T5he^bnerian Sundri/m en’s ^Vssocution,
THE INFLUENCE OF CHROMIUM IN CAST IRON
By J. W. Donaldson. D.Sc., A.I.C. (Associate Member).
Introduction.
I n a re c e n t in v e s tig a tio n c a rrie d o u t by th e a u th o r, an d p re se n te d as th e F re n c h E xchange P a p e r on b eh alf of th e I n s ti t u t e of B ritish F o u n d ry m en in 1928,1 th e influences of to ta l carbon, silicon, phosphorus a n d m anganese resp ectiv ely on th e carbon decom position and volum e changes of c a s t iro n w ere d eterm in ed . These in v e s tig a tio n s w ere c o n tin u e d to d e te r
m ine th e influence of v ario u s special elem ent- a d d itio n s , a n d in th e p re s e n t P a p e r th e influence of chrom ium on h e a t-re s is tin g an d o th e r p ro p e rtie s is considered.
The use of chrom ium in c a st iro n was first ex p e rim e n te d w ith by K e e p ,2 who ad d ed chro
m ium to th e e x te n t of 2 p er c e n t., an d found t h a t th e re w as a slig h t in crease in s tr e n g th up to 1 p er c e n t., th e n a decrease, w hile th e sh rin k age was in creased w hen less th a n 1 p e r c en t, was added. C am p io n ,2 in a la te r in v e s tig a tio n , found t h a t 1 p e r cen t, of chrom ium in creased th e s tr e n g th an d h ard n ess, an d also th e a m o u n t of com bined carbon. H u r s t1 s ta te s t h a t in an iron c o n ta in in g 1 p er c e n t, of silicon a n a d d itio n of 0.9 p e r c en t, chrom ium re n d e re d th e fra c tu re q u ite m o ttle d , an d t h a t 4 p e r c e n t, ad d ed to an iron c o n ta in in g 1.5 p er cen t, silicon ren d ered th e fr a c tu r e p erfectly w h ite, an d t h a t d ra s tic an n eal
ing a t 900 to 950 deg. C. fa ile d to produce g ra p h ite .
S m alley,5 in his sy stem atic in v e stig a tio n s w ith special elem en t a d d itio n s to c a s t iro n , added
1 J. W . Donaldson, Bull, de l'Assoc. Tech. de Fonderie, 1928 vol. 2, p. 257.
2 W. J . Ke e p, “ Cast Iron," 1916, p. 212.
J A. Campion, “ Foundry Trade Journal," 1918, vol. 20, p. 467.
4 J. E . Hu r st, M etallurgy of Cast Iron, p. 139.
i O. S m a lle y , Proc. Inst, of B rit. Found., 1922-23, vol 16
p.
495.chrom ium it- o rd in a ry grey c a st iro n an d to cy lin d er iro n . The influence of chrom ium a d d i
tio n s of 0.11 a n d 0.15 p e r cen t, respectively was to in crease th e stre n g th a n d h ard n ess slightly, w hile w ith th e la rg e r a d d itio n of 0.78 per cen t, to th e o rd in a ry iro n th e ten sile s tre n g th fell, th e
Duration
Of
Hear iris - Hours. Fi g. 1 . — He a t Tr e a t m e n t a t 4 5 0 d e g. C .tra n sv e rs e s tr e n g th was affected slig h tly , an d th e B rin ell h ard n ess in creased. A sim ilar a d d itio n of chrom ium to th e c y lin d er iro n produced a n in creased tra n sv e rs e s tre n g th an d B rin ell h a rd ness, b u t little change in th e ten sile stre n g th . E x p e rim e n ts by P iw o w arsk y 6 show t h a t w ith up
6 E . Piwowarsky, “ Foundry Trade Journal," 1925, vol. 31, pp. 331-34, 345-46.
to 0.5 p er c e n t, of chrom ium th e s tre n g th an d shock-resisting p ro p e rtie s of c a s t iro n a re in creased to th e e x te n t of a b o u t 10 p e r c e n t., while th e h ard n ess increases fro m 20 to 25 p er cen t.
H a m a s u m iV in v e s tig a tio n s show t h a t th e a d d i
tio n of 0.4 p er c e n t, of chrom ium raises th e ten
-Du r r t i o h Of He r t i h q - Ho u r s
F i g . 2 .— He a t Tr e a t m e n t a t 5 5 0 d e g. C .
sile s tr e n g th of c a s t iro n from 1 6 .5 to 2 2 .8 tons p er sq. in. w ith o u t m a te ria lly a lte rin g th e s tru c tu r e , an d t h a t c a s t iro n is n o t re n d e re d b r ittle by chrom ium u p to th is a m o u n t.
