30852555 Seminar Report on Cast Iron

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INTRODUCTION

I r on , t he s i l ve ry -w h i t i s h me ta l , i s t he mos t impor t an t o f me ta l s s i nce i t

f o rms the ba s i s o f t he s pec t rum o f s t e e l s and ca s t i r on . Today in

i ndus t r i e s s t e e l and ca s t i r on compr i s e w e l l ove r 80% by w e igh t o f C as t

i r on and s t ee l . Pu re i r on* i s no t an ea s y ma te r i a l t o p roduce . Pu re i r on

i s qu i t e s o f t , w eak and expens ive . I f c a rbon i s added in ce r t a in quan t i t y

i n i t , i t w i l l change i t s mechan ica l p rope r t i e s . A cco rd ing t o ca rbon

con t en t w e c l a s s i f i ed i r on ca rbon a l l oys i n to tw o ways :

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

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

C as 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 tw een

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 ,

manganes e , s u lphu r and phos pho rus i n s ubs t an t i a l amoun t s .

Wh y cas t i ron h as i t s name?

H ighe r ca rbon con t en t makes t hem more b r i t t l e . C as 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 aw n , e t c . bu t c an on ly be ‘ ca s t ’ i n to

de s i r ed shape and s i ze 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 ow ing i t t o s o l i d i fy .

D u e to p res en ce o f h igh carb on con ten t in i t mach in ab i l i t y i s p oor s o

cas t in g i s th e on ly an d exc lu s ive ly s u i tab le p roces s t o sh ap e th es e

a l l oys , kn ow n as C as t i ron .

C as 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 s ubs 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 ious s t ep s t o r emove undes i r ab l e

con t amin an t s s uch a s phos pho rus and s u lphu r . The me l t i ng un i t may be

cupo la , e l e c t r i c a r c , and i nduc t ion fu rnaces e t c . The common ca s t i r ons

a r e b r i t t l e and have l ow er s t r eng th p rope r t i e s t han s t ee 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)

E u tec 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 phas e

( l i qu id ) o f Eu tec t i c compos i t i on a t j u s t be fo r e 1147 o C . A nd th i s l i qu id

phas e w i l l t r an s fo rmed in to aus t en i t e and cemen t i t e phas es 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)

Hyp o E u tec t i c C as 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 tw o

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

1147 o C . A nd on ly l i qu id phas e w i l l t r an s fo rmed in to aus t en i t e and

cemen t i t e phas es 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 s en t above Eu tec t i c t empe ra tu r e l i ne i s know n a s p roeu t ec t i c o r

p r imary A us t en i t e .

Hyp er E u tec 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

tw o phas e s ( i . e . c emen t i t e , l i qu id o f Eu tec t i c compos i t i on ) a t j u s t

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

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

i s p r e s en t i n i t i a l l y i s know n a s p roeu t ec t i c o r p r imary cemen t i t e .

Coolin

Heatin

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

Development of Cast Iron

I n i t i a l l y , t he r e a r e tw o types o f C as t i r on ca l l ed Whi t e c a s t i r on and G rey ca s t i r on . I f c a rbon i s i n f o 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 f o 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 h e ca rbon in t he comb ined cemen t i t e f o rm ( f e r r i t e i s a s s umed to poss e s s neg l ig ib l e c a rbon ) . C emen t i t e i s a ha rd , b r i t t l e , w h i t e compound . The f r ac tu r ed su r f ace o f w h i t e c a s t i r on l ooks s i l ve ry - w h i t e due t o wh i t e c emen t i t e , and t ha t i s w h y th e name w h i t e ca s t i ron i s g iven .

G raph i t e i s s o 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 ac tu r e . C as t i r ons con t a in ing g r aph i t e ( a s f l akes ) a r e t hus , ca l l ed gray cas t i ron s . Unde r mic ro s cop ic g r aph i t e f l akes appea r a s i r r egu l a r s t r ands s uch a s ‘ co rn f l akes ’ . As s how n in F igu reu re .1

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

F rom tw o o r ig ina l c a s t i r ons , w h 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 s ence 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 ca 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 ty mos t ex t ens ive ly u s ed i n i ndus t ry ; due

t o i t s c e r t a in s pec i f i c p rope r t i e s . The compres 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 s e t o t he

p rope r t i e s o f t he s t e e l o f s imi 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 s u l t ed i n

f ou r more t ypes o f c a s t i r ons ca 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 ro s 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 akes embedded in 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 G ray i r on a r e

de t e rmin ed 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 ,

s hape and d i s t r i bu t ion o f g r aph i t e i nc lu s ions . Graph i t e f l akes have

w eaken 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 s o f t , pow de ry , and

b r i t t l e , and can be cons ide r ed 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 inu i ty 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 , s hape and d i s t r i bu t ion o f g r aph i t e

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

i s f u r the r d iv ided i n to M eahan i t e c a s t i r on (by mak ing f l akes f i ne r ) ,

S .G C as t i r on ( round s haped f l akes ) , M a l l eab l e C as t i r on , Mo t t l ed C as t

i r on , C h i l l ed C as t I r on e t c .

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Types of Cast Iron

The bes t me thod o f c l a s s i f y ing ca s t i r on i s ba s ed on t ype o f mic ro s t ruc tu r e :

Tw o ma in t ypes o f C as t i r on

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

II. Grey Cast I ron : C a rbon i s i n f o rm o f Graph i t e f l akes

Thes e a l l o the 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 s pec i a l t r e a tm en t ( hea t t r e a tm en 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 .

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III. Chi l led Cast i ron : S u 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 an s i t i on l aye r be tw een G rey ca s t i r on and w h 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 akes

V. Meahani te I ron: C as t i r on ha s ve ry f i ne f l akes 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 ide a s i nocu l an t i n me l t i n l ad l e o the rw i s e i t w ou ld ha s s o l i d i f i ed a s wh i t e c a s t i r on .

VI. Malleable I ron: Thes e cons i s t s o f s t r uc tu r e o f i r r egu l a r ly r ound g r aph i t e pa r t i c l e s c a l l ed t empe r ca rbon and s t r uc tu r e i s ob t a ined by hea t t r e a tmen t

VII. Spheroidal graphi te I r on s t r uc tu r e o f nodu le s embedded in s t e e l ma t r i x , nodu le s a r e o f more r egu l a r , shape and compac t sphe re s .

VIII. Compacted/Vermicular Cast I ron: The g r aph i t e he r e i s i n t e rmed ia t e be tw een f l akes and s phe 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. Alloy Cast I ron: P rope r t i e s and mic ro s t ruc tu r e o f c a s t i r on o r any o r f t he s e 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 .

Graphitisation

We d i s cus s ed i n ea 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 (w h i t e and g r ay ) on t he ba s i s o f c a rbon p r es en t i n i t . I n t h i s s ec t i on w e know tha t how cemen t i t e* and g r aph i t e** a r e f o rmed and a t wha t cond i t i on i n i r on ca rbon d i ag ram. The p rocess o f d i r ec t p r ec ip i t a t i on o f g r aph i t e f r om 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 roces s ca l l ed g r aph i t i s a t i on .

• I n i r on ca rbon d i ag ram g raph i t e (Equ i l i b r ium s t a t e ) i s more s t ab l e phas e t han cemen t i t e (M e 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 f o rm cemen t i t e* t han g r aph i t e** (becaus e 6 .67% C s hou ld s eg rega t e t o nuc l ea t e c emen t i t e whe rea s 100% s eg rega t ion o f c a rbon i s needed to 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 (F C C ) i s r e l a t i ve ly t o t ha t c emen t i t e ( comp lex o r tho rhombic*** ) , bu t d i f f e r s ubs t an t i a l l y f r om g raph i t e (H exagona l l aye r s t r uc tu r e ) .

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• C emen t i t e f o rms more ea s i l y f r om aus t en i t e o r f r om l i qu id becaus e 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 .

