B y Professor Cecil H. Desch, D .Sc., F.R.S. (Member)*
T he e x iste n c e o f in te r n a l stre ss in c astin g s is a fr e q u e n t cause o f fa ilu re , g iv in g rise to defects w hich a re fa m ilia r to ev ery fo u n d ry m a n . All m e ta ls a re liab le t o in te r n a l stre sse s w hen c a s t in to sh ap es w hich allow of th e cooling of d iffe re n t p a r ts a t u n e q u a l ra te s , b u t th e tr o u b le is a p t to p re s e n t itse lf in a m ore a c u te fo rm in th e non- fe rro u s fo u n d ry th a n in th e iro n fo u n d ry , ow ing to th e co n d itio n s o f so lid ificatio n . T he g r e a te r th e ch an g e of volum e d u r in g so lid ificatio n , an d th e g r e a te r th e solid c o n tr a c tio n d u r in g cooling, th e m ore sev ere th e stre sse s w hich will be developed. C a s t iro n c o n tr a c ts o n freezin g , b u t very s h o rtly a fte rw a rd s an e x p an sio n occurs, ow ing to th e lib e ra tio n of g ra p h ite , a n d th is e x p an sio n p a r tly n e u tra lis e s th e effect o f th e con
tr a c tio n , so t h a t stre s s is to t h a t e x te n t relieved.
S teel, o n th e o th e r h a n d , does n o t u n d e rg o an e x p an sio n , an d as its c a stin g te m p e r a tu r e is very h ig h , th e r e is a lo n g ra n g e o f te m p e r a tu r e ov er w hich c o n tra c tio n is o c c u rrin g in t h e solid s ta te , a n d th e lia b ility to in te r n a l stress is th e re fo re g re a t. C ooling stresses o f th is k in d will obviously be in g r e a te r p ro p o rtio n , as th e re d u c tio n of volum e on fre e z in g a n d th e solid c o n tra c tio n are g re a te r, b u t o th e r fa c to rs a re also involved. A very s o ft m e ta l, such as lead , w ill y ield t o stress so t h a t th e c a s tin g w ill d e fo rm m o re o r less, b u t will n o t crack . O n th e o th e r h a n d , a h a r d bronze o r n ic k e l alloy, w ith a h ig h e la s tic lim it, even a t te m p e ra tu re s above t h a t of th e atm o sp h ere , will be u n a b le t o y ield , an d is lik e ly t o develop cracks.
T able I gives th e c o n tr a c tio n o n fre e z in g an d th e m e a n coefficient o f li n e a r c o n tr a c tio n in th e solid s t a t e o f th e p rin c ip a l p u re m e ta ls, b u t few d a t a as to th e i r yield w hen h o t a re av ailab le.
S im ple sh rin k a g e is in itse lf a b le t o a c co u n t fo r q u ite co n sid erab le stresses, b u t in som e m e ta ls an d alloys th e effect is a c o e n tu a te d by t h e c ry s ta l
lin e n a tu r e o f th e m a te ria l. M etals su c h as copper, silv e r, iro n , le a d a n d a lu m in iu m cry sta llise in th e cubic sy stem , an d th e i r con
tr a c ti o n is th e sam e in all d ire c tio n s, b u t o th e rs ,
su c h as zinc, b ism u th , a n tim o n y a n d cadm ium , cry s ta llis e in less sim p le form s, a n d c o n tr a c t m ore in o n e d ir e c tio n th a n in a n o th e r. F o r in stan ce, t h e coefficient o f ex p an sio n (o r c o n tra c tio n ) of b is m u th is 16.2 x 10-' p a ra lle l t o th e ax is a n d o n ly 12.0 x 10-6 p e rp e n d ic u la r t o t h e axis, an d a lth o u g h a c c u ra te d e te r m in a tio n s of th i s k in d h a v e n o t b een m ad e fo r o th e r m e ta ls, i t is c e r
t a i n t h a t all m e ta ls a n d alloys w hich d o n o t c ry sta llise in th e cubic system m u s t show sim ilar differences. S u c h m e ta ls , fo r exam ple zinc, u su a lly fo rm c ry s ta ls w hich a re m uch longer in
Ta b l e L
MetaL
Coefficient of linear expansion.
