1

INTRODUCTION

I ron , t he s i l ve ry -wh i t i sh me t a l , i s t he mos t impor t an t o f me t a l s s i nce i t

f o rms t he ba s i s o f t he spec t rum o f s t e e l s and ca s t i r on . Today i n

i ndus t r i e s s t e e l and ca s t i r on compr i s e we l l ove r 80% by we igh t o f Cas t

i r on and s t e e l . Pu re i r on* i s no t an ea sy ma t e r i a l t o p roduce . Pu re i r on

i s qu i t e so f t , weak and expens ive . I f c a rbon i s added i n c e r t a i n quan t i t y

i n i t , i t w i l l change i t s mechan i ca l p rope r t i e s . Acco rd ing t o c a rbon

con t en t we c l a s s i f i ed i r on ca rbon a l l oys i n to two ways :

1 . S t ee l (Les s t han 2 .11%)

2 . Cas t i r on (2 .11 -6 .67%)

Cas t i r ons a r e ba s i ca l l y i r on -ca rbon a l l oys hav ing ca rbon be tween

2 .11% and 6 .67%. The i ndus t r i a l c a s t i r ons have ca rbon no rma l ly i n

t he r ange o f 2 .11% to 4 .0%, a long w i th o the r e l emen t s l i ke s i l i con ,

manganese , su lphu r and phospho rus i n subs t an t i a l amoun t s .

Why cas t i ron has i t s name?

Highe r c a rbon con t en t makes t hem more b r i t t l e . Cas t i r ons a r e b r i t t l e ,

and canno t be fo rged , r o l l ed , d r awn , e t c . bu t c an on ly be ‘ c a s t ’ i n to

de s i r ed shape and s i z e by pou r ing t he mo l t en a l l oy o f de s i r ed

compos i t i on i n to a mou ld o f de s i r ed shape and a l l owing i t t o so l i d i fy .

Due to presence o f h igh carbon content in i t mach inab i l i t y i s poor so

cas t ing i s the on ly and exc lus ive ly su i tab l e proces s to shape these

a l l oys , known as Cas t i ron .

Cas t i r ons i s made by r eme l t i ng p ig i r on ( C-3 .5%,S i -1 .9%,S-0 .06%,

P -1 .00%,Mn- 0 .70%) o f t en a long w i th subs t an t i a l quan t i t i e s o f s c r ap

i r on and s c r ap s t e e l , and t ak ing va r i ous s t eps t o r emove undes i r ab l e

con t aminan t s such a s phospho rus and su lphu r . The me l t i ng un i t may be

cupo l a , e l e c t r i c a r c , and i nduc t i on fu rnace s e t c . The common ca s t i r ons

a r e b r i t t l e and have l ower s t r eng th p rope r t i e s t han s t e e l s .

* P u r e I r o n - I r o n c o n t a i n s 9 9 . 9 8 % a l p h a f e r r i t e i n i t . P u r e i r o n p i l l a r s w e r e

m a n u f a c t u r e d a n d s i t u a t e d i n D e l h i a r o u n d 1 2 0 0 A D .

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Cast iron are also classified according to metallurgical point of view

Hypo Eutectic cast iron (2.11-4.3% carbon)

Eutectic* Cast iron (4.3% carbon)

Hyper Eutectic cast iron(4.3-6.67% carbon)

Eutec t i c Cas t i ron- I n t he eu t ec t i c c a s t i r on , t he r e i s on ly one phase

( l i qu id ) o f Eu t ec t i c compos i t i on a t j u s t be fo re 1147 o C. And t h i s l i qu id

phase w i l l t r an s fo rmed i n to aus t en i t e and cemen t i t e phase s a t 1147 o C

by eu t ec t i c r e ac t i on .

L(4.3%) Austenite (2.11% C) + Cementite (6.67% C)

Hypo Eutec t i c Cas t I ron - I n t he hypo eu t ec t i c c a s t i r on , t he r e a r e two

phase s ( i . e . au s t en i t e , l i qu id o f Eu t ec t i c compos i t i on ) a t j u s t be fo re

1147 o C. And on ly l i qu id phase w i l l t r an s fo rmed i n to aus t en i t e and

cemen t i t e phase s a t 1147 o C by eu t ec t i c r e ac t i on . Aus t en i t e wh ich i s

p r e sen t above Eu t ec t i c t empe ra tu r e l i ne i s known a s p roeu t ec t i c o r

p r ima ry Aus t en i t e .

Hyper Eutec t i c Cas t i ron - I n t he hype r eu t ec t i c c a s t i r on , t he r e a r e

two phase s ( i . e . c emen t i t e , l i qu id o f Eu t ec t i c compos i t i on ) a t j u s t

be fo re 1147 0 C. And on ly l i qu id phase w i l l t r an s fo rmed i n to aus t en i t e

and cemen t i t e phase s a t 1147 0 C by eu t ec t i c r e ac t i on . Cemen t i t e wh ich

i s p r e sen t i n i t i a l l y i s known a s p roeu t ec t i c o r p r ima ry cemen t i t e .

Iron bridge, made of cast iron Cover of sewerage system *Eutectic comes from Greek word Eutectus which means “That can be easily melted”

Cooling

Heating

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Development of Cast Iron

In i t i a l l y , t he r e a r e two t ypes o f Cas t i r on ca l l ed Whi t e c a s t i r on and Grey ca s t i r on . I f c a rbon i s i n fo rm o f c emen t i t e t hen wh i t e c a s t i r on fo rms and i f c a rbon i s i n fo rm o f g r aph i t e t hen g r aph i t e c a s t f o rms . Whi t e c a s t i r ons have a l l t he c a rbon i n t he combined cemen t i t e f o rm ( f e r r i t e i s a s sumed t o pos se s s neg l i g ib l e c a rbon ) . Cemen t i t e i s a ha rd , b r i t t l e , wh i t e compound . The f r ac tu r ed su r f ace o f wh i t e c a s t i r on l ooks s i l ve ry -wh i t e due t o wh i t e c emen t i t e , and t ha t i s why the name whi t e cas t i ron i s g iven .

Graph i t e i s so f t , b r i t t l e and g r ay , and t hus , impa r t s g r ay co lou r t o t he f r a c tu r e . Cas t i r ons con t a in ing g r aph i t e ( a s f l ake s ) a r e t hus , ca l l ed gray cas t i rons . Unde r m ic roscop i c g r aph i t e f l ake s appea r a s i r r egu l a r s t r ands such a s ‘ co rn f l ake s ’ . As shown in F igu reu re .1

Gray cast iron: 1 (a) Space model of flake graphite 1(b) Unetched photo-micrograph of gray cast iron

From two o r i g ina l c a s t i r ons , wh i t e c a s t i r on i s ve ry b r i t t l e and

unmach inab l e a s i t i s ve ry ha rd due t o p r e sence o f ha rd and b r i t t l e

c emen t i t e and t hus f i nds ve ry f ew app l i c a t i ons . I t i s t he g r ay ca s t i r on ,

t he common commerc i a l va r i e t y mos t ex t ens ive ly u sed i n i ndus t ry ; due

t o i t s c e r t a i n spec i f i c p rope r t i e s . The compre s s ive s t r eng th and

ha rdnes s o f g r ay ca s t i r on a r e qu i t e h igh and ve ry c lo se t o t he

p rope r t i e s o f t he s t e e l o f s im i l a r compos i t i on and ma t r i x s t r uc tu r e .

Whi l e deve lop ing g r aph i t i c c a s t i r ons o f supe r io r p rope r t i e s r e su l t ed i n

fou r more t ypes o f c a s t i r ons c a l l ed meahan i t e i r on , compac t ed i r on ,

ma l l e ab l e i r on and S .G . i r on . The mic ros t ruc tu r e o f g r ay i r ons cons i s t s

o f g r aph i t e f l ake s embedded i n t he s t e e l ma t r i x , i . e . , i n va ry ing

p ropo r t i ons o f f e r r i t e and pea r l i t e . The p rope r t i e s o f Gray i r on a r e

de t e rmined by t he p rope r t i e s bo th o f t he ma t r i x , and t he amoun t , s i z e ,

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shape and d i s t r i bu t i on o f g r aph i t e i nc lu s ions . Graph i t e f l ake s have

weaken ing and embr i t t l i ng e f f ec t s , a s g r aph i t e i s so f t , powde ry , and

b r i t t l e , and can be cons ide red i n app rox ima t ion a s vo id s o r c r acks ,

b r eak ing t he con t i nu i t y o f duc t i l e ma t r i x .

The p rope r t i e s o f g r ay i r on a r e de t e rmined by t he p rope r t i e s bo th o f t he

ma t r i x , and t he amoun t , s i z e , shape and d i s t r i bu t i on o f g r aph i t e

i nc lu s ions . Acco rd ing t o t he r e g r aph i t e f l ake s cond i t i on , g r ay ca s t i r on

i s f u r t he r d iv ided i n to Meahan i t e c a s t i r on (by mak ing f l ake s f i ne r ) ,

S .G Cas t i r on ( round shaped f l ake s ) , Ma l l e ab l e Cas t i r on , Mo t t l ed Cas t

i r on , Ch i l l ed Cas t I r on e t c .

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Types of Cast Iron

The be s t me thod o f c l a s s i fy ing ca s t i r on i s ba sed on t ype o f m ic ros t ruc tu r e :

Two ma in t ypes o f Cas t i r on

I . White Cast I ron : Ca rbon i s i n fo rm o f Whi t e c emen t i t e

I I . Grey Cast I ron : Ca rbon i s i n fo rm o f Graph i t e f l ake s

These a l l o t he r c a s t i r on excep t g r ay and wh i t e c a s t i r on a r e made by spec i a l t r e a tmen t ( hea t t r e a tmen t and by mix ing chemica l compos i t i on ) t o enhance i t s p rope r t i e s .

I I I . Chi l led Cast i ron : Su r f ace l aye r s a r e o f wh i t e c a s t i r on w i th i n t e r i o r o f g r ey ca s t i r on .

IV . Mott led i ron(Mixed I ron) : The t r ans i t i on l aye r be tween Grey ca s t i r on and wh i t e c a s t i r on i n ch i l l ed i r on i s mo t t l ed ca s t i r on and cons i s t s no rma l ly o f g r aph i t e f l ake s

V. Meahani te I ron: Cas t i r on ha s ve ry f i ne f l ake s o f g r aph i t e due t o add i t i on o f c a l c ium s i l i c i de a s i nocu l an t i n me l t i n l ad l e o the rwi se i t wou ld ha s so l i d i f i ed a s wh i t e c a s t i r on .

VI. Mal leable I ron: These cons i s t s o f s t r uc tu r e o f i r r egu l a r l y round g raph i t e pa r t i c l e s c a l l ed t empe r c a rbon and s t ruc tu r e i s ob t a ined by hea t t r e a tmen t

VII . Spheroidal graphi te I ron s t ruc tu r e o f nodu l e s embedded i n s t e e l

ma t r i x , nodu l e s a r e o f more r egu l a r , shape and compac t sphe re s .

VII I . Compacted/Vermicular Cas t I ron: The g raph i t e he r e i s i n t e rmed i a t e be tween f l ake s and sphe re s numerous rods o f g r aph i t e . S t r eng th and duc t i l i t y i s g r ea t e r t han g r ay ca s t i r on

IX. Al loy Cast I ron: Prope r t i e s and mic ros t ruc tu r e o f c a s t i r on o r

any o r f t he se i s mod i f i ed by add i t i on o f a l l oy ing e l emen t s .

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Graphitisation

We d i scus sed i n e a r l i e r s ec t i on t ha t c a s t i r on c l a s s i f i ed (wh i t e and g r ay ) on t he ba s i s o f c a rbon p r e sen t i n i t . I n t h i s s ec t i on we know tha t how cemen t i t e* and g r aph i t e** a r e fo rmed and a t wha t cond i t i on i n i r on ca rbon d i ag ram. The p roce s s o f d i r e c t p r ec ip i t a t i on o f g r aph i t e f rom l i qu id o r by decompos i t i on o f p r ev ious ly fo rmed cemen t i t e -p roce s s c a l l ed g r aph i t i s a t i on .

In i r on ca rbon d i ag ram g raph i t e (Equ i l i b r i um s t a t e ) i s more s t ab l e phase t han cemen t i t e (Me ta s t ab l e s t a t e ) bu t k ine t i c a l l y i t i s e a s i e r t o fo rm cemen t i t e* t han g r aph i t e** (because 6 .67% C shou ld s eg rega t e t o nuc l ea t e c emen t i t e whe rea s 100% seg rega t i on o f c a rbon i s needed t o nuc l ea t e g r aph i t e .

The c ry s t a l s t r uc tu r e o f aus t en i t e (FCC) i s r e l a t i ve ly t o t ha t c emen t i t e ( complex o r t ho rhombic*** ) , bu t d i f f e r subs t an t i a l l y f rom g raph i t e (Hexagona l l aye r s t r uc tu r e ) .

Cemen t i t e f o rms more ea s i l y f rom aus t en i t e o r f rom l i qu id because ene rgy r equ i r ed fo r d i f fu s ion i s much l e s s t han t ha t f o r g r aph i t e .

