By F. H. Hurren, A.I.C, Member.
The m alleable iro n in d u s try is one of th e la rg e s t a n d m ost im p o r ta n t b ran ch es of th e fo u n d ry tr a d e , b u t o u r know ledge h as n o t p rogressed to a n y th in g like th e e x te n t one m ig h t h ave expected.
The p ro d u c tio n of m alleable iro n c a stin g s as an in d u s try com m enced to w ard s th e la t te r en d of th e e ig h te e n th c e n tu ry , y e t p re s e n t-tim e fo u n d ers a re s till s tru g g lin g w ith m an y of th e sam e diffi
cu lties a g a in s t w hich m an y of th e ir fo re fa th e rs h a d to co n ten d . W h a t is th e re a so n ? I n p a r t i t m ay be d u e to th e laborious n a tu r e of research a n d th e v e ry m a n y v a ria b le fa c to rs w hich h ave to be considered. M alleable iro n is fu ll of o rig in a l sin a n d tro u b le s w hich, overcom e in one d ire c tio n , crop u p in a n o th e r. The m alleable tr a d e h a s suffered from a n excess of secrecy an d suspicion. M uch of th e rese a rc h h a s been c a rrie d o u t by p r iv a te firms a t th e ir own expense, a n d th e know ledge so g a in e d h as n o t been p ublished.
T he o u tp u t of m alleable c a stin g s in G re a t B r ita in is q u ite considerable, th e b u lk of i t b ein g th e w h ite -h e a rt v a rie ty , b u t i t h a s th e d is
a d v a n ta g e of b ein g d iv id ed a m o n g st a la rg e n u m b er of fo u n d ries, th e in d iv id u a l o u tp u t of each b ein g re la tiv e ly low. T he sm all fo u n d rie s c a n n o t afford th e expense of p riv a te research , a re n o t sufficiently well o rg an ised to co-operate, an d th e r e is a d e a r th of easily .available lite r a tu r e on th e su b je c t.
R esearch on m alleable iro n ta k e s tim e . I n v e s ti
g a tio n s in to c a stin g a n d a n n e a lin g c a n n o t be m ade in a few days n o r in a few w eeks. The B ritis h C ast Ir o n R esearch A ssociation will re n d e r in v alu ab le service to th e fo u n d ry tr a d e by a th o ro u g h in v e s tig a tio n of th e difficulties an d in tric a c ie s in se p a ra b le from th e p ro d u c tio n of m alleable iro n castin g s. T he scope of th e neces
sa ry research is enorm ous, a n d will ta k e y e a rs to b rin g to fr u itio n , b u t th e d efin ite know ledge gain ed will be well w o rth th e la b o u r involved.
The p re s e n t use of m alleable iro n is b u t a fra c tio n of th e p o te n tia l uses. M alleable occupies a p o sitio n in te rm e d ia te betw een g rey iro n an d steel sta m p in g s. I t is used w here g re y iro n is n o t of sufficient s tr e n g th an d d u c tility , b u t ow ing to its v a g a rie s is b ein g sdperseded by m ild-steel castings an d stam p in g s. U nless th e in h e r e n t difficulties in th e p ro d u c tio n of m alleable castin g s a re tack led in e a rn e s t, f u t u r e p ra c tic e w ill te n d m ore and m ore to its re p lacem en t by stam p in g s a n d steel c astin g s. I f o u r know ledge w ere such t h a t we could p roduce, w ith o u t fa il, a w h ite -h e a rt m al
leable iro n w hich was • so ft an d d u c tile ,, easily m achinable, free from h a rd patches, w ith a ten sile s tr e n g th of over 20 to n s p e r sq. in . an d a n elonga
tio n a p p ro a c h in g 10 p e r c e n t., an enorm ous d em an d would he se t up, an d m alleable iro n would assum e a p o sitio n second to none. Such a s ta te of affairs is n o t im possible. T ests have been m ade on w h ite -h e a rt m alleable g iv in g resu lts of 23 to n s p e r sq. in . u ltim a te stress, w ith 9 p er c e n t, elo n g atio n , b u t u n fo r tu n a te ly th ese resu lts c a n n o t be re p e a te d w ith c e rta in ty .
