Engineering Alloys(Ferrous)

8/10/2019 Engineering Alloys(Ferrous)

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Engineering Alloys

Ferrous systems


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Ferrous Alloys

Allotropy of Fe and its Alloys

Fe-Fe 3C Phase Diagram

Reactions in Fe-Fe 3C Diagram


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Ferrous Alloys

Classification of Fe-Fe 3C Diagram

Definition of Structures

Types of Ferrous alloys


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AllotropyMultiple crystal structure of the samechemical composition is calledpolymorphism or allotropy.Example: Iron, Graphite, etc.

The different crystal structures existsat different temperatures andpressure.


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910

1400

1535

T C

Time

Cooling curve of Fe

Liquid

iron(BCC)

iron(FCC)

iron(BCC)

L


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The temperature at which theallotropic changes takes place in ironis influenced by alloying elements.

The most important alloying element

is carbon.

A portion of the Iron-Carbon system

is called Fe - Fe 3C diagram.


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Fe-Fe 3C Phase Diagram

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There are three horizontal lines whichcorresponds to three isothermalreactions.

The first reaction occurs at 1493 C iscalled Peritectic reaction:


Liquid + iron Austenite ( )Cool ing

Heating


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The second reaction occurs at 1150 C and 4.3%C is called Eutectic reaction:

The eutectic mixture formed is called

ledeburite .

The mixture is not stable at roomtemperature.


Liquid Austenite + Cementite( ) (Fe 3C)

Cool ing

Heating


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The third reaction occurs at 725 C and 0.8%C is called Eutectoid reaction:

The eutectoid mixture formed iscalled pearlite .

Austenite( )

Ferrite + Cementite( ) (Fe 3C)

Cool ing

Heating


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Classification of Fe-Fe 3C Diagram

Alloys containing less than 2%C areknown as steels .

Steels containing less than 0.8%C iscalled hypo eutectoid steels andgreater than 0.8%C is called hyper

eutectoid steels .

Alloys containing more than 2%C are

called Cast irons . NEXT


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1. FerriteFerrite is an interstitial solidsolution of carbon in BCC Iron.

It is stable over -273 C to 910 C inpure iron.

The maximum solubility at roomtemperature is 0.008wt% and0.025wt% at 725 C



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The solubility is limited due to the

size difference between carbon atom(0.19A ) and the void size (0.17A ).

It is the softest phase present in Fe-Fe 3C diagram.



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2. Austenite( )It is an interstitial solid solution ofcarbon in FCC Iron.

It is stable over 910 C to 1410 C inpure iron.

Maximum solubility of carbon is 2wt%at 1150 C. This is due to the void size

(0.52A

).



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3. CementiteIt is nothing but Fe 3C phase. The

crystal structure is orthorhombic .

The carbon content is 6.67wt%

It is the hardest phase in the Fe- Fe 3CDiagram.



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4. PearliteIt is a very fine plate like or lamellarmixture of ferrite and Cementite.

It is the eutectoid mixture containing0.8wt%C and is formed at 725 C onvery slow cooling.

The mixture has good strength andtoughness than soft ferrite and hardCementite.



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Ferrite

Fe3C

Microstructure of Pearlite


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5. MartensiteThis is a metastable structure occursdue to rapid cooling of austenite.

The crystal structure is BCT (Bodycentered tetragonal)

It is formed through displacivetransformation while cooling fromFCC austenite.



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The microstructure of martensite willlook like needles.

It is the hardest phase and itshardness depends on the carboncontent (i.e for higher hardness

higher the carbon content and viceversa).


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Types of Ferrous alloys

STEELS

CAST IRONS

WROUGHT IRON


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1. Plain carbon (PC) steels

2. Effect of impurities on PC steels

2. Alloy steels

STEELS


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Classification of PC steels

1. Low carbon steels ( 0 - 0.1%C)

2. Mild steels (0.15 - 0.25%C)

3. Medium carbon steels (0.3 - 0.7%C)

4. High carbon steels ( 0.7 - 1.4%C)

Classification of Steels


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It has carbon content up to 0.1%

They are soft, ductile, tough,machinable, weldable and nothardenable by heat treatment.

It is used in the form of cold rolledsheets.