0
x 10‘.Change of volume on m elting.
P e r cent.
A lum inium 2 7 .4 6 .4
A ntim ony 10 1.4
B ism uth 13 3 .2
C admium 31 4 .7
Copper 16.5 4 .0
Gold 1 4.5 5.2
L ead 29 3 .4
M agnesium 29 —
X ickel . . 16
-Silver 24 5 .0
T in . . 22 2 .8
Zinc 26 6 .5
one d ir e c tio n th a n in an y o th e r, an d th e h ig h con
tr a c tio n in th e d ire c tio n of g re a te s t le n g th sets u p stre sse s w hich m ay be considerable. More
over, such c ry s ta ls grow m ost ra p id ly along th e
a v i s ' a n d e x e r t a m e asu rab le t h r u s t in so doing.
A consequence o f th is is t h a t w hen a castin g , such as a lo n g b a r, begins t o freeze, th e o u te r shell, w hich firs t solidifies, consists o f long crystals, th r u s ti n g a g a in s t o n e a n o th e r an d causing an a p p a r e n t ex p an sio n , le a v in g sm all cav ities in the in te r io r . F o r th is reaso n , m a n y brasses a n d o th er alloys h av e been reco rd ed as e x p a n d in g d u rin g fre e z in g , w h ilst a d e te rm in a tio n of th e d en sity p roves at h a t no re a l ex p a n sio n h as ta k e n place, b u t t h a t th e c a s t b a r is m in u te ly porous. An o u te r sh ell w hich h a s solidified in th is way is in a s t a t e o f stress, w hich is g re a te r th e g re a te r th e
difference in th e coefficients o f exp an sio n o f th e c ry s ta ls in tw o d ire c tio n s a t rig h t-a n g le s to one a n o th e r.
I t follow s t h a t a m ere know ledge of th e coefficients of ex p an sio n of m e ta ls an d alloys co u n ts fo r li ttle in d e c id in g how m u ch sh rin k a g e will be fo u n d in a c a stin g , a n d e m p iric a l m e a su re m en ts o f a c tu a l c a stin g s of sim p le form a re of g r e a te r valu e. T h e s h rin k a g e observed in th is w ay is th e n e t r e s u lt o f a n u m b e r of s e p a r a te changes, w hich m ay p a r tly n e u tr a lis e o n e a n o th e r.
As an exam ple, th e v alu es o f th e sh rin k a g e of lig h t alloys o f a lu m in iu m m ay 'be considered.
V a rio u s w rite rs h av e g iv en figures, w hich d iffer re m a rk a b ly li t t l e fro m one a n o th e r, as th e com
po sitio n is v a rie d w ith in w ide lim its , a lth o u g h ev ery fo u n d ry m a n know s t h a t th e differences in a c tu a l s h r in k a g e a re very la rg e , a n d t h a t ' th is acco u n ts i n la r g e m e a su re fo r th e p o p u la r ity of th e alloys w ith silicon, w hich show a sm aller sh rin k a g e th a n m o st alloys of th e k in d .