Basic Differences Between Malleable and Other Castings.
C onsider th e differences betw een m alleable iro n a n d o th e r c a stin g m a te ria ls. M alleable iro n has a f a r g r e a te r c o n tra c tio n , m ak in g i t liable to c o n tra c tio n cracks o r s e a r s ; i t h as a g re a te r s h rin k a g e in p assin g from th e liq u id to th e solid s ta te , n e c e ss ita tin g special m ethods of g a tin g and feeding. I t is p ra c tic a lly u n m ach in ab le in th e s ta te as cast, an d im possible to te s t in an y m an n er o th e r th a n a te s t to d e stru c tio n . I t has to u n d erg o a long h e a t- tr e a tm e n t, w hich a lte rs its dim ensions an d changes its chem ical com position an d m ic ro s tru c tu re . In c id e n ta lly , th is h e a t- tr e a t
m e n t in tro d u c e s a risk of d is to rtio n an d cracking.
A fte r h e a t- tr e a tm e n t, its u ti li ty can only be g au g ed by chem ical analysis an d its m a c h in a b ility . I n th e p re s e n t s ta te of knowledge, chem ical analysis gives no e x a c t c rite rio n of p ra c tic a l v alu e, as th e com position v aries according to th e th ick n ess of th e p a r t from w hich th e sam ple is ta k e n , an d to th e d e p th of d rillin g . I t is n o t know n w h a t should be th e p ro p o rtio n s of com bined an d fre e carbon in d ifferen t thicknesses of co rrectly -an n ealed m a te ria l. O pinions differ as
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to th e silicon c o n te n t, also re s p e c tin g su lp h u r an d m anganese. I t is know n w ith some degree of cer
t a i n ty t h a t phosphorus h as a d eleterio u s effect, an d should be k e p t as low as possible, p re fe ra b ly below 0.1 p e r c e n t. A nalyses of d iffe re n t m akes of m alleable c a stin g s w ill re v e a l silicon c o n te n ts v a ry in g betw een 0.35 a n d 1.1 p e r c e n t. ; su lp h u r betw een 0.15 an d 0.45 p e r c e n t., a n d m an g an ese fro m 0.10 to 0.60 p e r cen t. I t is a b su rd to dog
m atise, a n d a v ery long a n d c a re fu l rese a rc h is n eeded to find some d efin ite d a ta on chem ical com position. E v en th e n fo u n d ry m en w ould h av e to com prom ise. C om position w ould n a tu r a lly v a ry acco rd in g to th ick n ess of sectio n , a n d in th e sam e c a stin g i t is q u ite comm on to find sections v a ry in g from 1 in . dow n to | in. th ic k .
Present Position.
W h a t is th e e x te n t of o u r know ledge of th e science of p ro d u cin g m alleable iro n c a stin g s, an d how can th is know ledge be ap p lied to c o n tro l p ro d u c tio n in th e fo u n d ry ? W e know t h a t a h ig h silicon c o n te n t induces b r i ttle castin g s, w ith a d a rk g rey f r a c tu r e s im ila r to com m on iro n . I t is know n also t h a t a low silico n -co n ten t increases th e difficulty of a n n e a lin g ; in fa c t, w ith a low silicon a n d h ig h su lp h u r c o n te n t a n n e a lin g is a lm o st im possible. T he a u th o r h as su b jected sam ples of m alleable iro n c o n ta in in g 0.42 p e r c en t, silicon an d 0.45 p e r c e n t, s u lp h u r to a n n e a l
in g te m p e ra tu re fo r 21 days, an d a t th e end of t h a t tim e only a b o u t o n e-six tee n th of a n inch in d ep th was soft, th e re m a in d e r of th e section b ein g p ra c tic a lly u n c h an g ed .
Silicon Requirements.
