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CAST IRON� Cast iron - A ferrous alloys containing

� 2 < % C < 4 and 0.5 < % Si < 3

� Eutectic and Eutectoid reactions control the microstructure in

Cast Irons.

� Microstructure consists of ferrite and/or pearlite plus graphite flakes

2C (wt. % )

Cast IronSteel

Teutectoid

Teutectic

Cementite : Fe3C

Ferrite : α

Perlite : α + Fe3C

Ostenite: γ

Delta iron: δLedeburite.

γγγγ (ostenite)γγγγ+L L

δδδδ+L

Led

eburi

te

Per

lite

Iron Cementite Phase Diagram

≈1148oC

Tem

per

ature

(oC

)

In EngineeringCast iron

CE ≤≤≤≤ 4.3%

Figure 12.33 The iron-carbon phase diagram showing the relationship between the stable iron-graphite equilibria (solid lines) and the metastable iron-cementitereactions (dashed lines).

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Fe/C ve Fe/Fe3C Faz diyagramıFor steelsFor cast irons

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ADVANTAGES of CAST IRONS FOR DESING NEEDS

– High strength in compressive loading

– High brittleness

– Higher sliding wear resistance

– Good thermal conductivity

– Self lubrication

– Good machinability

– Vibration damping property

– Low cost

WEAKNESS FOR DESING NEEDS

– Low almost no ductility

– Low strength in tensile loading

IRON MAKING� Iron making (Blast furnace and Pig Iron)

� Couple furnace for cast iron

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Iron and steel making and Blast furnace

• The principle ores: (a)Hematite (Fe2O3),

(b)Magnetite (Fe3O4), (c) Siderite (FeCO3),

etc.

• Limestone: (CaCO3) to produce fluxes to

remove the impurities from the molten

metal and react with imruties forming fluid

fluxes and slag.

For 1 ton pig iron � 0,5 ton slug,

6 tons CO are produced

Pig

iron

Ore to Iron

•Coke: generates heat for chemical reaction

and produce CO to reduce iron oxide to

iron.

• Hot air: supplies oxygen to burn the

coke and produce CO.

•Air used is preheated by using CO leaving

the furnace thus necessary coke was

decreased about 70%.

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•Slug: taken at the intervals of 5-6 hrs. Later

used in making cements, fertilizer,

pavements, road ballasts, building materials,

insulations.

•CO: first washed and then use to power the

preheating system of air and fueling the other

furnaces in the plant.

Reduction of the iron oxide:

F2O3 (ore) + 3C � 2Fe + 3CO

Fe2O3 (ore)+ 3CO � 3CO2

Pig iron + Scrap+Limestone + Coke + Hot air.

Production of reducing gas:

C (coke) + O2 � CO2

CO2 + C � 2CO

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Blast furnace

Limestone, CaCO3

Iron oreHematite

magnetite

limonateCoke

MiningCoal

Hot air

Casting of

pig iron

Couple furnace

Cast iron production

COFuel for heating of air

and other furnaces

SlagLater used in

Road ballast

Cements

Filler materials

Steel production

Electric Arc Furnace

Vacuum Arc Remelting

Basic Oxygen Furnace

Open Hearth Furnace (S-M)

Vacuum Degassing

Vacuum Induction

Melting

Electro-slag Refining

Crushing

Palletizing

sintering

Processed to

change to coke

Carried with ladles (small cars)

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Continues Casting (Strand casting)

•First introduced in 1960s.

•Eliminates the Ingot teeming

process.

•Pouring the molten steel into a

water cooled mold with a hole in the

desired shape: rounds, rectangles,

etc.

•Usually cut into lengths and sold in

this form or rolled into bars, shapes

or sheets.

CAST IRON MICROSTRUCTURES

AND

CAST IRON TYPES

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Cast Irons

� Types of cast irons:

�Gray cast iron

�White cast iron

�Malleable cast iron

�Nodular (Ductile) cast iron

�Compacted graphite

(vermicular) cast iron

�Chilled cast iron

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The property of materials

depends on the microstructure

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GRAIN SHAPE AFTER SOLIDIFICATION DURING CASTING

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Nodular cast ion:

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™

is a trademark used herein under

license.