T he re s u lts of th ese in v e s tig a tio n s establish d e n ite ly t h a t 0 .5 p e r c e n t, of ch ro m iu m in th e
7 H. Hamasumi, “ Foundry Trade Journal," 1925, v o l. 32, p. 71,
presence of 1 to 2 p er c en t, of silicon increases
showed th e presence of free cem e n tite. Diffi
c u lty was also ex p erien ced in m ach in in g those irons.
T he irons w ere all c a s t in d ry -san d moulds in to b a rs 1 6 in . by l g in. d ia. The chrom ium was ad d ed in th e ladle, as 65 p e r c en t, ferro - chrom e, an d th e iro n s w ere re-m elted in
cru-F i g . 3 . — Pe r c e n t a g e Ch a n g e s.
cibles before c a stin g to re n d e r th em uniform . They w ere all c a st a t ap p ro x im a te ly th e same te m p e ra tu re a n d u n d e r sim ila r conditions.
Heat-Treatment Changes.
The m ethods of te s tin g w ere those ad o p ted in p revious te s ts w ith p lain c a s t irons. B ars from each o f th e irons w ere h e a te d in a n electric re sistan ce fu rn a c e fo r 8 ho u rs p er d ay to 450 deg.
C., cooled o v e rn ig h t, a n d re -h e a te d on th e fol
low ing d ay to th e sam e te m p e ra tu re . A t th e end o f each 5 d a y s’ o r 4 0 h o u rs ’ h e a tin g , the to ta l an d com bined carb o n of each iron was
Fi g. 4.—P . A s Ga s t. x 540.
Fi g. 5.— P . He a t Tr e a t e d f o r 200 h r s. a t 550 d e q. C. x 540.
e stim a te d , tlie B rin e ll-h a rd n e ss d eterm in ed , and, w here m a te ria l p e rm itte d , th e ten sile stre n g th d eterm in ed . S im ila r te s ts w ere c a rrie d o u t on b ars a t 550 deg. C. The re s u lts a re given in T ables I I a n d I I I an d re p re se n te d g rap h ically
Ta b l e II .—Heat Treatment at 450 deg. C.
Chromium irons. Duration of heating in hours.
Cr per cent. — 0 40 80 120 160 200
Plain T.C. per cent. 3.16 3.17 3.17 3.19 3.13 3.15 C.C. per cent. 0.68 0.64 0.48 0.43 0.38 0.38 Tens. M .S ... 16.6 16.2 15.7 15.3 15.4 15.5 Br. No. .. 223 212 197 183 183 179 Cr 1, 0.20 T.C. per cent. 3.19 3.18 3.16 3.16 3.13 3.21 per cent. C.C. per cent. 0.70 0.66 0.60 0.57 0.49 0.50 Tens. M .S... 17.0 16.8 16.4
.—
16.0 15.9Br. No. 235 223 212 197 197 197
Cr 2, 0.39 T.C. per cent. 3.17 3.18 3.17 3.18 3.16 3.20 per cent. C.C. per cent. 0.93 0.90 0.85 0.72 0.69 0.69 Tens. M.S.. . 18.4 17.9 17.5 17.4 17.2 17.3
Br. No. 248 235 212 207 201 207
Cr 3, 0.66 T.C. per cent. 3.16 3.14 3.16 3.14 3.10 3.09 per cent. C.C. per cent. 1.13 1.06 0.99 0.96 0.95 0.96
Tens. M .S... 17.8
— —
— —Br. No. 255 241 235 235 235 235
Cr 4, 0.78 T.C. percent. 3.24 3.30 3.20 3.26 3.28 3.29 per cent. C.C. per cent. 1.25 1.21 1.16 1.15 1.14 1.14
Tens. M .S... 15.8
—
— — —_
Br. No. 262 255 255 255 255 255
Cr 5, 0.90 T.C. per cent. 3.21 3.16 3.24 3.20 3.20 3.21 per cent. C.C. per cent. 1.53 1.51 1.54 1.53 1.52 1.54
Tens. M .S... 13.4 — — —
_ _
Br. No. 277 277 277 227 277 777
in F ig s. 1 an d 2, w hile th e p erc e n ta g e changes produced by all th e iro n s a re given in T able IV an d g ra p h ic a lly in F ig . 3.