Low carbon Steels


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Its microstructure consists of smallamount of pearlite.

Has Strength between 300 370 MPa and %elongation of 28 40.

Used in automobile and refrigeratorbodies, tin cans, corrugated sheets,

nails, welding rods, fan blades, etc.

Low carbon Steels

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This steel has carbon contentbetween 0.15 0.25%.

It is used in as rolled and air cooledconditions.

Microstructure consists of 25% offine pearlite and remaining ferrite.

Mild Steels


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Strength between 400 - 450 MPa.

% elongation between 26

30.

Used as structural steels, building

bars, grills, ship hulls, boilers, oilpipelines, beams, angles, etc.

Mild Steels

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They contains carbon contentbetween 0.3 0.7%.

They are medium hard, not soductile and malleable, mediumtough, slightly difficult to machineweld and harden.

They are also calledmachinery steels.

Medium carbon Steels


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They are used as:

1. Agricultural implements

2. Rail structures

3. Spring steels



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Agricultural implementsIt has carbon content between 0.3 0.5%.

The agricultural implements areused in hot forged and air cooledforms.

Microstructure consists of fineferrite pearlite mixture.



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Spring steelsCarbon content is between 0.5 0.65%.

They are quenched and tempered to havea high yield strength so that resilience( y /2E) of the steel is increased, because

the elastic modulus cannot be increased.

Tempering can increase the yield

strength about 1500 MPa.


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Carbon content between (0.7 1.4%C)

They are hard, wear resistant,brittle, difficult to machine, difficultto weld and can be hardened by

heat treatment.

These steel cannot be cold worked

and are hot worked.

High carbon Steels


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They are also called tool steels.

They are used for applications likeforging dies, punches, hammers,files, drill bits, razor blades, knives,

ball bearings, etc.

High carbon Steels

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1. Sulphur

In commercial steels it is kept below0.05%.

Because sulphur combines withiron to form iron sulfide ( FeS ).

Effect of impurities in Steels


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Iron sulfide forms a lowtemperature eutectic with ironwhich tends to concentrate on grain

boundaries.

When steel is hot worked at

elevated temperatures the steelbecomes brittle due to the meltingof the iron sulfide eutectic.This iscalled hot shortness.



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2. Manganese

It is kept between 0.03 to 1% in allcommercial P.C steels.

In the presence of manganese sulfertends to form manganese sulfide (MnS) rather than FeS.



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The MnS may pass out in the slag orremain as well distributed inclusionthroughout the structure.

When there is more manganesepresent than the amount required toform MnS , the excess combineswith the carbon to form Mn 3C whichis found associated with Fe 3C.



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3. Phosphorous

It is generally kept below 0.04%.

Small quantities dissolves in ferrite

an increasing the strength andhardness slightly.



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In large quantities, phosphorousreduces ductility, there byincreasing the tendency of the steelto crack when cold worked. This iscalled cold shortness.



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4. SiliconMost commercial alloys containbetween 0.05 to 0.3%.

Silicon dissolves in ferrite,increasing the strength of the steelwithout greatly decreasing ductility.

It promotes the de-oxidation ofmolten steel through the formationof SiO 2 .


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DEFINITIONAlloy steels are the steels containingother elements like Ni, Mn, Cr, W, Mo,

etc which are added to plain carbonsteels for enhancement of their one ormore properties.

They are classified into low alloysteels ( 10%).

ALLOY STEELS

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REASON FOR ALLOYING

To increase hardanability.

To improve mechanical properties

either at low or high temperatures.

ALLOY STEELS


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Increase wear resistance.

Improve corrosion resistance.

Improve magnetic properties, etc.

ALLOY STEELS

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EFFECT OF ALLOYING ELEMENTS

1. Solid solution strengthening.

2. Formation of carbides.

3. Formation of intermediatecompounds.

ALLOY STEELS


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ALLOY STEELS

4. Formation of inclusion.

5. Influence on critical temperaturesand eutectoid composition.


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Solid solution strengtheningMost of the alloying elements aresoluble in ferrite to some extendand form solid solutions.

The strengthening effect of P, Si,Mn, Ni, Mo, V, Cr, etc are indecreasing order.