• A s k ine t i c a l l y cemen t i t e c an fo rm more ea s i l y i t i s more p robab le t o ge t i n mic ro s t ruc tu r e F e r r i t e + C emen t i t e f r om aus t en i t e t hen F e r r i t e + G raph i t e a l s o t he l i qu id t o f o rm eu t ec t i c a l l y t o aus t en i t e + C emen t i t e and no t au s 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 becaus e g r aph i t e ha s l e s s f r ee ene rgy t han C emen t i t e . When g r aph i t e f o rm d i r ec t l y f r om l i qu id i s c a l l ed p r imary 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 r om l i qu id i s t akes 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 s o fo rma t ion o f g r aph i t e f r om aus t en i t e be tw een 738° C to 727° C wh ich r equ i r e s l ow coo l ing . Th i s g r aph i t e i s know n 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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C ommerc 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 ) w h ich becomes cen t r e o f g r aph i t e c rys t a l l i s a t i on and p romo te g r aph i t e f o rma t ion . When g r aph i t e f o rms f rom d i s s oc 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 . M e ta s t ab l e cemen t i t e above t empe ra tu r e 738° C decompos e t o A us ten i t e+ grap h i t e o r Ferr i t e + grap h i t e b e low 738° C . As w e know tha t s l ow coo l ing o f l i qu id ca s t i r on l e ads t o f o rma t ion o f g r aph i t e and f a s t coo l ing l e ads t o cemen t i t e . Th i s i s so becaus e t he fo rma t ion o f g r aph i t e f r om l i qu id o r au s t en i t e i s ve ry s low coo l ing p roces s and t akes p l ace on ly a t s ma l l unde r coo l ing .

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 w h i t e o r g r ay i r on , i . e . , w he the r c a rbon i s p r e s en t i n t he comb ined fo rm o r i n t he g r aph i t e f o rm a r e :

I . C h emica l comp os i t i on

I I . C oo l in g ra te .

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I. COMPOSITION

( a ) C arb on : H ighe r i s t he ca rbon , more i s g r aph i t e f o rmed and low er t he mechan i ca l p rope r t i e s . C a rbons l ow er t he me l t i ng po in t o f me ta 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 ) S i 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 s e 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 v e p ropo r t i ons o f comb ined ca rbon and f r ee g r aph i t e . I f s i l i con i s p r e s en 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 akes . S i l i con con t en t may va ry be tw een 1 .0% to 3 .5%.S i l i con s h i f t s t he g r aph i t e - eu t ec t i c l i ne upw ards . Thus du r ing coo l ing f rom l i qu id s t a t e , a l a rge r deg ree o f unde r coo l ing i s pos s ib l e w i th g r ea t e r chance t o f o rm g raph i t e be fo r e cemen t i t e f o rma t ion becomes poss ib l e .

( c ) Su lp h u r and Man gan es e : 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 s e s t he s i z e o f t he f l akes , H igh s u lphu r t ends t o r educe f l u id i t y and i s o f t en t he caus e o f b low ho le s i n ca s t i ngs . S u lphu r i s kep t l ow in amoun t o f . 06 t o . 12%.

S u lphu r i n ca s t i r on i s p r e s en t e i t he r a s F eS o r M nS . F eS t ends t o p romo te cemen t i t e f o rma t ion , i . e . , w h i t e c a s t i r on . Mn i s a mi ld ca rb ide fo rming e l emen t . The amoun t o f M n (one pa r t o f S t o 1 . 72 pa r t o f M n) w h ich combines w i th su lphu r t o f o rm M nS 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 ow n 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 s u 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 .

M anganes e i n exces s o f w ha t ha s f o rmed MnS , w eak ly r e t a rd s p r imar i l y g r aph i t i s a t i on . How eve 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 ) Ph os p h orus - M os t c a s t i r on con t a in phos pho rus be tw een . 1 t o .3%. I t s amoun t may be more t han . 9%, t hen i t f o rms i r on phos ph ide (F e3P ) , wh ich fo rm a t e rna ry eu t ec t i c w i th cemen t i t e and aus t en i t e . The t e rna ry Eu tec 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 s e t he f l u id i t y a l s o 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 . H ow eve r f o 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 phos pho rus l e s s t han 0 .3%, w h ich s ha l l be p r e s en t i n d i s s o lve s t a t e i n f e r r i t e .

( d ) C arb on equ iva l en t V a lu e : S i , P ha s s imi l a r e f f ec t on t he mic ro s 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 (C E) = To ta 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 ow er ( t han 4 .3%) , bu t i f C .E i s 4 . 3%, t hen , t he ca s t i r on i s eu t ec t i c c a s t i r on . C a rbon equ iva l en t va lue fo r a g iven coo l ing r a t e , de t e rmin es how c lo s e 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 k e ly t o f o rm. Th i s de t e rmin es p robab le 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 ing du r ing s o 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 the r t han g r aph i t e . Tha t i s , t h e h ighe r t he r a t e o f coo l ing fo r any g iven ca s t - i r on compos i t i on t he ' w h i t e r ' and more b r i t t l e t he ca 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 ion w i th t he cho i ce o f a s u i t ab l e i r on fo r t he p roduc t ion o f c a s t i ngs o f t h in s ec t i on . S uppos ing an i r on wh ich , w hen coo l ed s low ly , had a f i ne g r ey s t r uc tu r e con t a in ing s ma l l eu t ec t i c c e l l s w e re chos en fo r s uch a pu rpos e . I n t h in s ec t i ons i t w ou ld coo l s o 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 comp le t e ly wh i t e i r on w ou ld r e s u l t . S uch a s ec t i on w ou ld be b r i t t l e and u s e 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 s u 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 led 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 w h i t e f r ac tu r e and h igh B r ine l l va lue s . The t h i cke r s ec t i ons , hav ing coo l ed more s low ly , a r e g r aph i t i c and cons equen t ly s o 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 w h i t e s k in on t he s u 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 ca s t i ng t h in s ec t i ons , t hen , i t i s nece s s a ry t o choos e an i r on o f r a the r coa r s e r g r ey f r ac tu r e t han i s r equ i r ed i n t he f i n i s hed ca s t i ng . Tha t i s , t h e 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 s ed fo r t he p roduc t ion 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

Thes e 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 h e ca rbon i s p r e s en t i n t he comb ined cemen t i t e f o rm, w h ich makes t he f r ac tu r e o f t he s e a l l oys t o have du l l and w h i t e co lou r , and t ha t i s t he r ea s on o f t he i r name a s w h i t e i r ons . Typ ica l w h 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 % M n , 0 . 1 – 0 .4%P , 0 . 15%S , and ba l ance F e . F igu re . 3 i l l u s t r a t e s changes occu r r ing on coo l ing i n hypoeu tec t i c wh i t e c a s t i r on . A t r oom 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.

A l l w h i t e c a s t i r ons a r e hypoeu tec t i c a l l oys . The coo l ing o f a 2 . 50 pe r cen t c a rbon a l l oy w i l l now be de s c r ibed . 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 s o lu t i on o f c a rbon d i s s o 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 ing t akes p l ace un t i l t h e l i qu idus l i ne i s c ro s s ed a t x 2 . S o l id i f i c a t i on now beg in s by t he fo rma t ion o f au s 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 . A s t he t empe ra tu r e f a l l s , p r imary aus t en i t e con t inues t o s o l i d i fy , i t s compos i t i on mov ing dow n and to t he r i gh t a long the s o l i dus l i ne t ow ard po in t C . The l i qu id i n t he mean t i me i s becoming r i che r i n ca rbon , i t s compos i t i on a l s o mov ing dow n and to t he r i gh t a long the l i qu idus l i ne t ow ard 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 au s 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 r cen t o f t he a l l oy by w e 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 s o the rma l ly t o f o rm the eu t ec t i c mix tu r e o f au s t en i t e and cemen t i t e know n a s l edebu r i t e .