The a c tu a l silicon c o n te n t m ost desirab le is, in th e a u th o r ’s opin io n , still a m a t te r of c o n je c tu re . I t is s ta te d by v ario u s a u th o r itie s t h a t th e silicon should be low er fo r heavy sections th a n fo r lig h t sections. C om plications a rise w hen th e r e a re b o th heavjsi an d lig h t sections in th e sam e c a stin g , a n d in m o d ern e n g in e e rin g p ra c tic e th is is th e ru le r a t h e r th a n th e ex cep tio n . All th e r e is to w ork on is th e fa c t t h a t silicon below 0.4 p e r c en t, a p p e a rs to r e t a r d a n n e a lin g , an d above 0.9 p e r c en t, th e r e is in tro d u c e d a risk of low s tre n g th , loss of d u c tility , an d a s t a te very li tt le b e tte r th a n
g rey iro n . The only possible course is to find th e silicon ra n g e w hich b est su its th e g en eral r u n of w ork an d en d eav o u r to keep w ith in t h a t ran g e.
I n th e R o v er F o u n d ry a silicon c o n te n t ra n g in g fro m 0.55 to 0.75 p e r c en t, is called for.
Carbon Control.
C ontrol of silicon is of g r e a t u tility , b u t in th e a u th o r ’s o pinion w h a t is of even m ore im p o rtan ce is c o n tro l of carb o n c o n te n t. I n th e un -an n ealed c a s tin g th e carb o n should be wholly or m ain ly in th e com bined s ta te . T he la te s t th eo ry propounded is t h a t w hen a c e rta in te m p e ra tu re is reached d u rin g th e process of a n n e a lin g , th e com bined c arb o n or c e m e n tite b reak s down, a n d g ra p h ite or free carb o n is p re c ip ita te d . W ith in crease of te m p e r a tu r e th is p re c ip ita tio n of g ra p h ite continues u n til th e p ro p o rtio n of com bined carbon, falls to a b o u t 1.8 p e r cen t. F ro m th is p o in t, w ith con
tin u e d h e a tin g , d é c a rb u risa tio n ta k e s place. Now i t is obvious t h a t th e h ig h e r th e to ta l carbon con
t e n t in th e h a rd c a stin g th e longer m u st be th e p erio d of h e a tin g before th e p re c ip ita tio n of g r a p h ite ends an d d é c a rb u risa tio n commences. In o th e r w ords, to o b ta in good m alleable, a n increase in th e to ta l carb o n c o n te n t in th e h a rd c astin g n ecessita tes a lo n g er p eriod of a n n e a lin g . The a u th o r ’s view, born e o u t by h is personal e x p e ri
ence, is t h a t th e m ost d u ctile, s tro n g an d easily m ach in ab le c astin g s a re associated w ith a low to ta l-c a rb o n c o n te n t a f te r a n n e a lin g , th is carbon b ein g d is trib u te d as p a r t g ra p h ite or te m p e r carb o n a n d p a r t com bined as p e a rlite .
F ro m ex p e rim e n ts w hich h ave been c a rrie d o u t over a n u m b e r of y ears th e conclusion has been reach ed t h a t th e b e st re s u lts a re o b ta in e d when th e to ta l carbon c o n te n t in th e h a rd c astin g lies betw een 2.9 an d 3.2 p e r c e n t., an d i t is th o u g h t th is o p in io n w ill be confirm ed by others.
A nalysis of th e u su al b ra n d s of p ig -iro n supplied fo r m alleable w ork in v a ria b ly shows a p re p o n d e r
ance of supplies w ith a to ta l carbon c o n te n t above th e u p p e r lim it of 3.2 p e r c en t. I n m any instances th e a u th o r has found p ig -iro n s w ith a to ta l carbon c o n te n t of 3.6 p e r c e n t., an d even h ig h er. The m ak ers of m alleable p ig w ould confer a re a l boon on th e fo u n d ers if th e y would p ay m ore a tte n tio n
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to th e com position of th e m a te ria l, an d su p p ly an iro n w ith a m ax im u m to ta l carbon c o n te n t of 3.1 p e r cen t.
