THE INFLUENCE OF GRAPHITISATION OF CAST IRON

By Professor E. Diepschlag.

[ Ge r m a n Ex c h a n g e Pa p e r. ]

A t a n I n te r n a tio n a l F o u n d ry Congress held in P a ris in 1927, M. A ndré L evi1 m ade a re m a rk ­ able s ta te m e n t as to th e tran sm issio n of p ro p er­

ties in c a s t iro n . P ro ceed in g from th e well- know n f a c t t h a t tw o d ifferen t sam ples of cast iron of th e sam e chem ical com position often have very d iffe re n t m echanical p ro p ertie s, he e stab ­ lished t h a t , in a d d itio n to th e chem ical analysis of a m etallic m a te ria l, some o th e r c h a ra c te ristic facto rs m u s t be deriv ed to p re d ic t i t exactly, a n d in th e case of c a s t m etals th e previous th e rm a l h isto ry m u s t especially be ta k e n in to c o n sid eratio n . By th is, th e r a te of h e a tin g of th e m a te ria l, th e degree and tim e of su p e rh e a t­

ing an d th e r a te of cooling is understood. The chem ical com position an d th e previous th erm al h isto ry a re , in m an y cases of m etal alloys, two in d e p e n d e n t values, an d for a given chem ical com position th e m echanical p ro p ertie s m ay be p red icted w hen th e e a rlie r th e rm a l h istory is known.

H ow ever, in grey cast-iro n m a te ria ls, these two d e te rm in in g c h a ra c te ristic s are n o t in d ep en d e n t of each o th e r, as i t is n early im possible to m odify th e speed of cooling, fo r exam ple, w ith ­ o u t a t th e sam e tim e ch an g in g th e chemical com position. T he carb o n p re s e n t in a c a st iron is d eterm in ed by chem ical analysis to exist p a rtly as g ra p h ite an d p a rtly as com bined c a r­

bon, a n d th e p ro p o rtio n of th ese tw o p a rts is modified in re la tio n to th e th e rm a l conditions.

In th e case of cast iro n of a given chem ical com­

1 André L evi, “ L 'hédérité des fontes, La fonderie m oderne,”

19 2 7, page 3 9 9 . E x tra cted F o u n d r y T r a d e J o u r n a l , page 42, vol. x x x v ii.

k .

p o sitio n , th e m ech an ical p ro p e rtie s a re n o t ex­

have been v ery fully o u tlin ed . B u t, in th is con­

nection, i t w as recognised t h a t all im provem ents of th e m a trix a re m asked by th e existence of m ore or less larg e q u a n titie s of g ra p h ite being d is trib u te d th r o u g h o u t th e m ass in diverse o rie n ta tio n s.

Overcoming Inherency.

P . B a rd e n h e u e r an d K . L. Z eyen2 established from th e ir ex p e rim e n ts t h a t an an nealed chilled cast sam ple h a d b e tte r m echanical properties, even th o u g h i t h ad a f e r ritic m a trix , th a n a san d -cast sam ple h a v in g a p e a rlitic m a trix . They p o stu la te from th is t h a t th e s tru c tu r e of th e m etallic c o n s titu e n ts is only of m inor im p o rt­

ance in re la tio n to th e m echanical p ro p ertie s of grey c a s t iro n , an d t h a t , in th e m ain , th e form an d th e d is trib u tio n of th e g ra p h ite is decisive.

Thus, w ith re g a rd to th e q u a n tity , th e form and th e d is trib u tio n of th e g ra p h ite , only a fte r th e m ost fav o u rab le conditions a re c reated , th e question of th e fo rm a tio n of th e m a trix becomes im p o rta n t.

