HEAT RESISTING CAST-IRON - Proceedings of the Institution of British Foundrymen, Vol. 18 (192

French Exchange Paper.

By Andre Levi.

D u rin g th e la s t tw e n ty years th e influence of te m p e r a tu r e on c a st iro n has been m ade th e sub

je c t of n um erous P a p e rs an d researches. The g r e a te r p a r t of th e se P a p e rs m ay he described as th e o re tic a l research , th e o b ject so u g h t being to d e te rm in e th e actio n of te m p e ra tu re on th e physi

cal p ro p e rtie s of c a st iro n , an d above all to ex p la in th e causes w hich b rin g a b o u t th e m odi

ficatio n of th e se p ro p e rtie s.

I t seem s d esirab le to review th e p rin c ip a l works to u c h in g on th is q u estio n , w ith th e o b ject of a rr iv in g a t conclusions of a p ra c tic a l c h a ra c te r.

As a la rg e n u m b e r of c astin g s h av e to he sub

m itte d to h ig h te m p e ra tu re an d re s ist its effect, i t is th o u g h t t h a t a n e x a m in a tio n of such works m ay yield c e rta in in d ic a tio n s an d enable m a n u f a c tu rin g formulae to be em bodied fo rth w ith —- formulae of in te r e s t b o th to th e p ra c tic a l w orker an d th e fo u n d ry m an a g e r.

T he w ell-know n w orks of M M . C harpy and G re n e t h av e estab lish ed th e fa c t t h a t u n d e r th e a c tio n of h e a t grey c a st iro n undergoes a p e r

m a n e n t, n on-reversihle in crease in volum e.

These w orkers showed t h a t th is in crease was due to a g ra p h ite d ep o sit re s u ltin g from th e decom

po sitio n of th e c e m e n tite F e 3C = 3 F e + g ra p h ite . They also p roved t h a t th is re a c tio n took place by h e a tin g even below th e low est c ritic a l p o in t, i.e ., a t te m p e ra tu re s below 725 deg. C. As re g a rd s a p a r tic u la r k in d of c a st iron, th e am o u n t of th is in crease in volum e is th e g re a te r, th e slower th e h e a tin g . W ith a c o n sta n t h e a tin g speed th e r a p id ity of th e phenom enon increases in p ro p o rtio n to th e a m o u n t of silicon in th e iron.

This is ta n t a m o u n t to sa y in g t h a t th e a n n e a l

in g of th e m e ta l is accom panied by a n in crease in volum e d u e to th e fo rm a tio n of g ra p h ite . N evertheless, i t has been observed t h a t th e a u g m e n ta tio n in volum e in th e course of succes

sive h e a tin g s a n d coolings is m uch g re a te r th a n th e in crease w hich m ay r e s u lt fro m a n n e a lin g a n d th e decom position alone of F e 3C.

Outerbridge’s Conclusions.

I n 1904 O u te rb rid g e 1 co n d u cte d a series of te s ts w ith c a st-iro n b ars h e a te d a n d cooled sev eral tim e s in th e elec tric fu rn a c e . T hese e x p e rim e n ts led him to fo rm u la te th e follow ing c o n c lu s io n s: —

(1) T he in crease in volum e a n d also th e e x p a n sion a re n o t in d efin ite. A fte r each h e a tin g o p e ra tio n th e ex p an sio n s a p p e a r in d im in ish in g ra tio an d cease a lto g e th e r a f t e r a c e rta in n u m b e r of h eats.

(2) T he coefficient of d ila ta tio n in a m e ta l su b je c te d to re p e a te d h e a t te s ts does n o t re m a in c o n s ta n t. I t m ay re a c h a v a lu e low er th a n h a lf th e coefficient of d ila ta tio n in a n u n tr e a te d m etal.

(3) A h e a tin g , even if p rolonged, p roduces no effect unless i t is follow ed by cooling.