Figure 12.35 (a) Sketch and (b) photomicrograph of the flake graphite in gray cast iron (x 100).

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Effect of Cooling Rate on the

Cast Iron Microstructure• Different microstructures form depending on the cooling

rate from molten state,

• Fast cooling (not quenching) cementite formation (no time for graphite formation).

• Very slow cooling rates: graphitization due to decomposition of cementite into ferrite and graphite or pearlite/ferrite and graphite.

The Matrix:

• Slow cooling rates: γ�α + graphite

• Annealing: slow (furnace) cooling leading ferritic matrix.

• Normalizing: air (faster) cooling leading pearlitic matrix.

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• Depending on the cooling rate, different cast irons.

– Slow cooling: Gray cast iron.

• Ferritic,

• Ferritic/pearlitic,

• Pearlitic.

– Fast Cooling: White cast iron.

• Malleable (Tempered) cast iron: Annealing Heat treatment of White cast iron.

– Nodular cast ion: with the addition of spherodizing agents, such as Mg (and Ce).

– Compacted graphite (vermicular) cast iron: Shape of the graphites in between flakes and spheres.

Cooling rate and Types of Cast Iron

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™

is a trademark used herein under license.

Figure 12.34 The transformation diagram for austenite in a cast iron.

True equilabrium

Metastable

Quenching and tempering: Tempered martensite.

Austempering: Bainitic matrix or surface hardening.

• Si is a graphite stabilizing agent.

• Cr and Bi, cementite stabilizing agents.

• Si reduces the amount of C in the eutectic composition.

• Therefore, Si content evaluated in Carbon Equivalentconcept.

Effects of Alloying Elements

on the Cast Iron Microstructure

Matrix structure also effected by the composition of iron:

Si, graphite and ferrite stabilizer,

0.05% Sn and 0.5% Cu, pearlite stabilizer.

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)equivalent Carbon :(CE Si%3

1 C% CE% +=

Carbon equivalent and Si

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• Interconnected graphite flakes connected where the nucleation

originated: nucleation sites.

• The graphite flakes behaves like small cracks in the cast iron

causing stress concentration. Therefore low tensile strength and

brittleness with an elongation of 1% or less.

• As the flake size (finer graphite) decreases tensile strength

increases. The finer the graphite is, the stronger the cast iron is.

• At lower CE, the nominal strength is higher.

Notch effect

causes stress

concentration at

the sharp edges of

the graphite flakes

Gray cast iron

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αααα+Fe3C

γγγγ+Fe3C

γγγγ

γγγγ+S

S

S+Fe3C

Lede

burite

Tra

nsfo

rmed

Lede

burite

����

����

����

����

1

2

3

4

Cast iron

•Fast cooled structure: White cast iron. •Due to cementite: hard andbrittle.

Eutectic reaction:

Liquid ⇒ ledeburite microstructure

(γ+Fe3C)

White cast iron

1

2

3

4

Liquid

γ

Eutectic

Fe3C

Proeutectic γ

Eutectic γ

Eutectic Fe3CEutectoid

Perlite

Dr.C.Ergun, Asst.Prof

Mak214-E

� Microstructure consits of cementite network and

pearlite

� Extreme brittleness

� High hardness

� Wear resistance,

� Can not machined by tools, only by grinding

� An intermediate product for malleable iron

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White cast iron

Dr.C.Ergun, Asst.Prof

Mak214-E

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Inoculation:

� Mg is a carbide stabilizer in other words white iron forms.

� Inoculation with ferrosilicon alloys (50 - 85 % Si) and small

amounts of Ca, Al, Sr or Ba.

� So nucleation sites for graphite to grow is provided by inoculation

of molten metal.

� Also this effect fades with time.

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• Addition of Mg (or Ce) to high CE liquid iron to formspehroidal graphite during solidification.

• Best ductility.

• Steps:

�Desulfurization: S flake stabilizer.

�Nodulizing.

� Inocculation

Ductile or Nodular Cast Iron

BEST DUCTILITY

and TOUGHNESS

Ferritik-Perlitik KDD

Artan Soğuma Hızı

Ferritik KDD Perlitik KDDKüresel grafit

Perlit

α

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Desulfurization: S flake stabilizer. So for low S is needed.