L iqu id (4 .3%) A us t en i t e ( 2 . 11%) + C emen t i t e ( 6 . 67%)

S ince t he r eac t i on t akes p l ace a t a r e l a t i v e 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 s e mix tu r e r a the 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 unus ua l f o r l edebu r i t e t o be

Heating

Cooling

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s epa ra t ed comp le t e ly , w i th t he eu t ec t i c au s t en i t e added to 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 mas s ive , f r e e cemen t i t e .

A s t he t empe ra tu r e f a l l s , b e tw een x3 and x4 , t he s o lub i l i t y o f c a rbon in au s t en i t e dec r ea s e s , a s i nd i ca t ed by t he Acm l i ne CJ . Th i s c aus e 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 w h ich i s depos i t ed upon the cemen t i t e a l r eady p r e s en 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 eac t i on i s o the rma l ly t o f o rm pea r l i t e . D u r ing s ubs equen t coo l ing t o r oom t empe ra tu r e , t he s t r uc tu r e r ema ins e s s en t i a l l y unchanged .

The t yp i ca l m ic ro s t ruc tu r e o f w h 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 an s fo rmed aus t en i t e (P ea r l i t e ) i n a w h i t e i n t e rdend r i t i c ne tw ork o f c emen t i t e a s show n 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:

H ard and w ea 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 s e s a s t he ca rbon con t en t i nc r ea s e s .

H a rdnes s B r ine l l 375 t o 600 .

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

C ompres s ive s t r eng th 200000 to 250000 .

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Limitations

B ecaus e o f ex t r eme b r i t t l en e s s and l a ck o f mach inab i l i t y , w h i t e i r ons

f i nd l im i t ed eng inee r ing app l i ca t i ons .

Application

The pa r t s w he re r e s i s t ance t o w ea r i s t he mos t impor t an t r equ i r emen t s uch a s l i ne r s o f c emen t mixe r s , ba l l m i l l s , pumps , w ea r ing p l a t e s . P a r t s o f s and - s l i nge r s , c e r t a in t ype o f d r aw ing d i e s , ex t ru s ion nozz l e s , g r ind 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 ind ing . A l a rge t onnage o f w h i t e c a s t i r ons i s u s ed a s a s t a r t i ng ma te r i a l f o r t he p roduc t ion o f ma l l e ab l e ca s t i r on pa r t s .

• B rake shoes

• S ho t b l a s t i ng nozz l e s

• M il l l i ne r s

• C rus he r s

• P ump impe l l e r s and o the r ab r a s ion r e s i s t an t pa r t s .

II. GRAY CAST IRON

I r on -ca rbon a l l oys con t a in ing f l akes o f g r aph i t e embedded in s t e e l ma t r i x , w h ich show a g r ay -b l ack i s h co lou red f r ac tu r e due t o g r aph i t e ’—th e f r ee f oam o f c a rbon , a r e ca 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 r ac t e r o f g r aph i t e f l akes i n i t . A typ 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 akes i n mic ro s t ruc tu r e , F igu re 10 . Th i s mic ro s t ruc tu r e r ep r e s en t s t he i r appea rance on a p l ane s u r f ace , bu t f l akes a r e t h r ee d imens iona l p l a t e s , s ome t imes connec t ed .

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Figure. – 10 Microstructure of gray cast iron

COMPOSITION OF GRAY IRONS

The g r ay ca s t i r ons a r e hypoeu tec 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 tw een 2 .4% to 3 .8%. The amoun t o f c a rbon does no t exceed 3 .8%, a s more t he ca 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 akes , r e s u l t i ng i n poo r mechan ic a l p rope r t i e s . C a rbon i s kep t a t l e a s t 2 . 4%. S o 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 tw een1 .2% to 3 .5%. I t b e ing a g r aph i t i s e r con t ro l s a long w i th ca rbon and the r a t e o f coo l ing , t he na tu r e o f s t e e l ma t r i x . I n s uch i r on , g r aph i t i s a t i on o f a l l t h e cemen t i t e excep t t he eu t ec to id cemen t i t e t akes 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 ing 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 r a 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 ing r a t e . Thes e a l l oys s o l i d i fy by

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f i r s t f o rming p r imary aus t en i t e . The g r aph i t i z a t i on p rocess i s added by h igh ca rbon con t en t , h igh t empe ra tu r e and t he p rope r amoun t o f g r aph i t i z ing e l emen t s mos t ly 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 r aph i t e equ i l i b r ium d i ag ram ( Figu re .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

D ur ing con t inuous coo l ing , 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 becaus e o f t he dec r ea s e i n s o lub i l i t y o f c a rbon in au s 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 ly on t he ma t r i x i n w h ich t he g r aph i t e i s embedded . I f t he compos i t i on and coo l ing r a t e a r e s uch t ha t t he eu t ec to id cemen t i t e a l s o g r aph i t i z e s , t hen t he ma t r i x w i l l b e en t i r e ly F e 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 M a t r ix w i l l b e en t i r e ly pea r l i t i c . The g r aph i t e - f e r r i t e mix tu r e i s t he so f t e s t and w eakes t g r ay i r on , t he s t r eng th and ha rdness i nc r ea s e w i th t he i n i nc r ea s e i n ca rb ide , r e ach ing a max imum w i th t he pea r l i t i c g r ay i r on .

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P ea 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 ing 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 r ac t e r and amoun t o f g r aph i t e f l akes . 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 s ed on t he min imum t ens i l e s t r eng th pos s e ss ed by a ca s t i r on . i . e . , i s ba s ed 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)

P ea r l i t i c g r ay i r on hav ing h igh phos pho rus (0 .3 -0 .5%) u s ed fo r p i s ton r i ngs . H igh w ea 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 P ea r l i t e and un i fo rmly d i s t r i bu t ed phos ph ide eu t ec t i c w i th f ew f l akes o f g r aph i t e .

B ea r ings ma t ing w i th ha rdened (o r no rma l i s ed ) s t e e l s ha f t a r e o f g r ay i r on w i th a round 85% P ea 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 has 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 ing : ( 3 . 2 -3 .8% C , 1 . 7 -2 .6% S i 0 . 4 -0 .7% M n , 0 . 1% T i , 0 . 3 -0 .5% C u) .

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FORMATION OF FLAKES

N orma l ly commerc i a l g r ay i r on i s e i t he r hypoeu tec t i c o r eu t ec t i c i n na tu r e . N eg lec t i ng t he p r e s ence dend r i t e s o f p r imary aus t en i t e i n hypoeu tec t i c i r on , wh ich impos es cons t r a in t s l a t e r on i n t he r ad i a l g row th 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 s ucces s ive s t ages i n t he fo rma t ion o f g r aph i t e f l akes f r om the eu t ec t i c l i qu id p r e s en t . O nce g r aph i t e ha s nuc l ea t ed ( i t o ccu 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 akes p l ace a t nuc l e i F igu re 13a , f r om each o f w h ich i s f o rmed a r ough ly s phe r i ca l l ump ca l l ed t he eu t ec t i c c e l l . I t g row s in an app rox ima te ly r ad i a l manne r , w he re t he r e i t s imu l t aneous g row th o f au s t en i t e and g r aph i t e , t h e l a t t e r be ing i n con t inuous con t ac t w i th t he l i qu id . The f l akes 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 inuous b r anched s ke 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 k e a c abbage . When the r a t e o f coo l ing i s i nc r ea s ed , 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