(4) T he in crease of volum e co rresponds to a decrease in specific g ra v ity . I n th e course of his te s ts O u te rb rid g e fo u n d t h a t a f t e r 16 successive h e a ts th is specific g ra v ity h a d changed fro m 7.13 to 6.86, t h a t is, a d ecrease of n e a rly 4 p e r c e n t.

(5) I n c o n tin u in g his te s ts on th in p la te s h e a te d by th e B u n sen b u rn e r, he sim ila rly fo u n d an in crease of w eight.

(6) A m icroscopic e x a m in a tio n of th ese p la te s showed th e existence, a f te r tr e a tm e n t, of a series of crack s w hich w ould re n d e r th e p la te s perm eab le to w a te r a n d a ir.

T he tw o ex p e rim e n ts described below, w hich a re p a rtic u la rly s trik in g , enabled O u te rb rid g e to illu s

t r a t e p ra c tic a lly th e phenom enon he h a d s tu d ie d . To show th e d iffe re n t a c tio n of te m p e ra tu re on c a s t iro n a n d Steel, O u te rb rid g e su b je c te d to re p e a te d h e a ts a steel tu b e in to w hich he p oured a c a st-iro n p lu g . A fte r a few h e a ts th e steel tu b e was broken by th e sw elling of th e c a st iro n . A second tu b e of w ro u g h t iro n was filled w ith m olten c a s t iro n an d th e tw o ends w ere p la n e d level.

A fte r th r e e h e a ts i t w as fo u n d t h a t th e iro n was p ro je c tin g from th e tu b e a t each end ow ing to its h a v in g swollen v

R esu m in g his te s ts in 1908, O u te rb rid g e 2 suc

ceeded in g e ttin g th e specific g ra v ity of c a st iron to fa ll from 7.13 to 6.01, w hich is e q u iv alen t to a decrease of n e a rly 13 p e r c en t.

H e p o in ts o u t, m oreover, t h a t th e phenom enon of in crease of volum e ap p e a rs to he m ore ra p id in s o ft m e ta l th a n in h a rd . W ith m e ta l c o n ta in  in g 0.5 p e r c e n t, com bined carb o n , he o b tain ed an in creased volum e of 40 p er cen t, w ith 100 rep eated h e a ts a t 650 deg., th e sam e re s u lt b ein g obtain ed w ith th is m e ta l a f te r 27 h e a ts a t 790 deg.

To sum up, th e w ork of C h arp y , G re n e t and O u te rb rid g e ap p ears to prove t h a t th e increase in volum e is produced by re p e a te d h e a ts only if th e y a re followed by cooling; t h a t th is phenom enon occurs even a t te m p e ra tu re s below 725 deg. ; th a t i t is followed by a decrease in specific g ra v ity and by a n in crease in w eig h t of th e s a m p le ; an d th a t i t in v a ria b ly causes th e fo rm atio n of fissures, w hich produce p o ro sity in th e sam ple tr e a te d . I t is n ecessary to n o te t h a t in no case can th e decom position of th e c e m e n tite alone ex p lain th e in crease in volum e by 40 p e r c en t, fo u n d by O u te rb rid g e .

F o r th is reaso n he is disposed to a tt r ib u te th is p henom enon to th e a ctio n of th e occluded gases in th e m etal. H e supposes t h a t these gases u n d erg o a t h ig h te m p e ra tu re s a n expansion which in t u r n produces a p e rm a n e n t expansion of th e m in u te cav ities in w hich th e y are confined. This ex p an sio n w ould produce in th e m etal th e porosity an d th e a p p a r e n t in crease in volum e which have been described above.

The Work of Rugan and Carpenter.