� High quality starting material;

� Melting in furnace to remove S;

� Mix the liquid iron with desulfurizing agents, calcium

carbide.

Nodulizing:

� Add Mg near 1500oC to spherodize the graphite (also

remove S and O) in the molten metal,

� Residual 0.03% Mg is enough for nodulization.

� Since Mg vaporizes at 1150oC, ferrosilicon used to Mg recoveries.

� Fading (non violent vaporization or oxidation) of Mg

should be controlled with pouring the molten metal within

a few minutes, otherwise turns to gray iron.

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(a)Annealed ferritic ductile iron

(b)As-cast ferritic-pearlitic ductile iron

(c) Normalized perlitic ductile iron

with.

• Compared to gray iron, excellent strength, ductility and

toughness.

• Compared to malleable iron, higher ductility and strength, but

slightly lower toughness due to higher Si content.

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• Produced by heat treatment of white iron.

• During malleablizing, cementite dissolves and graphite clumps or

nodules looking like popcorn forms.

• Rounded graphite provides a combination of strength and ductility.

• Steps for mallebilization:

�Starting material: white cast iron with a CE of 3 %.

�First stage graphitization (FSG): at about 925oC to decompose

cementite into austenite and graphite. (Fe3C� γ + graphite)

�The austenite decomposes in the subsequent cooling from FSG

temperature with two different structures.

�Ferritic Malleable cast iron

�Pearlitic Malleable cast iron

Malleable cast iron

BETTER DUCTILITY

and TOUGHNESS

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(a)White cast iron prior to heat treatment.

(b)Ferritic malleable iron with graphite nodules and small MnS

inclusions.

(c) Pearlitic malleable iron drawn to produce a tempered martensite

matrix.

Ferritik TDD

Ferritik-Perlitik TDD

Artan Soğuma Hızı

Perlitik TDD

Temper grafiti

Perlit

α

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Ferritic Malleable Iron:

• The casting is cooled 5-15oC/h through the eutectoid temperature to second stage graphitization (SSG).

• The γ transforms to α and excess C diffuses to graphite nodules.

• Exceptional toughness.

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Pearlitic Malleable Iron:

• Cooled in air or oil.

• Pearlitic if air cooled, martensitic if oil quenched.

• Both hard and brittle.

• To improve ductility, the pearlitic malleable iron is drawn below eutectoid T.

Drawing process

• Tempers martensite or spheroidies the pearlite, thus reducing the amount of combined C or cementite.

• Thus the strength of pearlitic malleable iron decreases and ductility and toughness increase.

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• Graphite shape; intermediate between flakes and spheres.

• Nodulizing the molten metal with a residual Mg content of 0.015%

(<0.03%)

• May also form with fading of ductile iron.

• Compared to gray cast iron; better strengths and ductility good thermal

conductivity, and vibration damping characteristics.

• Similar treatment to that of ductile iron.

�Low S starting material,

�Inoculation to nucleate graphite.

�Pouring shortly after inoculation to prevent fading (loosing the

spheoridizing effect of Mg with time due to its evaporation).

Compacted Graphite(Vermecular)Cast Irons

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Chilled iron; surface white iron, center gray iron for low cost wear

resistant components.

The eutectic reaction:

• Solidification between two

eutectics: a graphite containing

cast irons: Such as gray, ductile or

compacted graphite iron.

• Solidification below the lower

eutectic temperature: white cast

iron.

• If solidification starts above lower

and finish below it, mottled iron, a

mixture of white and gray iron –not

desirable.

• The chill depth: tests

for CE measurements.

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f T

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mso

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earn

ing

, In

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Figure 12.36 The effect of the cooling rate or casting size on the tensile properties of two gray cast irons.

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Coding System

Yokes: BağlantıGear: Dişli

Drum: arka fren dış kabıHausing: gövde

Knuckle: oynak nokta

Cap:kapakHub: merkez kısım

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Gray Cast Iron

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Nodular Cast Iron

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Malleable Cast Iron