As k ine t i c a l l y c emen t i t e c an fo rm more ea s i l y i t i s more p robab l e t o ge t i n m ic ros t ruc tu r e Fe r r i t e + Cemen t i t e f rom aus t en i t e t hen Fe r r i t e + Graph i t e a l so t he l i qu id t o fo rm eu t ec t i c a l l y t o aus t en i t e + Cemen t i t e and no t aus t en i t e t han f e r r i t e + g r aph i t e . I f k ine t i c f a c to r a r e f avou rab l e t hen g r aph i t e c an fo rm because g r aph i t e ha s l e s s f r e e ene rgy t han Cemen t i t e . When g raph i t e f o rm d i r ec t l y f rom l i qu id i s c a l l ed p r ima ry g r aph i t i s a t i on . The fo rma t ion o f g r aph i t e f rom l i qu id i s t ake s p l ace i n a na r row r ange o f t empe ra tu r e (1153 -1147°C) and a l so fo rma t ion o f g r aph i t e f rom aus t en i t e be tween 738°C to 727°C wh ich r equ i r e s l ow coo l i ng . Th i s g r aph i t e i s known a s s econda ry g r aph i t e .

The line Q’C’R’ (1153°C) in Figure. 1.2 is for Eutectic reaction

L Austenite + graphite (Primary graphitisation)

The line is for the Eutectoid reaction

Austenite Ferrite + graphite (Primary graphitisation)

* Cementite-It is an interstitial compound of fixed carbon percentage of 6.67% carbon ** Graphite and Diamond are purest form of carbon present in nature*** For Fe3C- Complex orthorhombic structure with 12 Fe atoms and 4 carbon atoms per unit cell at melting point 1227°C. Crystal structure= Radius of solute atom/radius of solvent atom=0.63

Cooling

Heating

Heating

Cooling

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Commerc i a l c a s t i r ons con t a in s f i ne pa r t i c l e o f i nc lu s ion ( l i ke S i ) wh ich becomes cen t r e o f g r aph i t e c ry s t a l l i s a t i on and p romote g r aph i t e f o rma t ion . When g raph i t e f o rms f rom d i s soc i a t i on o f c emen t i t e i s c a l l ed s econda ry g r aph i t i z a t i on . Me ta s t ab l e c emen t i t e above t empe ra tu r e 738°C decompose t o Austen i t e+ graph i t e o r Ferr i t e + graph i t e b e low 738°C . As we know tha t s l ow coo l i ng o f l i qu id ca s t i r on l e ads t o fo rma t ion o f g r aph i t e and f a s t coo l i ng l e ads t o c emen t i t e . Th i s i s so because t he fo rma t ion o f g r aph i t e f rom l i qu id o r aus t en i t e i s ve ry s l ow coo l i ng p roce s s and t ake s p l ace on ly a t sma l l unde r coo l i ng .

FACTORS EFFECTING FORMATION OF CAST IRONS

The ma in f ac to r s e f f ec t i ng t he fo rma t ion o f wh i t e o r g r ay i r on , i . e . , whe the r c a rbon i s p r e sen t i n t he combined fo rm o r i n t he g r aph i t e f o rm a r e :

I . Chemica l compos i t i on

I I . Cool ing ra te .

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I. COMPOSITION

( a ) Carbon : H ighe r i s t he c a rbon , more i s g r aph i t e f o rmed and l ower t he mechan i ca l p rope r t i e s . Ca rbons l ower t he me l t i ng po in t o f me t a l and ac t a g r aph i t i s e r t o f avou r t he fo rma t ion o f g r ay ca s t i r on .

( b ) Si l i con : S i l i con i s a s t r ong g r aph i t i s e r and i nc r ea se s t he f l u id i t y . I t con t ro l s t he r e l a t i ve p ropo r t i ons o f combined ca rbon and f r ee g r aph i t e . I f s i l i con i s p r e sen t du r ing t he so l i d i f i c a t i on ca rbon p r ec ip i t a t e s a s g r aph i t e f l ake s . S i l i con con t en t may va ry be tween 1 .0% to 3 .5%.S i l i con sh i f t s t he g r aph i t e - eu t ec t i c l i ne upwards . Thus du r ing coo l i ng f rom l i qu id s t a t e , a l a rge r deg ree o f unde r coo l i ng i s pos s ib l e w i th g r ea t e r chance t o fo rm g raph i t e be fo re c emen t i t e f o rma t ion becomes pos s ib l e .

( c ) Su lphur and Manganese : Su lphu r r e t a rd s g r aph i t i s a t i on and i nc r ea se s t he s i z e o f t he f l ake s , H igh su lphu r t ends t o r educe f l u id i t y and i s o f t en t he c ause o f b lowho le s i n c a s t i ngs . Su lphu r i s kep t l ow in amoun t o f . 06 t o . 12%.

Su lphu r i n c a s t i r on i s p r e sen t e i t he r a s FeS o r MnS . FeS t ends t o p romote cemen t i t e f o rma t ion , i . e . , wh i t e c a s t i r on . Mn i s a m i ld c a rb ide fo rming e l emen t . The amoun t o f Mn (one pa r t o f S t o 1 .72 pa r t o f Mn) wh ich combines w i th su lphu r t o fo rm MnS pa r t i c l e s i n l i qu id i r on and r i s e s t o be t op o f me l t t o be r emoved , ha s no t been ab l e t o have i t s own e f f ec t o f c emen t i t e f o rma t ion , no r t he l o s t su lphu r cou ld exe r t i t s e f f ec t o f c emen t i t e f o rma t ion t hus , i nd i r ec t l y he lp s t o g ive g r ay i r on .

Manganese i n exce s s o f wha t ha s fo rmed MnS , weak ly r e t a rd s p r ima r i l y g r aph i t i s a t i on . Howeve r , i t h a s s t r ong cemen t i t e s t ab i l i s i ng e f f ec t on eu t ec to id g r aph i t i s a t i on .

( d ) Phosphorus - Mos t c a s t i r on con t a in phospho rus be tween . 1 t o . 3%. I t s amoun t may be more t han . 9%, t hen i t f o rms i ron phosph ide (Fe3P) , wh ich fo rm a t e rna ry eu t ec t i c w i th c emen t i t e and aus t en i t e . The t e rna ry Eu t ec t i c i s c a l l ed s t ead i t e . S t ead i t e i s b r i t t l e and ha s a me l t i ng po in t o f a round 960 deg ree . Th i s i nc r ea se t he f l u id i t y a l so he lp s i n g iv ing good ca s t ab i l i t y t o t he t h in and i n t r i c a t e c a s t i ng , whe re l ow me l t i ng f l u id cou ld ea s i l y f l ow . Howeve r fo r t h i ck and h igh s t r eng th ca s t i r on ca s t i ng , b r i t t l e s t e ad i t e c an be avo ided by ma in t a in ing phospho rus l e s s t han 0 .3%, wh ich sha l l be p r e sen t i n d i s so lve s t a t e i n f e r r i t e .

( d ) Carbon equ iva l en t Va lue : S i , P ha s s im i l a r e f f ec t on t he mic ros t ruc tu r e , t he i r e f f ec t i n t e rm o f c a rbon i s impor t an t .

The ca rbon equ iva l en t va lue (CE) = To t a l C% + 1 /3 (S i %+P %)

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The ca rbon con t en t o f c a s t i r on may be l ower ( t han 4 .3%) , bu t i f C .E i s 4 . 3%, t hen , t he c a s t i r on i s eu t ec t i c c a s t i r on . Ca rbon equ iva l en t va lue fo r a g iven coo l i ng r a t e , de t e rmines how c lo se i s t o g iven compos i t i on o f c a s t i r on t o t he eu t ec t i c and t hus how much f r ee g r aph i t e , i t i s l i ke ly t o fo rm . Th i s de t e rmines p robab l e s t r eng th o f a s ec t i on o f c a s t i ng .

I I . The Effect of Rate of Cool ing on the Structure of Cast Iron

A h igh r a t e o f coo l i ng du r ing so l i d i f i c a t i on t ends t o f avou r t he fo rma t ion o f c emen t i t e r a t he r t han g r aph i t e . Tha t i s , t he h ighe r t he r a t e o f coo l i ng fo r any g iven ca s t - i r on compos i t i on t he 'wh i t e r ' and more b r i t t l e t he c a s t i ng i s l i ke ly t o be . Th i s e f f ec t i s impor t an t i n connec t i on w i th t he cho i ce o f a su i t ab l e i r on fo r t he p roduc t i on o f c a s t i ngs o f t h in s ec t i on . Suppos ing an i r on wh ich , when coo l ed s l owly , had a f i ne g r ey s t ruc tu r e con t a in ing sma l l eu t ec t i c c e l l s we re chosen fo r such a pu rpose . I n t h in s ec t i ons i t wou ld coo l so r ap id ly t ha t c emen t i t e wou ld fo rm in p r e f e r ence t o g r aph i t e and a t h in s ec t i on o f comple t e ly wh i t e i r on wou ld r e su l t . Such a s ec t i on wou ld be b r i t t l e and u se l e s s .

Th i s e f f ec t i s i l l u s t r a t ed by ca s t i ng a ' s t epped ba r ' o f i r on o f a su i t ab l e compos i t i on . H e r e , t h e t h i n s e c t i o n s h a v e c o o l e d s o q u i c k l y t h a t s o l i d i f i c a t i o n o f c e m e n t i t e h a s o c c u r r e d , a s i n d i c a t e d b y t h e w h i t e f r a c t u r e a n d h i g h B r i n e l l v a l u e s . T h e t h i c k e r s e c t i o n s , h a v i n g c o o l e d m o r e s l o w l y , a r e g r a p h i t i c a n d c o n s e q u e n t l y s o f t e r . D u e t o t h e c h i l l i n g e f f e c t e x e r t e d b y t h e m o u l d , m o s t c a s t i n g s h a v e a h a r d w h i t e s k i n o n t h e s u r f a c e . T h i s i s o f t e n n o t i c e a b l e w h e n t a k i n g t h e f i r s t c u t i n a m a c h i n i n g o p e r a t i o n .

F i g u r e 3 I l l u s t r a t i n g t h e e f f e c t s o f t h i c k n e s s o f s e c t i o n , a n d h e n c e r a t e o f c o o l i n g o n t h e s t r u c t u r e o f a g r e y i r o n . T h e t h i n n e s t p a r t o f t h e s e c t i o n h a s c o o l e d q u i c k l y e n o u g h t o p r o d u c e a w h i t e i r o n s t r u c t u r e , w h i l s t t h e c o r e o f t h e t h i c k e s t p a r t h a s a g r e y i r o n s t r u c t u r e . T h e r e l a t i o n s h i p s b e t w e e n s e c t i o n a l t h i c k n e s s a n d m i c r o s t r u c t u r e a r e s i m i l a r t o t h o s e i n d i c a t e d i n F i g u r e . . o n t h e o p p o s i t e p a g e . B o t h m i c r o g r a p h s x 3 0 0 a n d e t c h e d i n 2 % n i t a l . M a c r o s e c t i o n x 3 .

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coo l ed so qu i ck ly t ha t so l i d i f i c a t i on o f c emen t i t e ha s occu r r ed , a s i nd i ca t ed by t he wh i t e f r a c tu r e and h igh Br ine l l va lue s . The t h i cke r s ec t i ons , hav ing coo l ed more s l owly , a r e g r aph i t i c and consequen t l y so f t e r . Due t o t he ch i l l i ng e f f ec t exe r t ed by t he mou ld , mos t c a s t i ngs have a ha rd wh i t e sk in on t he su r f ace . Th i s i s o f t en no t i c eab l e when t ak ing t he f i r s t cu t i n a mach in ing ope ra t i on . I n c a s t i ng t h in s ec t i ons , t hen , i t i s nece s sa ry t o choose an i r on o f r a t he r coa r se r g r ey f r a c tu r e t han i s r equ i r ed i n t he f i n i shed ca s t i ng . Tha t i s , t he i r on mus t have a h ighe r s i l i con con t en t t han t ha t u sed fo r t he p roduc t i on o f c a s t i ngs o f heavy s ec t i on .

Figure 4 The effect of thickness of cross-section on the rate of cooling, and hence upon the microstructure of a grey cast iron.

Now we discussed types of cas t i ron in de ta i led:

I. WHITE CAST IRONS

These a r e i r on -ca rbon a l l oys hav ing more t han 2 .11% ca rbon and a l l t he c a rbon i s p r e sen t i n t he combined cemen t i t e f o rm , wh ich makes t he f r a c tu r e o f t he se a l l oys t o have du l l and wh i t e co lou r , and t ha t i s t he r e a son o f t he i r name a s wh i t e i r ons . Typ i ca l wh i t e c a s t i r on con t a in s 2 .5 – 3 .5% C , 0 . 4 – 1 .5% S i , 0 . 4 – 0 .6 % Mn , 0 .1 – 0 .4%P, 0 . 15%S, and ba l ance Fe . F igu re . 3 i l l u s t r a t e s changes occu r r i ng on coo l i ng i n hypoeu t ec t i c wh i t e c a s t i r on . A t room t empe ra tu r e wh i t e c a s t i r on i s m ix tu r e o f pea r l i t e and cemen t i t e .

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Figure. 5 The metastable iron—iron carbide phase diagram.