O u te rb rid g e ’s te s ts w ere resum ed in 1908 and 1911 by R u g a n a n d C a rp e n te r. W ith o u t e n te rin g in to th e d e ta ils of th e ex p erim en ts c a rrie d o u t by th ese a u th o rs, th e ir conclusions are sum m arised : (1) I n a g reem en t w ith O u terb rid g e, R u g a n a n d C a rp e n te r find t h a t th e in crease in volum e does n o t occur unless th e sam ple is sub

2 “ Iron A g e .”

je c te d to a lte r n a te h e a tin g s a n d coolings. H e a t

in g alone does n o t cause a n in crease in v o lu m e ; (2) v a rio u s te s ts show ed t h a t th e e x te n t of th e in creased volum e is a fu n c tio n of th e chem ical com position of th e m e ta l. T h e ' in creased volum e fo u n d in th r e e d iffe re n t m e ta ls v a rie s fro m 35 to 38 p e r c e n t., a n d re m a in s c o n s ta n t a f t e r a b o u t a h u n d re d h e a t s ; (3) only g re y c a st iro n s show th is phenom enon. T he presence of g r a p h ite is th e r e fo re re g a rd e d as a n in d isp en sa b le elem en t in in c re a sin g th e v o lu m e; (4) in creases of volum e a re a u g m e n te d by a n in c re a se in th e silicon c o n te n t, a n d th is ta k e s place alm o st in p ro p o rtio n to th e c o n te n t of t h a t e le m e n t; (5) only a p a r t of th is in crease in volum e c a n be a t t r i b u te d to 'the a n n e a lin g of F e 3C ; (6) in creases of w e ig h t ta k e a course a p p ro x im a te ly s im ila r to t h a t of th e v o lu m e ; (7) th e g r e a te r th e Si content,- th e g r e a te r is t h e ’ final in crease in w e ig h t; (8) th e a u th o rs co n sid er t h a t th e y h av e th u s d e m o n s tra te d t h a t increases in w e ig h t a n d volum e a re solely d i e to p h en o m en a of o x id a tio n , w hich o p e ra te specially on th e fre e c arb o n , th e silicon, th e iro n , an d p a rtic u la rly on th e s ilic o -fe r rite ; an d (9) m icro

scopic e x a m in a tio n shows t h a t th is o x id a tio n tak es place very sp ecially alo n g th e lamellae of g ra p h ite , w hich finally d isa p p e a rs e n tire ly .

T he in crease in volum e is said to be due to fo u r c a u s e s : To th e p a r t ia l o x id a tio n of th e G r (w hich phenom enon dim in ish es w hen Si in creases a n d d is

ap p e a rs e n tire ly w hen S i = 6 p e r c e n t.) ; to th e p robable o x id a tio n of th e S i e x is tin g in th e fo rm of s ilic o -fe rrite c a u sin g th e fo rm a tio n of F e + S i 0 2; to th e o x id a tio n of th e iro n unco m b in ed w ith silicon ; a n d finally, to a s lig h t e x te n t, to th e ex p an sio n of th e h y d ro g en , occluded in th e m etal.

To sum u p , R u g a n a n d C a rp e n te r co n sid er t h a t it is p rim a rily a q u estio n of a process of o x id a tio n .

I n a second m o n o g rap h (1911) th e sam e w rite rs seek to e x am in e th e influence of th e d iffe re n t elem ents on th e phenom enon of ex p an sio n . T hey a d m it, as th e re s u lt of th ese te s ts , t h a t phosphorus slig h tly d im in ish es th e in crease of volum e, t h a t th e presence of m an g an e se r e ta r d s i t considerably, an d t h a t s u lp h u r is w ith o u t influence.

F in a lly , as re g a rd s th e a c tio n of th e occluded gases, th e s e w rite rs c o n sid er t h a t th is a c tio n is

negligible in re g a rd to m etals c o n ta in in g m ore th a n 3 p e r cen t, silicon. O n th e o th e r h a n d , w ith Si b etw een 1.75 an d 3 p e r c e n t., th e action of th e g ases would effect a n exp an sio n of 1 to 2 p e r cent.

