The Presidential Address
NOTES ON CARBON IN FOUNDRY IRONS AND CASTINGS : UTILITY OF ITS CONTROL AND
PRACTICAL DIFFICULTIES.
By M. Siegle, Professor of Metallurgy (Nancy), Late Manager of the Im phy and Decuzeville Works.
[T his P a p e r w as p re se n te d on b eh alf of th e A ssociation T echnique de F o n d e rie de 1’ ra n e e .]
F ro m th e in d u s tria l p o in t of view th e m ost im p o rta n t p ro p e rtie s of cast-iro n a re its re s is t
ance to ten sile stress and tra n sv e rs e , th e ease w ith which i t c a n be m achined, and its re sista n c e to shock. Of th ese p ro p e rtie s th e first th r e e depend m ore p a rtic u la rly on th e greyness or w hiteness of th e iro n , ju d g ed by th e a p p earan ce of th e f r a c tu re . R esistan ce to shock also depends upon th is fe a tu re , b u t is larg ely influenced by th e phosphorus p re s e n t. B u t th e iro n becomes g rey er as th e g ra p h itic carb o n increases, which hap p en s w hen (1) th e to ta l carbon co n te n t increases, (2) w hen th e silicon is h ig h er, o r (3) when th e r a t e of cooling from s ta r tin g c astin g is dim inished. M anganese also affects th e ap p earan ce of iro n used b y th e Siem ens an d basic B essem er processes; its action is c o n tra ry to t h a t of silicon or carbon, t h a t is, i t te n d s to reduce th e fo rm atio n of g ra p h ite . I n cast-irons, in which a c e rta in q u a n tity of g ra p h ite alm ost alw ays occurs, so long as th e m anganese p re s e n t does n o t exceed 1 o r 1.25 p e r cen t, th e r e will be little effect on th e p re c ip ita tio n of g ra p h ite .
As is g en erally known, carb o n an d silicon g e t in th e iro n in th e b la s t fu rn ace, an d in g re a te r p ro p o rtio n w hen th e fu rn a c e is on h o t-b last, w hich gives g rey iro n of good castin g q u a lity . T he p a r t played by th e to ta l carbon is p a rtic u la rly in te r e s t
in g an d will be d ealt w ith from th e p ra c tic a l p o in t of view in th e following notes.
The Influence of Silicon on the Formation of Graphite and the Mechanical Properties of a Cast-Iron
with a Fixed Carbon Content.
P ro fesso r T u rn e r, in “ The M e ta llu rg y of I r o n ,”
has given th e resu lts of his ex p erim en ts in a d d in g g ra d u a lly in creasin g pro p o rtio n s of silicon to
cast-u cast-u
iron, a t th e sam e tim e k eep in g th e carb o n c o n te n t as c o n s ta n t as possible. B y carefu lly h e a tin g w h ite iro n w ith wood charcoal in a crucible to a high te m p e ra tu re , he succeeded in o b ta in in g an iro n absolutely fr e e from silicon. T his w h ite iron w as th e n re-m elted w ith g ra d u a lly in c re a sin g p ro p o rtio n s of ferro -silico n , w ith th e r e s u lt t h a t an in creasin g ly g rey iro n was o b ta in e d . As th e silicon w as in creased th e p ro p o rtio n of g ra p h itic carbon
S / i / C iW C O N T E N T
w h i t e, m o t t l e d o b e y v e r y g r e y
Fig. 1 .— Te n s il e St r e n g t h of Ir o n s WITH 2 PER CENT. CARBON AND Va r y in g Am o u n ts of Sil ic o n acc o rd in g to Pr o f. Tu r n e r. also becam e g re a te r. T he o rig in a l w h ite iro n , l^ard an d b rittle , g ra d u a lly becam e g re y e r, so fte r, a n d to u g h e r. An excess of silicon, how ever, produced too m uch g ra p h ite , w ith th e re s u lt t h a t th e iron a g a in becam e m ore fra g ile .