So if, on th e one side, th e re la tio n sh ip betw een th e m echanical p ro p e rtie s an d th e chem ical com­

position really re fe rs to th e ra tio betw een these and th e q u a n tity a n d form of th e g ra p h ite , and, on th e o th e r h a n d , as p o stu lated , a transm ission of th e s tru c tu r e of th e pig an d scrap iron ta k in g place in n o rm al re m e ltin g m ethods, founders m ust s tu d y th ese m e ta llu rg ic a l questions, not m erely as to th e final p ro d u ct, b u t w ith sim ilar assid u ity to th e q u a lity an d p ro p ertie s of th e m etallic raw m a te ria ls em ployed. The m ost im p o rta n t raw m a te ria l in th is case is th e pig- iron. T his m u st n o t only be ev a lu a te d accord­

ing to its chem ical com position, b u t r a th e r w ith reference to th e ty p e an d q u a n tity of th e g ra p h ite c o n te n t. The g ra p h ite is m ainly re ­ sponsible fo r th e ten sile s tre n g th , an d these pro­

p erties are m odified by re m e ltin g —only slightly, never com pletely.

2 P. B ardenheuer and K. L. Zeyen, “ Stahl und Eisen, 8 (1928),

How Blast-Furnace Conditions affect Final Product.

in th e ch a rg e u n til solidification, an d so this

of g ra p h ite o u t of th e liq u id m ass only ta k e s

between th e analysis of th e r e s u lta n t alloy and

nuclei a p p e a r in th e reg io n of in c re a sin g speed

favoured by th e expansion of volume which is a c h a ra c te ristic of th is tra n sfo rm a tio n , an d th e g ra p h ite s e p a ra tio n s m ain ly o rig in a te in th e nidæ of th is netw o rk s tru c tu r e . The q u ality of th is g ra p h ite differs from t h a t sep arated p rim a rily as well in its c o n stitu tio n as in its physical p ro p ertie s. W. A. R o th established t h a t g ra p h ite in p ig-iron is a m odification which he calls /8-graphite, an d th e g ra p h ite which is

Fi g. 1 .

se p a ra te d o u t fro m th e solid solution differs from th is, a n d is identified by him as a-graphite.

B o th m odifications differ in th e ir ig n itio n te m ­ p e ra tu re s an d in th e ir specific g ra v ity .

Importance of Nuclei.

I n th e solidification of pig-iron, th e num ber of m ixed c ry sta ls an d th e ir ■extent depends upon th e n u m b er of nuclei, w hich la t te r are influenced by c e rta in in h e re n t conditions. The num ber and

size of ausbenite c ry sta ls especially depends upon th e r a te of cooling of th e m ass ; if i t is slow, a n u m b er of w ell-developed c ry sta ls w ill grow, b u t if i t is ra p id , how ever, a g r e a te r num ber of sm aller cry stals of ir re g u la r form grow. The g ra p h ite cry stals, being s e p a ra te d d u rin g th e solidification of th e re m a in in g liq u id mass, will grow (according to B a rd e n h e u e r5) a t every c ry s ta l face w ith a b o u t th e sam e r a te of

crystal-Fi g. 2.

lisa tio n , assum ing th em to be se p a ra te d , u n til n e a r th e solidification p o in t, if liq u id is still p re s e n t. H ow ever, w ith a n increased u n d er­

cooling, re m a rk a b le differences in th e grow th from th e single faces of th e crystals are observed. The faces, h a v in g a g r e a te r ra te of c r y s ta llis a tio n ,. r e s tr a in th e o th e rs which grow m ore slowly, th e cry sta ls h a v in g in fe rio r facets

6 B ardenheuer, “ Stahl und E isen ,” 47 (1927), page 861.

because of re ta rd e d gro w th in one or tw o d irec­

tio n s. I n th e first case, w ith slow cooling and slig h t under-cooling, th e g ra p h ite cry stals grow to ro u n d ed g ra in s. I n th e second case, w ith a serious under-cooling an d a h ig h e r r a te of cool­

ing, g ra p h ite cry stals grow in th e fo rm of th in leaves of v a ry in g size. These sm all leaves s u r­

ro u n d th e su rfa c e of th e e x is tin g a u s te n ite

Fi g. 3 .

cry stals an d have a tw isted form , w hich in section is w orm -like.

i Situation Summarised.