W hen th e silicon is below 1 p e r c en t, th e increased volum e due to th e actio n of th e gases would am o u n t to 10 p e r cen t, of th e to ta l increased volume.

W h ile m ak in g c e rta in reserv atio n s as re g a rd s th e asc rip tio n of th e ex p an sio n to phenom ena of oxida

tio n (as w ill be seen la te r, th is p o in t h as been stro n g ly co n tested ), th e a u th o r re ta in s th e e x p e ri

m e n ta l fa o t w hich ap p ears to him to sta n d out fro m th e te s ts of R u g a n an d C a r p e n te r : The elem en ts w hich fa v o u r th e fo rm atio n of g ra p h ite accelerate th e phenom enon of increase in v o lu m e;

th o se w hich oppose th is fo rm atio n r e ta rd its occur

rence. T his first conclusion, w hich th e a u th o r had alre a d y fo rm u la te d in 1923,3 h as been confirm ed by e x p e rim e n ta l researches m ade by A ndrew an d H y m a n , w hich w ill be ex am in ed la te r.

I t m ay be assum ed fo rth w ith t h a t th e p ro p o r

tio n a lity betw een th e S i-co n ten t a n d th e ex p an sion of th e m etal, estab lish ed by R u g a n an d C a r

p e n te r, w ould be a ttr ib u ta b le r a th e r to th e pro

p e r ty of th e Si in p ro m o tin g th e fo rm atio n of g ra p h ite th a n to th e chem ical p ro p e rtie s th e m selves of t h a t elem en t. I n fa c t, th e p e n e tra tio n of th e a ir in to th e mass of th e m e ta l will be all th e easier in p ro p o rtio n as th e n u m b er an d th e dim ensions o f th e g ra p h ite lam ellae a re g re a te r.

T h e e x p e rim e n ts ex am in ed above, and, m oreover, m ost of those c a rrie d o u t la te r hy v ario u s w orkers, h av e all refe re n c e to th e action on th e m etal of te m p e ra tu re s com prised betw een 650 an d 850 deg.

C., t h a t is to say, a p p ro x im a te to o r above th e tr a n s fo r m a tio n p o in ts.

B efore pro ceed in g to ex am in e th ese exp erim en ts, i t a p p e a rs in te re s tin g to an aly se a stu d y dealing w ith th e a c tio n of su p e rh e a te d steam on c a st n o n , t h a t is, w ith th e effect of re la tiv e ly low te m p e ra tu r e s , 425 to 500 deg. C.

A v ery com plete s tu d y of th is question has been m ad e by C am pbell an d G lassfo rd .4

3 “ L a F o n d e r ie M o d ern e,” A pril an d N o v em b er, 1923.

‘ E ig h t h C o n g r ess on T e s tin g M eth o d s, " R ev u e de J le t a llu r g ie ,” 1912.

These w rite rs, h a v in g observed th e r a p id corro

sion of iro n c a s tin g su b je c te d to t h e actio n of su p e rh e a te d steam — a corrosion accom panied by an in crease in volum e—so u g h t to d e te rm in e : (1) T he n a tu r e of (the) corro sio n ; (2) W hy c e r ta in m e ta ls do n o t c o rro d e ; a n d (3) w h a t m u s t be do n e to p re v e n t its occurence.

I t m ay be n o te d , m oreover, t h a t R u g a n an d C a rp e n te r h a d a lre a d y fo u n d t h a t cocks p lu n g ed in to s u p e rh e a te d ste a m a t 360 deg. e x p a n d e d , b u t th e y considered t h a t th is p h enom enon w as d iffe re n t from t h a t of e x p an sio n a t a h ig h te m p e ra tu re . T he w ork of C am pbell a n d G lassford, how ever, a p p e a rs to th e a u th o r to b rin g o u t c e rta in v ery in te r e s tin g resem blances betw een th e s e tw o pheno

m en a. A first s tu d y co n sists of a m icro g ra p h ie e x a m in a tio n of a dome-cock a n d flange w hich h as been in use o n th e d isc h a rg e sid e of th e s u p e r

h e a te r of a s e p a r a te fu r n a c e fo r tw o y ears. I t shows considerable s tr u c tu r a l m odifications in th e m e ta l.