ALLOY STEELS


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ALLOY STEELS

Alloying element(wt%)

H a r d n e s s

P SiMn Ni

Mo

VCr

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Formation of carbidesSome of the alloying elementscombine with carbon in steel and

form respective carbides.

They increase wear resistance.

In the absence of carbonconsiderable amount dissolve inferrite.

ALLOY STEELS


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Mn, Cr, W, Mo and V are examplesof carbide forming elements.

Chromium and vanadium areoutstanding in hardness and wearresistance.

ALLOY STEELS

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Formation of intermediate compounds

Some of the elements formintermediate compounds with ironlike Fe 3W2, FeS, etc.

These phases increases thebrittleness of steel.

ALLOY STEELS

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Formation of inclusions

Some elements may combine withoxygen and forms oxides whenadded to steel

Example: Si, Al, Mn, Cr, etc.

ALLOY STEELS

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INFLUENCE ON CRITICAL TEMPERATUREAND EUTECTOID CARBON

Some elements like Ti, Mo, Si, W, Cr, etc increases the critical temperature.

Other elements like Ni, Mn reduces thecritical temperature are called austenitestabilizers.

ALLOY STEELS


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ALLOY STEELS

Alloying element(wt%) C r i

t i c a l t e m p e r a

t u r e

Ti MoSi W

Cr

Ni Mn725 C


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Some elements like Mo, Cr, Si tendto reduce the austenite region asthe amount increases are calledferrite stabilizers.

All the alloying elements shift theeutectoid carbon content to lowervalues ( i.e, < 0.8%)

ALLOY STEELS


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They have high corrosion resistancein most of the usual environmentconditions, hence the name stainlesssteel.

High corrosion resistance is due tothe presence of Chromium which

forms a passive and self healingChromium oxide thin film on thesurface when exposed to oxidationenvironments.

STAINLESS STEELS


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The minimum amount of Cr in solidsolution should be >13% forsufficient corrosion resistance in

most general environmentconditions.

When Cr is added to steel itcombines with carbon (17 times theamount of Carbon) and formscomplex chromium carbides.

STAINLESS STEELS


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The remaining chromium goes inthe solid solution.

Therefore the Cr present in thesolid solution form is

= Total Cr (17 * %C)

STAINLESS STEELS


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Types of Stainless steels

Martensitic stainless steel

Ferritic stainless steel

Austenitic stainless steel

STAINLESS STEELS


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Martensitic stainless steelThey have Cr content in solidsolution form < 13%

( i.e, %Cr (17* %C) < 13% )

These steels show austenitic phase

at high temperatures and hence canbe hardened by martensitictransformation, hence the name martensitic stainless steel.

STAINLESS STEELS


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Therefore, these steels contains 12 -18 %Cr and 0.15 - 1.2 %C.

They are hard, wear resistant andmagnetic in character.

Used for springs, ball bearings,valves, razor blades, surgicalinstruments, etc.

STAINLESS STEELS

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Ferritic stainless steelThey have Cr content in solidsolution form > 13%

( i.e, %Cr (17* %C) > 13% )

Since Cr is a ferrite stabilizer as the

Cr content increases the ferritephase becomes stable over theentire temperature range and hencecalled ferritic stainless steel.

STAINLESS STEELS


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They cannot be hardened bymartensitic transformation.

Cold working can be done toincrease the strength and hardness.

These steels contains 13 - 27 %Cr and < 0.2 %C.

STAINLESS STEELS


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They have higher oxidation andcorrosion resistance compare tomartensitic stainless steel.

They are soft, ductile and magneticin character.

Used for vessels in chemical andfood industries, pans, pipes insugar industries, etc.

STAINLESS STEELS

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Austenitic stainless steelThis group contains at least a totalof 24% Cr, Ni & Mn and the amountof Cr alone is at least 18% with thecarbon content between 0.03 to0.25%.

The presence of austeniticstabilizers makes the austenitephase stable at room temperatureand hence called austenitic stainless steel.

STAINLESS STEELS


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The equilibrium phases in this steelis + + carbide at roomtemperature and homogeneous

austenite above 900

C , however,due to fast cooling only austenitephase is present at roomtemperature.