C o n sid eratio n of T ab le 11 an d F ig . 1, which d eal w ith th e e x p e rim e n ts c a rrie d o u t a t 450 deg. C., show t h a t as th e chrom ium c o n te n t in
creases th e sta b ility of th e com bined carbon u n d e r te m p e ra tu re is increased. I n th e plain iro n decom position ta k e s place to th e e x te n t of 44 p e r cen t, an d is m ore or less com plete a fte r 160 h o u rs ’ h e a tin g . W ith th e a d d itio n of 0.2
T a b l e I I I .—Heat Treatment at 5 5 0 deg. C.
Chromium irons. Duration of heating in hours.
Cr per cent. — 0 40 80 120 160 200
Plain T.C. percent. 3.16,3.13 3.16,3.15 3.15 3.14 C.C. per cent. 0.68 0.12 0.11 0.09 0.12 0.12 Tens. M.S... 16.6 15.8 15.1 14.8 14.6 14.8
Br. No. 223 138 129 129 125 129
Cr 1, 0.20 T.C per cent. 3.193.13 3.17 3.15 3.17 3.17 per cent. C.C. per cent. 0.70 0.54 0.40 0.30 0.32 0.30 Tens. M.S.. . 17.0 16.3 15.8 15.3 15.1 15.1 Br. No. .. 235 197 171 152 152 152 Cr 2, 0.39 T.C. per cent. 3.17 3.16 3.22 3.20 3.15 3.21 per cent. C.C. per cent. 0.93 0.57 0.53 0.49 0.51 0.49 Tens. M .S... 18.4 17.9 17.4 16.8 16.4 16.4 Br. No. .. 248 207 174 170 170 170 Cr 3. 0.66 T.C. per cent. 3.16 3.17 3.10 3.12 3.15 3.13 per cent. C.C. percent. 1.13 1.02 0.89 0.79 0.80 0.80
Tens. M .S... 17.8 — — — — 16.3
Br. No. 255 235 217 212 212 212
Cr 4, 0.78 T.C. per cent. 3.24 3.23 3.27 3.21 3.19 3.22 per cent. C.C. per cent. 1.25 1.18 1.11 1.09 1.11 1.11
Tens. M.S... 15.8 — — — — 15.0
Br. No. 262 255 248 248 248 248
Cr 5, 0.90 T.C. per cent. 3.21 3.20 3.20 3.22 3.25 3.20 per cent. C.C. per cent. 1.53 1.53 1.53 1.52 1.51 1.53
Tens. M.S.. . 13.4 — — — 13.2
Br. No. 277 277 277 277 277 277
per cent, of chrom ium decom position is m ore u n ifo rm a lth o u g h th e a m o u n t is less, 28 per c e n t., an d is com plete a f te r 160 h o u rs’ h e a tin g . The 0.39 per c en t, chrom ium iro n shows a 26 per cent, decom position a f te r 120 h o u rs, w hile a d d i
tio n s of 0.60 and 0.78 p er cen t, chrom ium reduce
th e a m o u n t of decom position stili f u r th e r to 15
c arb o n c o n te n t a n d th e B rin ell-h ard n ess rem ains th e sam e, th e te n sile s tre n g th d im in ish in g by 1.5 p er cen t. (T able I I I an d F ig . 2). The o th er chrom ium a d d itio n s of 0.20, 0.39, 0.66 a n d 0.78 p er cen t, chrom ium produce com bined carbon decom positions of 57, 47, 29 an d 11 p er cent, respectively, w hich is considerably less th a n th e 82 p er c en t, decom position in th e p la in iron.
T his decrease in th e com bined carb o n c o n te n t is reflected in th e s tre n g th a n d h a rd n e s s p ro p e rtie s, p a rtic u la rly w ith a d d itio n s of over 0.1
p er cen t, chrom ium . The r a te of decom position is slower in th e chrom ium iro n s, equilibrium co n d itio n s being a tta in e d a f te r a p p ro x im ately 100 h o u rs’ h e a tin g , w h ereas in th e p lain iron th e decom position t h a t ta k e s place is com plete a f t e r 40 hours.