I n th e first p lace, a t h i n c o a tin g o f o x id e is form ed ro u n d th e p a rtic le s of g r a p h ite ; th e n th e r e a re crevices w hich become w id er a n d w id e r; finally, as th e p a r ts in d ir e c t c o n ta c t w ith th e ste a m a re ap p ro ach ed , th e fo rm a tio n of a m ix tu r e of oxides a n d g ra p h ite is fo u n d , c o n s titu tin g a pow dery m ass devoid o f s tre n g th .

T h is first e x a m in a tio n , th e n , in d ic a te s p ro g re s

sive o x id a tio n process o f a v ery p ro n o u n ced c h a ra c te r, w hich ta k e s p lace alo n g th e g ra p h ite lam ellae.

Campbell and Glassford’s Findings.

I n stu d y in g th e sam e p h enom enon o n b a rs sub

je c te d to th e a ctio n o f su p e rh e a te d ste a m (6.75 kg.

(14.85 lbs.) a t 425 deg C. fo r 30 days a t first, theD 90 days a fte rw a rd s) C am pbell a n d G lassford cam e to th e follow ing conclusions : —

(1) U n d e r th e a c tio n of ste a m , sam ples o f w h ite c a st iro n , ste e l an d m alleable c a s t iro n show only one a n d th e sam e ty p e of o x id a tio n . T h e sam ples h av e a sk in re g u la rly o xidised, to a v e ry slig h t d e p th , a n d th e o x id e fills-u n ifo rm ly all th e crevices w hich m ay e x is t o n th e su rfa c e of t h e sam ple.

(2) U n d e r th e sam e tr e a tm e n t sam ples of grey c a s t iro n c o n ta in in g less th a n 1 p e r cen t. S i show

su p erficial o x id a tio n sim ilar to th e specim ens of w h ite c a st iro n o r steel, b u t in ad d itio n a slight p e n e tr a tio n of oxides follow ing th e la rg e s t p lates of g ra p h ite is found.

(3) W ith S i betw een 1.25 an d 2.50 p e r cent, th e r e a re th e sam e ap p earan ces, b u t th e p e n e tr a tio n of th e o x id a tio n ap p e a rs to grow w ith th e in crease of th e silicon c o n te n t a n d extends in d e p th .

T h e a u th o rs a p p e a r to conclude, th e re fo re , w ith R u g a n a n d C a rp e n te r t h a t th e processes of oxida

tio n in crease p ro p o rtio n a lly w ith th e in crease of Si. I n closing, how ever, th e y m ake one rem ark w hich is considered especially im p o rta n t: —

“ I t ap p e a rs n a tu r a l,” th e y observe, “ to con

clude t h a t th e in crease of th e silicon c o n te n t is re la te d to th e increased corrosion. Y et th e very good re sista n c e of a sam ple co n ta in in g 2 p e r cent.

S i m ig h t ju s tif y a n o th e r conclusion, viz., t h a t the fin e r th e g ra p h ite lam ellae are, the less is the corrosion. T h is conclusion is in accordance w ith p ra c tic a l experience, w hich shows th a t good gun iro n (grey special c a st iron) with- 2 p e r c e n t Si an d G r low) resists th e actio n of su p e rh e a te d steam v ery w ell.” I n sh o rt, th e two la s t researches a p p e a r to in d ic a te t h a t th e in crease in volume is du e to a process of corrosion, th at- is to say, oxida

tio n .

Okochi and Sato’s Views.

