FMP 221 Lecture 4

Lecture - 4Lecture - 4

Allotropic forms of ironAllotropic forms of iron Iron - Carbon equilibrium diagramIron - Carbon equilibrium diagram

AllotropyAllotropy - - Elements that occur in more than one Elements that occur in more than one

crystallographic form or lattice form is called allotropy and crystallographic form or lattice form is called allotropy and

the metal in which such changes occur is known as the metal in which such changes occur is known as

allotropic. allotropic.

Allotropy is a specific form of polymorphism, Allotropy is a specific form of polymorphism, whichwhich is the is the

existence of a substance in more than one crystal form.existence of a substance in more than one crystal form.

The different crystal structures are known as polymorphs. The different crystal structures are known as polymorphs.

The term polymorphism is applied to The term polymorphism is applied to compoundscompounds as well as well

as to as to elementselements..

Iron represents perhaps the best-known example for allotropy.

There are three allotropic forms of iron, known as alpha, gamma & delta.

ALLOTROPIC FORMS OF IRON

DELTA IRON

GAMMA IRON

ALPHA IRON -Non-magnetic

APLHA IRON-Magnetic

Heating and cooling curves for pure ironHeating and cooling curves for pure iron

DELTA IRON (δ ) - As molten iron cools down it crystallizes at 1535°C into its delta allotrope, which has a body-centered cubic (BCC) crystal structure. GAMMA IRON (γ ) ) - It exists between 9100 C and 14010C and has a FCC structure. eg: Austenite .

ALPHA IRON ()- It has a BCC structure and occurs in two forms.

a. Ferro magnetic alpha solution – magnetic in nature.

b. Paramagnetic solution. It exists between 7700C and 9100c and is non- magnetic in nature.

Four main phases of steelFour main phases of steel

FerriteFerrite CementiteCementite

PearlitePearliteAusteniteAustenite

FerriteFerrite

Grains or crystals of solid Grains or crystals of solid solution of carbon in Alpha solution of carbon in Alpha iron as the solubility is iron as the solubility is negligible,negligible, ferrite is more close ferrite is more close to pure iron. It is soft and to pure iron. It is soft and ductile.ductile.

CementiteCementite

When carbon goes into definite When carbon goes into definite combination with iron, it forms iron combination with iron, it forms iron carbide (Fecarbide (Fe33C) which is called as C) which is called as

cementite. cementite. It contains 6.67 % It contains 6.67 % carbon and 93.33.% iron.carbon and 93.33.% iron.

PearlitePearlite

It is a mechanical mixture of about It is a mechanical mixture of about 87 % ferrite and 13 % cementite. 87 % ferrite and 13 % cementite.

Contains a Contains a 0.8% carbon 0.8% carbon and it is and it is mechanical aggregate of ferrite mechanical aggregate of ferrite and cementite. and cementite.

It is a constituent of steel It is a constituent of steel consisting of alternate layers of consisting of alternate layers of ferrite (alpha-iron) and cementite ferrite (alpha-iron) and cementite (iron Carbide Fe(iron Carbide Fe33C) and is formed C) and is formed

on cooling austenite at 723on cooling austenite at 723ooC. C.

This produces a This produces a tough structure tough structure and is responsible for the and is responsible for the mechanical properties of mechanical properties of unhardened steel.unhardened steel.

AusteniteAustenite

It is the solid solution of carbon It is the solid solution of carbon or iron carbide (Feor iron carbide (Fe33C) in gamma C) in gamma

iron having, up to 1.7% carbon. iron having, up to 1.7% carbon.

A nonmagnetic solid solution of A nonmagnetic solid solution of ferric carbide or carbon in iron, ferric carbide or carbon in iron, used in making corrosion-used in making corrosion-resistant steelresistant steel. .

Iron-Carbon Equilibrium Diagram

The iron-carbon (or iron carbide) equilibrium diagram concerns transformations that occur in alloys having compositions for pure iron to cementite (6.67 percent carbon).

Any point in the diagram represents a definite alloy at definite temperature.

The carbon contents are shown on the horizontal axis and temperature on the vertical axis.

Whenever an alloy is heated or cooled so that a line on the diagram is crossed, a phase change occurs.