They are soft, ductile and non-magnetic in character.

STAINLESS STEELS


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Their corrosion resistance issuperior to other stainless steels.

They have low thermal conductivityand high coefficient of expansion.

Used for food and chemical plants,utensils, sanitary fittings, etc.

STAINLESS STEELS

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CAST IRON Definition

White cast iron

Malleable cast iron

Gray cast iron

Nodular cast iron


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Definition

Cast irons are the alloys of Ironand carbon with 2 to 6.67% C in it.

Most commercial alloys containcarbon only between 2.5 to 4%.

CAST IRON


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It cannot be forged, rolled, drawn orpressed into the desired shape.

They are formed by melting andcasting and hence the name cast iron.

CAST IRON

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1. White cast ironThey are hypoeutectic alloyspresent in the Fe-Fe 3C diagram.

The fracture surface of the whitecast iron looks white in colourhence the name white cast iron.

Carbon is present in the combinedform (Fe 3C).

WHITE CAST IRON


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The microstructure consisting ofdendritic network of Cementite(Fe 3C).

Due to the presence of large amountof Cementite, it is hard and wearresistant but extremely brittle anddifficult to machine.

WHITE CAST IRON


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WHITE CAST IRON

Microstructural Changes during cooling of a

hypoeutectic white cast iron

Liquid

Primary-Dendrites

Fe3CPearlite Eutectic

(a)

(d)

(b)

(c)


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At eutectic temperature, the liquidundergoes a eutectic reaction toform ledeburite ( i.e, Austenite +

Fe 3C).

At the eutectoid temperature, all theaustenite, primary ( pro-eutectic ) aswell as eutectic, transforms topearlite ( i.e, ferrite+ Fe 3C).

WHITE CAST IRON


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Therefore, the final microstructureconsists of dendritic areas oftransformed austenite ( i.e, pearlite) in a matrix of transformedledeburite (Fe 3C + pearlite).

WHITE CAST IRON


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Properties & applicationsIt is hard, wear resistant, extremelybrittle and difficult to machine.

Used in high wear resistanceapplications such as liners forcement mixtures, ball mills, etc.

Used for making malleable castiron.

WHITE CAST IRON

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2. Malleable cast ironThey are produced from white castiron by a malleablizing heattreatment.

The silicon present (1%Si) is lowenough to prevent the formation ofgraphite flakes during casting, butadequate to keep the subsequentannealing time within reasonablelimit.

MALLEABLE CAST IRON

MALLEABLE CAST IRON


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Heat treatment consists of heatingwhite cast iron to 900- 950 C andholding at this temperature for along time (24 hrs to several days) followed by cooling to room

temperature.

MALLEABLE CAST IRON

MALLEABLE CAST IRON


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MALLEABLE CAST IRON

Cooling900 C

Time

T e m p e r a

t u r e

Malleablizing Heat treatment Cycle

MALLEABLE CAST IRON


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At 900 C the structure consists ofaustenite and Cementite.

Cementite being a metastable phasedecomposes to austenite andgraphite with a long holding time.

The free carbon precipitates in theform of spheroidal particles calledtemper-carbon.

MALLEABLE CAST IRON


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MALLEABLE CAST IRON

Microstructure Malleable Cast Iron

Temper -carbonPearlite

MALLEABLE CAST IRON


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Therefore, the microstructure atroom temperature consists oftemper-carbon in the matrix of

pearlite.

However, the cooling rate is veryslow (or Si in white cast iron ismore ), the Cementite from pearlitemay also decomposes to give amatrix of temper-carbon in ferrite

matrix .

MALLEABLE CAST IRON

MALLEABLE CAST IRON


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Properties & applicationsThe temper carbon doesn t break upthe continuity of the tough ferrite (orpearlite) matrix, this results in higher

strength (700MPa) and ductility (10-15%) than gray cast iron.

More expensive than gray cast iron.

Used in automobile crankshafts, pipefittings, etc.

MALLEABLE CAST IRON

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GRAY CAST IRON


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3. Gray cast ironCast irons containing graphite in theform of flakes are called gray cast irons, because the fracture surface looks black(gray) in colour.