P rofessor T u r n e r ’s d ia g ra m , show ing th e re la tio n betw een silicon c o n te n t an d te n s ile s tre n g th , is show n in F ig . 1. One of th e n o tab le f e a tu re s in th is series of e x p erim en ts is th e v ery low pro p o rtio n o f to ta l carb o n , w hich is of th e o rd e r r>f
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2 p er c en t. The su b seq u en t effects on th e a p p e a r
ance of f r a c tu r e a n d m e c h a n i c a l p ro p e rtie s, fol
low ing increased p ro p o rtio n s of silicon, would be th e sam e if ta k e n on iro n s w ith a fixed carbon c o n te n t g re a te r th a n 2 p e r cent.
Usual Carbon Content in Pig Irons.
As P ro fesso r T u rn e r h a s shown, th e iro n from a b last fu rn a c e ra re ly c o n ta in s less th a n 2 p e r c en t, carbon or m ore th a n 4.5 p er cen t. B u t it is c-leai th a t , if th e carb o n can v ary from 2 to 4.5 per ce n t., th e p ro p o rtio n of g ra p h ite w hich will be p re c ip ita te d m u st be affected, d e p e n d in g also on th e silicon p re s e n t an d th e r a te of cooling. T ak in g , for exam ple, th e o rd in a ry b last-fu rn ace pig c a st in san d . I f two sam ples M an d N (F ig . 2) of equal carb o n are com pared, t h a t w ith th e h ig h e r silicon c o n te n t w ill show th e m ore g ra p h ite and will be th e g rey er of th e tw o. On th e o th e r h an d , if tw o sam ples (M an d 0 ) of eq u al silicon c o n te n t be com pared, th e n th a t w ith th e h ig h e r carbon (M) will be th e g rey er and, consequently, show m ore g ra p h ite .
I f a n iron P w ith th e sam e carbon as O an d th e sam e silicon as N is broken, i t will c o n ta in less g ra p h ite th a n any o th er betw een th ese lim its.
T hus M will be th e greyest iron and P th e w h ite st an d O an d N will h ave th e m ost sim ilar fra c tu re s . Suppose th e re a re tw o sam ples of iro n 0 a n d N whose co n ten ts of silicon and carb o n a re such t h a t , in th e pigs, have exactly th e sam e g r a p h ite c o n te n t. This sim ila rity of a p p earan ce w ould n o t be m a in ta in e d u n d e r all conditions, b u t would be a lte re d by subsequent tr e a tm e n t. I f th e iro n w ere rem elted an d cast in to la rg e blocks, as, for exam ple, th e rolls fo r a rolling mill, w hich would cool slowly, th e n th e iron w ith h ig h e r carbon con
t e n d (N) would ap p e a r grey er th a n 0 . E a c h in d i
v id u a l in crem en t of p re c ip ita tio n of g r a p h ite h as an u n d o u b ted influence on th e m echanical p ro p e rtie s of th e iron. The g ra p h ite m ay be segre
g a te d in a few places, o r i t m ay be evenly d is tr i
b u te d . M r. Adam son, in a P a p e r re a d b efo re th e M an ch ester Association of E n g in eers, d istin g u ish es betw een -‘ fine g ra p h ite ,” “ s ta r g r a p h i te ,” and
“ flake g r a p h ite .” T h ere is, too, th e com bined carbon, t h a t is, th e to ta l carbon less t h a t o ccu rrin g
as g ra p h ite , which also m u st be ta k e n in to con
s id e ra tio n . I t is r a th e r su rp ris in g t h a t irons, w idely d iffe re n t in to ta l carb o n , should p re se n t th e sam e a p p e a ra n c e of f r a c tu r e an d sim ilar m echanical p ro p e rtie s w hen th e silicon c o n te n t is such as to com pensate th e difference in carb o n . I t is, how ever, a f a c t w hich will be tr e a te d m ore fu lly f u r th e r on. I t w ould a p p e a r t h a t th e p e r
ce n ta g e of g ra p h ite , insoluble in b o ilin g acids and d e te rm in e d by d ire c t analysis, docs n o t a lto g e th e r correspond w ith th e m ore o r less g ra p h itic a p p e a r
ance of th e fra c tu re . I t is, how ever, th is aspect, to which g r e a t p ra c tic a l im p o rta n c e h a s alw ays been a tta c h e d , t h a t seems to offer th e c le a re st in d ic a tio n of th e m echanical p ro p e rtie s of a
cast-Fi g. 2 .— Gr a p h it e Va r ia t io n s Wh e n C and Si v a r y.
iron, an d its u tilis a tio n in th e fo u n d ry as a m ix er to give a d efin ite analysis o r fo r q u a lity .