Q u ite a n o th e r p o in t of view has since been advanced by c e rta in J a p a n e s e scien tists, whose w ork i t is proposed to ex am in e.5

A first in v e s tig a tio n was c a rrie d o u t by Professors O kochi a n d S ato , th e expansion curves being ta k e n by m eans of th e C hev en ard d ilato m eter.

T he follow ing w ere th e p rin c ip a l conclusions reach ed as a r e s u lt of th ese t e s t s : (1) The influ

ence of o x id a tio n is of little im p o rta n c e in reg ard to th e in crease in v o lu m e; (2) th e expansion, d u rin g th e first h e a t, is d u e to th e decom position of F e 3C ; (3) d u rin g th e re p e a te d h e a tin g an d cooling, a f t e r th e fo rm e r th e in crease ta k e s p lace tw ice : (a) A t th e p o in t A c; (b) a t th e h ig h e r te m p e ra t u r e s ; an d (4) th e in crease a t th e p o in t Ac (about 815 deg.) is d u e to th e effect caused by the pressure

5 S ee th e C om m u n ication to th e Lahge C ongress (1922) by M r. T a rio Kdfcuta.

of th e occluded gases in re la tio n to th e c o n tr a c tio n

e x p a n d in g m e ta l undergoes a sh a rp c o n tra c tio n a t th e p o in t A c ; likewise, in th e course of cooling a n d th e sh rin k a g e w hich accom panies it, a sharp ex p an sio n occurs a t th e p o in t A r. Now th e m etal is n o t a hom ogeneous su b stan ce, th e d is trib u tio n of th e g ra p h ite b ein g i r r e g u l a r : th e re a re th ic k p la te s a t c e rta in p o in ts w ith th in lamellae close beside th e m . The c o n tra c tio n an d expansions w hich occur a t th e Ac an d A r p o in ts, an d which a r e d u e to th e dissolution a n d d ep o sit of a cer

ta i n q u a n tity of free carbon, a re v ery d ifferen t in e x te n t acco rd in g as th e y a re produced in th e n eighbourhood of th ic k or th in p lates of g ra p h ite . The re s u lt is very considerable te n sio n s a t th e d iffe re n t p o in ts of one a n d th e sam e sam ple of m etal. I t is to th ese ten sio n s, th e actio n of which is specially a p p a re n t in th e v ic in ity of th e g ra p h ite p la te s fo rm in g ac u te an g led fissures in th e m etal, t h a t K ik u ta a ttr ib u te s th e phenom enon of in crease in volum e.

H e expresses his conclusions as fo llo w s: The co n tin u o u s ex p an sio n of g rey cast iro n in vacuo d u rin g th e re p e a te d h e a tin g s an d coolings is due to th e d iffe r e n tia l expansion, to th e d iffe r e n t m icro

p ro p o rtio n s of the sam ple w hich produce num erous fissures or ca vities in th e neighbourhood of th e p la tes of g ra p h ite.

M oreover, he adds t h a t : “ I n a n oxidising atm o sp h ere th e exp an sio n is accelerated by th e oxides w hich a re form ed an d w hich fill up th e fissures or c a v itie s o rig in a te d d u rin g th e Ar tr a n s f o r m a tio n .” The a ctio n of o x id atio n on th e in crease of volum e—c o n tra ry to w h at is accepted by R u g a n an d C a rp e n te r—w ould th e re fo re ap p e a r to be a secondary a n d in d ire c t actio n .

I t m ay be n o ted , how ever, t h a t if th is th eo ry is capable of e x p la in in g th e in crease in volum e by h e a tin g a t 725 deg. C. or above, i t does n o t in any w ay e x p la in th e in crease in volum e a t th e te m p e ra tu r e of 425 deg., as s tu d ie d by Cam pbell an d G lassford. F o r, even a d m ittin g t h a t th e action

W dokumencie Proceedings of the Institution of British Foundrymen, Vol. 18 (1924-1925) (Stron 72-95)