The form and q u a n tity of th e g ra p h ite crystals in p ig-iron can be d iffe re n tia te d in th e follow ing m a n n e r :— (1) G ra p h ite in larg e s tr a ig h t plates (h y p er-eu tectic g ra p h ite ) s e p a ra te d w hilst th e m a jo r p o rtio n of th e mass is liq u id ; (2) g ra p h ite in com pact ro u n d ed g ra in s se p a ra te d n e a r th e eu tectic line if th e r a te of cooling is slow and th e degree of under-cooling is s lig h t; (3) g ra p h ite

in th in leaves, w ith sp ira ls co rresp o n d in g to th e su rface of th e a u s te n ite c ry sta ls, a n d s e p a ra te d in c o n d itio n of u nder-cooling ; (4) g ra p h ite in a very fine s ta te a n d equally d issem in ated (eu tectic g ra p h ite ) s e p a ra te d if th e c o n s titu tio n of th e alloy is fav o u rab le ; a n d (5) g r a p h ite in so ft an d porous flakes, o ften a d ja c e n t to th e c e m e n tite , a n d s e p a ra te d from th e solid so lu tio n d u rin g cooling.

Fi g. 4.

Identification of Graphite Types.

These d iffe re n t k in d s of g ra p h ite m ay be recog­

nised on th e su rfa c e of p ig -iro n m icrosection.

H ow ever, th e y a re n o t u su ally s e p a ra te d u n d e r th e ir “ fa v o u ra b le ” co n d itio n s, a n d , th e re fo re , all so rts of v a rie tie s an d tr a n s itio n a l s ta te s m ay be f o u n d ; th e y a re n o t alw ays seen in th e i r well- defined form s, a n d som etim es i t m ay be difficult to recognise th e k in d of g ra p h ite , especially as

q u ite o fte n in iro n alloys n o t m erely one k in d of g ra p h ite b u t sev eral e x ist. F o r in stan ce, K . Ish ik a w a h as described tw o ty p e s of g ra p h ite in c a s t iro n w hich he d e sig n a te d as p la ty g r a p h ite a n d g ra p h ite in w h irl. T he g ra p h ite in w h irl, he says, is th e m ost desirab le k in d fo r c a s t iro n , because i t h a s a m o d e ra te s tre n g th a n d its s tr u c tu r e is v ery hom ogeneous th r o u g h ­ o u t th e whole c a stin g . To produce th is stru e

-Fi g. 5.

tu r e Ish ik a w a su g g ested a com position c o n ta in ­ in g a b o u t 3.2 p e r c e n t C a n d 1.7 p e r c e n t. Si.

T his b in d of g ra p h ite is id e n tic a l w ith th e g ra p h ite in t h i n leaves. M . H a m a su m i6 has stu d ie d th e q u estio n of th e o rig in of th e g ra p h ite in w h irl, w ith th e re s u lt t h a t th is s tr u c tu r e is no t alw avs assu red even if th e in d ic a tio n s of

6 M. H am asum i, “ Sc. R ep. Ser. I .” v ol. xiii, X o. 2. See F o u n d r y T r a d e J o u r n a l , page 71, et seq„ vol. xxxii.

Ish ik aw a a re closely observed. H e supposes, th e re fo re , t h a t th e re a re still o th e r in ­ fluences o p e ra tin g , a n d he re fe rs to a P a p e r of K . H o n d a a n d T. M u ra k a m i , who s ta te d t h a t gas, especially carbon m onoxide, plays a p a r t in th e s e p a ra tio n of g ra p h ite . One of th e re su lts of H am asu m i is t h a t th e r e a re tw o k in d s of cu rly g ra p h ite , g ra p h ite in w h irl and a w orm -like8 g ra p h ite . H e says th e one or th e o th e r is se p a ra te d according

Fi g. 6 .

to th e r a te of cooling. I t m ay be seen by th is t h a t th ese a u th o rs a lre a d y saw th e possibility of reco g n isin g d iffe re n t k in d s of g ra p h ite .