Al l wh i t e c a s t i r ons a r e hypoeu t ec t i c a l l oys . The coo l i ng o f a 2 .50 pe rcen t c a rbon a l l oy w i l l now be de sc r i bed . The a l l oy , a t x 2 i n F igu re . 5 , ex i s t s a s a un i fo rm l i qu id so lu t i on o f c a rbon d i s so lved i n l i qu id i r on . I t r ema ins i n t h i s cond i t i on a s coo l i ng t ake s p l ace un t i l t he l i qu idus l i ne i s c ro s sed a t x 2 . So l i d i f i c a t i on now beg ins by t he fo rma t ion o f aus t en i t e c ry s t a l s con t a in ing abou t 1 pe r cen t c a rbon . As t he t empe ra tu r e f a l l s , p r ima ry aus t en i t e con t i nues t o so l i d i fy , i t s compos i t i on mov ing down and t o t he r i gh t a l ong t he so l i dus l i ne t oward po in t C . The l i qu id i n t he mean t ime i s becoming r i che r i n c a rbon , i t s compos i t i on a l so mov ing down and t o t he r i gh t a l ong t he l i qu idus l i ne t oward po in t E . A t t he eu t ec t i c t empe ra tu r e , 1147°C the a l l oy cons i s t s o f aus t en i t e dend r i t e s con t a in ing 2 pe r cen t c a rbon and a l i qu id so lu t i on , con t a in ing 4 .3 pe r cen t c a rbon . The l i qu id accoun t s f o r ( 2 .5—2.0 ) / ( 4 .3—2.0 ) o r 22 pe rcen t o f t he a l l oy by we igh t . Th i s l i qu id now unde rgoes t he eu t ec t i c r e ac t i on i so the rma l ly t o fo rm the eu t ec t i c m ix tu r e o f aus t en i t e and cemen t i t e known a s l edebu r i t e .

L iqu id (4 .3%) Aus t en i t e ( 2 .11%) + Cemen t i t e ( 6 .67%)

S ince t he r e ac t i on t ake s p l ace a t a r e l a t i ve ly h igh t empe ra tu r e , 1edebu r i t e t ends t o appea r a s a coa r se m ix tu r e r a t he r t han t he f i ne mix tu r e t yp i ca l o f many eu t ec t i c s . I t i s no t unusua l f o r l edebu r i t e t o be

Heating

Cooling

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sepa ra t ed comple t e ly , w i th t he eu t ec t i c aus t en i t e added t o t he p r imacy aus t en i t e dend r i t e s , l e av ing beh ind l aye r s o f mass ive , f r e e c emen t i t e .

As t he t empe ra tu r e f a l l s , be tween x3 and x4 , t he so lub i l i t y o f c a rbon i n aus t en i t e dec r ea se s , a s i nd i ca t ed by t he Acm l i ne CJ . Th i s c ause s p r ec ip i t a t i on o f p roeu t ec to id cemen t i t e , mos t o f wh ich i s depos i t ed upon t he cemen t i t e a l r e ady p r e sen t . A t t he eu t ec to id t empe ra tu r e , 727°C , t he r ema in ing aus t en i t e con t a in ing 0 .8 pe r cen t c a rbon and cons t i t u t i ng ( 6 . 67—2.5 ) / ( 6 .67—0.8 ) , o r 70 pe r cen t o f t he a l l oy , unde rgoes t he eu t ec to id r e ac t i on i so the rma l ly t o fo rm pea r l i t e . Dur ing subsequen t coo l i ng t o room t empe ra tu r e , t he s t r uc tu r e r ema ins e s sen t i a l l y unchanged .

The t yp i ca l m ic ros t ruc tu r e o f wh i t e c a s t i r on , cons i s t i ng o f dend r i t e s o f t r ans fo rmed aus t en i t e (Pea r l i t e ) i n a wh i t e i n t e rdend r i t i c ne twork o f c emen t i t e a s shown in F igu re . 9 .

Figure 6 Changes during cooling of hypoeutectic white cast iron

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Figure 7 Microstructure of white cast iron

F i g u r e 8 . C h a n g e s o n c o o l i n g , o f w h i t e c a s t i r o n s ( s c h e m a t i c ) . ( a ) D e n d r i t e s o f a u s t e n i t e f o n t : w h i c h g e t b r o k e n b y s e c o n d a r y c e m e n t i t e . A u s t e n i t e c h a n g e s t o P e a r l i t e a t e u t e c t o i d t e m p e r a t u r e . ( b ) C o m p l e t e l e d e b u r i t e f o r m s b y e u t e c t i c r e a c t i o n . C o a r s e l e d e b u r i t e f o r m s a s t e m p e r a t u r e i s h i g h . S e c o n d a r y c e m e n t i t e f o r m s r e d u c i n g s i z e o f a u s t e n i t e p a r t i c l e s w h i c h a t e u t e c t o i d t e m p e r a t u r e c h a n g e s t o P e a r l i t e t o r e s u l t i n c o m p l e t e t r a n s f o r m e d l e d e b u r i t e . ( c ) C e m e n t i t e b e i n g a c o m p o u n d , f e a s t s a s p l a t e , a s p r i m a r y c e m e n t i t e . A m o u n t o f t e r t i a r y c e m e n t i t e i n a l l t h e s e c a s e s i s n e g l i g i b l y s m a l l , t h u s . M i c r o s t r u c t u r e

i s s a m e a f t e r e u t e c t o i d r e a c t i o n a n d a t r o o m t e m p e r a t u r e .

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F i g u r e . 9 M i c r o s t r u c t u r e o f w h i t e c a s t i r o n s , ( a ) M i c r o s t r u c t u r e o f h y p o e u t e c t i c w h i t e c a s t i r o n . C a r b o n i s c l o s e t o 2 . 1 1 % , a s i t h a s m a j o r a m o u n t o f b r o k e n d e n d r i t e s o f P e a r l i t e a n d l e s s t r a n s f o r m e d l e d e b u r i t e , ( b ) M i c r o s t r u c t u r e o f h y p e r e u t e c t i c w h i t e c a s t i c o n h a v i n g m o r e c a r b o n t h a n ( a ) a s t h e a m o u n t o f b r o k e n d e n d r i t e s i s l e s t , ( c ) e u t e c t i c c a s t i r o n h a v i n g o n l y t r a n s f o r m e d l e d e b u r i t e , ( 4 ) H y p e r e u t e c t i c w h i t e i r o n . P r e s e n c e o f p l a t e s o f p r i m a r y c e m e n t i t e i n d i c a t e s t h i s .

Properties:

Hard and wea r r e s i s t an t

The ha rdnes s and b r i t t l ene s s i nc r ea se s a s t he c a rbon con t en t i nc r ea se s .

Ha rdnes s B r ine l l 375 t o 600 .

Tens i l e s t r eng th 20000 t o 70000 p s i .

Compres s ive s t r eng th 200000 t o 250000 .

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Limitations

Because o f ex t r eme b r i t t l ene s s and l a ck o f mach inab i l i t y , wh i t e i r ons

f i nd l im i t ed eng inee r i ng app l i c a t i ons .

Application

The pa r t s whe re r e s i s t ance t o wea r i s t he mos t impor t an t r equ i r emen t such a s l i ne r s o f c emen t m ixe r s , ba l l m i l l s , pumps , wea r ing p l a t e s . Pa r t s o f s and - s l i nge r s , c e r t a i n t ype o f d r awing d i e s , ex t ru s ion nozz l e s , g r i nd ing ba l l s . Mos t pa r t s a r e s and -ca s t and don ’ t r equ i r e much mach in ing , wh ich can be done by g r i nd ing . A l a rge t onnage o f wh i t e c a s t i r ons i s u sed a s a s t a r t i ng ma t e r i a l f o r t he p roduc t i on o f ma l l e ab l e c a s t i r on pa r t s .

Brake shoes Sho t b l a s t i ng nozz l e s Mil l l i ne r s Crushe r s Pump impe l l e r s and o the r ab ra s ion r e s i s t an t pa r t s .

II. GRAY CAST IRONI ron -ca rbon a l l oys con t a in ing f l ake s o f g r aph i t e embedded i n s t e e l ma t r i x , wh i ch show a g r ay -b l ack i sh co lou red f r ac tu r e due t o g r aph i t e ’—the f r ee foam o f c a rbon , a r e c a l l ed g r ay ca s t i r ons . The s t r eng th o f g r ay i r on depends on t he s t r eng th o f s t e e l ma t r i x and t he s i z e and cha rac t e r o f g r aph i t e f l ake s i n i t . A t yp i ca l f e a tu r e o f g r ay i r on i s t ha t g r aph i t e i s i n t he fo rm o f f l ake s i n m ic ros t ruc tu r e , F igu re 10 . Th i s m ic ros t ruc tu r e r ep re sen t s t he i r appea rance on a p l ane su r f ace , bu t f l ake s a r e t h r ee d imens iona l p l a t e s , some t imes connec t ed .

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Figure. – 10 Microstructure of gray cast iron

COMPOSITION OF GRAY IRONS

The g ray ca s t i r ons a r e hypoeu t ec t i c c a s t i r ons , t he t o t a l c a rbon con t en t l i e s be tween 2 .4% to 3 .8%. The amoun t o f c a rbon does no t exceed 3 .8%, a s more t he c a rbon , more t he eu t ec t i c l i qu id , wh ich y i e ld s more g r aph i t e a s f l ake s , r e su l t i ng i n poo r mechan i ca l p rope r t i e s . Ca rbon i s kep t a t l e a s t 2 . 4%. So t ha t c a s t i r on ha s good f l u id i t y and ca s t ab i l i t y . S i l i con i s kep t be tween1 .2% to 3 .5%. I t be ing a g r aph i t i s e r con t ro l s a l ong w i th c a rbon and t he r a t e o f coo l i ng , t he na tu r e o f s t e e l ma t r i x . I n such i r on , g r aph i t i s a t i on o f a l l t he c emen t i t e excep t t he eu t ec to id cemen t i t e t ake s p l ace . The gene ra l i s ed r ange o f compos i t i on o f g r ay i r ons i s :

Total carbon : 2.4—3.8%

Silicon : 1.2—3.5%

Manganese : 0.5—1.0%

Sulphur : 0.06—0.12%

Phosphorus : 0.1—0.9%

In manufac tu r i ng o f g r ay ca s t i r ons , t he t endency o f c emen t i t e t o s epa ra t e i n to g r aph i t e and aus t en i t e o r f e r r i t e i s f avou red by con t ro l l i ng a l l oy compos i t i on and coo l i ng r a t e . These a l l oys so l i d i fy by

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f i r s t f o rming p r ima ry aus t en i t e . The g r aph i t i z a t i on p roce s s i s added by h igh ca rbon con t en t , h i gh t empe ra tu r e and t he p rope r amoun t o f g r aph i t i z i ng e l emen t s mos t l y s i l i con .

Figure.11 Iron-graphite equilibrium diagram

With p rope r con t ro l o f above f ac to r s a l l oy w i l l f o l l ow the s t ab l e i r on -g raph i t e equ i l i b r i um d i ag ram ( Figure .11 ) f o rming aus t en i t e and g r aph i t e a t t he eu t ec t i c t empe ra tu r e o f 1154°C a t any r a t e any cemen t i t e wh ich i s f o rmed w i l l g r aph i t i z e r ap id ly

Dur ing con t i nuous coo l i ng , t he r e i s add i t i ona l p r ec ip i t a t i on o f c a rbon because o f t he dec rea se i n so lub i l i t y o f c a rbon i n aus t en i t e . t h i s c a rbon i s p r ec ip i t a t e a s g r aph i t e

S t r eng th o f g r ay ca s t i r on depends a lmos t en t i r e l y on t he ma t r i x i n wh ich t he g r aph i t e i s embedded . I f t he compos i t i on and coo l i ng r a t e a r e such t ha t t he eu t ec to id cemen t i t e a l so g r aph i t i z e s , t hen t he ma t r i x w i l l be en t i r e l y Fe r r i t i c . I f g r aph i t i z a t i on o f t he eu t ec to id cemen t i t e i s p r even t ed , t he Ma t r i x w i l l be en t i r e l y pea r l i t i c . The g r aph i t e - f e r r i t e m ix tu r e i s t he so f t e s t and weakes t g r ay i r on , t he s t r eng th and ha rdnes s i nc r ea se w i th t he i n i nc r ea se i n c a rb ide , r e ach ing a max imum wi th t he pea r l i t i c g r ay i r on .

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Pea r l i t i c ma t r i x i s ob t a ined by p rope r con t ro l o f a l l oy compos i t i on r a t e o f coo l i ng o r hea t t r e a tmen t . P rope r t i e s o f g r ay i r on depend on t he na tu r e o f ma t r i x , t he s i z e , cha rac t e r and amoun t o f g r aph i t e f l ake s . The c l a s s i f i c a t i on o f c a s t i r ons i s ba sed on t he min imum t ens i l e s t r eng th pos se s sed by a c a s t i r on . i . e . , i s ba sed on p rope r ty and no t t he compos i t i on .

Figure 12 Microstructure of gray irons. (a) Pearlitic gray iron, (b) Ferreto pearlitic gray iron x 250. (c) Gray phosphoric cast iron (CE, = 4.2%) (C = 3.4%, Si = 2.4%, Mn = 0.45%, S = 0.02%, P = 1.0%. showing ternary phosphide eutectic. Steadite, (d) Characteristic Herring bone structure of pseudo-binary eutectic (of dark Fe3P and ferrite)

Pea r l i t i c g r ay i r on hav ing h igh phospho rus (0 .3 -0 .5%) u sed fo r p i s t on r i ngs . H igh wea r r e s i s t ance i s ob t a ined i n r i ngs due t o t i ne Pea r l i t e and un i fo rmly d i s t r i bu t ed phosph ide eu t ec t i c w i th f ew f l ake s o f g r aph i t e .