W hen a n alloy c o n sists o f tw o c o n s titu e n ts of very d iffe re n t physical p ro p e rtie s , stre sse s m ay easily be s e t u p . T h e g u n -m e ta l a n d sim ila r alloys m ostly c o n ta in th e e u te c to id of cop p er and ti n , a h a rd , b r i ttle m a te r ia l. T he difference in c o n tra c tio n betw een th is a n d th e a d jo in in g solid so lu tio n is so g r e a t t h a t sm all crack s o fte n s t a r t fro m th e p a tc h e s of e u te c to id . R a p id cooling will d im in ish th e size of an y su ch inclusions, an d th u s lessen th e c o n c e n tra tio n o f s tre ss . A lth o u g h ra p id cooling m ig h t be ex p ected to cause th e rm a l stresses, th is is o fte n m o re th a n co m p en sated fo r by th e sm a lle r size of th e c ry s ta ls a n d by th e ir less r e g u la r a rr a n g e m e n t, c a u sin g a m o re even d is trib u tio n of th e sm all re s id u a l stresses. I t is fo r th is reaso n t h a t ch ill c a stin g s m ay be m ore fre e fro m stre ss th a n s a n d c a stin g s, a lth o u g h a t a first g lan ce th e c o n tr a ry m ig h t be ex p ected to be th e case. W h en th e chill is v e ry severe, as in p re s su re d ie-castin g , th e r e m a y be g r e a t sh rin k a g e stresses, a n d th is h a s been observed w ith a lu m in iu m alloys. P is to n s , w hen d ie -c a st u n d e r p ressu re, h av e been know n t o s e p a r a te in to tw o co n cen tric shells, com pletely d is tin c t frc m one a n o th e r, ow ing to sh a rp c o n tra c tio n stre sse s betw een th e tw o ra p id ly -c h illed su rfaces. O rd i
n a rily , w hen th e feed in g of th e d ie-castin g is p ro perly p ro v id ed fo r, th e m e ta l o b ta in e d is com
p a ra tiv e ly fre e fro m in te r n a l stre ss , unless tthe alloy is o f su c h a k in d t h a t a tr a n s fo r m a tio n m ay c o n tin u e in th e solid s ta te a f t e r solidification.
S u ch a c o n d itio n a p p e a rs to be responsible fo r th e w a rp in g o f som e zinc base d ie-castin g s in th e course o f tim e . T he alloys of a lu m in iu m w ith zinc c o n ta in a n u n sta b le phase, a n d th e g ra d u a l c h an g e to th e sta b le co n d itio n sets up stress. As a n ex am p le, d ie -c a st te n sile te s t pieces o f a n alloy of 93 p e r c en t, zinc, 4 p e r c e n t, alu m in iu m , an d 3 p e r c e n t, copper, show ed a d is tin c t fa llin g off of te n s ile s tr e n g th a f t e r one y e a r, b u t th e resist
ance t o shock w as v ery g re a tly reduced, fa llin g in th e sam e tim e fro m 76 to 8 ft.-lb . sq. in ., alth o u g h chem ical a c tio n w as p re v e n te d . This effect m u st be a t t r i b u te d t o in te r n a l stress.
M an y alloys pass th ro u g h a b rittle ran g e d u rin g th e process of cooling, an d w ith in c e rta in lim its of te m p e r a tu r e th e y a re easily fra c tu re d . All alloys o f c o p p er an d zinc h a v e th is p ro p e rty . F o r c a s t brasses of th e a lp h a -b e ta ty p e th is ra n g e lies in th e neig h b o u rh o o d of 300 to 500 d eg C., an d a sim ila r ra n g e is fo u n d in th e ti n bronzes. A dvan
ta g e is ta k e n of th is f a c t in knocking off th e ru n n e r s of castin g s, b u t i t is obviously d an g ero u s to s h a k e o r h a n d le c astin g s w hile w ith in th is ra n g e o f te m p e ra tu re .
A ny local h e a tin g sub seq u en tly t o c astin g is liab le to s e t u p stresses. B u rn in g -o n a t defective places is a prolific source of tro u b le . I t m ay h a p p e n t h a t th e b u rn t-o n p o rtio n m ay ap p e a r to be q u ite so u n d , a n d t h a t cra c k in g will occur a t a l a t e r sta g e . M onel-m etal castin g s which have been tr e a te d in th is w ay som etim es crack violently som e te n m in u te s a f t e r m a k in g th e weld. Such tr o u b le can o n ly be avoided by an n ealin g , ta k in g c a re t h a t t h e m e ta l cools from th e w elding te m p e r a tu r e slowly e n o u g h to allow o f th e g rad u al re lie f o f stre ss. L a rg e c astin g s h av e som etim es s p lit su d d en ly w ith o u t know n cause, a n d th e p re
sence o f dissolved gases h a s been suspected as bein g responsible. M uch w ork is now in progress o n th e rem oval of gas fro m c a stin g m e ta l, an d i t re m a in s to be seen w h eth er th e m easu res which a re recom m ended w ill lead to th e p ro d u ctio n of
c a stin g s w hich a re less lia b le to c ra c k as th e re s u lt of in te r n a l ten sio n .