These re su lts, o b ta in e d by chem ical and m echanical te s ts, a re confirm ed by microscopic e x a m in a tio n . F ig s. 4 a n d 5 show th e s tru c tu r e of th e p la in iro n as c a s t a n d a f t e r 200 h o u rs’
h e a t tr e a tm e n t a t 550 deg. C. A fte r tr e a tm e n t Fig. 8 . — C n . 5 . As Ca s t, x 5 4 0 .
th e g ra p h ite is in creased in size, an d th e re is o x id atio n of th e iron in th e v icin ity of th e g ra p h ite . In th e 0.39 p er cen t, chrom ium iron, F igs. 6 an d 7, c a s t and h e a t-tre a te d , th e a m o u n t of p e a rlite u n affected is considerably g r e a t e r ; th e g ra p h ite shows little change w ith li tt le or no o x id a tio n of iro n . The tw o micro- photos of th e 0.90 p er cen t, chrom ium iron, F ig . 8, as cast, a n d F ig . 9, h e a t-tre a te d , show p ra c tic a lly no v a ria tio n in s tru c tu re .
Fi g. 9 .—Ck. 5. He a t Tr e a t e d f o r
200 h r s. at 550 d e g. C. x 540.
Sum m ing u p g en erally , i t will be n o ted , F ig . 3, th a t as th e chrom ium c o n te n t of grey c a s t iron is increased, th e influence of th e chrom ium is to stab ilise th e com bined carbon, both a t 450 an d 550 deg. C., com plete s ta b ility being a tta in e d w ith 0.90 p er cen t, of chrom ium . This increased sta b ility of th e com bined carb o n u n d er te m p e ra tu re is no d o u b t due to th e presence, according to th e re c e n t w ork of W estgren,
l
1
P h ra g m e n an d N egresco,“ of chrom ium in th e c e m e n tite of th e p e a rlite an d affects th e heat- re s istin g p ro p e rtie s of th e irons en ab lin g them to r e ta in both th e ir s tre n g th an d hardness.
Volume Changes.
The volum e changes were c a rrie d o u t on b ars acc u ra te ly m achined to 6 in . long by 1 in.
141
o t s m is :o :s
Humbsr Of 8 Hour Hear/rigs
Fi g. 1 0 .—
Volum e
Ch a n g e s.d ia. These w ere h e a te d in an electric-resistance fu rn a c e to 550 deg. fo r e ig h t hours p er day, cooled over n ig h t an d re h e a te d th e following day. A t th e end of every 40 h o u rs’ h e a tin g th e b ars w ere m easured an d th e volum es determ in ed . The re s u lts a re g iv en in T able V an d F ig . 10.
C o n sid eratio n of these resu lts show t h a t while th e p la in iro n has a volum e increase, all the irons c o n ta in in g chrom ium show co n tractio n a fte r prolonged h e a tin g . The irons co n tain in g
8 A. Westgren, G. Phragmen, and Tr. Negresco, Journ.
Iron and Steel Inst., 1928, vol. 1, p. 383.
0.20 an d 0.39 p er c en t, of chrom ium c o n tra c t to th e e x te n t of 0.15 an d 0.27 p er c e n t, respec
tiv ely . F u r t h e r a d d itio n of chrom ium to 0.66 p er c en t, reduces th e c o n tra c tio n to 0.06 per c e n t., w hile still f u r th e r a d d itio n s to 0.78 and
P e g s . C e H t
Fi g. 1 1 .— Pl a i n Ir o n.
(1) As Ca s t.
(2 ) He a t Tr e a t e d f o r 2 0 0 h r s. a t 4 5 0 d e g. C .
(3 ) He a t Tr e a t e d f o r 2 0 0 h r s. a t 5 5 0 deg. C.
0.90 p e r c en t, in crease th is red u ced co n tractio n to 0.18 an d 0.23 p e r c e n t, resp ectiv ely .
These re s u lts a p p e a r to be v ery e r r a ti c an d a t first, i t is difficult to conceive why th e in crease of chrom ium from 0.39 to 0.66 p e r cent, should produce a low er volum e c o n tra c tio n .
I t is possible t h a t th e e x p la n a tio n m ay be found
F ro m th e re s u lts o b ta in e d i t w ould, th e re fo re , a p p e a r t h a t tw o m a in fa c to rs o p e ra te in th e volum e changes of those chrom ium irons. F ir s t th e r e is th e c o n tra c tio n phenom enon due to th e chrom ium , as y e t n o t e x p la in a b le ; an d second, th e ch an g e due to th e s tr u c tu r a l n a tu r e of th e
Fi g. 1 2 .— C h r o m i u m I r o n , 0 . 3 9 p e r c e n t . C r.