Bea r ings ma t i ng w i th ha rdened (o r no rma l i s ed ) s t e e l sha f t a r e o f g r ay i r on w i th a round 85% Pea r l i t e . ( 3 . 2 -3 .6 C , 1 .6 -2 .4% S i . 0 . 6 -0 .9% Mn) . I f sha f t ha s no t been hea t t r e a t ed , t hen t he compos i t i on o f t he bea r i ng : ( 3 .2 -3 .8% C , 1 .7 -2 .6% S i 0 .4 -0 .7% Mn, 0 .1% T i , 0 . 3 -0 .5% Cu) .

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FORMATION OF FLAKES

Norma l ly commerc i a l g r ay i r on i s e i t he r hypoeu t ec t i c o r eu t ec t i c i n na tu r e . Neg l ec t i ng t he p r e sence dend r i t e s o f p r ima ry aus t en i t e i n hypoeu t ec t i c i r on , wh ich imposes cons t r a in t s l a t e r on i n t he r ad i a l g rowth o f t he eu t ec t i c c e l l , F igu re .13 i l l u s t r a t e s t he succe s s ive s t age s i n t he fo rma t ion o f g r aph i t e f l ake s f rom the eu t ec t i c l i qu id p r e sen t . Once g r aph i t e ha s nuc l ea t ed ( i t occu r s w i th in t he i n t e rdend r i t i c l i qu id and no t on aus t en i t e dend r i t e a rms ) , so l i d i f i c a t i on t ake s p l ace a t nuc l e i F igu re 13a , f r om each o f wh ich i s f o rmed a rough ly sphe r i c a l l ump ca l l ed t he eu t ec t i c c e l l . I t g rows i n an app rox ima te ly r ad i a l manne r , whe re t he r e i t s imu l t aneous g rowth o f aus t en i t e and g r aph i t e , t he l a t t e r be ing i n con t i nuous con t ac t w i th t he l i qu id . The f l ake s bend , tw i s t and b r anch a s dep i c t ed i n F igu re13d .The re i s a con t i nuous b r anched ske l e ton o f g r aph i t e i n e ach eu t ec t i c c e l l l i ke a c abbage . When t he r a t e o f coo l i ng i s i nc r ea sed , t he r e i s more

Figure.13 (a), (b), (C): Stages in the formation of graphite flakes, (d) Growth of flake graphite eutectic cell

Unde r coo l i ng , t hen t he ske l e ton i s b r anched more f r equen t l y w i th t he r ap id r ad i a l g rowth o f t he c e l l and t hus , f i ne r g r aph i t e f l ake s a r e obse rved . The d i ame te r o f t he eu t ec t i c c e l l dec r ea se s a s t he number o f c e l l s pe r un i t vo lume i nc r ea se , and t h i s r e su l t s i n h ighe r t en s i l e s t r eng th , t hough t he soundnes s o f t he c a s t i ng i s a f f ec t ed adve r se ly . The number o f nuc l e i c an be i nc r ea sed by i nocu l an t s a s we l l a s by su lphu r ( su lphu r p romote s cons t i t u t i ona l supe rcoo l i ng , i nc r ea s ing t he f r equency o f b r anch ing i . e . , c e l l dens i t y a s we l l a s p roduces coa r se r f l ake s ) . Supe rhea t i ng o r ho ld ing t ime o f mo l t en me t a l r educes t he number o f nuc l e i .

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HEAT TREATMENT OF GRAY IRON

The s t r e s s - r e l i ev ing i s p robab ly t he mos t f r equen t l y app l i ed hea t t r e a tmen t t o g r ay i r ons . I n t he a s - ca s t s t a t e , c a s t i ngs have r e s idua l s t r e s se s deve loped due t o d i f f e r en t i a l coo l i ng and d i f f e r en t i a l con t r ac t i on , e spec i a l l y i n non -un i fo rm c ros s - s ec t i oned ca s t i ngs . These s t r e s se s a r e comple t e ly r emoved by soak ing a t 650°C , bu t g r a in g rowth i s s e r i ous a t and above 600°C . Annea l i ng o f g r ay i r on i s done t o g r aph i t i s e c a rb ide , and t o homogen i se t he c a s t i ngs . I t so f t ens , i nc r ea se s duc t i l i t y and mach inab i l i t y o f g r ay i r on . Cas t i ngs a r e soaked fo r up t o 10 hou r a t 850 -950°C . Norma l i s i ng may be done t o i nc r ea se t he s t r eng th and ha rdnes s o f c a s t i r on by hea t i ng a t 900 -930°C fo r a soak ing t ime o f 2 .5 m /min o f max imum th i cknes s o f c a s t i ng and t hen a i r coo l i ng . Ha rden ing can be done by hea t i ng t o and soak ing a t 800 -850°C , and t hen quench ing i n wa t e r , o i l , ho t s a l t ba th , t hough fo r t h rough - ha rden ing , o i l i s common ly u sed a s wa t e r quench ing may cause d i s t o r t i on and c r ack ing . Tempe r ing i s done a t 150 t o 650°C . Tab l e 1 i l l u s t r a t e s ha rdnes s o f c a s t i r ons ba sed on t he mic ros t ruc tu r e . Tab l e 2 i l l u s t r a t e s compos i t i on o f some g ray i r ons w i th some app l i c a t i ons .

Table 1 Hardness of Gray Iron based on Matrix Microstructure

Nature of matrix

Ferritic Soft Low grade

Pearlitic Iron

Low Alloy

Pearlitic

Austenitic Martensitic Tempered Martensite

Hardness BHN

110-140 140-160 160-220 200-250 140-160 350-450 260-350

Table 2 Composition of Gray Irons with Applications

Applications C Si Mn P S Ni Cr Tensile Strength(MPa)

Break DrumPiston RingCylinder and PistonsHeavy CastingsClutch Casting

3.303.503.25

3.253.20

1.92.92.25

1.252.10

0.650.650.65

0.500.80

0.150.500.15

0.350.17

.08

.06

.10

.10

.05

1.25 0.5

0.32

150

To

350275

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Properties of Grey Cast iron:

1. Low cost of production: I n f a c t , g r ay i r on , be ing t he l e a s t expens ive ca s t i ng ma t e r i a l , i s a lways cons ide red f i r s t when a c a s t me t a l i s be ing chosen fo r a p roduc t , un l e s s mechan i ca l and phys i ca l p rope r t i e s o f g r ay i r on a r e i nadequa t e .

2. Low melting point: ( 1150°—1250°C) o f c a s t i r ons , s eve ra l hund red deg ree s l e s s t han s t e e l , r equ i r e s s imp le fu rnace s l i ke p i t f u rnace , c ruc ib l e fu rnace , cupo l a , e t c . wh i ch a r e s imp le , i nexpens ive t o fun and ma in t a in . The con t ro l o f impur i t i e s i s no t c r i t i c a l he r e a s i n s t e e l me l t i ng .

3. Good Castability: Cas t i r ons have exce l l en t f l u id i t y and t ake good mou ld - impre s s ions e a s i l y . Cas t i r ons ; a s compa red t o s t e e l s so l i d i t y ma in ly a t t he cons t an t eu t ec t i c t empe ra tu r e—a c r i t e r i on u sed fo r choos ing a l l oy compos i t i ons hav ing be s t c a s t ab i l i t y . Graph i t e hav ing l ow dens i t y i s vo luminous . I t s l a rge vo lume compensa t e s fo r t he sh r i nkage . Gray i r on , t hus , does no t need sh r i nkage a l l owance a t a l l t o t ake a lmos t exac t c a s t i ng impre s s ions .

4. Good machinability of gray cast iron i s due t o e a sy and d i s con t i nuous ch ip fo rma t ion due t o b r i t t l e g r aph i t e f l ake s . Graph i t e s e rve s a s a so l i d l ub r i c an t dec r ea s ing coe f f i c i en t o f f r i c t i on . I t smea r s t he cu t t i ng t oo l a l l owing f r ee s l i d ing o f ch ip s i nc r ea s ing t hus , t oo l l i f e t oo . (Whi t e c a s t i r ons , due t o h igh ha rdnes s , a r e unmach inab l e ) .

5. Good wear resistance of gray i ron i s due t o g r aph i t e a c t i ng a s so l i d l ub r i c an t l aye r , avo id ing t he r eby me ta l t o me t a l d i r e c t con t ac t . On o the r hand , wh i t e c a s t i r ons a r e wea r r e s i s t an t due t o ’ t he i r h igh ha rdnes s .

6. High damping capacity i s due t o t he g r aph i t e f l ake s , wh i ch b r eaks t he con t i nu i t y o f t he me t a l l i c ma t r i x , and t hus , v ib r a t i ons a r e no t a l l owed t o t r ans f e r f rom one s i de o f f l ake t o o the r , i . e . , g r aph i t i c c r acks qu i ck ly dampen t he v ib r a t i ons and r e sonance o sc i l l a t i ons . Gray i r on su i t s t hus t he mach ine beds a s compa red t o s t e e l s .

7. High compressive strength of g r ay i r on - a lmos t 3 t o 5 t imes o f i t s t en s i l e s t r eng th (110 -350 N /mm2) , and a lmos t equa l t o t ha t o f s t e e l s makes i t su i t ab l e fo r app l i c a t i ons , whe re componen t s a r e sub j ec t ed t o compre s s ion such a s mach ine beds , e t c .

8. High thermal conductivity, and have ab i l i t y t o w i th s t and t he rma l shocks .

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9. Good resistance to atmospheric corrosion due t o h igh s i l i con and pe rhaps o the r f a c to r s , t han mi ld s t e e l s .

10. Notch-insensitive: La rge number o f f l ake s i n g r ay i r on ac t s a s no t ches i n sp i t e o f t he se no t ches , i f g r ay i r on ha s t he r equ i r ed s t r eng th , t hen add i t i ona l no t ch o r no t ches sha l l have mino r , o r no e f f ec t , i . e . , g r ay i r on i s no t ch - in sens i t i ve ; whe rea s i n s t e e l s . A no t ch ha s qu i t e a damag ing e f f ec t a s i t a c t s a s s t r e s s - r a i s e r t o make t he s t e e l even b r i t t l e .

Table 3 Properties of Grey Cast Iron

Some other properties of Grey cast iron ASTM Chemical composition: C=2.7-4%, Mn=0.8%, Si=1.8-3%, S=0.07% max, P=0.2% max Property Value in metric unit Value unit Density 7.06 *10³-7.34 *10³ kg/m³ 441-458 lb/ft³ Modulus of elasticity 124 GPa 18000 ksi Thermal expansion (20 ºC) 9.0*10-6 ºCˉ¹ 5.0*10-6 in/(in* ºF) Specific heat capacity 840 J/(kg*K) 0.2 BTU/(lb*ºF) Thermal conductivity 53.3 W/(m*K) 370 BTU*in/(hr*ft²*ºF) Electric resistivity 1.1*10-7 Ohm*m 1.1*10-5 Ohm*cm Tensile strength 276 MPa 40000 psi Elongation 1 % 1 % Shear strength 400 MPa 58000 psi Compressive yield strength Min. 827 MPa Min. 120000 psi Fatigue strength 138 MPa 20000 psi Hardness (Brinell) 180-302 HB 180-302 HB Wear resistance Low Corrosion resistance Low Weldability Low Machinability Good Castability High

Limitations:

Apar t f r om low duc t i l i t y and t oughnes s , g r ay i r ons a r e s ec t i on s ens i t i ve , i . e . , d epend ing on t he s ec t i on t h i cknes s o f t he c a s t i ng , t he m ic ros t ruc tu r e and t hus , t he p rope r t i e s va ry . Th i ck s ec t i ons have l ow s t r eng th (due t o f e r r i t i c ma t r i x ) and ca r e ha s t o be t aken , when de s ign ing t he ca s t i ngs .

Applications: Gray ca s t i r ons have ex t ens ive app l i c a t i ons . The h igh damping capac i t y and h igh compre s s ive s t r eng th make t hem su i t ab l e fo r t he beds and

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base s o f power fu l mach ine s and f r ames . Good wea r r e s i s t ance , good mach inab i l i t y and damping capac i t y make t hem su i t ab l e fo r app l i c a t i ons l i ke l ocomot ive and i n t e rna l combus t i on eng ine cy l i nde r b locks and heads , p i s t ons r i ngs , cy l i nde r s . The ea se o f c a s t i ng and l ow cos t makes t hem su i t ab l e fo r coun t e r -we igh t s f o r e l eva to r s , i ndus t r i a l f u rnace doo r s ;

Fly whee l s Guards and f r ames a round haza rdous mach ine ry Gear hous ings Pump hous ings Steam tu rb ine hous ings Moto r f r ames Sewer cove r s Enc losu re s fo r e l e c t r i c a l equ ipmen t s .