Variation of Carbon and Silicon in Pig Irons.
As is g en erally know n, th e cold-blast iro n s are , as a ru le, low in carb o n an d s ilic o n ; e x tr a hot- b last irons, such as ferro-silicon, r u n n in g u p to 10 to 12 p e r cen t, silicon, a re n o t g en erally h ig h in carbon. Iro n s h a v in g a g ra n u la r a p p earan ce—
due to excess of g ra p h ite — a re those c a rry in g 4 to 4.25 p e r cen t, carbon, w ith 3 to 2.75 p e r c en t, of silicon. The carbon an d silicon c o n te n ts in a series of ta p p in g s follow, m ore o r less, th e o p e ra tio n of th e b la st fu rn a c e as i t ru n s “ h o t ” o t
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“ cold b u t i t h a p p e n s t h a t as one of th e s e ele
m en ts increases th e o th e r decreases.
A ccording to th e q u a lity of th e ores an d coke, th e n a tu r e of th e slag, an d according to c e rta in undefined v a ria tio n s in w orking a n in d iv id u a l b la s t fu rn a c e or fu rn a c e s in a n o th e r region, will yield p ig which, w hen c a s t in san d , is exactly sim ilar in a p p e a ra n c e of fr a c tu r e a n d sold u n d e r th e sam e tr a d e num b er, y e t h av in g very d iffe re n t p ro p o rtio n s of to ta l carb o n an d silicon.
As an exam ple, a b last fu rn a c e d u r in g tw o s e p a ra te periods a t an in te rv a l of several m onths yielded very grey irons, which w ere deliv ered to
Fi g. 3 .— Sil ic o n Co n ten t of Fo u n d r y Ir o n s Cl a s s if ie d as No. 2.
a custom er and gave e n tire s a tisfa c tio n , an d w hich, based on th e a p p earan ce of th e fr a c tu re , w ere classified as a No. 2 from its fr a c tu re . Y et th e av erag e silicon c o n te n t d u rin g th e first p erio d w as to ta lly different from t h a t d u rin g th e second p eriod. T his is shown in F ig . 3, w hich gives th e n u m b er of casts a g a in s t th e silicon of th e tw o periods A and B. This was only possible because th e iro n d u rin g th e first p eriod (A) h a d a h ig h carbon an d low silicon co n ten t, w hereas d u rin g th e second p eriod (B) th e re la tiv e p ro p o rtio n s w ere reversed. A q u a n tity of th e iro n pro d u ced in th e second period (B)—w ith silicon 3 to 3.5 p e r cen t.
—w as classified an d sold as a N o. 5, th o u g li th e to ta l carb o n w as a c tu a lly low.
T h e re a re c e rta in b la s t fu rn a c e s w hich, th a n k s to th e o res an d th e coke av ailab le, easily t u r n o u t iro n c a rr y in g a b o u t 4 p e r c en t, carb o n , w hilst o th e rs, even w ith h o t w orking, c a n n o t yield m etal w ith m ore th a n 3.25 to 3.5 p e r c e n t, of carbon.