Now i t would be v ery v alu ab le to h av e a m ethod of d e te rm in a tio n av ailab le by w hich th e d iffe re n t k in d s of g ra p h ite could be recognised

7 K . H onda and T. Murakami, “ Sc. R e p .,” vol. x. N o. 4.

8 K ikum e. E d. Foundry Trade Journal.

an d q u a n tita tiv e ly d e te rm in e d . T he a n a ly tic m eth o d fo r th e d e te rm in a tio n of g ra p h ite only allows th e e v a lu a tio n of th e to ta l q u a n tity of g r a p h ite c o n ta in e d in th e alloy. A ll e x p e ri­

m e n ts to iso late th e sin g le c ry sta ls of g ra p h ite o u t of th e alloy w ere a b o rtiv e , an d th u s i t has been im possible to m easu re th e size a n d q u a n tity of th e c ry sta ls, o r, fo r exam ple, to d eriv e a basis fo r th e m e a su re m e n t of size, fo rm an d n u m b er

Fi g. 7 .

of th e c ry s ta ls p re v a ilin g in th e alloy. D r.

F r a n z R o ll3 d e te rm in e d th e size an d form of th e g r a p h ite cry sta ls by observing on th e su rfa c e of a m icrosection some single lamellse of g r a p h ite by m e a s u rin g th e i r size an d position a n d a f te r rem oving a th in la y e r re m e a su rin g an d so on u n til he a sc e rta in e d th e whole size

» “ D ie G iesserei,” 1928, page 1270.

of th e c ry sta l. As a re s u lt, he was able to d e te rm in e by th is m eth o d th e space occupied by th e g ra p h ite . T his m eth o d m ay c e rta in ly th ro w lig h t on th is q u estio n , b u t i t c a n n o t generally be em ployed because i t is too laborious.

Experimental Evidence.

T h erefo re, an a tte m p t h a d to be m ade to supply a n e x p la n a tio n of th e g ra p h itic s tru c tu r e

Fi g. 8 .

fro m its a p p e a ra n c e u n d e r th e m icroscope a fte r th e m a n n e r of H am asu m i. To m a k e a n e x p e ri­

m e n t fo r reco g n isin g an d d e te rm in in g th e d iffe re n t k in d s of g ra p h ite , i t w as first tr ie d to m ake sam ples c o n ta in in g th e g r a p h ite in specific well-defined form s, a n d th is w as done in itia lly w ith o u t co n sid erin g th e m eans w hich h a d to be em ployed.

F ig . I 10 is a m icrosection show ing well-defined p la ty g ra p h ite . T his g ra p h ite in te r r u p ts th e m etallic s tr u c tu r e of th e alloy in an im p o rta n t m a n n e r, a n d i t seem s probable t h a t i t is s e p a ra te d w hen th e m ass, o r m ost of it , is still liq u id . T his s tr u c tu r e is o b tain ed , as th e a u th o r ’s e x p e rim e n ts h av e show n, if th e iro n cools dow n slowly u n d e r an argillaceous slag.

Fi g. 9 .

The p la te s of g ra p h ite a re la rg e r an d b e tte r defined as th e r a t e of cooling is slow.

F ig . 2 illu s tra te s a sam ple h a v in g th e g ra p h ite in t h i n leaves. To o b ta in th is s tr u c tu r e a c e rta in re la tio n sh ip betw een th e com position of th e alloy an d th e r a t e of cooling m u s t be d e te r­

m ined . I t is n o t easy to s ta te w ith e x istin g

10 All th e m icrophotographs show n are m agnified to 120 dia­

m eters and are unetched.

know ledge a n e x a c t re la tio n betw een these two facto rs.

F ig . 3 shows a m icro p h o to g rap h of an alloy of th e sam e com position as F ig . 2, ex cep t t h a t b efore c a stin g 2 p e r c en t, of copper w as added.

The g ra p h ite is p re s e n t as th in , crooked leaves, b u t in a d d itio n th e r e a re n u m ero u s nid æ of g ra p h ite .

F ig . 4 shows th e g ra p h ite in its finest d is

tri-F io . 10.

b u tio n , t h a t is, th e g ra p h ite e u te c tic . To o b tain th is s tr u c tu r e , a chem ical com position of th e alloy confined w ith in c e rta in lim its is necessary, b u t personal e x p e rim e n ts h av e shown t h a t th e re a re o th e r obscure co n d itio n s to be ta k e n into account.