III. CHILLED CAST IRON

Chi l l ed - i ron ca s t i ngs a r e made by ca s t i ng t he mo l t en me t a l aga in s t ch i l l e r s wh ich r e su l t i n a su r f ace o f wh i t e c a s t i r on . Ch i l l ed i r on ha s su r f ace l aye r s o f wh i t e i r on , wh i l e t he s t r uc tu r e o f t he co re i s t ha t o f g r ay i r on . Norma l ly , ch i l l ed i r on ca s t i ngs a r e ob t a ined by ca s t i ng t he mo l t en a l l oy i n me t a l mou ld . Ch i l l i ng t o c e r t a i n dep th (12 t o 30 mm) i s because o f t he f a s t coo l i ng ( ch i l l i ng ) ob t a ined due t o h igh t he rma l conduc t i v i t y o f me t a l mou ld , The compos i t i on o f mo l t en a l l oy i s so chosen t ha t no rma l coo l i ng r e su l t s i n g r ay i r on i n t he who le s ec t i on , bu t f a s t coo l i ng o f t he who le su r f ace , o r a pa r t o f t he su r f ace y i e ld wh i t e i r on t he r e . The f a s t coo l i ng ob t a ined by emp loy ing me ta l o r g r aph i t e p l a t e s—ca l l ed ch i l l s i n t he s and mou ld . A ch i l l ed ca s t i r on o f f o l l owing compos i t i on can ge t ch i l l ed ea s i l y :

C = 2.8—3.6%; Si =0.5 to 0.8%; Mn = 0.4 — 0.6%

Where the deeper ch i l l i s needed can be increased by increas ing the th i cknes s o f the ch i l l p la t e s . I t i s pos s ib l e to choose the compos i t i on o f the cas t i ron so tha t the normal coo l ing ra te a t the sur face i s ju s t f a s t enough to y i e ld wh i t e i ron there , and the s l ower coo l ing ra te be low th i s sur face produces mot t l ed or mot t l ed and gray i ron . Presence o f graph i t i s er decrease s the ch i l l dep th and the carb ide forming e l ement s increase the ch i l l dep th .

Element Chill depth Hardness of chill depth

Other Facts

CSi

Mn as

Decrease DecreaseDecrease

IncreaseSlightly IncreaseIncrease

24

MnSDissolved

MnP

Ni

Cr

Mo

Cu

IncreaseDecrease

Decrease

Increase

Increase

Decrease unto 4% then increase

Slightly IncreaseIncrease Added unto 5%

Increase

Increase

Increase

Increase

0.1 % P decrease chill depth by 2.5% for constant C and Si

Refines Carbides Chilled structure and core. Helps to pearlitic structure in thick sections.

1-4% Cr as chromium carbide increases hardness and wear resistance 12-35% for corrosion and oxidation resistance at high temperaturesChilled layer has more resistance to spalling, heat checking, Chipping It decrease mottled layer

Table 4 Effect of Elements on Chill Depth, etc. of Chilled Iron

Gene ra l , ch i l l dep th i s i nc r ea sed by i nc r ea s ing ca rb ide fo rming e l emen t s and dec rea s ing ca rbon and s i l i con . Cas t i r on me l t i s a l l owed so so l i d i fy i n mou ld o f shape o f wedge . F igu re . ( 14b ) . The Coo l ing r a t e i s f a s t e r a t mo ld wa l l s , wh i ch p r even t s g r aph i t i s a t i on t o y i e ld wh i t e c a s t i r on . The coo l i ng r a t e dec rea se s a s t he c en t r e o f t he c a s t i ng i s app roached , a l l owing g r aph i t i s a t i on t o t ake p l ace t o y i e ld g r ay i r on . F igu re . ( 14a ) i l l u s t r a t e s changes i n ha rdnes s o f t he s t ep -ba r t e s t p i e ce wh ich i s due t o changes i n m ic ros t ruc tu r e s . The dep th o f ch i l l dec r ea se s and t he ha rdnes s o f t he ch i l l ed zone i nc r ea se s w i th i nc r ea s ing ca rbon con t en t . The dep th o f ch i l l i s dec r ea sed w i th i nc r ea s ing s i l i con con t en t . Phospho rus dec rea se s t he dep th o f ch i l l . Wi th ca rbon and s i l i con cons t an t , an i nc r ea se o f 0 .1 % Phospho rus w i l l dec r ea se t he dep th o f ch i l l abou t 0 .1 i n . N icke l r educes t he ch i l l dep th and r e f i ne s t he c a rb ide s t ruc tu r e . Ch romium i s u sed i n sma l l amoun t t o con t ro l ch i l l dep th . Manganese dec rea se s t he dep th o f ch i l l un t i l t he fo rma t ion o f Manganese su lph ide a f t e r t ha t i nc r ea se s ch i l l dep th and ha rdnes s . Mo lybdenum improves t he r e s i s t ance o f t he ch i l l ed f ace t o spa l l i ng , p i t t i ng , ch ipp ing and hea t check ing .

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Figure. 14(a) Effect of elements on chill depth. (b) Step bar test piece for chill depth cast iron having 3-

3.3% C.

Properties: Cas t i ngs have some good p rope r t i e s due t o wh i t e i r on

su r f ace wh ich a r e h igh wea r and ab ra s ion r e s i s t ance , and some good p rope r t i e s due g r ay i r on co re wh ich a r e damping capac i t y , l ow no t ch s ens i t i v i t y .

Application:

Chilled cast irons used as Rail-freight car wheel Cane-crushing rolls Road rollers Grinding balls Liners Stamp shoes and dies Sprockets Ploughshares many other heavy-duty machinery parts

IV. MOTTLED IRON

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In a ch i l l ed ca s t i r on ca s t i ng , su r f ace l aye r s a r e o f wh i t e i r on and t he co re i s o f g r ay i r on , bu t i n t he

Figure. 15 Microstructure of mottled cast iron

t r an s i t i on r eg ion , t he s t r uc tu r e cons i s t s bo th o f g r ay and wh i t e i r on , i . e . , h a s g r aph i t e f l ake s , Pea r l i t e and s econda ry f r e e c emen t i t e , i . e . , m ixed i r on o r c a l l ed mo t t l ed i r on , The i n t e rmed i a t e coo l i ng r a t e f o r c e r t a i n c a rbon and s i l i con con t en t s cou ld no t g r aph i t i s e t he f r e e s econda ry cemen t i t e , Due t o i nco r r ec t f ound ry con t ro l f o r c e r t a i n compos i t i ons , The non un i fo rm f l ake s i nc r ea se b r i t t l ene s s o f t he c a s t i ngs , apa r t f r om the ex t r a b r i t t l ene s s due t o t he p r e sence o f s econda ry cemen t i t e . Mo t t l ed ca s t i r ons , t hus , don ’ t f i nd app l i c a t i ons . I f c a rbon and s i l i con con t en t o f t he c a s t i r on i s i nc r ea sed , t hen t he ca s t i ng sha l l so l i d i fy a s g r ay i r on . The t h i cknes s o f t he mo t t l ed zone i n ch i l l ed i r on can be r educed by i nc r ea s ing bo th t he g r aph i t i s e r and t he ca rb ide fo rming e l emen t s i n t he c a s t i r on . F igu re . 15 i l l u s t r a t e s m ic ros t ruc tu r e o f mo t t l ed ca s t i r on .

V. MEAHANITE CAST IRON

The mo l t en ca s t i r on i s t r e a t ed w i th c a l c ium s i l i c i de s a s i nocu l an t s t o p roduce a f i ne g r aph i t i c s t r uc tu r e . The f l ake s a r e un i fo rmly d i s t r i bu t ed t o g ive h igh mechan i ca l p rope r t i e s (Tens i l e s t r eng th = 25 — 40 Kg /mm 2 ) . The compos i t i on i s so chosen t ha t wh i t e f r a c tu r e i s ob t a ined i n t he absence o f any t r e a tmen t , i . e . , t he c a s t i r on i s l ow in s i l i con con t en t , mode ra t e ly l ow in c a rbon con t en t abou t 2 .5 -3%. Ca l c ium s i l i c i de s a c t a s g r aph i t i s e r , so t ha t r e su l t i ng ca s t i ng i s g r ay and merchan t ab l e . Meahan i t e c a s t i r on f i nds app l i c a t i ons a s a g r ay i r on w i th h igh mechan i ca l s t r eng th , such a s fo r heavy mach ine beds and f r ames .

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VI. MALLEABLE CAST IRON

Cemen t i t e ( i r on ca rb ide ) i s a c tua l l y a me t a s t ab l e phase . The re i s a t endency fo r c emen t i t e t o decompose i n to i r on and ca rbon , bu t unde r no rma l cond i t i ons i t t ends t o pe r s i s t i nde f i n i t e l y i n i t s o r i g ina l f o rm . Up t o t h i s po in t , c emen t i t e ha s been t r e a t ed a s a s t ab l e phase ; howeve r , t h i s t endency t o fo rm f r ee c a rbon i s t he ba s i s f o r t he manufac tu r e o f ma l l e ab l e c a s t i r on . The r eac t i on Fe 3 C3Fe + C i s f avou red by e l eva t ed t empe ra tu r e s , t he ex i s t ence o f so l i d non me ta l l i c impur i t i e s , h ighe r c a rbon con t en t s , and t he p r e sence o f e l emen t s t ha t a i d t he decompos i t i on o f Fe 3 C On the i r on—iron ca rb ide equ i l i b r i um d i ag ram fo r t he me t a s t ab l e sy s t em, shown in F igu re . 16 , a r e supe r imposed t he phase bounda r i e s o f t he s t ab l e i r on -ca rbon (g r aph i t e ) sy s t em a s do t t ed l i ne s . The pu rpose o f ma l l e ab i l i z a t i on i s t o conve r t a l l t he combined ca rbon i n wh i t e i r on i n to i r r egu l a r nodu l e s o f t ampe r c a rbon (g r aph i t e ) and f e r r i t e . Commerc i a l l y , t h i s p roce s s i s c a r r i ed ou t i n two s t eps known a s t he f i r s t and s econd s t age s o f t he annea l .

Whi t e i r ons su i t ab l e fo r conve r s ion t o ma l l e ab l e i r on a r e o f t he fo l l owing r ange o f compos i t i on :

Table 5 Composition of Malleable Iron

In t he f i r s t - s t age annea l i ng , t he wh i t e - i r on ca s t i ng i s s l owly r ehea t ed t o a t empe ra tu r e be tween 1660 and 1750°F . Dur ing hea t i ng , t he pea r l i t e i s conve r t ed t o aus t en i t e a t t he l ower c r i t i c a l l i ne . The aus t en i t e t hus fo rmed d i s so lve s some add i t i ona l c emen t i t e a s hea t ed t o t he annea l i ng t empe ra tu r e

Components PercentageCarbon 2.00-2.65Silicon 0.90-1.40Manganese 0.25-0.55Phosphorus Less than 0.18Sulphur 0.05

28

F i g u r e . 1 6 T h e s t a b l e i r o n - G r a p h i t e s y s t e m ( d o t t e d l i n e s ) s u p e r i m p o s e d o n t h e m e t a s t a b l e i r o n — i r o n c a r b i d e s y s t e m .

Figu re 16 show tha t t he aus t en i t e o f t he me t a s t ab l e sy s t em can d i s so lve more ca rbon t han can aus t en i t e o f t he s t ab l e sy s t em. The re fo re , a d r i v ing fo r ce ex i s t s f o r t he c a rbon t o p r ec ip i t a t e ou t o f t he aus t en i t e a s f r e e g r aph i t e . Th i s g r aph i t i z a t i on s t a r t s a t t he ma l l e ab i l i s i ng t empe ra tu r e . The i n i t i a l p r ec ip i t a t i on o f a g r aph i t e nuc l eus dep l e t e s t he aus t en i t e o f c a rbon , and so more i s d i s so lved f rom the ad j acen t c emen t i t e , l e ad ing t o fu r t he r c a rbon depos i t i on on t he o r i g ina l g r aph i t e nuc l eus . The g r aph i t e nuc l e i g row a t app rox ima te ly equa l r a t e s i n a l l d i r e c t i ons and u l t ima t e ly appea r a s i r r egu l a r nodu l e s o r sphe ro id s u sua l l y c a l l ed t empe r c a rbon (F igu re . 17 ) . Tempe r c a rbon g r aph i t e i s f o rmed a t t he i n t e r f ace be tween p r ima ry ca rb ide and s a tu r a t ed aus t en i t e a t t he f i r s t - s t age annea l i ng t empe ra tu r e , w i th g rowth a round t he nuc l e i by a r e ac t i on i nvo lv ing d i f fu s ion and ca rb ide decompos i t i on . Nuc l ea t i on and g r aph i t i z a t i on a r e a cce l e r a t ed by t he p r e sence o f sub mic roscop i c pa r t i c l e s t ha t c an be i n t roduced i n to t he i r on by t he p rope r me l t i ng p r ac t i c e . H igh s i l i con and ca rbon con t en t s p romote nuc l ea t i on and g r aph i t i z a t i on , bu t t he se e l emen t s mus t be r e s t r i c t ed t o c e r t a i n max imum l eve l s s i nce t he i r on mus t so l i d i fy a s wh i t e i r on . The re fo re , g r aph i t i z i ng nuc l e i a r e be s t p rov ided by p rope r annea l i ng p r ac t i c e

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F i g u r e . 1 7 M a l l e a b l e i r o n , u n e t c h e d . I r r e g u l a r n o d u l e s o f g r a p h i t e c a l l e d t e m p e r c a r b o n , b o x . ( b ) F e r r i t i c m a l l e a b l e i r o n , t e m p e r c a r b o n b l a c k ) i n a f e r r i t e m a t r i x .