F ig s. 4 an d 5 show th e d a ily analyses for O Si a n d M n d u rin g a r u n of tw o weeks on a b last fu rn a c e w o rk in g on m a n g a n ife ro u s iron, a fria b le coke a n d m agnesium b e a rin g ores. I n s p ite of a
Fi g. 4 .— Re su l t of Two We e k s Wo r k in g w i t h Re e in e d Ir o n Co n t a in in g 2 p e r c e n t. Ma n g a n e s e a n d Sh o w in g t h e Av e r a g e Da il y Carbon a n d Sil ic o n Co n t e n t.
m anganese c o n te n t above 2 p e r c en t.— which should h av e aid ed th e c a rb o n isa tio n of th e iro n —th e re su ltin g m e ta l was low in carbon. T his re fe rs to th e sam e tw o weeks d e ta ile d in F ig . 5, w hen th e carbon was p a rtic u la rly low, in sp ite of th e fa c t t h a t th e silicon was g en erally h ig h e r, an d th e iro n was g re a tly la ck in g in flu id ity .
Fracture Resemblance in Relation to the C and Si Content of Cast Pig.
F ro m a v ery larg e n um ber of analyses of hem a
t i t e fo u n d ry irons (phosphorus less th a n 0.12 p er
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c en t.) from a b la s t fu rn a c e o p e ra tin g u n d e r fre q u e n t changes of r a t e of w orking, a d ia g ra m was draw n (F ig . 6) in w hich th e cu rv ed lines c o rre spond to th e com position of san d -cast pigs which would give sim ilar f r a c tu re s o r com m ercial classi
fication. T his m ethod proved e n tire ly s a tisfa c to ry to a works b asin g th e ir b uying on it. F ig . 6 shows t h a t irons w ith the sam e carbon^ c o n te n t, say, fo r exam ple, 3 p e r cent, on line X X 1, w ill be w hite, m o ttled , g rey, o r very g rey in p ro p o rtio n as th e silicon p re s e n t increases. I t is also tr u e
F i g . 5 .— S a m e a s F i g . 4, b u t f o r a F u r t h e r P e r i o d .
th a t those irons which have th e same silicon, of ab o u t 2 per cent, (line YY1), will be m o ttled , grey, or very grey according to th e in creasin g p e rc e n ta g e of carbon. A b last fu rn ace p ro d u cin g a w h ite iron of 2 pen- cent, silicon and only slig h tly car- bu rised is n o t usual, indeed, i t is n o t w ith in th e w r ite r ’s experience.
The increasing grevness of th e fr a c tu r e fo u n d in P ro fesso r T u rn e r’s experim ents when d ealin g w ith iron of a co n sta n t to ta l carbon c o n te n t of 2 p e r c en t, an d in creasin g silicon would be confirm ed in every case in which th e to ta l carbon is c o n sta n t.
M oreover, th e m ore th e carbon is ra ise d , th e less
will be th e p ro p o rtio n of silicon necessary to p ro duce a given greyness.
I t is su rp ris in g th a t , in P ro f. T u rn e r’s e x p e ri
m ents, 2 p er c en t, silicon w ith 2 p e r c e n t, carbon sufficed to give a m o ttle d iro n of h ig h ten sile s tre n g th , th o u g h i t is possible t h a t th is re s u lt was produced by some special c o n d itio n in m ethod used fo r th e p ro d u c tio n of th e c a st iro n . Iro n s sold as No. 2 d u rin g th e periods ( a ) and (b) (F ig. 2) correspond to th e com positions shown by th e d o tte d
I 2 3
TOTAL CARBONX
Fi g. 6 . — Te x t u r e o f Sa n d-c a s t He m a t i t e Pi g- Ir o n i n Re l a t i o n t o t h e C a n d S i Co n t e n t.
a re a in F ig . 7. T he d o tte d lin e Z Z 1 of F ig . 6 shows th a t, in b la s t fu rn a c e p ra c tic e , th e h ig h est to ta l carbon c o n te n t possible to e x is t fo r each silicon c o n te n t is dim inished as th e p ro p o rtio n of silicon increases. I t is, in fa c t, well know n t h a t th e presence of silicon reduces th e po ssib ility of in creasin g th e carb o n in th e m olten s t a t e ; a t any ra te , u n d e r norm al conditions of w orking. B u t below th is m axim um c arb o n —co rresp o n d in g to a given p e rcen tag e of silicon— an in fin ite v a rie ty of co n ten ts is possible, exam ples of which a re f r e q u en tly found in p ractice.