A re m a rk a b le case of c ra c k in g of castin g s th ro u g h in te rn a l stress is t h a t of th e valve c a st
ings used in th e c o n stru c tio n of th e C atsk ill A que
d u c t fo r a supply of w a te r to New Y ork, an a c co u n t of w hich was pub lish ed in 1915. These ca stin g s w ere of th e alloy u su ally called m anganese bronze, an d w ere very la rg e , som etim es u p to 10 to n s in w eig h t. A lth o u g h th e y h a d been sub
je c te d to h y d ra u lic te s t p ressu res of 200 o r 300 lb.
p e r sq. in. ¡for h a lf an h o u r o r m ore, th e y developed leaks some m o n th s la te r u n d e r v ery sm all p ressu res, a n d th e s e leak s becam e la rg e r in course of tim e. A n an aly sis o f one of th e castin g s g a v e :— C opper, 58.5; zinc, 3 9 .1 ; ti n , 1.0;
iron, 1.4 p e r c e n t.; lead , tr a c e , an d m anganese, A
none. T he s tru c tu r e was t h a t of a n a p p ro x i
m a te ly 60 : 40 brass, composed of b e ta crystals c o n ta in in g n u m ero u s iso lated a lp h a cry stals.
M any of th e valves a n d o th e r o b jects h a d been re p a ire d by “ b u rn in g -in ,” cracks an d o th e r d e
fe c ts b ein g filled by m e ltin g in some of th e same m a te ria l. S ound welds w ere o b ta in e d in th is way, a n d th e castin g s h a d every a p p e a ra n c e of being in good c o n d itio n . I t was only a f te r an in te rv a l of tim e t h a t fa ilu r e o ccu rred . The m a tte r w as in v e s tig a te d by th e U .S . B u re a u of S ta n d a r d s in 1916, an d e x p e rim e n ts w ere m ade by c a stin g fram es of th e sh a p e shown in F ig . 1, c u ttin g th ro u g h a t th e p o in t A, an d b u rn in g in a su p p ly o f th e sam e m e ta l, m a k in g a weld. By
saw ing th ro u g h th is arm of th e fram e, a fte r h a v in g rem oved th e p o u rin g g a te , i t w as possible to m e a su re th e stress in th e c a stin g by m easu r
in g th e ch an g e in d istan ce betw een tw o gauge p o in ts previously m a rk e d . Tensile stresses ra n g ing from 2.8 to 4.3 to n s p e r sq. in. w ere found to e x is t. C alcu latio n s showed t h a t a stress four tim e s as g r e a t m ig h t h av e been ex pected th eo re tic a lly , so t h a t in all p ro b a b ility th e re had been some p la s tic y ield in g of th e alloy, th e elas
tic lim it b ein g exceeded. T his would leave th e o th e r p a r t s of th e c a stin g n e a r to th e re p a ir in a s t a te ' of severe in te rn a l stress, sim ila r to th a t w hich is fo u n d in a severely cold-worked m etal.
Fi g. 2.
T he su b seq u en t cra c k in g would th e n be of e x a c tly th e sam e k in d as th e “ se a so n -c ra c k in g ” of cold d ra w n tu b e s an d rods. I t would n o t occur im m ed iately , b u t a f t e r an in te rv a l, as in season- crack’ing. T h e fa ilu re was n o t in any way due to th e weld m e ta l itse lf, which gave no rm al tests an d s tru c tu r e , b u t to in te r n a l stress, which could n o t be rem oved by su b seq u en t an n ealin g , owing to th e la rg e size of th e castin g s.