The r a t e o f annea l i ng depends on chemica l compos i t i on , nuc l ea t i on t endency , and t empe ra tu r e o f annea l i ng . The t empe ra tu r e o f f i r s t - s t age annea l i ng exe r t s cons ide rab l e i n f l uence on t he number o f t empe r -ca rbon pa r t i c l e s p roduced . I nc r ea s ing annea l i ng t empe ra tu r e a cce l e r a t e s t he r a t e decompos i t i on o f p r ima ry ca rb ide and p roduces more g r aph i t e pa r t i c l e s pe r un i t a r ea . Howeve r , h igh f i r s t - s t age annea l i ng t empe ra tu r e s r e su l t i n exce s s ive d i s t o r t i on o f c a s t i ngs du r ing annea l i ng and t he need fo r - s t r a i gh t en ing ope ra t i ons a f t e r hea t t r e a tmen t Annea l i ng t empe ra tu r e s a r e ad ju s t ed t o p rov ide max imum p rac t i c a l annea l i ng r a t e s and min imum d i s t o r t i on and a r e t he r e fo re con t ro l l ed be tween 1650 and 1750°F . The wh i t e - i r on ca s t i ng i s he ld a t t he f i r s t -s t age annea l i ng t empe ra tu r e un t i l a l l mas s ive ca rb ide s have been decomposed . S ince g r aph i t i z a t i on i s a r e l a t i ve ly s l ow p roce s s , t he c a s t i ng mus t be soaked a t t empe ra tu r e fo r a t l e a s t 20 h , and l a rge l oads may r equ i r e a s much a s 72 h . The s t ruc tu r e a t comple t i on o f f i r s t - s t age g r aph i t i z a t i on cons i s t s o f t empe r - ca rbon nodu l e s d i s t r i bu t ed t h roughou t t he ma t r i x o f s a tu r a t ed aus t en i t e .Af t e r f i r s t - s t age annea l i ng , t he c a s t i ngs a r e coo l ed a s r ap id ly a s p r ac t i c a l t o abou t 1400°F i n p r epa ra t i on fo r t he s econd s t age o f t he annea l i ng t r e a tmen t . The f a s t coo l i ng cyc l e u sua l l y r equ i r e s 2 t o 6 h , depend ing on t he equ ipmen t u sed . I n t he s econd - s t age annea l i ng , t he c a s t i ngs a r e coo l ed s l owly a t a r a t e o f 5 t o 15°F /h t h rough t he c r i t i c a l r ange a t wh ich t he eu t ec to id r e ac t i on wou ld t ake p l ace . Dur ing t he s l ow coo l i ng , t he c a rbon d i s so lved i n t he aus t en i t e i s conve r t ed t o g r aph i t e on t he ex i s t i ng t empe r - ca rbon pa r t i c l e s , and t he r ema in ing aus t en i t e t r ans fo rms i n to f e r r i t e . Once g r aph i t i z a t i on i s comple t e , no fu r t he r s t r uc tu r a l changes t ake p l ace du r ing coo l i ng t o room t empe ra tu r e , and t he s t r uc tu r e cons i s t s o f t empe r - ca rbon nodu l e s i n a f e r r i t e ma t r i x (F igu re . 17 ) . Th i s t ype i s known a s s t anda rd o r Fe r r i t i c ma l l e ab l e i r on . The changes i n m ic ros t ruc tu r e du r ing t he ma l l e ab i l i s i ng cyc l e a r e shown schema t i ca l l y i n F igu re . 18 .

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.

F i g u r e . 1 8 T h e c h a n g e s i n m i c r o s t r u c t u r e a s a f u n c t i o n o f t h e m a l l e a b i l i s i n g c y c l e r e s u l t i n g i n t e m p e r c a r b o n i n a f e r r i t e m a t r i x .

TYPES OF MALLEABLE CAST IRONS

1. Ferrite malleable iron: The s t ruc tu r e cons i s t s o f nodu l e s o f

t empe r c a rbon embedded i n f e r r i t e ma t r i x (due t o s l ow coo l i ng i n eu t ec to id t empe ra tu r e r ange ) . As t he se nodu l e s b r eak t he con t i nu i t y t o l e s se r damag ing ex t en t o f t ough f e r r i t e . The ca s t i ngs a r e coo l ed a t s l owly a t t he r a t e o f 5 t o 15 °F /h r . Th rough t he c r i t i c a l r ange a t wh ich t he eu t ec to id r e ac t i on wou ld t ake p l ace . Dur ing t he s l ow coo l i ng t he ca rbon d i s so lved i n t he aus t en i t e i s conve r t ed t o g r aph i t e on t he ex i s t i ng t empe r –ca rbon pa r t i c l e s & r ema in ing aus t en i t e t r ans fo rms i n to f e r r i t e . Once g r aph i t i z a t i on i s comple t e no fu r t he r s t r uc tu r a l changes t ake s p l ace du r ing coo l i ng t o room t empe ra tu r e and t he s t r uc tu r e cons i s t o f t empe r - c a rbon nodu l e s i n a f e r r i t e ma t r i x . Th i s known a s f e r r i t e ma l l e ab l e C . I .

Properties In t he fo rm o f compac t nodu l e s , t he t empe r c a rbon does no t

b r eak up t he con t i nu i t y o f t he t ough Fe r r i t i c ma t r i x . Th i s r e su l t s i n a h ighe r s t r eng th and duc t i l i t y t han exh ib i t ed by g r ay ca s t i r on . The g r aph i t e nodu l e s a l so s e rve t o l ub r i c a t e cu t t i ng t oo l s , wh i ch accoun t s f o r t he ve ry h igh mach inab i l i t y o f ma l l e ab l e i r on .

The Fe r r i t i c ma l l e ab l e i r on shows h ighe r s t r eng th and duc t i l i t y t han g r ay ca s t i r on .

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Graph i t e nodu l e s l ub r i c a t e t he cu t t i ng t oo l s l e ad ing t o good mach inab i l i t y o f ma l l e ab l e i r on .

Application Fe r r i t i c ma l l e ab l e i r on ha s been u sed fo r p ipe f i t t i ngs , expans ion j o in t s , r a i l i ng ca s t i ng on b r i dges , n -ho i s t a s s emb l i e s , bea r i ng b locks , va lve s , f a rm equ ipmen t , cha in s , au tomob i l e pa r t s , i n gene ra l ha rdware , r educ ing gea r hous ings , r e a r - ax l e hous ings , hubs , hooks , shack l e s , l e ads , yokes , nu t s , muf f l e r s , f l anges , coup l i ngs .

2 . Pearlitic malleable iron: To ob t a in pea r l i t i c ma t r i x , 1% manganese i s added t o c a s t i r on , o r s econd - s t age g r aph i t i s a t i on i s r ep l aced by a quench , u sua l l y a i r , wh i ch coo l s t he c a s t i ngs t h rough t he eu t ec to id r ange f a s t enough t o r e t a i n combined ca rbon t h roughou t t he ma t r i x . The amoun t o f Pea r l i t e f o rmed depends upon t he t empe ra tu r e a t wh i ch t he quench s t a r t s and r a t e o f coo l i ng . I f t he a i r quench p roduces a f a s t enough coo l i ng r a t e t h rough t he eu t ec to id r ang , t he ma t r i x w i l l be comple t e ly pea r l i t i c . A fu l l y f e r r i t i c ma l l e ab l e i r on may be conve r t ed i n to pea r l i t i c ma l l e ab l e i r on by r ehea t i ng above t he l ower c r i t i c a l t empe ra tu r e , f o l l owed by r ap id coo l i ng . A t h ighe r t empe ra tu r e c a rbon w i l l be d i s so lved f rom the g r aph i t e nodu l e s and subsequen t coo l i ng r e t a i n s t he combined ca rbon . The coo l i ng f rom t empe ra tu r e o f f i r s t - s t age g r aph i t i s a t i on ( cu rve I I i n F igu re 19 ) . Norma l ly , a f t e r a i r coo l i ng , t he pea r l i t i c ma l l e ab l e c a s t i r on ca s t i ngs a r e hea t ed t o h ighe r t empe ra tu r e s ( c a l l ed d r awing p roce s s ) a t 550 - 650°C o r so t o sphe ro id i s e t he Pea r l i t e t o improve t he mach inab i l i t y , duc t i l i t y and t oughnes s w i th s l i gh t d rop i n ha rdnes s and s t r eng th . Pea r l i t i c ma l l e ab l e i r on can be ha rdened and t empe red . We ld ing o f pea r l i t i c ma l l e ab l e i r on i s r a r e ly u sed due t o t he fo rma t ion o f b r i t t l e and l ow s t r eng th wh i t e i r on unde r t he we ld bead . P r e sence o f l a rge r amoun t o f s i l i con i n wh i t e c a s t i r on ca s t i ngs he lp s t o g r aph i t i s e i t du r i ng ma l l e ab l e hea t t r e a tmen t . Bu t a t h i ck ca s t i ng hav ing h ighe r s i l i con may r e su l t i n g r ay i r on i n t he c en t r e wh i l e c a s t i ng i t . As t he c a s t i ng shou ld he o f wh i t e i r on up t o t he c en t r e be fo re ma l l e ab l e hea t t r e a tmen t i s g iven , s i l i con con t en t ha s t o be kep t l ow in t he compos i t i on , wh ich makes g r aph i t i s a t i on du r ing ma l l e ab l e t r e a tmen t a l ong and d i f f i cu l t p roce s s .

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Figure 19 Typical Malleabilising iron

Application: Due t o h igh s t r eng th and ha rdnes s , pea r l i t i c ma l l e ab l e i r on i s u sed fo r c am sha f t s , c r ank - sha f t s , ax l e s , d i f f e r en t i a l hous ing i n au tomob i l e i ndus t ry , r o l l s , pumps , nozz l e s , gea r s , l i nks , sp rocke t s , e l eva to r b r acke t s i n conveye r equ ipmen t , hammers , wrenches , shows , sw i t ch gea r pa r t s , f i t t i ngs fo r h igh and l ow vo l t age t r ansmi s s ion and d i s t r i bu t i on sy s t em, j aws o f un ive r sa l - j o in t sha f t s , l i nks and ro l l e r s o f conveye r cha in s , bush ings , coup l i ngs b r ake - shoes . Ma l l e ab l e i r ons a r e u sed ch i e f l y fo r t h in wa l l ed ca s t i ngs because t he r e a r e r e s t r i c t i ons i n

s ec t i on t h i cknes s .

Properties: The ma l l e ab l e c a s t i r ons have r ea sonab l e duc t i l i t y , h igh s t r eng th , t oughnes s and even a r e bendab l e . The ma in r ea sons o f u s ing ma l l e ab l e i r ons a r e l ow cos t and ea se o f mach in ing w i th above p rope r t i e s . Ma l l e ab l e i r on ha s l im i t a t i ons o f s ec t i on t h i cknes s , l owe r damping capac i t y and impac t r e s i s t ance .

VII. SPHEROIDAL GRAPHITE IRON (S.G. IRON) Thi s c a s t i r on a l so known a s nodu l a r c a s t i r on . I n an o rd ina ry g r ey ca s t i r on g r aph i t e i s p r e sen t a s ' f l ake s ' wh i ch t end t o have sha rp -edged r ims . S ince t he se f l ake s have neg l i g ib l e s t r eng th t hey ac t a s w ide - f aced d i s con t i nu i t i e s i n t he s t r uc tu r e wh i l s t t he sha rp -edged r ims i n t roduce r eg ions o f s t r e s s - concen t r a t i on . I n SG ca s t i r on t he g r aph i t e f l ake s a r e

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r ep l aced by sphe r i c a l pa r t i c l e s o f g r aph i t e (F igu re . 20 a ) , so t ha t t he me t a l l i c ma t r i x i s much l e s s b roken up , and t he sha rp s t r e s s r a i s e r s a r e e l im ina t ed .

Figure 20 a) A Spheroidal-graphite cast iron. Here the graphite has been made to precipitate in nodular form by adding a nickel-magnesium alloyb) A compacted graphite cast iron. Unetched to show the rounded edges of the graphite flakes,

The fo rma t ion o f t h i s Sphe ro ida l g r aph i t e i s e f f ec t ed by add ing sma l l amoun t s o f c e r i um o r magnes ium to t he mo l t en i r on j u s t be fo re c a s t i ng .

S ince bo th o f t he se e l emen t s have s t rong ca rb ide - fo rming t endenc i e s , t he s i l i con con t en t o f t he i r on mus t be h igh enough ( a t l e a s t 2 . 5%) i n o rde r t o p r even t t he fo rma t ion o f wh i t e i r on (by ch i l l i ng ) i n t h in s ec t i ons . Magnes ium i s t he more w ide ly u sed , and i s u sua l l y added ( a s a n i cke l -magnes ium a l l oy ) i n amoun t s su f f i c i en t t o g ive a r e s i dua l magnes ium con t en t o f 0 .1% in t he i r on . SG ca s t i r ons p roduced by t he magnes ium p roces s have t ens i l e s t r eng th s o f up t o 900 N /mm 2 o r even h ighe r i n some hea t - t r e a t ed i r ons . The t e rm 4SG i ron ' r e a l l y de sc r i be s a f ami ly o f c a s t i r ons , wh ich i nc lude some a l l oy i r ons , bu t i n a l l c a se s t r e a tmen t by i nocu l an t s i s emp loyed t o p roduce Sphe ro ida l -g r aph i t e

pa r t i c l e s . Some SG i ron p roduced by u s ing t he fo l l owing subs t ances i n s t e ad o f c e r i um o r magnes ium: ca l c ium, c a l c ium ca rb ide , c a l c ium f l uo r ide , l i t h i um, s t r on t i um, ba r i um and a rgon . Those i r ons cons i s t i ng o f g r aph i t e nodu l e s i n a f e r r i t e ma t r i x w i l l have h igh duc t i l i t y and

t oughnes s wh i l s t t hose cons i s t i ng o f g r aph i t e nodu l e s i n a pea r l i t e ma t r i x w i l l be cha rac t e r i s ed by h igh s t r eng th . Some o f t he se i r ons a r e hea t - t r e a t ed t o g ive even be t t e r mechan i ca l p rope r t i e s . Thus , Amer i can mo to r i ndus t ry ha rdens some o f t he i r SG i ron gea r s by t he u se o f ' i n t e r rup t ed aus t empe r ing ' . Th i s i nvo lve s aus t en i t i s i ng t he gea r s a t 9000C fo r 3 .5 h i n a n i t r ogen a tmosphe re fo l l owed by quench ing t o 235°C and ho ld ing a t t ha t t empe ra tu r e fo r 2 hou r . S ince t r ans fo rma t ion f rom aus t en i t e occu r s i so the rma l ly a t 235°C the r e i s l i t t l e d i s t o r t i on i n

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shape . I t i s c l a imed t ha t SG i ron hypo id r i ng and p in ion gea r s a r e compa rab l e w i th t hose o f s t e e l i n t e rms o f f a t i gue and a l so have a g r ea t e r t o r s i ona l s t r eng th .

