite and cementite by eutectoid reaction on

cooling in 7270C

Gamma(austenite) alpha+cementite

The eutectoid of ferrite and cementite is known as

“Pearlite”

cooling

heating

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If the carbon % is less than 2.14, that type of alloy

comes under the category of steels.

If the carbon % greater than 2.14, that alloy

comes under the category of cast irons

STEELS:

There are major categories of steels.

i. Hypoeutectoid steels

ii. Hypereutectoid steels

iii. Eutectoid steels

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Steels with carbon content from 0.025% to 0.8%

are called hypoeutectoid steels.

Steels with a carbon content of 0.8% is known as

eutectoid steels

Steels with a carbon content greater than 0.8%

are called hypereutectoid steels

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Hypoeutectoid steels There are solid state transformations in this steels.

They are the transformation of gamma iron to alpha

iron and the decomposition of austenite.

The limiting composition for getting pearlite is

0.0025%C.

With carbon content less than this amount, no pearlite

will be formed. The alloy will contain only ferrite

grains.

Steels containing carbon between 0.025-0.8% would

contain varying amount of ferrite and pearlite and

their relative proportions depend on carbon content

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Schematic representations of the microstructures for an

iron–carbon alloy of hypoeutectoid composition

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Point C: At about 8750C, point c, the

microstructure will consist entirely of grains of

the 𝛾-phase, as shown in fig.

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Point D: about 775oC, which is within the 𝛼 +𝛾 phase region, both these phases will coexist as in

the schematic microstructure.

Most of the small particles will form along the

original 𝛾 grain boundaries

𝜶

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Micrograph of hypoeutectoid steel (0.34%C)

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Hypereutectoid steels

At eutectoid temperature, the composition of

austenite is 0.8% carbon

On further cooling, the entire amount of

austenite will transform to pearlite

Hence, the final microstructure consists of

pearlite and proeutectoid cementite

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Schematic representations of themicrostructures for an iron–

carbon alloy of hypereutectoid composition

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Micrograph of hypereutectoid steel

(1.34%C)

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Pearlite + proeutectoid cementite

Eutectoid steel

On cooling at eutectoid point (0.8%C-7270C), all

austenite will transform into 100% pearlite.

So, the microstructure at room temperature will

reveal alternate layers of ferrite and cementite,

called pearlite

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Schematic representations of themicrostructures for an

iron–carbon alloy of eutectoidcomposition (0.76 wt% C) above and

below the eutectoid temperature.

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The microstructure for this eutectoid steel that is

slowly cooled through the eutectoid temperature

consists of alternating layers or lamellae of the two

phases (𝜶 and Fe3C) tat form simultaneously during

the transformation

Point b, is called pearlite because it has the appearance

of mother of pearl when viewed under the microscope

at low magnifications

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Photomicrograph of a eutectoid

steel showing the pearlitemicrostructure consisting of Alternating layers of -ferrite (the light phase) and Fe3C (thin layers most of which

appear dark).

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Microstructure of a eutectoid steel The pearlite exists as grains, often termed

‘‘colonies’’

The thick light layers are the ferrite phase,

and the cementite phase appears as thin

lamellae most of which appear dark.

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The evolution of the microstructure of hypoeutectoid and

hypereutectoid steels during cooling, in relationship to the Fe-Fe3C

phase diagram

Steels

E

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Cast iron

Cast Irons Hypoeutectic cast iron

Eutectic cast iron

Hypereutectic cast iron

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Hypoeutectic cast iron

In this case, a structure below 11470C consists of

proeutectic austenite and ledeburite (eutectic

mixture consisting of austenite and cementite).

On further cooling, in the temperature range

11470C – 7230C, excess carbon comes out as

cementite from proeutectic and eutectic austenite.

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Cementite

Austenite

Within the temp range of 1147 to 727oC upon cooling

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Cementite network

Ferrite

When it cools to room temp

Eutectic cast iron (ledeburite

structure @1147oC)

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Austenite + Cementite

When the same cast iron is cooled to room temp, austenite

transforms to pearlite with cementite

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Pearlite formed from austenite Cementite

Hypereutectic Cast Iron In this case the structure just below 11470C

consists of proeutectic cementite and ledeburite.

On further cooling in the temperature range

11470C – 7270C, excess carbon comes out as

cementite from the eutectic austenite.

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Calculation of relative amounts of

phases in Fe-Fe3C diagram

The relative amount of proeutectoid ferrite and

pearlite in 0.2 percent carbon steel:

0.025%C 0.8%C0.2%C

X YZ

Percent of ferrite: 𝒀𝒁

𝑿𝒀𝟏𝟎𝟎 = (

𝟎.𝟖−𝟎.𝟐

𝟎.𝟖−𝟎.𝟎𝟐𝟓)100 =77.4%

Percent of Pearlite: 𝒁𝑿

𝑿𝒀𝟏𝟎𝟎 = (

𝟎.𝟐−𝟎.𝟎𝟐𝟓

𝟎.𝟖−𝟎.𝟎𝟐𝟓)100 =22.6%

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Calculation of relative amounts of

phases in Fe-Fe3C diagram

The relative amount of pearlite and cementite

in 1.2 percent carbon steel:

0.8%C 6.67%C1.2%C

X YZ

Percent of pearlite: 𝒀𝒁

𝑿𝒀𝟏𝟎𝟎 =

𝟔.𝟔𝟕−𝟏.𝟐

𝟔.𝟔𝟕−𝟎.𝟖100 =?%

Percent of cementite: 𝒁𝑿

𝑿𝒀𝟏𝟎𝟎 =

𝟏.𝟐−𝟎.𝟖

𝟔.𝟔𝟕−𝟎.𝟖100 =?%

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