U nder coo l ing , t hen t he s ke l e ton i s b r anched more f r equen t ly w i th t he r ap id r ad i a l g row th o f t he ce l l and t hus , f i ne r g r aph i t e f l akes a r e obs e rved . The d i ame te r o f t he eu t ec t i c c e l l d ec r ea s e s a s t he number o f c e l l s pe r un i t vo lume inc r ea s e , and t h i s r es u l t s i n h ighe r t en s i l e s t r eng th , t hough the s oundnes s o f t he ca s t i ng i s a f f ec t ed adve r s e ly . The number o f nuc l e i c an be i nc r ea s ed by i nocu l an t s a s w e l l a s by s u lphu r ( s u lphu r p romo te s cons t i t u t i ona l s upe rcoo l ing , i nc r ea s ing t he f r equency o f b r anch ing i . e . , c e l l d ens i t y a s w e l l a s p roduces coa r s e r f l akes ) . S upe rhea t i ng o r ho ld ing t ime o f mo l t en me ta 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 ly app l i ed hea t t r e a tm en t t o g r ay i r ons . I n t he as - 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 s e s deve loped due t o d i f f e r en t i a l coo l ing and d i f f e r en t i a l con t r ac t i on , e s pec i a l l y i n non -un i fo rm c ro ss - s ec t i oned ca s t i ngs . Thes e s t r e s s e s a r e comp le t e ly r emoved by soak ing a t 650° C , bu t g r a in g row th i s s e r ious a t and above 600° C . Annea l ing 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 s e t he ca s t i ngs . I t s o f t en s , i nc r ea s e s duc t i l i t y and mach inab i l i t y o f g r ay i r on . C as t i ngs a r e soaked fo r up t o 10 hou r a t 850 -950° C . N orma l i s ing may be done t o i nc r ea s e 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 s oak 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 ing . H a rden ing can be done by hea t i ng t o and s oak ing a t 800 -850° C , and t hen quench ing i n w a te r , o i l , ho t s a l t b a th , t hough fo r t h rough - ha rden ing , o i l i s common ly u s ed a s w a te r quench ing may caus e d i s to r t i on and c r ack ing . Temper ing i s done a t 150 t o 650° C . Tab le 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 s ed on t he mic ro s 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 r ay i r ons w i th some app l i ca 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 ac t , g r ay i r on , be ing t he l e a s t expens ive ca s t i ng ma te r i a l , i s a lw ays cons ide r ed f i r s t w hen a ca s t me ta l i s be ing chos en fo r a p roduc t , un l e s s mechan ica 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 r a l hund red deg ree s l e s s t han s t ee l , r equ i r e s s imp le fu rnaces l i ke p i t f u rnace , c ruc ib l e f u rnace , cupo la , e t c . w h ich a r e s imp le , i nexpens ive t o f un 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: C as 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 ea s i l y . C as t i r ons ; a s compared t o s t e e l s s o 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 us ed 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 . G raph i t e hav ing l ow dens i t y i s vo luminous . I t s l a rge vo lume compens a t e s f o r t he s h r inkage . Gray i r on , t hus , does no t need sh r inkage a l l ow ance 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 ea s y and d i s con t inuous ch ip fo rma t ion due t o b r i t t l e g r aph i t e f l akes . G raph i t e s e rves a s a s o l i d l ub r i can 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 s mea r s t he cu t t i ng t oo l a l l ow ing 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 can t l aye r , avo id ing t he r eby me ta l t o me ta l d i r ec t con t ac t . O n o the r hand , w h i t e c a s t i r ons a r e w ea 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 akes , wh ich b r eaks t he con t inu i ty o f t he me ta 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 ow ed to t r an s f e r f r om one s ide 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 the v ib r a t i ons and r e s onance os c i l l a t i ons . G ray i r on s u i t s t hus t he mach ine beds a s compared 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 s u i t ab l e f o r app l i ca t i ons , w he re componen t s a r e sub j ec t ed t o compres 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 ac to r s , t h an mi ld s t e e l s .

10. Notch-insensitive: La rge number o f f l akes 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 s e 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 s ha 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 s ens 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 es 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:

A par t f r om low duc t i l i t y and t oughness , 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 ca s t i ng , t he mic ro s 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 des ign ing t he ca s t i ngs .

Applications: G ray ca s t i r ons have ex t ens ive app l i ca t i ons . The h igh damp ing capac i t y and h igh compres s ive s t r eng th make t hem s u i t ab l e f o r t he beds and

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bas e s o f pow er fu l mach ines and f r ames . G ood w ea r r e s i s t ance , good mach inab i l i t y and damp ing capac i t y make t hem su i t ab l e f o r app l i ca t i ons l i ke l ocomot ive and i n t e rna l combus t ion eng ine cy l i nde r b locks and heads , p i s tons r i ngs , cy l i nde r s . The ea s e o f c a s t i ng and l ow cos t makes t hem s u i t ab l e f o r coun te r - w e 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 ;

• F ly w hee l s

• G uards and f r ames a round haza rdous mach ine ry

• G ear hous ings

• P ump hous ings

• S team tu rb ine hous ings

• M oto r f r ames

• S ew er cove r s

• Enc lo s u re s f o r e l ec t r i c a l equ ipmen t s .

III. CHILLED CAST IRON

C hi l l ed - i r on ca s t i ngs a r e made by ca s t i ng t he mo l t en me ta l aga in s t ch i l l e r s w h ich r e s u l t i n a s u r f ace o f wh i t e c a s t i r on . C h i l l ed i r on ha s s u 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 r e i s t ha t o f g r ay i r on . N orma 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 ta l mou ld . C h i l l i ng t o ce r t a in dep th (12 t o 30 mm) i s becaus e o f t he f a s t coo l ing ( ch i l l i ng ) ob t a ined due t o h igh t he rma l conduc t iv i t y o f me ta l mou ld , The compos i t i on o f mo l t en a l l oy i s s o chos en t ha t no rma l coo l ing r e s u l t s i n g r ay i r on i n t he w ho le sec t i on , bu t f a s t coo l ing o f t he w ho le su r f ace , o r a pa r t o f t he s u r f ace y i e ld w h i t e i r on t he r e . The f a s t coo l ing 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 ow ing 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 t he deepe r ch i l l i s needed can be i nc r ea s ed by i nc r ea s ing t he t h i cknes s o f t he ch i l l p l a t e s . I t i s pos s ib l e t o choos e t he compos i t i on o f t he ca s t i r on s o t ha t t he no rma l coo l ing r a t e a t t he s u r f ace i s j u s t f a s t enough to y i e ld wh i t e i r on t he r e , and t he s low er coo l ing r a t e be low th i s s u r f ace p roduces mo t t l ed o r mo t t l ed and g r ay i r on . P r e s ence o f g r aph i t i s e r dec r ea s e s t he ch i l l d ep th and t he ca rb ide fo rming e l emen t s i nc r ea s e t he ch i l l d ep th .

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Element Chill depth Hardness of chill depth

Other Facts

CSi

Mn as MnS

Dissolved MnP

Ni

Cr

Mo

Cu

Decrease DecreaseDecreaseIncreaseDecrease

Decrease

Increase

Increase

Decrease unto 4% then increase

IncreaseSlightly IncreaseIncreaseSlightly 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