Tens i l e s t r eng th s o f t he o rde r o f 1600 N /mm2 (w i th an e longa t i on o f . 1%) can be ob t a ined by aus t empe r ing SG i ron a t 2500C , fo l l owing an i n i t i a l au s t en i t i s i ng a t 9000C; wh i l s t h ighe r aus t empe r ing t empe ra tu r e s up t o 4500C wi l l y i e l d ba in i t i c s t r uc tu r e s o f l ower s t r eng th s (900 -1200 N /mm 2 ) bu t e l onga t i ons up t o 14%. SG i ron c r anksha f t s c a s t t o nea r f i na l shape a r e l e s s expens ive and some 10% l i gh t e r t han equ iva l en t f o rged componen t s . They a r e hea t - t r e a t ed i n a s im i l a r way t o t he gea r s men t i oned above .

Properties of S.G. Iron:

S .G . i r ons have h ighe r mechan i ca l p rope r t i e s , a lmos t equa l t o c a s t c a rbon s t e e l s ( t hus u sed fo r p ipe s ) , such a s t en s i l e s t r eng th , duc t i l i t y and t oughnes s (Tab l e 6 ) , combined w i th f avou rab l e p rope r t i e s o f g r ay ca s t i r ons , l i ke good mach inab i l i t y , damp ing capac i t y , h igh wea r r e s i s t ance , r e a sonab l e c a s t ab i l i t y , bu t do no t su f f e r f rom the de f ec t s o f g r ay i r ons such a s g rowth and f i r e c r aze s , when u sed a t e l eva t ed

t empe ra tu r e s , and i s l e s s s ec t i on - sens i t i ve .

Table 6 Properties of S.G. Iron

Grade Minimum Tensile

strengthMN/m2

Minimum, 0.2% Proof stress MN/m2

BHN % Elongations

Matrix HeatTreatment

350/22 350 220 130 22 Ferrite Annealed420/12 420 270 150 12 Mainly Ferritic Annealed500/7 500 320 275 7 Ferrite+Pearlite Annealed800/7 800 480 320 2 Tempered

martensiteQuenched and Tempered

400/40 400 200 130 40 Austenite Cast

STEPS IN PRODUCTION OF S.G. IRON

1. Desulphurisat ion : Su lphu r he lp s t o fo rm g raph i t e a s f l ake s .

Thus , t he r aw ma te r i a l f o r p roduc ing i r on shou ld have l ow su lphu r ( l e s s t ha t 0 . 1%) , o r r emove su lphu r f rom i ron du r ing me l t i ng , o r by mix ing i r on w i th a de su lphu r i s i ng agen t such a s c a l c ium ca rb ide , o r soda a sh ( sod ium ca rbona t e ) .

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2. Nodul is ing : Magnes ium i s added t o r emove su lphu r and oxygen

s t i l l p r e sen t i n t he l i qu id a l l oy and p rov ide s 0 .04% magnes ium, wh ich cause s g rowth o f g r aph i t e t o be Sphe ro ida l . Magnes ium t r ea tmen t de su lphu r i s e s t he i r on t o be low 0 .02% S be fo re a l l oy ing w i th i t . Magnes ium and such e l emen t s have s t rong a f f i n i t y fo r su lphu r , and t hus s cavenge su lphu r f rom the mo l t en a l l oy ’ s an i n i t i a l s t ep o r , p roduc ing S .G . i r on . These add i t i ons a r e expens ive t o i nc r ea se t he cos t o f S .G . i r on p roduced . Thus , su lphu r S mo l t en a l l oy (o r t he r aw ma te r i a l u sed ) , be fo re nodu l i s i ng , shou ld be kep t l ow .

Magnes ium i s added when me l t i s nea r 1500°C bu t magnes ium vapo r i s e s a t 1 150°C . Magnes ium, be ing l i gh t e r f l oa t s on t he t op o f t he ba th , and be ing r eac t i ve bu rn o f f a t t he su r f ace . I n such ca se s magnes ium i s added a s N i -Mg , N i -S i -Mg a l l oy o r magnes ium coke t o r educe v io l ence o f r e ac t i on and t o have s av ing i n magnes ium. Magnes ium me ta l c an be added a s me t a l i t s e l f The me thod o f add i t i on i nc lude l ad l e t r ans f e r , cove red l ad l e t e chn ique , po rous p lug s t i r r i ng , and i n -mou ld t e chn ique . Add i t i on o f magnes ium and f e r ro s i l i con i s done sho r t l y be fo re c a s t i ng .

3 . Inoculation : As magnes ium i s c a rb ide fo rmer , f e r ro s i l i con i s

added immed ia t e ly a s i nocu l an t . Reme l t i ng cause s r eve r s i on t o f l ake g r aph i t e due t o t he l o s s o f magnes ium. S t i r r i ng o f mo l t en a l l oy a f t e r add i t i on o f nodu l i s i ng e l emen t evo lve s a l o t o f ga s , wh i ch ge t s d i s so lved i n l i qu id a l l oy , and fo rms b low-ho l e s i n so l i d c a s t i ng . The con t r ac t i on du r ing so l i d i f i c a t i on o f nodu l a r c a s t i r on ca s t i ngs i s much g rea t e r t han o f g r ay i r on ca s t i ngs , wh ich needs ca r e fu l de s ign o f mou lds t o avo id sh r i nkage cav i t i e s i n so l i d i f i ed ca s t i ngs .

In sp i t e o f t he se d r awbacks , nodu l a r c a s t i r on i s r ep l ac ing g r ay i r on and s t e e l s i n app l i c a t i ons . A nodu l e o f g r aph i t e ( hav ing min imum su r f ace a r ea pe r un i t vo lume) weakens t he s t e e l ma t r i x t o a l e s se r ex t en t t han g r ay i r on f l ake s . The nodu l e s don ’ t a c t ve ry much a s s t r e s s - r a i s e r s . F igu re .19 I l l u s t r a t e s r ange o f c a rbon and s i l i con i n S .G . i r on . One o f t he compos i t i on o f S .G . i r on can be :

C = 3 .7%, S i = 2 .5%, Mn = 0 .3% S = 0 .01%, P = 0 .01%, Mg = 0 .04%

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Range of carbon and silicon for S.G irons. Figure 21

Helpful Neutral InhibitorsMg,Ce,Ca,Ba,Li,Zr Fe,C,Ni,Si,Mo Al,Ti,Sb,As,Pb,Bi

Table 7 illustrates effect of some elements in the production of S.G. iron.

Figure. 22 illustrates various C.E.V. depending on the maximum thickness of the casting in sand moulds and in metal moulds.

The ma t r i x o f a s - ca s t S .G . i r on depends on t he compos i t i on and t he r a t e o f coo l i ng . Comple t e Fe r r i t i c o r ma t r i x hav ing a max imum o f 10% Pea r l i t e , s t i l l c a l l ed Fe r r i t i c ma t r i x , pos se s se s max imum duc t i l i t y , t oughnes s and mach inab i l i t y , F igu re . 23 (a ) . A l a rge ly pea r l i t i c ma t r i x 23 (b ) , ob t a ined i n a s - ca s t , o r by no rma l i s i ng ( a i r coo l i ng f rom 850 t o 900°C) makes S .G . i r on s t ronge r bu t l e s s duc t i l e . O i l o r wa t e r quench ing f rom 900° -950°C y i e ld s ma r t ens i t e ma t r i x , wh i ch i s t empe red t o de s i r ed s t r eng th , ha rdnes s and t oughnes s . Aus t en i t i c

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duc t i l e ma t r i x , F igu re .21 (d ) (wh ich can be r e t a i ned up t o 25°C) i s ob t a ined by a l l oy ing (15 -36% Ni , 1 . 8 -6% Cr ) t he c a s t i r on t o have h igh co r ro s ion r e s i s t ance and good c r eep r e s i s t ance a t h igh t empe ra tu r e s . F igu re .21 (c ) i l l u s t r a t e s , bu l l s eye S .G . i r on , whe re f e r r i t e i n immed ia t e v i c in i t y o f g r aph i t e i s p r e sen t i n ma in ly Pea r l i t e ma t r i x .

Figure.23 Microstructures of S.G. irons. (a) Ferrite S.G. iron. x 250, (b) Pearlitic S.G. Iron. x 500, (c) Bull’s eye S.G. Iron. x 100, (d) Austenitic S.G. iron (Ni-Resist 21.06% Ni, 2.20% Cr,0.06% Mg) as cast. X 500 (nital)

Application of S.G. Iron:

S .G . i r on i s u sed fo r gea r pumps fo r p roce s s ing and t r anspo r t o f su lphu r i c a c id , pumps and va lve s i n s ea wa t e r app l i c a t i ons , componen t s u sed i n s t e am se rv i ce s , and i n t he hand l i ng o f a l ka l i , c aus t i c and ammon ia - ca l so lu t i ons , and fo r pumping and hand l i ng o f sou r c rude o i l s i n pe t ro l eum indus t ry . O the r w ide app l i c a t i ons a r e -

Crank - sha f t s

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Pi s tons and cy l i nde r heads i n au tomob i l e and d i e se l eng ine s Pre s su re c a s t i ngs l i ke gea r s and ro l l e r s l i de s Stee r i ng knuck l e s Rocke r a rms Pape r m i l l d rye r r o l l s Bea r ing

VIII. COMPACTED/VERMICULAR CAST IRON

Thi s i s t he l a t e s t member t o j o in t he f ami ly o f c a s t i r ons i n wh ich g r aph i t e occu r s a s worm- l i ke b lun t - edged s t ubby f l ake s ( rounded rods , wh ich a r e i n t e r connec t ed w i th in eu t ec t i c c e l l ) ; embedded i n s t e e l ma t r i x , F igu re .24 The fo rma t ion o f compac t ed i r on depends on t he chemica l compos i t i on , s ec t i on t h i cknes s . and t he p roce s s u sed fo r p roduc t i on . Norma l ly , 10 -20% o f t he sphe ro ida l g r aph i t e may be p r e sen t , wh i ch r equ i r e s C .E .V . o f 4 .00 , and f l ake -g raph i t e shou ld be avo ided . I n one o f t he p roduc t i on me thods , n i t r ogen (—0.015%) i s added t o l i qu id a l l oy i n l ad l e by add ing n i t r i de Fe r ro -manganese

Figure.24 Microstructure of compacted vermicular cast iron.

(80% Mn, 4% N, r e s t Fe ) . Th i s me thod g ive s non -un i fo rmi ty o f s t r uc tu r e and unsoundnes s i n c a s t i ngs . I n ano the r me thod , an a l l oy (4 -5% Mg, 8 .5 -10 .5% T i , 4 -5 .5% Ca , 1 -1 .5% Al , 0 . 2 -0 ,5% Ce , 48 -52% S i , r e s t Fe ) i n amoun t s 0 .6 -1 .6% i s added , a s add i t i ons a r e made t o p roduce S .G . i r on . Su lphu r con t en t o f i r on shou ld no t be more t han 0 .035%. Th i s me thod i s s ec t i on - sens i t i ve a s sphe ro id s ge t f o rmed i n t h in

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sec t i ons . The compac t ed g r aph i t e pe rmi t s s t r eng th . S t i f fne s s and

duc t i l i t y t ha t exceeds t hose o f g r ay i r on .

Properties:

Compac t ed ca s t i r on t o r e t a i n good damping capac i t y , and t he rma l conduc t i v i t y . I t s r e s i s t ance t o c r az ing , t r a ck ing and d i s t o r t i on i s supe r io r t o bo th S .G . i r on and g r ay i r on . As t he sh r i nkage du r ing ca s t i ng i s l e s s t han i n S .G . i r on . Th i s c a s t i r on hav ing i n f e r i o r mechan i ca l p rope r t i e s hu t s im i l a r p roduc t i on cos t s a s S .G . I ron ha s l im i t ed r ep l acemen t po t en t i a l t o S .G . i r on pa r t s . Howeve r , because o f g r ea t e r s t r eng th and t oughnes s , i t c an r ep l ace more expens ive a l l oyed g r ay ca s t i r ons .

Applications:

Compacted cast iron is used for making thick sections.

Hydrau l i c va lve s Ingo t mou lds Cyl inde r heads Exhaus t man i fo ld s , Brake d rums Disc s and p i s t on r i ngs a r e made f rom th i s i r on a s i t ha s good

e l eva t ed t empe ra tu r e p rope r t i e s .

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Figure. 25 Types of cast irons (a) Gray iron, (b) White iron, (c) Malleable iron, (d) S.G. iron, compacted graphite iron.