Due to the presence of high siliconcontent (2-3%Si), the alloy will formaustenite and graphite (in the form ofirregular elongated and curved flakes) at

the eutectic temperature.

GRAY CAST IRON


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GRAY CAST IRON

Graphite Flakes Pearliteor

Ferrite

Microstructure Gray Cast Iron

GRAY CAST IRON


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The microstructure consists of graphite

embedded in a matrix of ferrite or pearlite.

Gray cast iron is brittle ( % elongation )

due to the presence of graphite flakeswhich has sharp tips and act like internalcracks or stress raisers.

The properties of gray cast iron dependson the morphology and size of graphite

flakes and matrix

GRAY CAST IRON

GRAY CAST IRON


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Properties & applicationsExcellent machinability due to thepresence of graphite flakes.

It has excellent fluidity and takes themould impression quite well.

The wear resistance of gray castiron is very good, as the graphiteflakes acts as lubricant.

GRAY CAST IRON


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NODULAR CAST IRON


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4. Nodular cast iron

Spheroidal graphite iron ductile iron ornodular

iron is the cast iron which has thegraphite present in the form of tinyballs or spheroids.

The fairly high silicon content(2.5%Si) promotes graphitizationduring solidification.

NODULAR CAST IRON

NODULAR CAST IRON


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The modifier (i.e, magnesium andcerium) makes the growth rate of

graphite to be approximately same inall directions.

The graphite nodules in this iron ismore spherical than that of tempercarbon.

NODULAR CAST IRON


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NODULAR CAST IRON

GraphiteSpheroids Pearlite

Microstructure Nodular Cast Iron

NODULAR CAST IRON


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Properties & applications

It has tensile strength of (400-700MPa) and %elongation between 10-18.

It is the major engineering material asit combines the engineering advantageof steel with the processingeconomics of cast iron.

NODULAR CAST IRON

NODULAR CAST IRON


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Used in agricultural implements,automotive crankshaft, pistons,gears, chuck bodies, elevatorbuckets in mining, etc.

NODULAR CAST IRON

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WROUGHT IRON

Definition

Manufacturing process


WROUGHT IRON


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DEFINITION

Wrought iron is essentially a two-component metal consisting ofhigh-purity iron and slag ( slag iscomposed mainly of iron silicate).

WROUGHT IRON

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WROUGHT IRON


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Manufacturing processi. To melt and refine the base metal.

ii. To produce and keep molten aproper slag.

iii. To disintegrate the base metal andmechanically incorporate the slag ofdesired amount into the base metal.

WROUGHT IRON

WROUGHT IRON


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In the Byer

s process of manufacturingconsists of the following steps:

The raw materials such as pig iron,scrap, etc. are melted in cupolas.

The pig iron is purified to a highlyrefined state in a Bessemer converterand then transferred to the ladle of theprocessing machine.

WROUGHT IRON

WROUGHT IRON


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Iron silicate slag made by meltingtogether iron oxide and certainsiliceous materials in an open-hearthfurnace is poured into a ladle andmoved directly below the processingmachine.

The base metal kept at 1535 C ispoured at a predetermined rate into theladle containing the molten slag whichis at about 1260 C.

WROUGHT IRON

WROUGHT IRON


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Since the slag is maintained at lowertemperature than the base metal themetal solidifies rapidly.

Due to this rapid solidification thedissolved gases in the liquid isliberated in the form of smallexplosion and this explosion shattersthe metal into small fragments whichsettle at the bottom of the slag ladle.

WROUGHT IRON

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Since the iron is in the weldingtemperature the fragments sticktogether to form a sponge like ball

of iron globules coated with silicateslag.

This globules are passed throughgrooved rollers as a result they getconverted into puddle bars.

WROUGHT IRON

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WROUGHT IRON


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The carbon content is between 0.02 -0.03% and slag content between 1-3%.

It possess high resistance tocorrosion.

It is never cast, and all shaping is doneby hammering, pressing, forging, etc.

WROUGHT IRON

WROUGHT IRON


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Strength can be increased by coldworking and aging.

Owing to the nature of slagdistribution the tensile strength andductility is greater along the rollingdirection.

It is used in oil industries, shipbuilding under ground service

WROUGHT IRON

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Engineering Alloys(Ferrous)