A fte r h av in g c re a te d th ese s ta n d a rd form s, a tte n tio n was n e x t d ire c te d to th e q u estio n as to w h e th e r th e g rey q u a litie s of b la s t-fu rn a c e

sm elted p ig -iro n show a g ra p h ite fo rm atio n on th e s u rfa c e of a m icrosection sim ilar to those of th e s ta n d a r d form s. A la rg e n um ber of sam ples from d iffe re n t b la st-fu rn a c e p la n ts w ere pro cu red an d ex am in ed , w ith th e re s u lt t h a t th e m a jo rity d id n o t p e rm it of a ra tio n a l classifica­

tio n as to w h e th e r sev eral k in d s of g ra p h ite were co-existing. D ifficulty was th u s p re se n te d in de­

cid in g w hich was p re d o m in a tin g , or w h eth er th e

Fi g. 1 1 .

g ra p h ite w as a tr a n s itio n a l p ro d u c t, how ever, an e x a c t c h a ra c te ris a tio n was deem ed to be use­

less. A c e rta in n u m b er of specim ens did show a c h a ra c te ris tic s tr u c tu r e , an d a selection of th ese a re in clu d ed in la te r illu s tra tio n s .

Interesting Structures from Diverse Sources.

F ig . 5 shows a m icro of a sam ple of a Swedish charcoal p ig -iro n c o n ta in in g C 4.2, Si 0.64, and

M n 0.1 p e r c e n t. T he g ra p h ite is in th in , curly leaves. A sim ila r s tr u c tu r e is show n in F ig . 6, w hich is a sam ple of p ig -iro n p ro d u ced in a sm all b la s t fu rn a c e .

F ig . 7 illu s tra te s a sam ple of a n E n g lish pig- iro n c o n ta in in g C 3.32, S i 3.28 a n d M n 0.58 p er cen t. B esides g ra p h ite in t h i n leaves, g lo b u lar g ra p h ite is also seen. A sim ila r k in d of s tru c ­ tu r e is show n in F ig . 8, a p ig -iro n w ith S i 2.64

Fi g. 12.

p e r c e n t., fro m a G erm an p la n t. A n otew orthy m icrosection show ing p la ty g ra p h ite is re p ro ­ duced in F ig . 9. F ig . 10 shows th e s t r u c tu r e of a n I n d ia n p ig -iro n w ith w ell-defined e u tec tic g ra p h ite .

Condition of Graphite and Transverse Strength.

N o d o u b t th e m ech an ical p ro p e rtie s of these alloys a re definitely d e p e n d e n t u p o n th e ty p e of

g ra p h ite c o n ta in e d in th e s tru c tu r e of th e alloy.

P la ty g r a p h ite very larg ely in te r r u p ts th e m etallic c o n tin u ity of th e alloy, a n d sam ples w ith g r a p h ite of th is ty p e w ill h ave very low s tr e n g th p ro p e rtie s. The s tre n g th values in ­ crease w ith a n in creasin g refin em en t an d c u rli­

ness of th e g ra p h ite . As a n illu s tra tio n , some exam ples from p ra c tic e m ay be c ite d . Test- b ars, 650 m m . long by 30 mm . d ia. (26 in. by

Fi g, 13.

1.2 in .), w ere c a st an d te s te d in th e u su al m a n n e r. T he first te s t-b a r c o n ta in e d g ra p h ite in p la ty fo rm , as show n in F ig . 11. T his s tru c ­ tu r e was o b ta in e d by m e ltin g th e alloy u n d e r an argillaceous slag an d cooling i t down slowly.

The second te s t-b a r c o n tain ed g ra p h ite in th e form shown in F ig . 12, t h a t is, in cu rly striae.

The s tr u c tu r e of th e th i r d b a r is shown in F ig . 13. I t w as o b ta in e d by a s till slower cool­

in g th a n previously. T he striae a re th in n e r.

to en d eav o u r to produce a p ig -iro n w ith th e