IX. ALLOYED CAST IRONS

One o r more o f t he e l emen t s l i ke , N i , C r , Cu , S i , Mo , V e t c . (> 3%) a r e added i n to g r aph i t e f r e e , o r g r aph i t e -bea r i ng ca s t i r ons t o improve co r ro s ion , e l eva t ed t empe ra tu r e and wea r and ab ra s ion r e s i s t ance p rope r t i e s .

1. Ni-hard: In t he wh i t e i r on compos i t i on , 3 -5% Ni and 1 -3% Cr a r e added , p roduc ing a m ic ros t ruc tu r e cons i s t i ng o f mass ive con t i nuous ca rb ide s i n t he ma t r i x o f ma r t ens i t e and some r e t a ined aus t en i t e on coo l i ng a f t e r so l i d i f i c a t i on . Mar t ens i t e i s ob t a ined due t o i nc r ea sed ha rdenab i l i t y , due t o t he p r e sence o f t he se e l emen t s , wh i ch a long w i th c a rbon l ower t he M f t empe ra tu r e t o be low room t empe ra tu r e t o r e t a i n some aus t en i t e . Ha rdnes s a t t a i ned i s 550 -700 BHN. As n i cke l i s ha l f a s power fu l a g r aph i t i s e r a s s i l i con , t he r i sk o f g r aph i t i s a t i on i s p r even t ed by add ing ca rb ide - fo rmer , ch romium. The poo r impac t s t r eng th and f a t i gue r e s i s t ance due t o t he con t i nuous ne twork o f c a rb ide s c an be improved by i nc r ea s ing N i and Cr con t en t . The mod i f i ed N i -ha rd hav ing 4 -8% Ni and 4 -15% Cr , a f t e r hea t t r e a tmen t ha s a m ic ros t ruc tu r e o f d i s con t i nuous ca rb ide s i n t he ma t r i x o f t empe red mar t ens i t e and Ba in i t e .

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Figure . 26 Ni-Hard cas t i ron micrography

Propert ies:

Very good u su ry s t r eng th un t i l 700°C

These ca s t i r ons have exce l l en t wea r r e s i s t ance .

2. Ni-Resist: Ni (13 -36%) and Cr (1 .8 -6%) a r e added t o p roduce aus t en i t i c ma t r i x w i th f l ake o r Sphe ro ida l g r aph i t e , t o ge t good co r ro s ion r e s i s t ance . The l a t t e r o f f e r s be t t e r mechan i ca l p rope r t i e s bu t a r e more expens ive . N i be ing aus t en i t e s t ab i l i s e r makes t he ma t r i x aus t en i t i c , and t hus , t he se a r e c a l l ed aus t en i t i c c a s t i r ons . The concen t r a t i on o f t he e l emen t s depends on t he na tu r e o f t he co r ro s ion env i ronmen t . Ch romium in combina t i on w i th n i cke l f o rms an e f f ec t i ve ox ida t i on r e s i s t an t s ca l e . N i - r e s i s t s combine good co r ro s ion r e s i s t ance , exce l l en t e ro s ion r e s i s t ance t o t he f l ow o f l i qu id s w i th hea t r e s i s t i ng p rope r t i e s . Some Ni - r e s i s t s con t a in 5 .5 -8 .0% coppe r . Though , t he se a l l oys cou ld be u sed up t o 800°C , bu t a f t e r s t ab i l i s a t i on a t 950°C . These a l l oys cou ld be u sed a t t empe ra tu r e s h ighe r t han 800°C . Ave rage p rope r t i e s a r e :

T .S . = 247 — 485 MN&2

BHN = 120 - 250

%E =2-15%

Impor t an t app l i c a t i ons a r e gea r pumps ( fo r p roce s s ing and t r anspo r t o f su lphu r i c a c id ) , pumps and va lve s i n s ea -wa t e r app l i c a t i ons , pa r t s u sed i n s t e am and fo r hand l i ng o f a l ka l i , c aus t i c , f o r pumping and hand l i ng

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of sou r c rude o i l s i n pe t ro l eum indus t ry . Fu rnace pa r t s , cy l i nde r l i ne r s , exhaus t man i fo ld s , e t c .

F igure . 27 Ni -Res is t cast i ron micrography

3. Silal and Nicrosilal: Si l a l i s t he cheapes t ox ida t i on and g rowth -r e s i s t an t c a s t i r on , pa r t i cu l a r l y t he l ow ca rbon ca s t i r on r e s i s t s up t o 750°C . Thy compos i t i on on an ave rage i s :

C = 2 .3%; S i=5 .5 -7 .0%; Mn=0 .5 -0 .8%; S=0 .06%; P=0 .1 -0 .3%

T.S . = 139—263 Nmm - 2

BHN 220 — 255

The mic ros t ruc tu r e o f S i l a l cons i s t s o f f e r r i t e and f i ne g r aph i t e ‘D’ t ype f l ake s . These ca s t i r ons a r e ve ry b r i t t l e . S i l i con i nc r ea se ox ida t i on r e s i s t ance by fo rming a r e s i s t i t ox ide f i lm , and w i th more s i l i con , an impe rmeab l e s i l i c a t e f i lm . N ic ros i l a l i s N i -Cr added . S i l a l wh i ch g ive s aus t en i t i c ma t r i x r educ ing t he b r i t t l ene s s , and can be u sed a t 650 -900°C . The compos i t i on i s :

C 1 .5 -2 .0%; S i = 4 .5 -5 .0%, Mn = 0 .6 -1 .0%; S = 0 .10%; p <0 .1%,

N i = 18— 23%; Cr = 2 -2 .4%

T.S . 139 -247 Nmm - 2

BHN = 150 -200

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Nic ros i l a l o f f e r s exce l l en t co r ro s ion r e s i s t ance . Common app l i c a t i ons a r e : I ngo t -mou lds . Cy l i nde r heads exhaus t man i fo ld s , a l umin ium me l t i ng c ruc ib l e s , r e t o r t s , g l a s s -mou lds , ga s - t u rb ine pa r t s .

Table (8) Comparative qualities of cast irons

Comparative qualities of cast ironsName Nominal

composition [% by

weight]

Form and condition

Yield strength

ksi (0.2% offset)]

Tensile strength

[ksi]

Elongation [% (in

2 inches)]

Hardness [Brinell scale]

Uses

Grey cast iron

(ASTM A48)

C 3.4, Si 1.8, Mn 0.5

Cast — 25 0.5 180 Engine cylinder blocks, flywheels, gears, machine-tool bases

White cast iron

C 3.4, Si 0.7, Mn 0.6

Cast (as cast)

— 25 0 450 Bearing surfaces

Malleable iron

(ASTM A47)

C 2.5, Si 1.0, Mn 0.55

Cast (annealed)

33 52 12 130 Axle bearings, track wheels, automotive crankshafts

Ductile or nodular

iron

C 3.4, P 0.1, Mn 0.4, Ni 1.0, Mg 0.06

Cast 53 70 18 170 Gears, camshafts, crankshafts

Ductile or nodular

iron (ASTM A339)

— cast (quench tempered)

108 135 5 310 —

Ni-hard type 2

C 2.7, Si 0.6, Mn 0.5, Ni 4.5, Cr 2.0

Sand-cast — 55 — 550 High strength applications

Ni-resist type 2

C 3.0, Si 2.0, Mn 1.0, Ni 20.0, Cr 2.5

Cast — 27 2 140 Resistance to heat and corrosion

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HEAT TREATMENT OF CAST IRONS

The common heat t rea tments g iven to cas t i rons are :

I . STRESS-RELIEVING TREATMENT

Res idua l - s t r e s se s deve lop du r ing so l i d i f i c a t i on and d i f f e r en t i a l coo l i ng and t hus c ause d i f f e r en t i a l con t r ac t i on . The rma l g r ad i en t s and r e s idua l -s t r e s se s a r e more p ronounced i n c a s t i ngs w i th non -un i fo rm c ros s -s ec t i ons . Phase t r ans fo rma t ions a ccompan i ed w i th vo lume changes agg rava t e t he s i t ua t i on fu r t he r . Cas t i ngs a r e s l owly hea t ed t o a t empe ra tu r e 480 -650°C , no rma l ly a t 600°C and t hen fu rnace coo l ed t o

200°C , fo l l owed by a i r coo l i ng .

II . ANNEALING

The a im i s t o decompose ca rb ide s and Pea r l i t e f rom the a s c a s t -s t r uc tu r e . Th i s g ive s g r aph i t e i n Fe r r i t i c ma t r i x . Gray ca s t i r on and S .G . i r ons ge t so f t ened i nc r ea s ing duc t i l i t y and mach inab i l i t y . Whi t e c a s t i r on ge t s ma l l e ab l i s ed .

A typ i ca l two s t age p roce s s pa r t i cu l a r l y fo r S .G . i r on cou ld be u sed : F i r s t au s t en i t i s i ng a t 900°C and t hen coo l t o t r ans fo rm to Pea r l i t e t o 675°C and t hen f e r r i t i z a t i on o f Pea r l i t e i s done a t 760°C . A i r coo l i ng may be done un l e s s c a s t i ng i s su scep t i b l e t o r e s i dua l - s t r e s se s .

III . NORMALISING

I t i s hea t i ng t he c a s t i ngs t o t empe ra tu r e s above t he c r i t i c a l r ange , soak ing a t i t and coo l i ng i n s t i l l a i r a s i nduced by l a rge f ans . Norma l i s i ng g ive s h ighe r ha rdnes s and s t r eng th by ob t a in ing f i ne pea r l i t i c ma t r i x . Tab l e 9 gives normalising temperature range for some cast irons.

Malleable Iron High Strength Gray Iron

Low strength Gray iron

S.G. Iron

Temperature Range

800-830°C 810-870°C 840-900°C 820-900°C

Table 9

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IV. HARDENING AND TEMPERING

Harden ing and t empe r ing i nduce h ighe r s t r eng th s , and good wea r r e s i s t ance . The t ime and t empe ra tu r e aus t en i t i s i ng depends on t he o r i g ina l ma t r i x o f t he c a s t i r on . The t empe ra tu r e i s up t o 50°C above c r i t i c a l t empe ra tu r e r ange , bu t t ime i s impor t an t i n l ow combined -ca rbon -ma t r i x and t hus , soak ing i s con t i nued t i l l d e s i r ed amoun t o f c a rbon ha s been d i s so lved i n aus t en i t e f rom f r ee g r aph i t e . H igh s i l i con ca s t i r ons a r e l e s s r e spons ive t o quench ing and p rone t o c r ack ing a s s i l i con r educes so lub i l i t y o f c a rbon i n aus t en i t e nece s s i t a t i ng h igh t empe ra tu r e s o f aus t en i t i s i ng , bu t wh ich can cause c r ack ing due t o more s eve re quench ing . Wa te r quench ing o f c a s t i ngs ( complex shapes and d i f f e r en t s ec t i oned ) c ause s quench c r acks . O i l quench i s no rma l ly u sed o r even a i r -quench , i f l a rge amoun t s a l l oy ing e l emen t s a r e p r e sen t . Tempe r ing improves t en s i l e s t r eng th , r educ ing ha rdnes s , t hough depends on t empe r ing t empe ra tu r e and t ype o f i r on .

V. MARTEMPERING

I t r educes chances o f d i s t o r t i on and c r acks . Th in -wa l l ed cy l i nde r l i ne r s f o r d i e se l eng ine s (BHN needed 390 -430 ) a r e ma r t empe red . The ca s t i ng i s quenched i n a ho t s a l t ba th , o r o i l kep t s l i gh t l y above M s , t empe ra tu r e ( f rom aus t en i t i s i ng t empe ra tu r e ) t i l l t he c en t r e o f t he c a s t i ng t oo a t t a i n s t he ba th t empe ra tu r e , and t hen a i r coo l ed . Tempe r ing may be done a s u sua l .

VI. AUSTEMPERING

Cas t i ngs aus t en i t i s ed a t 850 -950°C , quenched i n to , s a l t o r o i l ba th , kep t a t t empe ra tu r e 450 -250°C fo r a round 4 h r s . Lower S .G . i r on i s tw ice a s s t r ong w i th s ame t oughnes s . As i t app roaches p rope r t i e s o f s t e e l s , c r ank sha f t s , c amsha f t s , gea r s o f S .G . i r on a r e u sed i n aus t empe red s t a t e .

VII. SURFACE HARDENING

I t i s an economica l me thod t o ge t wea r r e s i s t ance i n s e l ec t ed a r ea s . Excep t i ng wh i t e and h igh ly a l l oyed ca s t i r ons , mos t c a s t i r ons cou ld be su r f ace ha rdened by i nduc t i on , f l ame , l a s e r e t c . Fe r r i t i c ma t r i x i s no t u sed , nece s s i t a t i ng a pea r l i t i c ( even bu l l s eye ) , o r t empe red mar t ens i t i c ma t r i x . F l ame ha rden ing r equ i r e s combined ca rbon o f 0 .5 -0 .7% in ma t r i x . I nduc t i on ha rden ing i s good fo r mass p roduc t i on . E l ec t ron beam, p l a sma and l a se r a r e i nc r ea s ing ly u sed me thods fo r su r f ace

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ha rden ing . Gene ra l Mo to r s have been u s ing g r ay i r on d i e se l eng ine cy l i nde r l i ne r s , wh i ch a r e l a s e r ha rdened .

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References

1. Physical Metallurgy by Vijendra Singh (Standard publications)2. A Introduction to Physical Metallurgy by Sidney H Avner (Tata McGraw-Hill

Publications)3. Physical Metallurgy for Engineers by Clark Donald 4. Engineering metallurgy Raymond Higgins5. http://en.wikipedia.org/wiki/Cast_iron