The iron-carbon

equilibrium diagram

Melting point of pure iron

Solubility limit of carbon in γ iron at 1130ºC

Equilibrium diagram for combinations of carbon in a solid solution Equilibrium diagram for combinations of carbon in a solid solution

of iron. of iron.

The diagram shows iron and carbons combined to form Fe-FeThe diagram shows iron and carbons combined to form Fe-Fe33C at C at

the 6.67%C end of the diagram. the 6.67%C end of the diagram.

The left side of the diagram is pure iron combined with carbon, The left side of the diagram is pure iron combined with carbon,

resulting in steel alloys. resulting in steel alloys.

Three significant regions can be made relative to the steel portion of Three significant regions can be made relative to the steel portion of

the diagram. the diagram.

They are the eutectoid E, the hypo eutectoid A, and the They are the eutectoid E, the hypo eutectoid A, and the

hypereutectoid B. hypereutectoid B.

The composition of 2 % carbon is very important in the alloys The composition of 2 % carbon is very important in the alloys of iron and carbon. of iron and carbon.

An alloy containing less than 2 % carbon solidifies entirely as An alloy containing less than 2 % carbon solidifies entirely as austenite and is known as steel. austenite and is known as steel.

When the carbon content is above 2 % the alloy is known as When the carbon content is above 2 % the alloy is known as cast iron.cast iron.

Steels are classified into steel containing 0.77 percent carbon are known as eutectoid steel and steel containing less than 0.77 percent carbon are known as hypoeutectoid steel and those containing mores than 0.77 percent carbon are known as hypereutectoid steel.

The iron carbon equilibrium diagram can be divided into the following simple diagrams.

Peritectic transformation Alpha iron transforms into austenite

Eutectic transformation Austenite and cementite is formed from liquid

Solid solution Austenite is formed

Eutectoid transformation Pearlite is formed

Peritectic transformation – The Peritectic transformation takes

place at 1492ºC as shown by HJB.

, CoolingAlpha iron () + liquid Austenite

Heating

Peritectic transformation

Eutectic transformationEutectic transformationEutectic transformation takes place at 1130 ºC having 4.3 % Eutectic transformation takes place at 1130 ºC having 4.3 % carbon, when the liquid transforms into eutectic mixture of carbon, when the liquid transforms into eutectic mixture of cementite and austenite (Ledeburite). In the micro structure cementite and austenite (Ledeburite). In the micro structure austenite is visible at room temperature since it is stable upto austenite is visible at room temperature since it is stable upto 723ºC. 723ºC.

CoolingCoolingLiquid Liquid Austenite +CementiteAustenite +Cementite

HeatingHeating

Eutectoid transformationEutectoid transformation

Austenite containing 0.8 percent carbon as completely converted Austenite containing 0.8 percent carbon as completely converted to pearlite at 723ºC to pearlite at 723ºC

CoolingCoolingAustenite Austenite Ferrite +CementiteFerrite +Cementite

HeatingHeating

Upper critical temperature (point)Upper critical temperature (point) AA33

Upper critical temperature (point)Upper critical temperature (point) AACMCM

Lower critical temperature (point) ALower critical temperature (point) A11

Magnetic transformation temperature AMagnetic transformation temperature A22

Critical temperatures

Upper critical temperature (point)Upper critical temperature (point) AACMCM is the temperature, is the temperature, below which cementite starts to form as a result of ejection from below which cementite starts to form as a result of ejection from austenite in the hypereutectoid alloys.austenite in the hypereutectoid alloys.

ACM

Upper critical temperature (point) AUpper critical temperature (point) A33 is the temperature, is the temperature,

below which ferrite starts to form as a result of ejection below which ferrite starts to form as a result of ejection from austenite in the hypoeutectoid alloys.from austenite in the hypoeutectoid alloys.

Lower critical temperature (point) ALower critical temperature (point) A11 is the temperature of the is the temperature of the austenite-to-pearlite eutectoid transformation. Below this temperature austenite-to-pearlite eutectoid transformation. Below this temperature austenite does not exist.austenite does not exist.

Magnetic transformation temperature AMagnetic transformation temperature A22 is the temperature is the temperature

below which α-ferrite is ferromagnetic.below which α-ferrite is ferromagnetic.