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UNIT 6 (PHASE &PHASE

TRANSFORMATIONS)

What is a Phase?A phase is a homogenous, physically distinct and mechanically

separable portion of the material with a given chemical composition and structure.

Chemical Composition of Phases: It is the chemical composition of each phase in

the system In a system having more than one phase, each phase will have a unique

chemicalcomposition which will be different from each other, and will also be

different fromthe overall composition Not to be confused with overall

composition

Limitations to use of Phase Diagrams

Phase Diagrams are also known as Equilibrium Diagrams Rate of Transformation is missing

TTT (Time-Temperature- Transformation) diagrams are a complement to Phase Diagrams

What is a solid solution?

When foreign atoms are incorporated into a crystal structure, Whether in

substitutional or interstitial sites, the resulting phase is a solid solution of the

matrix material (solvent) and the foreign atoms (solute)

Substitutional Solid Solution: Foreign (solute) atoms occupy normal lattice sites

occupied by matrix (solvent) atoms,

E.g. Cu-Ni; Ge-Si

Interstitial Solid Solutions: Foreign (solute) atoms occupy interstitial sites, e.g.,

Fe-C

Phase Transformations:

Phase transformations: change in the number or character of phases.

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Most phase transformations begin with the formation of numerous small particles of

the new phase that increase in size until the transformation is complete

For Iron transformation to occur, must cool to below 727C

IRON CABIDE DIAGRAM

Cooling Curve of Pure Iron

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TTT(TIME, TEMPERATURE AND TRANSFORMATION) DIAGRAM):

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Definition and Properties of Structures

Ferrite () An interstitial solid solution of carbon dissolved in BCC iron

Iron Iron Carbide Phase Diagram

-iron.

Carbon solubility 0.025 wt.% max. at 723 C, 0.008 wt.% min. at 0 C.

The softest structure that appears on the iron iron carbide diagram.

Average properties: 40,000 psi TS, 40 % elong. in 2 inch, < RC 0 or < RB 90 hardness.

Austenite ()

An interstitial solid solution of carbon dissolved in FCC -iron.

Carbon solubility 2.00 wt.% max. at 1130 C, 0.80 wt.% min. at 723 C.

Not stable at room temperature; can be made stable under certain conditions.

Average properties: 150,000 psi TS, 10 % elong. in 2 inch, RC 40 hardness,

high toughness.

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Cementite (Fe3C)

An interstitial inter metallic compound of iron carbide with an orthorhombic structure.

Its chemical formula is Fe C and contains 6 67 wt % carbon

Iron Iron Carbide Phase Diagram

Definition and Properties of Structures

Pearlite

The eutectoid mixture of fine plate-like lamellar mixture of ferrite and cementite.

Formed from austenite that contains 0.80 wt.% carbon during slow cooling at 723 C.

Average properties: 120 000 psi TS 20 % elong in 2 inch RC 20 hardness

Fe3C 6.67 wt.% carbon.

The hardest and brittle structure that appears on the iron iron carbide diagram.

Average properties: 5,000 psi TS, high compressive strength.

Ladeburite

The eutectic mixture of austenite and cementite.

Formed from liquid that contains 4.30 wt.% carbon during slow cooling at 1130 C.

Not stable below 723 C, where austenite of ladeburite transformed into pearlite.

The structure is then called transformed ladeburite.

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The IronIron Carbide (FeFe3C) Phase DiagramIn their simplest form, steels are alloys of Iron (Fe) and Carbon (C). The Fe-C phase

diagram is a fairly complex one, but we will only consider the steel part of the diagram,

Up to around 7% Carbon.

Phases in FeFe3C Phase Diagram-ferrite - solid solution of C in BCC Fe

Stable form of iron at room temperature.

The maximum solubility of C is 0.022 wt%

Transforms to FCC -austenite at 912 C

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-austenite - solid solution of C in FCC Fe

The maximum solubility of C is 2.14 wt %.

Transforms to BCC -ferrite at 1395 C

Is not stable below the eutectoid temperature

(727 C) unless cooled rapidly

-ferrite solid solution of C in BCC Fe

The same structure as -ferrite

Stable only at high T, above 1394 C

Melts at 1538 C

Fe3C (iron carbide or cementite)

This inter metallic compound is Meta stable, it Remains as a compound

indefinitely at room T, but decomposes (very slowly, within several years)

Into -Fe and C (graphite) at 650 - 700 C

Fe-C liquid solution

A few comments on FeFe3C system

C is an interstitial impurity in Fe. It forms a solid solution with , , phases of

iron Maximum solubility in BCC -ferrite is limited (max.

0.022 wt% at 727 C) - BCC has relatively small interstitial positions

Maximum solubility in FCC austenite is 2.14 wt% at 1147 C - FCC has larger

interstitial positions

Mechanical properties: Cementite is very hard and brittle -can strengthen steels.

Mechanical properties also depend on the microstructure, that is, how ferrite and

cementite are mixed.

Magnetic properties: -ferrite is magnetic below 768 C, austenite is non-magnetic

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