Influence of Total Carbon.
In co nnection w ith th is s u b je c t n u m ero u s re su lts of ex p e rim e n ts could be g iven, illu s tr a tin g th e effect of to ta l carb o n , an d th e tw o follow ing have been ta k e n as ty p ic a l exam ples. Two sam ples of h a rd c a stin g s w ere a n n e a le d u n d e r p ra c tic a lly id e n tic a l c o n d itio n s of tim e an d te m p e r a tu r e . O ne co n tain ed 3.06 p e r c en t, to ta l carb o n in th e h a rd sta te , an d th e o th e r 3.38 p e r c en t, to ta l carbon.
R e p re s e n ta tiv e sam ples w ere ta k e n fo r an aly sis a f t e r a n n e a lin g . T he re su lts a re given in T ab le I.
T a b l e I.—Result of Annealing Two Samples— One with High Total Carbon.
No. 1. No. 2.
Before annealing. Per cent. Per cent.
Total carbon 3.06 3.38
After annealing.
Total carbon 0.58 2.62
Comb. 0.14 1.04
Free „ 0.44 1.58
Silicon 0.56 0.61
I t will be n o ticed t h a t No. 1 only c o n ta in e d 0.58 p e r c en t, to ta l carb o n , w hereas No. 2 c o n ta in e d 2.62 p e r cen t. No. 1 h a d lost 2.48 p e r c e n t, carb o n d u rin g a n n e a lin g , a g a in s t a loss of only 0.76 p e r c en t, carb o n in No. 2. N o. 1 was so ft, easily m achinable, an d b e n t th ro u g h a n angle of 90 deg.
before show ing signs of cra c k in g . No. 2 was so ft an d easily m achinable, b u t f r a c tu re d a f te r b e n d in g th ro u g h an an g le of a b o u t 15 deg. I t h a s been fo u n d re p e a te d ly t h a t m alleable c astin g s w hich a re so ft b u t n o n -d u c tile h av e a h ig h to ta l-c a rb o n con
te n t, an d u n d e r c e rta in c o n d itio n s only v ery slig h t d e c a rb u risa tio n ta k e s place even w ith an u n d u ly prolonged an n eal.
Graphite Precipitation.
Some tim e ago th e R over w orks w as v e ry m uch tro u b le d by a c e rta in c a s tin g show ing a dull s u r face w ith black spots in sm all a re a s a f t e r m ach in in g . T his occurred on ab o u t 25 p e r c en t.
of th e c a stin g s su pplied, th e castin g s g iving no difficulty in m ach in in g , b u t being very b rittle . A nalysis rev ealed a h ig h c o n te n t of fre e carbon, a n d m icro-sections showed g ra p h ite in a fine s ta te of division. R e p e a te d re-a n n e a lin g s a t d iffe re n t te m p e ra tu re s fo r le n g th y p erio d s h a d h u t little effect on th e carbon c o n te n t. T he a u th o r’s o rig in a l o p in io n w as t h a t th e tro u b le was caused by h ig h to ta l carb o n in th e h a rd c a stin g , b u t in th e lig h t of la t e r know ledge would assume t h a t a h ig h te m p e ra tu re of a n n e a lin g w as p a rtly respon
sible. T h e b e st re s u lts fro m m alleable castin g s a re o b ta in e d w hen th e fre e o r te m p e r carbon is in th e n o d u la r form . I t has been p roved recently by N o rth c o tt t h a t th e ro u n d ed o r n o d u la r form of te m p e r carbon is produced below 1,000 deg. C., a n d above th is te m p e ra tu re th e g ra p h ite assumes th e flaky s tr u c tu r e fa m ilia r in g rey iron. I t is reaso n ab le to c o n jec tu re t h a t in th e castings ju s t re f e rre d to , n o t only was th e to ta l carbon con
t e n t h ig h , b u t t h a t som e of th e castings h a d been su b jected to a n a n n e a lin g te m p e ra tu re exceeding 1,000 deg. O., an d g ra p h ite h a d se p a ra te d in a finely d iv id ed s ta te .