A m ore obscure co n d itio n of stress, in which th e w rite r h a s ta k e n p a rtic u la r in te re st, is th a t which m a n ife sts itself betw een th e individual
cry s ta l g ra in s of a c a s t m e ta l. I t is well know n to all who h av e been concerned w ith th e c a stin g of m a rin e p ro p ellers in m an g an e se bronze o r s im ila r alloys t h a t occasionally a b r ittle c a stin g is o b ta in e d , w hich m ay f r a c tu r e on a blow, in v a ria b ly show ing a coarse in te rc ry s ta llin e fra c tu r e . A fr a g m e n t of a b ro k en p ro p eller of th is k in d is show n in F ig . 2. I n th is case f r a c tu r e o ccu rred w hen a blade of th e p ro p eller h ap p en ed to s trik e a flo a tin g p la n k . T he c ry s ta l g ra in s of such c a stin g s a re occasionally of enorm ous size, th e sh ap e of one, w hich was of th e size of a tu r k e y ’s egg, b ein g show n in F ig . 3. I n th e w r ite r ’s ex p erien ce, c a stin g s w hich have behaved
in th is w ay h av e alw ays consisted e ith e r of th e b e ta c o n s titu e n t only, or of b e ta w ith a re la tiv e ly sm all p ro p o rtio n of a lp h a , and th e y have c o n ta in e d a h ig h p ro p o rtio n of a lu m inium . The la te P ro f . H u n tin g to n noticed t h a t v ery coarse c ry s ta llin e alloys of th is k in d would d is in te g ra te if placed in m e rc u ry , a n d th is te s t h as proved u sefu l. T he f r a g m e n t of p ro peller in q u estio n , if im m ersed in m ercu ry , will b re a k u p in th e course of th ir t y seconds in to a loose m ass like sa n d , each p a rtic le b ein g a single c ry s ta l g ra in . O th e r specim ens d is in te g ra te m ore slowly, a n d th e process m ay ta k e m in u te s o r hours. O th ers, a g a in , a lth o u g h w ith g rain s as
coarse as th e specim ens in q u estion, are not loosened by th e a c tio n of m ercu ry , b u t are evenly a m a lg a m a te d w ith o u t p e n e tra tio n along th e g ra in b o u n d aries. T he effect stro n g ly recalls th e use of m erc u ry or of m e rc u ric sa lts as a te s t fo r season- c ra c k in g . Cold-w orked brasses a re som etim es in a c o n d itio n in which sp o n tan eo u s crack in g m ay occur, p e rh a p s m o n th s a f t e r m a n u fa c tu re , on ex p o su re to corrosive fum es or by a change of te m p e ra tu re . T he m erc u ry te s t produces sudden crack in g , w hich is evidence of in te rn a l stress.
R ea so n in g fro m analogy, i t m ay he supposed th a t th e c a stin g s w hich respond to th e a ctio n of m er
cu ry in th is way a re also in a co n d itio n of stress, an d th is conclusion is alm ost c e rta in ly justified.
A com parison m ay be m ade w ith th e action of t i n o r solder on brasses a t high te m p e ra tu re s, to which a tte n tio n was first d ire c te d bv D ickenson, who described a b e ta b rass (so-called m anganese bronze) of th e follow ing com position :—
C opper, 55.75; zinc, 36.77; m anganese, 3.87;
a n d a lu m in iu m , 2.56 p e r c e n t., which cracked ow ing to th e p e n e tr a tio n of m o lten solder betw een th e g ra in s . I t was show n t h a t th e solder could only p e n e tr a te w hile th e brass was u n d e r ten sile stress, an d th e alloy could be b ro u g h t in to con
t a c t w ith liq u id solder w hen th e specim en was u n d e r com pression o r on th e com pression side of a b e n t b a r w ith o u t p ro d u cin g an y effect. F a ilu re u n d e r ten sio n was o b ta in e d in a n um ber of brasses, a n d in m ost in stan ces th e p e n e tra tio n could be seen to follow th e b o undaries of th e b e ta g ra in s. The m ore re g u la r th e polyhedral stru c tu r e o f th e brasses th e m ore re a d ily such failu re o ccu rred . A v ery sim ila r k in d of f r a c tu r e is seen w hen gases p e n e tr a te in to m etals, as for exam ple, w hen h y d ro g en is forced by electrolysis in to cop