G ene ra l , ch i l l d ep th i s i nc r ea s ed by i nc r ea s ing ca rb ide fo rming e l emen t s and dec r ea s ing ca rbon and s i l i con . C as t i r on me l t i s a l l ow ed s o s o l i d i fy i n mou ld o f shape o f w edge . F igu re . ( 14b ) . The C oo l ing r a t e i s f a s t e r a t mo ld w a l l s , w h ich p r even t s g r aph i t i s a t i on t o y i e ld w h i t e c a s t i r on . The coo l ing r a t e dec r ea s e s a s t he cen t r e o f t he ca s t i ng i s app roached , a l l ow ing 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 ece w h ich i s due t o changes i n mic ro s t ruc tu r e s . The dep th o f ch i l l d ec r ea s e s and t he ha rdnes s o f t he ch i l l ed zone i nc r ea s e 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 s ed w i th i nc r ea s ing s i l i con con t en t . P hos pho rus dec r ea s e 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 s e o f 0 . 1 % P hos pho rus w i l l d ec r ea s e 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 d ep th and r e f ine s t he ca rb ide s t r uc tu r e . C h romium i s us ed i n s ma l l amoun t t o con t ro l ch i l l d ep th . M anganes e dec r ea s e s t he dep th o f ch i l l un t i l t h e f o rma t ion o f M anganes e su lph ide a f t e r t ha t i nc r ea s e s ch i l l d ep th and ha rdnes s . M o lybdenum improves t he r e s i s t ance o f t he ch i l l ed f ace t o s pa 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: C as t ings have s ome good p rope r t i e s due t o w h i t e i r on

s u r f ace w h ich a r e h igh w ea r and ab ra s ion r e s i s t ance , and s ome good p rope r t i e s due g r ay i r on co re w h ich a r e damp ing 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

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IV. MOTTLED IRON

I n 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 w h i t e i r on and t he co r e 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 w h i t e i r on , i . e . , h a s g r aph i t e f l akes , P ea r l i t e and s econda ry f r ee cemen 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 ia t e coo l ing r a t e f o r c e r t a in ca 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 ee s econda ry cemen t i t e , D ue t o i nco r r ec t f ound ry con t ro l f o r c e r t a in compos i t i ons , The non un i fo rm f l akes i nc r ea s e b r i t t l ene s s o f t he ca s t i ngs , apa r t f r om the ex t r a b r i t t l en e s s due t o t he p r e s ence o f s econda ry cemen t i t e . M o t t l ed ca s t i r ons , t hus , don ’ t f i nd app l i ca t i ons . I f c a rbon and s i l i con con t en t o f t he ca s t i r on i s i nc r ea s ed , t hen t he ca s t i ng s ha 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 ca s t i r on . F igu re . 15 i l l u s t r a t e s mic ro s t ruc tu r e o f mo t t l ed ca s t i r on .

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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 ca l c ium s i l i c ide 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 akes a r e un i fo rmly d i s t r i bu t ed t o g ive h igh mechan ica l p rope r t i e s (Tens i l e s t r eng th = 25 — 40 K g /mm 2 ) . The compos i t i on i s so chos en t ha t w h i t e f r ac tu r e i s ob t a ined i n t he abs ence o f any t r ea tmen t , i . e . , t h e ca 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 ca rbon con t en t abou t 2 . 5 -3%. C a l c ium s i l i c ide s ac t a s g r aph i t i s e r , s o t ha t r e s u l t i ng ca s t i ng i s g r ay and merchan tab l e . M eahan i t e c a s t i r on f i nds app l i ca t i ons a s a g r ay i r on w i th h igh mechan ica l s t r eng th , such a s f o r heavy mach ine beds and f r ames .

VI. MALLEABLE CAST IRON

C emen t i t e ( i r on ca rb ide ) i s a c tua l l y a me ta s t ab l e phas e . The re i s a t endency fo r c emen t i t e t o decompos e 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 in i t e ly i n i t s o r ig ina l f o rm. U p to t h i s po in t , c emen t i t e ha s been t r ea t ed a s a s t ab l e phas e ; how eve r , t h i s t endency to f o rm f r ee ca rbon i s t he ba s i s f o r t he manu fac tu r e o f

ma l l e ab l e ca s t i r on . The r eac t i on F e 3 C3F e + 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 s ence o f e l emen t s t ha t a id t he decompos i t i on o f F e 3 C On the i r on—iron ca rb ide equ i l i b r ium d i ag ram fo r t he me ta s t ab l e s y s t em, s how n in F igu re . 16 , a r e s upe r impos ed t he phas e 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 ) s ys t em a s do t t ed l i ne s . The pu rpos e 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 h e comb ined ca rbon in w h i t e i r on i n to i r r egu l a r nodu le s o f t ampe r ca rbon (g r aph i t e ) and f e r r i t e . C ommerc i a l l y , t h i s p roces s i s c a r r i ed ou t i n tw o s t ep s know n a s t he f i r s t and s econd s t ages o f t he annea l .

Wh i t e i r ons s u i t ab l e f o 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 ow ing r ange o f compos i t i on :

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

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Table 5 Composition of Malleable Iron

In t he f i r s t - s t age annea l ing , t he w h i t e - i r on ca s t i ng i s s l ow ly r ehea t ed t o a t empe ra tu r e be tw een 1660 and 1750°F . D ur ing hea t i ng , t he pea r l i t e i s conve r t ed t o au s t en i t e a t t he l ow er 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 s o 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 ing t empe ra tu r e

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 .

F igu re 16 show tha t t he aus t en i t e o f t he me ta s t ab l e s ys t em can d i s s o lve more ca rbon than can aus t en i t e o f t he s t ab l e s y s t em. The re fo r e , a d r iv ing fo r ce ex i s t s f o r t he ca rbon to 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 ee 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 eab 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 s o more i s d i s s o lved f rom the ad j acen t c emen t i t e , l e ad ing t o f u r the r c a rbon depos i t i on on t he o r ig 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 ec t i ons and u l t ima te ly appea r a s i r r egu l a r nodu le s o r s phe ro id s u s ua l l y ca l l ed t empe r ca rbon (F igu re . 17 ) . Temper ca rbon g r aph i t e i s

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f o rmed a t t he i n t e r f ace be tw een p r imary 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 ing t empe ra tu r e , w i th g row th a round the nuc l e i by a r eac t i on i nvo lv ing d i f fu s ion and ca rb ide decompos i t i on . N uc l ea t i on and g r aph i t i z a t i on a r e acce l e r a t ed by t he p r e s ence o f s ub mic ro s cop ic pa r t i c l e s t ha t c an be i n t roduced in 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 romo te nuc l ea t i on and g r aph i t i z a t i on , bu t t he s e e l emen t s mus t be r e s t r i c t ed t o ce r t a in max imum l eve l s s i nce t he i r on mus t s o l i d i fy a s w h i t e i r on . The re fo r e , g r aph i t i z ing nuc l e i a r e be s t p rov ided by p rope r annea l ing p r ac t i c e

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 ing 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 ing . The t empe ra tu r e o f f i r s t - s t age annea l ing exe r t s cons ide r ab l e i n f luence 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 ing t empe ra tu r e acce l e r a t e s t he r a t e decompos i t i on o f p r imary 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 . H ow eve r , h igh f i r s t - s t age annea l ing t empe ra tu r e s r e s u l t i n exces s ive d i s to r t i on o f c a s t i ngs du r ing annea l ing and t he need fo r - s t r a igh t en ing ope ra t i ons a f t e r hea t t r e a tmen t A nnea l ing 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 ing r a t e s and min imu m d i s to r t i on and a r e t he r e fo r e con t ro l l ed be tw een 1650 and 1750° F . The w h 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 ing t empe ra tu r e un t i l a l l mas s ive ca rb ide s have been decompos ed . S ince g r aph i t i z a t i on i s a r e l a t i v e ly s low p roces s , t he ca s t i ng mus t be soaked a t t empe ra tu r e f o 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 r uc tu r e a t comp le 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 le 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 .A f t e r f i r s t - s t age annea l ing , t he ca s t i ngs a r e coo l ed a s r ap id ly as p r ac t i c a l t o abou t 1400° F in p r epa ra t i on fo r t he s econd s t age o f t he annea l ing t r e a tmen t . The f a s t coo l ing cyc l e us ua l l y r equ i r e s 2 t o 6 h , depend ing on t he equ ipmen t u s ed . I n t he s econd - s t age annea l ing , t he ca s t i ngs a r e coo l ed s low ly a t a r a t e o f 5 t o 15° F /h t h rough the c r i t i c a l r ange a t w h ich t he eu t ec to id r eac t i on

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w ou ld t ake p l ace . Dur ing t he s low coo l ing , t he ca rbon d i s s o 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 an s fo rms in to f e r r i t e . Once g r aph i t i z a t i on i s comp le t e , no fu r the r s t r uc tu r a l changes t ake p l ace du r ing coo l ing t o r oom 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 le 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 know n a s s t anda rd o r F e r r i t i c ma l l eab l e i r on . The changes i n mic ro s t ruc tu r e du r ing t he ma l l eab i l i s i ng cyc l e a r e s how n s chema t i ca l l y i n F igu re . 18 ..