Manufacturing Desiderata.
T he m a n u fa c tu re of m alleable iron castings Comprises tw o d is tin c t o p eratio n s, th e prod u ctio n of th e h a rd castin g s, a n d th e an n ealin g process.
T he difficulties m et w ith in th e c a stin g sta g e can b e overcom e by c a re an d comm onsense. Any fa u lts due to- th e a c tu a l m a n u fa c tu re of th e c a s t
in g a re u sually discovered d u rin g m ach in in g , and before th e a rtic le is p u t in to service. U n fo r
tu n a te ly , i t is d u rin g th e an n ealin g process th a t tro u b le is m ost likely to develop, an d castings w hich appear- p erfectly sa tisfa c to ry fa il in service, ow ing to s tr u c tu r a l w eaknesses induced by incor
r e c t com position. I t h as been p a r t of th e a u th o r ’s d u ty fo r m an y y ears to in v estig ate causes of fa ilu re in service, an d he can say, w ith o u t h e s ita tio n , t h a t th e m a jo rity of fa ilu re s have been d u e to irre g u la ritie s of an n ealin g , a n d in m ost in stan ces d u e to lack of d u c tility caused by excessive carb o n c o n te n t. T he text-book th eo ry of a n n e a lin g , a n d th e one w hich is g en erally accepted, is t h a t th e changes occur in two
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s e p a r a te phases :— (1) T he e lim in a tio n of carhon fro m th e o u te r p o rtio n s o f th e c a s tin g ; a n d (2) a so fte n in g of th e iro n by p re c ip ita tio n of a n n e a l
in g carbon.
Chemistry of Annealing Process.
The u n iv e rsa l p ra c tic e in th is c o u n try is to p ack th e c a stin g s w ith a m ix tu r e o f re d an d black h e m a tite ores. I t is assum ed t h a t a t h ig h te m p e ra tu re s oxygen is lib e ra te d from th e o re an d re a c ts w ith c a rb id e from th e o u te r su rfa c e of th e c a stin g fo rm in g carb o n m onoxide a n d fre e f e r r ite . T he carbon m onoxide com bines w ith f u r th e r oxy
g en, to fo rm c a rb o n dioxide, w hich in tu r n re a c ts w ith carb o n from th e c a stin g , fo rm in g carb o n m onoxide a n d lib e ra tin g fe r rite . T hese re a c tio n s a re c o n tin u o u s, a n d a t a la te r stag e, fre e o r te m p e r carb o n is p re c ip ita te d . T h is is som e
w h a t a t v a ria n c e w ith a la te r th e o ry t h a t , a t a c e rta in te m p e ra tu re , th e r e is a p a r t ia l conversion of c e m e n tite in to a u s te n ite . On f u r th e r h e a tin g th e c e m e n tite b reak s down, a n d g r a p h ite is p r e c ip ita te d . C o n tin u ed h e a tin g re s u lts in d é c a r
b u risa tio n a t th e expense of th e a u s te n ite an d g ra p h ite .
F ro m a n u m b e r of e x p e rim e n ts m ade, a n d th e ex p erien ce g ain e d in th e p ro d u c tio n of th o u s a n d s of to n s o f m alleable c a stin g s d u r in g th e p a s t fifteen y ears, th e a u th o r doubts if th e e a rlie r an d g en erally accepted th e o ry fu lly ex p lain s th e chem ical physics of a n n e a lin g . F ro m te s ts m ad e, n o t u n d e r la b o ra to ry c o n d itio n s, b u t in a c tu a l w orks p ra c tic e , sev eral a p p a r e n t c o n tra d ic tio n s arise . T hese m ay be w o rth e n u m e ra tio n .
(1) C astin g s o f th e sam e p a tte r n , o f sim ila r o rig in a l com position, a n n ealed in th e sam e oven u n d e r p ra c tic a lly id e n tic a l co n d itio n s, h av e u ltim a te com positions v a ry in g w idely in carb o n co n te n t.