p e r o r iro n . T he gases p e n e tr a te along th e g ra in b o u n d aries an d m ay re a d ily cause ru p tu re . B rasses w hich on slow cooling give an a lp h a and b e ta s tr u c tu r e fr a c tu r e m ore re a d ily in co n tac t w ith m erc u ry o r w ith m olten m etals w hen quenched from a sufficiently h ig h te m p e ra tu re to give th e m th e s tru c tu r e of a hom ogeneous b eta alloy. T he w rite r has m ade a la rg e num ber of e x p e rim e n ts fo r th e p urpose o f discovering th e e x a c t co n d itio n s u n d e r which d is in te g ra tio n could
be b ro u g h t a b o u t by m ercu ry . A com plete b e ta s tru c tu r e is th e m ost fav o u rab le, an d it a p p e a rs t h a t th e presence of some m e ta l o th e r th a n zinc is u su ally necessary. I n one series of e x p e ri
m ents brasses w ere m ade to h av e a sim ple b e ta s tr u c tu r e by v a ry in g t h e zinc a n d a d d in g a suffi
c ie n t q u a n tity of a lu m in iu m , m an g an e se o r s ili
con. I t was fo u n d t h a t 2 p e r c en t, of alu m in iu m was necessary to p roduce d is in te g ra tio n , b u t th is m ig h t be rep laced by a r a t h e r la rg e r q u a n tity , p e rh a p s 3 p e r c e n t, of silicon. On th e o th e r h a n d , m an g an ese did n o t te n d to develop th is ty p e of b rittle n e ss , an d i t is of in te r e s t to notice t h a t in th e J a p a n e s e n avy m uch a tte n tio n has b een given to alloys c o n ta in in g as m uch as 4 p e r c e n t.
l i s
m an g an e se fo r th e castin g s of propellers, and it is u n d ersto o d t h a t th e s e a re n o t so liab le to ex
h ib it g r e a t b rittle n e ss. T h a t th e q u estio n is not m erely one of chem ical com position is shown by th e b eh av io u r of a c a stin g w hich was exam ined recen tly . The an aly sis of th is c astin g w as: — C opper, 64.40; zinc, 26.55; iron, 1.55; m anganese, 1.84; an d a lu m in iu m , 5.30 p e r c e n t., w ith only m in u te q u a n titie s of o th e r m etals. The s tru c tu r e w as t h a t of a b e ta b rass an d th e m echanical p ro p e rtie s in th e cold w ere ex cellen t, alth o u g h i t was possible to p roduce a b r ittle in te rc ry sta llin e fra c tu r e by m o d e ra te h e a tin g . M ercu ry did n o t cause d is in te g ra tio n unless th e su rfa c e to which th e m e rc u ry was ap p lied was in ten sio n . The alu m in iu m in th is alloy is re m ark ab ly h ig h , and n ev erth eless d ire c t d is in te g ra tio n by m ercury w ith o u t stre ss w as n o t produced. W hen th e su r
h ib it g r e a t b rittle n e ss. T h a t th e q u estio n is not m erely one of chem ical com position is shown by th e b eh av io u r of a c a stin g w hich was exam ined recen tly . The an aly sis of th is c astin g w as: — C opper, 64.40; zinc, 26.55; iron, 1.55; m anganese, 1.84; an d a lu m in iu m , 5.30 p e r c e n t., w ith only m in u te q u a n titie s of o th e r m etals. The s tru c tu r e w as t h a t of a b e ta b rass an d th e m echanical p ro p e rtie s in th e cold w ere ex cellen t, alth o u g h i t was possible to p roduce a b r ittle in te rc ry sta llin e fra c tu r e by m o d e ra te h e a tin g . M ercu ry did n o t cause d is in te g ra tio n unless th e su rfa c e to which th e m e rc u ry was ap p lied was in ten sio n . The alu m in iu m in th is alloy is re m ark ab ly h ig h , and n ev erth eless d ire c t d is in te g ra tio n by m ercury w ith o u t stre ss w as n o t produced. W hen th e su r