Chemical Classification of Pig-Irons.
P ig -iro n h as long been sold (su b ject to th e phos
phorus c o n te n t b ein g reasonable) e n tire ly on tho
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a p p e a ra n c e of th e f r a c tu r e . Tlxe g re y e s t n'°*i s ’ called q u a lity -g iv in g (a m élio ran tes), Nos. 2 an d 3 in th e F re n c h classification, a re th e d earest. 5 system is still largely used b o th fo r h e m a tite s an d for th e phosphoric iro n s of E a s te r n F ra n c e .
Chem ical analysis, how ever, is com ing m ore an d m ore in to fav o u r. T h e elem en t silicon is easily d e te rm in a b le q u a n tita tiv e ly , an d , sin ce its in fluence on th e p re c ip ita tio n of g ra p h ite is well know n, m e ta llu rg ists in several c o u n trie s a ie en d eav o u rin g to estab lish , fo r u n iv e rsa l use, a system of num bers based on th e p e rc e n ta g e of sili
con in th e iro n . I t is hoped by th is m ean s to o b ta in g re a te r accuracy in w orking, an d to elim
in-Fi g. 7 .— Av era ge Co m p o s it io n of t h e Ir o n s at t h e Two Pe r io d s A a n d B of Fi g. 3 , all Cl a s s i f i e d as N o . 2 from t h e Fr a c t u r e.
a te th e personal elem en t which p re d o m in a te s in th e ju d g m e n t by inspection of th e f r a c tu r e of a sam ple.
T he ad o p tio n of th is scheme w ould u n d o u b ted ly lessen th e chances of d isp u te an d litig a tio n . M ore
over, a system of classification based on chem ical analysis becomes an absolute necessity w hen th e iron is c a s t in m etal moulds, as i t is in p ig -c a stin g m achines. F ig . 8 ou tlin es g rap h ically a few classi
fications. A is th e classification ad o p te d in th e S ta te s, w herein th e b a r corresponds to th e a v erag e of each n u m b er, and on th is figure th e r e is a 10 p er c en t, allow ance up or down. B show s th e R u ssian classification decided u pon a t a C o n fer
ence held in R ig a in 1902. T he classification C was p u t fo rw a rd fo r G erm any by P ro fesso r W iist.
D a n d E a re classifications s e t o u t by tw o F re n c h w orks who sell on th e silico n -co n ten t basis. S everal of th e system s so f a r proposed a re open to th e o b jectio n s t h a t th e y give th e sam e n u m b er to irons w ith widely d iffe re n t silicon c o n te n ts, an d , a d d i
tio n a lly , none of th e se classifications h a s ta k e n a n y a c co u n t of th e carbon in th e iro n . I f th e carb o n re m a in e d c o n s ta n t or v a rie d d ire c tly in p ro p o rtio n to th e silicon, all would be w e ll; b u t, as h a s been seen, th is is b y no m eans th e case.
P ro fesso r W iist, who h a s w ritte n several a rtic le s
F i g . 8 . — Ex a m p l e o f t h e Cl a s s i f i c a t i o n o f Fo u n d r y Ir o n s b a s e d SOLELY ON T H E S i C O N T E N T .
d e a lin g w ith th e com position of fo u n d ry irons, calls a tte n tio n to discrepancies in th e p e rc e n ta g e of silicon in d eliveries of iro n sold u n d e r th e sam e n um b er. As an exam ple, he quotes a co n sig n m e n t of 80 wagons of iro n , sold u n d e r th e sam e n um ber. A nalyses from th is c o n sig n m en t showed a v a ria tio n in silicon ra n g in g fro m 1.4 to 3.66 p e r c en t. As he p o in ts o u t, i t would be im possible to o b tain c o n sisten t re s u lts in th e cupola w ith th is m a te ria l. Sam ples ta k e n fro m several wagons of a n o th e r consignm ent, sold as No. 3, showed silicon v ary in g fro m 2.9 to 3.8 p e r c e n t., w hich is con
siderably above th e a m o u n t le g itim a te ly belonging to t h a t n u m b er, a n d he fo u n d t h a t th e b la s t f u r nace h a d supplied a b e tte r n u m b er th a n h a d been o rd ered .