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 r uc tu r e cons i s t s o f nodu le s o f

t empe r ca rbon embedded in f e r r i t e ma t r i x ( due t o s l ow coo l ing i n eu t ec to id t empe ra tu r e r ange ) . As t he s e nodu le s b r eak t he con t inu i ty t o l e s s e 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 ow ly a t t he r a t e o f 5 t o 15 ° F /h r . Th rough the c r i t i c a l r ange a t w h ich t he eu t ec to id r eac t i on w ou ld t ake p l ace . Dur ing t he s low coo l ing t he ca rbon d i s s o 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 an s fo rms in to f e r r i t e . O nce g r aph i t i z a t i on i s comp le t e no fu r the r s t r uc tu r a l changes t akes p l ace du r ing coo l ing t o r oom 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 le s i n a f e r r i t e ma t r i x . Th i s know n as f e r r i t e ma l l eab l e C . I .

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Properties

• I n t he fo rm o f compac t nodu le s , t he t empe r ca rbon does no t b r eak up t he con t inu i ty o f t he t ough F e r r i t i c ma t r i x . Th i s r e s u 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 le s a l s o s e rve t o l ub r i ca t e cu t t i ng t oo l s , w h ich accoun t s f o r t he ve ry h igh mach inab i l i t y o f ma l l eab l e i r on .

• The F e r r i t i c ma l l eab l e i r on show s 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 .

• G raph i t e nodu le s l ub r i ca 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 eab l e i r on .

Application

F er r i t i c ma l l eab l e i r on ha s been u s ed 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 idges , n -ho i s t a s s emb l i e s , bea r ing 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 rdw are , 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 ings .

2. Pearlitic malleable iron: To ob t a in pea r l i t i c ma t r i x , 1% manganes e i s added to ca 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 , us ua l l y a i r , w h ich coo l s t he ca s t i ngs t h rough the eu t ec to id r ange f a s t enough to r e t a in comb ined ca rbon th roughou t t he ma t r i x . The amoun t o f P ea r l i t e f o rmed depends upon the t empe ra tu r e a t w h ich t he quench s t a r t s and r a t e o f coo l ing . I f t he a i r quench p roduces a f a s t enough coo l ing r a t e t h rough the eu t ec to id r ang , t he ma t r i x w i l l b e comp le 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 eab 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 ow er c r i t i c a l t empe ra tu r e , f o l l ow ed by r ap id coo l ing . A t h ighe r t empe ra tu r e ca rbon w i l l b e d i s s o lved f rom the g r aph i t e nodu le s and s ubs equen t coo l ing r e t a in s t he comb ined ca rbon . The coo l ing 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 ) . N orma l ly , a f t e r a i r coo l ing , t he pea r l i t i c ma l l e ab l e ca 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 ( ca l l ed d r aw ing p roces s ) a t 550 - 650° C o r s o t o s phe ro id i s e t he P ea 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 rdness and s t r eng th . P ea r l i t i c ma l l eab l e i r on can be ha rdened and

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t empe red . We ld ing o f pea r l i t i c ma l l eab l e i r on i s r a r e ly u s ed 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 w h i t e i r on unde r t he w e ld bead .

P r e s ence 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 ing ma l l e ab l e hea t t r e a tmen t . B u t a t h i ck ca s t i ng hav ing h ighe r s i l i con may r e s u l t i n g r ay i r on i n t he cen t r e w h i l e c a s t i ng i t . A s t he ca s t i ng shou ld he o f w h i t e i r on up t o t he cen t r e be fo r e ma l l e ab l e hea t t r e a tm en 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 , w h ich makes g r aph i t i s a t i on du r ing ma l l eab l e t r e a tmen t a l ong and d i f f i cu l t p roces s .

Figure 19 Typical Malleabilising iron

Application:

D ue to h igh s t r eng th and ha rdnes s , pea r l i t i c ma l l eab l e i r on i s u s ed fo r c am s ha 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 , s p rocke t s , e l eva to r b r acke t s i n conveye r equ ipmen t , hammers , w renches , show s , s w i t ch gea r pa r t s , f i t t i ngs f o r h igh and l ow vo l t age t r an s mis s ion and d i s t r i bu t ion s ys t em, j aw s o f un ive r s a l - j o in t s ha f t s , l i nk s and ro l l e r s o f conveye r cha in s , bus h ings , coup l ings b r ake - s hoes . M a l l eab l e i r ons a r e

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us ed ch i e f ly f o r t h in w a l l ed ca s t i ngs becaus e 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 eab l e ca s t i r ons have r ea s onab le duc t i l i t y , h igh s t r eng th , t oughnes s and even a r e bendab le . The ma in r ea s ons o f u s ing ma l l eab l e i r ons a r e l ow cos t and ea s e o f mach in ing w i th above p rope r t i e s . M a l l eab 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 ow er damp ing capac i t y and impac t r e s i s t ance .

VII. SPHEROIDAL GRAPHITE IRON (S.G. IRON) Th i s ca s t i r on a l s o know n a s nodu la 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 s en t a s ' f l ake s ' w h ich t end t o have sha rp - edged r ims . S ince t he s e f l akes have neg l ig ib l e s t r eng th t hey ac t a s w ide - f aced d i s con t inu i t i e s i n t he s t r uc tu r e w h i l s t t he sha rp - edged r ims i n t roduce r eg ions o f s t r es s - concen t r a t i on . I n S G ca s t i r on t he g r aph i t e f l akes a r e r ep l aced by sphe r i ca l pa r t i c l e s o f g r aph i t e (F igu re . 20 a ) , s o t ha t t he me ta 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 in a 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 S phe ro ida l g r aph i t e i s e f f ec t ed by add ing s ma l l amoun t s o f c e r ium o r magnes ium to t he mo l t en i r on j u s t be fo r e ca s t i ng .

S ince bo th o f t he s e e l emen t s have s t r ong 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%) in 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 . M agnes ium i s t he more w ide ly u s ed , and i s u s ua 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 idua l

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magnes ium con ten 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 en s 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 s ome hea t - t r e a t ed i r ons . The t e rm 4S G i ron ' r e a l l y de s c r ibe s a f ami ly o f c a s t i r ons , w h ich i nc lude s ome a l l oy i r ons , bu t i n a l l c a s e s t r e a tm en t by i nocu l an t s i s emp loyed to p roduce Sphe ro ida l - g r aph i t e

pa r t i c l e s . Some S G i ron p roduced by us ing t he fo l l ow ing s ubs t ances i n s t ead o f c e r ium o r magnes ium: ca l c iu m, ca l c ium ca rb ide , c a l c iu m f luo r ide , l i t h ium, s t r on t ium, ba r ium and a rgon . Thos e i r ons cons i s t i ng o f g r aph i t e nodu le s i n a f e r r i t e ma t r i x w i l l h ave h igh duc t i l i t y and

t oughnes s w h i l s t t hos e cons i s t i ng o f g r aph i t e nodu le s i n a pea r l i t e ma t r i x w i l l b e cha r ac t e r i s ed by h igh s t r eng th . S ome o f t he s e 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 ic a l p rope r t i e s . Thus , A mer i can mo to r i ndus t ry ha rdens s ome o f t he i r SG i ron gea r s by t he us e o f ' i n t e r rup t ed aus t empe r ing ' . Th i s i nvo lves au s 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 tmos phe re fo l l ow ed by quench ing t o 235° C and ho ld ing a t t ha t t empe ra tu r e f o r 2 hou r . S ince t r an s fo rma t ion f rom aus t en i t e occu r s i s o the rma l ly a t 235° C the r e i s l i t t l e d i s to r t i on i n s hape . I t i s c l a imed tha t S G i ron hypo id r i ng and p in ion gea r s a r e comparab l e w i th t hos e o f s t e e l i n t e rms o f f a t i gue and a l s o have a g r ea t e r t o r s iona 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 ion o f . 1%) can be ob t a ined by aus t empe r ing SG i ron a t 2500C , f o l l ow ing 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 w i l l y i e ld ba in i t i c s t r uc tu r e s o f l ow er s t r eng th s ( 900 -1200 N /mm 2 ) bu t e longa t ions up t o 14%. SG i ron c r anks ha 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 igh 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 imi l a r way to t he gea r s men t ioned above .