(2) Loss of c a rb o n ta k e s p la c e n o t only when th e p a c k in g m edium is o x id isin g , b u t also w ith re d u c in g o r n e u tr a l m edia.
(3) Loss of c arb o n does n o t a p p e a r to> be d ire c tly p ro p o rtio n a l to mass.
(4) W h ilst g e n e ra lly sp eak in g , loss of carbon increases w ith in creased tim e of a n n e a lin g , n o tab le exceptions occur.
(5) I n t e r m i tt e n t rises an d falls in te m p e ra tu re o fte n re s u lt in a lower carbon c o n te n t th a n a continuously m a in ta in e d a n n e a lin g te m p e ra tu re .
(6) O th er elem ents, nam ely silicon, su lp h u r, a n d m anganese, h av e a pronounced effect on th e final re s u lt.
(7) B e tte r re s u lts -are o b ta in a b le by a len g th y a n n e a l a t a co m p arativ ely low te m p e ra tu re , th a n a sh o rte r a n n eal a t h ig h te m p e ra tu re .
Detailed Consideration.
I t has been found on fre q u e n t occasions in te s tin g sam ples from th e a n n e a lin g ovens, th a t tw o c a stin g s o f th e sam e p a t t e r n ta k e n fro m a d ja c e n t stacks, show considerable v a ria tio n s in carb o n c o n te n t. D rillin g s h av e been ta k e n th o ro u g h ly to re p re s e n t th e sam ples, an d to be com parable. In s ta n c e s m ig h t be m entioned of to t a l c arb o n c o n te n ts of 2.06 an d 1.48 p e r cen t, resp ectiv ely , w hen i t m ig h t be reaso n ab ly expected t h a t th e re s u lts would be sim ilar. W h a t is th e e x p la n a tio n ? Is th e u ltim a te com position
influenced by th e c a stin g te m p e ra tu re ?
I f th e process of a n n e a lin g involves a supply of oxygen from th e m ixed ores used as a pack in g m edium , w hy is i t t h a t loss of carbon occurs when n e u tr a l o r re d u c in g m edia a re used ? Y et such is th e case, according to ex p erim en ts personally conducted. A ty p ic a l in s ta n c e m ay be cited.
S am ples c a s t from th e sam e sh a n k of m etal w ere p ack ed resp ectiv ely in th e s ta n d a rd m ix tu re of h e m a tite ores, in coarse coke, an d in well b u r n t san d . T he lids an d jo in ts of th e te s t can s were lu te d w ith a m ix tu re of sand an d clay, to exclude a ir as m uch as possible. The to ta l carbon in th e h a rd s t a te w as 3.3 p e r cen t. A te m p e ra tu re of 950 deg. C. w as m a in ta in e d , in a gas-fired f u r nace, fo r 120 ho u rs, followed by a slow cooling.
T horoughly re p r e s e n ta tiv e sam ples w ere analysed w ith re s u lts shown in T able I I . These show t h a t w ith th e m ixed ores th e r e w as a loss of 1.5 p e r c en t, c a rb o n ; w ith coke -a loss of 1.0 p e r c e n t., a n d w ith sa n d p ack in g , 0.90 p er c e n t, carbon.
I f a n n e a lin g is d e p e n d e n t on th e oxygen derived from th e ore, how h as th e e lim in a tio n of carbon been a rriv e d a t w hen a n o x id isin g atm o sp h ere was n o t e x is te n t? R e p e titio n of th is ex p erim en t by
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tr e a tm e n t in one of th e w orks a n n e a lin g ovens was a tte n d e d w ith sim ila r re s u lts .
T h a t loss of carb o n is n o t p ro p o rtio n a l to th e m ass of th e c a stin g h a s been evidenced on n u m e r
ous occasions in o rd in a ry w orks p ra c tic e , a n d also in a series of e x p e rim e n ts c a rrie d o u t some few
ous occasions in o rd in a ry w orks p ra c tic e , a n d also in a series of e x p e rim e n ts c a rrie d o u t some few