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A ccording to W iist, “ a com parison of th e com
position of iro n s of th e sam e n u m b er an d from th e sam e fu rn a c e shows, w h a t h as been know n fo r a long tim e, t h a t a No. 3 iron is o fte n of b e tte r q u a lity th a n a No. 1 ; sim ila rly , a No. 4 h as fre q u en tly proved b e tte r th a n a No. 3. In view of these re su lts, i t is obvious t h a t th e cu sto m ary m ethod of classification—by th e ap p e a ra n c e of th e f r a c tu re —is e n tire ly u n reliab le, an d should be su p e rse d e d .”
T he a u th o r does n o t a lto g e th e r su p p o rt th is view ; in his opin io n , th e im p o rtan ce of carbon is fa r too
3 ^
■06 06 04
S L / L P H U R
Fi g. 9 .— Pb o po s e d Cl a s s if ic a t io n of Fo u n d r y Ir o n s Ba se d on S i a n d S Co n t e n t (Cl a s s if ic a t io n s h o w n i n Fi g. 8 c o m pl e te d).
g re a t to be e n tire ly ign o red . I t is p ro b ab le t h a t , am ong those ta p p in g s delivered u n d e r th e sam e index num ber, an d of which W iist o n ly e stim a te d th e silicon, those which w ere p a rtic u la rly h ig h in silicon w ere low in carbon, a n d those h ig h in carbon were low in silicon.
Sulphur Content.
The extrem ely h a rm fu l influence of su lp h u r on cast iron is o ften h a r d to define w ith precision, because of th e lack of in fo rm a tio n re g a rd in g th e other elem ents p resen t. G enerally sp eak in g , th e su lp h u r c o n te n t of pig-iron is v ery low in th e grey irons, pro d u ced by h o t w orking, an d increases in cold-worked w hite irons. A d d itio n ally , SOme
fo u n d ry p ig -iro n classifications, based on th e silicon c o n te n t of th e ty p e o u tlin e d in F ig . 8, specify th e m axim um su lp h u r c o n ten t. F o r in stan ce, T able I. shows th is w ith respect to classi
fications C an d D of F ig . 8.
T h u s an iron c o n ta in in g 2.60 p er cent. Si and 0.08 p er cent. S. in classification C would be desig
n a te d a No. 3, an d n o t No. 1. F ig . 9 o u tlin e s these co n d itio n s. The su lp h u r co n te n ts a re shown as abscissae, w ith decreasing values to th e rig h t, a n d th e silicon co n te n ts as o rd in a te s, w ith in creas
ing values from th e base lin e upw ards.
I n F ra n c e d u rin g th e w ar th e price of fo u n d ry irons was c o n tro lled ; w here th e phosphorus con
te n t v a rie d little , th e p ric e was fixed on th e basis of th e silicon an d su lp h u r co n te n ts. F ig . 10 o u t
lines th is, using th e sam e scheme as F ig . 9. The d e a re s t irons were those which c a rrie d th e m ax i
mum of silicon an d th e m inim um of su lp h u r.
W)
Table I . — Classifications B ased on Silicon, but T a kin g S u lp h u r C o n ten t in to C onsideration.
Classification C. Classification D.
of Fig . 8. of Fig. 8
Si. greater S less Si. greater S less
than than than than
0. 3.50 0.03 I
1. 2.50 0.03 y 2.40 0.02
2. 1.80 0.05 J
3. 1.20 0.07
4. — .— 2.10 0.032
5. — — 1.70 0.050
G enerally speaking, F ig s. 9 an d 10 in d ic a te a base which “ hall m a r k s ” th e iro n in d e p en d e n tly of th e silicon c o n te n t. The nearn ess of e q u a lity of
G enerally speaking, F ig s. 9 an d 10 in d ic a te a base which “ hall m a r k s ” th e iro n in d e p en d e n tly of th e silicon c o n te n t. The nearn ess of e q u a lity of