Properties of S.G. Iron:

S .G . i r ons have h ighe r mechan ica l p rope r t i e s , a lmos t equa l t o c a s t c a rbon s t ee l s ( t hus u s ed fo r p ipe s ) , s uch 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 le 6 ) , comb ined 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 w ea r r e s i s t ance , r e a s onab le ca s t ab i l i t y , bu t do no t s u f f e r f r om the de f ec t s o f g r ay i r ons such a s g row th and f i r e c r aze s , w hen u s ed 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 - s ens i t i ve .

Table 6 Properties of S.G. Iron

Grade Minimum Tensile

strengthMN/m2

Minimum, 0.2% Proof

BHN % Elongations

Matrix HeatTreatment

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stress MN/m2

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 : S u lphu r he lp s t o f o rm g raph i t e a s f l akes .

Thus , t he r aw ma te r i a l f o r p roduc ing i r on s hou ld have l ow s u lphu r ( l e s s t ha t 0 . 1%) , o r r emove su lphu r f r om i ron du r ing me l t i ng , o r by mix ing i r on w i th a de s u lphu r i s ing agen t s uch as c a l c ium ca rb ide , o r s oda a s h ( s od ium ca rbona t e ) .

2. Nodul is ing : M agnes ium i s added to r emove s u lphu r and oxygen

s t i l l p r e s en t i n t he l i qu id a l l oy and p rov ides 0 . 04% magnes ium, w h ich caus e s g row th o f g r aph i t e t o be S phe ro ida l . Magnes ium t r ea tmen t de s u lphu r i s e s t he i r on t o be low 0 .02% S be fo r e a l l oy ing w i th i t . M agnes ium and s uch e l emen t s have s t r ong a f f i n i t y f o r s u lphu r , and t hus s cavenge s u lphu r f r om 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 . Thes e add i t i ons a r e expens ive t o i nc r ea s e 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 s ed ) , be fo r e nodu l i s i ng , s hou ld be kep t l ow .

M agnes ium i s added w hen me l t i s nea r 1500° C bu t magnes ium vapo r i s e s a t 1 150° C . M agnes 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 s uch ca s e s magnes ium i s added a s N i -Mg , N i -S i -M g 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 ta 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 an s 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 . A dd 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 r e ca 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 as i nocu l an t . R eme l t i ng caus e s r eve r s ion 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 lves a l o t o f ga s , w h ich ge t s d i s s o lved i n l i qu id a l l oy , and fo rms b low -ho le s i n so l i d ca s t i ng . The con t r ac t i on du r ing s o l i d i f i c a t i on o f nodu la r c a s t i r on ca s t i ngs i s much

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g rea t e r t han o f g r ay i r on ca s t i ngs , w h ich needs ca r e fu l de s ign o f mou lds t o avo id sh r inkage cav i t i e s i n so l i d i f i ed ca s t i ngs .

In s p i t e o f t he s e d r aw backs , nodu la r c a s t i r on i s r ep l ac ing g r ay i r on and s t ee l s i n app l i ca t i ons . A nodu le 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) w eakens t he s t e e l ma t r i x t o a l e s s e r ex t en t t han g r ay i r on f l akes . The nodu le 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 . O ne 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%

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.

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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 ing . C omple t e F e r r i t i c o r ma t r i x hav ing a max imum o f 10% P ea r l i t e , s t i l l c a l l ed F e r r i t i c ma t r i x , pos s es s e 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 ing f rom 850 to 900° C ) makes S .G . i r on s t r onge r bu t l e s s duc t i l e . O i l o r w a te r quench ing f rom 900° -950° C y i e ld s mar t ens i t e ma t r i x , w h ich 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 duc t i l e ma t r i x , F igu re .21 (d ) (w h ich can be r e t a ined up t o 25° C ) i s ob t a ined by a l l oy ing (15 -36% N i , 1 . 8 -6% C r ) t he ca 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 , w he 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 s en t i n ma in ly P ea 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)

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Application of S.G. Iron:

S .G . i r on i s us ed fo r gea r pumps fo r p roces s ing and t r an s po r t o f s u lphu r i c a c id , pumps and va lve s i n s ea w a te r app l i ca t i ons , componen t s u s ed i n s t e am s e rv i ce s , and i n t he hand l ing o f a lka l i , c aus t i c and ammon ia - ca l s o lu t i ons , and fo r pumping and hand l ing o f s ou 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 ca t i ons a r e -

• C rank - sha f t s

• P i s tons and cy l inde r heads i n au tomob i l e and d i e s e l eng ines

• P res s u re ca s t i ngs l i ke gea r s and ro l l e r s l i de s

• S tee r ing knuck le s

• R ocke r a rms

• P ape r mi l l d rye r r o l l s

• B ear ing

VIII. COMPACTED/VERMICULAR CAST IRON

Th i 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 w h ich g r aph i t e occu r s a s worm- l ike b lun t - edged s tubby f l akes ( r ounded rods , w h 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 in 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 roces s us ed fo r p roduc t ion . Norma l ly , 10 -20% o f t he s phe ro ida l g r aph i t e may be p r e s en t , w h ich r equ i r e s C .E .V . o f 4 . 00 , and f l ake - g r aph i t e shou ld be avo ided . I n one o f t he p roduc t ion me thods , n i t r ogen (—0.015%) i s added to l i qu id a l l oy i n l ad l e by add ing n i t r i de F e r ro -manganes e

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Figure.24 Microstructure of compacted vermicular cast iron.

(80% M n, 4% N , r e s t F e ) . Th i s me thod g ive s non -un i fo rmi ty o f s t r uc tu r e and uns oundnes s i n ca s t i ngs . I n ano the r me thod , an a l l oy (4 -5% M g , 8 . 5 -10 .5% T i , 4 -5 .5% C a , 1 -1 .5% A l , 0 . 2 -0 ,5% C e , 48 -52% S i , r e s t F e ) 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 . S u 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 - s ens i t i ve a s sphe ro id s ge t f o rmed in t h in s ec 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 f ne s s and

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

Properties:

C ompac ted ca s t i r on t o r e t a in good damp ing capac i t y , and t he rma l conduc t iv 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 to r t i on i s s upe r io r t o bo th S .G . i r on and g r ay i r on . As t he sh r inkage 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 io r mechan ic a l p rope r t i e s hu t s imi l a r p roduc t ion cos t s a s S .G . I r on ha s l im i t ed r ep l acem en t po t en t i a l t o S .G . i r on pa r t s . How eve r , becaus e 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.

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• H ydrau l i c va lve s

• I ngo t mou lds

• C yl inde r heads

• Exhaus t man i fo ld s ,

• B rake d rums

• D is cs and p i s ton r i ngs a r e made f rom th i s i r on a s i t h a s good e l eva t ed t empe ra tu r e p rope r t i e s .