(1 ) A s Ca s t.
(2 ) He a t Tr e a t e d f o r 2 0 0 h r s. a t 4 5 0 d e g. C.
(3 ) He a t Tr e a t e d f o r 2 0 0 h r s. at 5 5 0 d e g. C.
irons. T his la t te r p o in t, th e g ro w th of a purely g rey c a s t iro n as com pared w ith th e g ro w th of a m o ttle d c a st iro n of a sim ila r com position and th e o m ittin g of elem ents, such as chrom ium , w hich m ay p roduce co m p licatio n , re q u ire s f u r th e r
O-ISi
in v e s tig a tio n , an d m ig h t also ex p lain th e e rra tic volum e changes t h a t a re know n to occur in irons of a bord er-lin e com position.
SSSM-X3WX UO / &0J 3yniUH3dlM3l U/ 30M3H3JJIH ^ 3 J ¿JOJ 0 3 S y i j K> *s- H 3 J S7V J rn X £
Elevated Temperature Tests.
The s tre n g th of p la in a n d chrom ium cast irons a t elevated te m p e ra tu re s has been d e a lt w ith by th e a u th o r in his P a p e r on “ S pecial Cast
I r o n s ,” 10 a n d th e curves o b ta in e d fo r th e p lain
a d d itio n s of chrom ium h ad any influence on cast iron.
T est-pieces, 25 mm . long by 20 mm. d ia ., were m achined from each iro n , buffed, polished an d im m ersed in th e follow ing solutions fo r 28 days a t 60 deg. F . (15 deg. C .). The solutions w ere N /1 0 hydrochloric acid, N /1 0 n itric acid, N 110 acetic acid, ta p w a te r, sea w a te r, an d N /1 0
Crtzorfmrt - %
F ig .
14.—C orrosion T ests .
am m onium chloride. A t th e end of th e period th e test-p ieces w ere w ashed, d rie d in alcohol an d th e loss in w eig h t d eterm in ed . T hree te s ts were c a rrie d o u t on each iro n in each solution, an d from th e av erag e w eights o b tain ed th e loss in w eig h t p e r 100 sq. cm. d eterm in ed . The resu lts w ere given in T able V I an d F ig s. 14 an d 15.
The g e n e ra l influence of th e chrom ium a d d i
Summary.
C hrom ium added to g rey c a st iron in q u a n titie s u p to 0.90 per c en t, increases th e sta b ility of th e com bined carbon a t te m p e ra tu re s up to 550 deg. C. T his increase in sta b ility is probably due to th e presence of chrom ium c a r
bide in th e cem e n tite of th e p e a rlite , an d is re flected in th e increased s tre n g th b o th a t o rd in ary
CHeorfiurt - %
F ig .
15.—C oerosion T ests .
a n d elev a ted te m p e ra tu re s an d in th e B rinell h ard n ess. B e tte r h e a t-re sistin g irons are, th e r e fore, o b tain ed , b u t th e re is difficulty in m achin
ing irons w ith over 0.4 p er cen t, of chrom ium . The influence of chrom ium on volum e changes a t te m p e ra tu re s below th e c ritic a l ra n g e is e rr a tic , b u t th e g en e ra l effect is to produce a c o n tra c tio n . The n a tu r e of th is c o n tra c tio n is difficult to e x p lain or define. W ith chrom ium up to 0.4 p e r c en t, th e th e rm a l c o n d u ctiv ity is in
creased by a b o u t 8 per cent.
S m all a d d itio n s of chrom ium red u ce very slig h tly th e corrosion of g rey c a s t iro n . The effect is m o st n o ticeab le w ith sea w a te r, an d to a lesser e x te n t w ith s tro n g acids. W ith weak acid, ta p w a te r, an d am m onium ch lo rid e little a d v a n ta g e is g ain ed .
T he a u th o r th a n k s M r. Ja m e s B row n, C .B .E ., a n d th e o th e r d ire c to rs of S c o tts’ S h ip b u ild in g an d E n g in e e rin g C om pany, L im ite d , G reenock, fo r perm ission to c a rry o u t th e above in v e stig a tio n s.