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

O ne o r more o f t he e l emen t s l i ke , N i , C r , C u , S i , M o , V e t c . (> 3%) a r e added in to g r aph i t e f r ee , o r g r aph i t e - bea r ing 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 w ea r and ab ra s ion r e s i s t ance p rope r t i e s .

1. Ni-hard: I n t he w h i t e i r on compos i t i on , 3 -5% N i and 1 -3% C r a r e added , p roduc ing a mic ro s t ruc tu r e cons i s t i ng o f mas s ive con t inuous ca rb ide s i n t he ma t r i x o f mar t ens i t e and s ome r e t a ined aus t en i t e on coo l ing a f t e r s o l i d i f i c a t i on . M ar t ens i t e i s ob t a ined due t o i nc r ea s ed ha rdenab i l i t y , due t o t he p r es ence o f t he s e e l emen t s , w h ich a long w i th ca rbon low er 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 in

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s ome aus t en i t e . H a rdness a t t a in ed i s 550 -700 B H N . As n i cke l i s ha l f a s pow er fu l a g r aph i t i s e r a s s i l i con , t he r i s k 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 inuous ne tw ork o f c a rb ide s can be improved by i nc r ea s ing N i and C r con t en t . The mod i f i ed N i -ha rd hav ing 4 -8% N i and 4 -15% C r , a f t e r hea t t r e a tmen t ha s a mic ro s t ruc tu r e o f d i s con t inuous 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 B a in i t e .

Figure . 26 Ni-Hard cas t i ron micrography

Propert ies:

• V ery good u s u ry s t r eng th un t i l 700° C

• Thes e ca s t i r ons have exce l l en t w ea r r e s i s t ance .

2. Ni-Resist: N i (13 -36%) and C r (1 .8 -6%) a r e added to p roduce aus t en i t i c ma t r i x w i th f l ake o r S phe 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 ica 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 au s t en i t i c , and t hus , t he s e a r e ca 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 . C h romium in comb ina t ion 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 comb ine good co r ro s ion r e s i s t ance , exce l l en t e ro s ion r es 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 N i - r e s i s t s con t a in 5 .5 -8 .0% coppe r . Though , t he s e a l l oys cou ld be us ed 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 . Thes e a l l oys cou ld be u s ed 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 :

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T.S . = 247 — 485 M N &2

B H N = 120 - 250

%E =2-15%

Impor t an t app l i ca t i ons a r e gea r pumps ( fo r p roces s ing and t r an s po r t o f s u lphu r i c a c id ) , pumps and va lve s i n s ea -w a te r app l i ca t i ons , pa r t s u s ed i n s t e am and fo r hand l ing o f a lka l i , c aus t i c , f o r pumping and hand l ing o f s ou r c rude o i l s i n pe t ro l eum indus t ry . F u 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 .

Figure. 27 Ni-Resist cast iron micrography

3. Silal and Nicrosilal: S i l a l i s t he cheapes t ox ida t i on and g row th -r e s i s t an t c a s t i r on , pa r t i cu l a r ly 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 r age i s :

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

T.S . = 139—263 N mm - 2

B H N 220 — 255

The mic ro s 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 akes . Thes e 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 s e 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 rme ab le s i l i c a t e f i lm . N ic ro s i l a l i s N i -C r added . S i l a l w h ich g ive s au s 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 s ed a t 650 -900° C . The compos i t i on i s :

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

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N i = 18— 23%; C r = 2 -2 .4%

T.S . 139 -247 N mm - 2

B H N = 150 -200

N ic ro s 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 . C ommon app l i ca t i ons a r e : I ngo t -mou lds . C y l inde r heads exhaus t man i fo ld s , a lumin iu m me l t i ng c ruc ib l e s , r e to r t s , g l a s s -mou lds , ga s - tu 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

R es idua l - s t r e s s e 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 ing and t hus caus e 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 s e s a r e more p ronounced in ca s t i ngs w i th non -un i fo rm c ro s s -s ec t i ons . P has e t r an s fo rma t ions accompan ied w i th vo lume changes agg rava t e t he s i t ua t i on fu r the r . C as t i ngs a r e s l ow ly hea t ed t o a t empe ra tu r e 480 -650° C , no rma l ly a t 600° C and then fu rnace coo l ed t o

200° C , f o l l ow ed by a i r coo l ing .

II . ANNEALING

The a im i s t o decompos e ca rb ide s and P ea r l i t e f r om 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 F e 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 tw o s t age p roces s pa r t i cu l a r ly f o r S .G . i r on cou ld be u s ed : F i r s t au s t en i t i s i ng a t 900° C and then coo l t o t r an s fo rm to P ea r l i t e t o 675° C and then f e r r i t i z a t i on o f P ea r l i t e i s done a t 760° C . A i r coo l ing may be done un l e s s c a s t i ng i s s us cep t ib l e t o r es idua l - s t r e s s e s .

III . NORMALISING

I t i s hea t i ng t he ca 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 , s oak ing a t i t and coo l ing i n s t i l l a i r as i nduced by l a rge f ans . N orma l i s ing g ive s h ighe r ha rdness 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 g ive s no rma l i s i ng t empe ra tu r e r ange fo r s ome ca s t i r ons .

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

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Table 9

IV. HARDENING AND TEMPERING

H arden ing and t empe r ing i nduce h ighe r s t r eng th s , and good w ea r r e s i s t ance . The t ime and t empe ra tu r e au s t en i t i s i ng depends on t he o r ig ina l ma t r i x o f t he ca 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 ix and t hus , soak ing i s con t inued t i l l d es i r ed amoun t o f c a rbon has been d i s s o lved i n au s t en i t e f r om 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 s pons 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 in au s t en i t e neces s i t a t i ng h igh t empe ra tu r e s o f au s t en i t i s i ng , bu t wh ich can caus e c r ack ing due t o more s eve r e quench ing . Wa te r quench ing o f c a s t i ngs ( comp lex s hapes and d i f f e r en t s ec t i oned ) caus e s quench c r acks . O i l quench i s no rma l ly us ed 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 s en t . Temper 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 to 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 s e l eng ines (B HN needed 390 -430 ) a r e mar t empe red . The ca s t i ng i s quenched in a ho t s a l t b a th , o r o i l k ep t s l i gh t l y above M s , t empe ra tu r e ( f r om aus t en i t i s i ng t empe ra tu r e ) t i l l t h e cen t r e o f t he ca s t i ng t oo a t t a in s t he ba th t empe ra tu r e , and t hen a i r coo l ed . Temper ing may be done a s u s ua l .

VI. AUSTEMPERING

C as t ings aus t en i t i s ed a t 850 -950° C , quenched in to , s a l t o r o i l b a th , kep t a t t empe ra tu r e 450 -250° C fo r a round 4 h r s . Low er S .G . i r on i s tw ice a s s t r ong w i th s ame toughnes s . As i t app roaches p rope r t i e s o f s t e e l s , c r ank s ha f t s , c ams ha f t s , g ea r s o f S .G . i r on a r e u s ed i n au s t empe red s t a t e .

VII. SURFACE HARDENING

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I t i s an economic a l me thod to ge t w ea r r e s i s t ance i n s e l ec t ed a r ea s . Excep t ing w h 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 s u r f ace ha rdened by i nduc t ion , f l ame , l a s e r e t c . F e r r i t i c ma t r i x i s no t u s ed , neces 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 comb ined ca rbon o f 0 . 5 -0 .7% in ma t r i x . I nduc t ion ha rden ing i s good fo r mas s p roduc t ion . E l ec t ron beam, p l a s ma and l a s e r a r e i nc r ea s ing ly us ed me thods fo r su r f ace ha rden ing . G ene ra l M o to r s have been us ing g r ay i r on d i e s e l eng ine cy l i nde r l i ne r s , w h ich 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 Higgins

5. http://en.wikipedia.org/wiki/Cast_iron