THE ANNEALING OF WHITE IRONS TO PRODUCE NODULAR GRAPHITE

th e p r e c i p it a ti o n of t h e c a rb o n a r e m a d e , s u p p le ­ m e n ta ry to th o se of F i s h e r, 1 H a tf ie ld2 a n d O h a rp y a n d G r e n e f.3 T h e seco n d p a r t o f t h e P a p e r d e sc rib e s e x p e r im e n ts o n a ty p ic a l g r e y c a s t iro n of good q u a lity to d e te r m in e t h e m e c h a n is m o f th e fo rm a tio n o f flak e y o r p r im a r y g tra p h ite as com ­ m only fo u n d in g re y iro n s.

PART I.

THE ANNEALING OF WHITE IRONS TO PRODUCE NODULAR GRAPHITE.

I t is t h e g e n e r a l e x p e rie n c e in t h e m a n u f a c tu r e of m a llea b le c a s tin g s t h a t t h e lo w e r t h e a n n e a lin g te m p e r a tu r e , p ro v id e d t h a t c o m p le te a n n e a lin g is o b ta in e d , th e b e t t e r th e c a s tin g . F u r t h e r , th e a n n e a lin g t e m p e r a t u r e h a s b e e n c o n sid e re d t o h a v e som e effect o n t h e fo rm o f th e p r e c ip ita te d c a rb o n . A n in v e s tig a tio n w as t h e r e f o r e c a r r ie d o u t to d e te rm in e e x a c tly w h a t effec t d if f e r e n t a n n e a lin g te m p e r a tu r e s h a d o n t h e s h a p e o f t h e te m p e r- c arb o n p r e c i p it a te d d u r i n g th e a n n e a lin g o f w h ite iro n . T e m p e r-c a rb o n m a y b e d e fin ed as t h e finely d iv id e d g r a p h it e fo u n d in m a lle a b le c a s t iro n , occasio n ally in h e a t - tr e a te d g re y iro n s, o r in b a d ly h e a t- tr e a te d ste e ls.

Experimental Details.

T h e iro n u s e d in th e s e e x p e r im e n ts w as a ty p ic a l 6am p le o f w h ite iro n u se d in th e W h ite H e a r t ,

1. F ish er, T ra n s. A m erican F o n n d ry m en ’s A sso c ia tio n , 30, 395. See a ls o W h ite an il A rch er, ib id ., 27 . 331.

2. H a tfield , “ J o u rn a l Iron a n d S teel I n s t it u t e ,” 1907, 2, 79.

3. " E n g i n e e r in g ,” 1902, 73 , 626.

E

E u r o p e a n o r R e a u m u r p ro c e s s o f m a lle a b le cas iro n m a n u f a c f f ir e , a n d c o n ta in e d C c 2 .9 ; P • >

S i 0 . 6 1 ; S 0 . 3 8 , a n d M n 0 . 1 1 p e r c e n t. 1' ig -

à

show s th e m ic r o - s tr u c tu r e , a n d i t w ill be se e n o b e q u ite n o r m a l f o r t h i s cla ss o f m a te r ia l, as l c o n s is ts o f p e a r l it e ( d a rk ) a n d c e m e n tite ( " n *®'’

all t h e c a rb o n -being in t h e c o m b in e d fo rm . _ 1 m e ta l w as a n n e a le d in a silic a t u b e re s is ta n c e r u r n a c e t h e te m p e r a t u r e o f w hich c o u ld be a by m e a n s of f o u r v a r ia b le re s is ta n c e s , a n l e a d i r e c t fro m a C a m b rid g e in d ic a to r c o n n e c te d t o a p la tin u m — p la tin u m - r h o d iu m th e rm o -c o u p le . i t w as fo u n d m o re c o n v e n ie n t to h e a t f o r sb o r t p e rio d s o f fro m six to n in e h o u rs a t a t im e i n s t e a d o f o n e c o n tin u o u s a n n e a lin g . I n o r d e r t o r e d u c e o x id a tio n a n d s u b s e q u e n t d é c a r b u r is a tio n , t h e sp e c im e n s in t h e o r ig in a l e x p e r im e n ts w e re a n n e a le d w ith a p a c k in g of C h in a c la y . A s t h is d id n o t h a v e t h e d e s ire d e ffe c t, l a t e r e x p e r im e n ts w ere c a r r ie d o u t u n d e r re d u c e d p r e s s u r e .

Sum m ary of Results.

T h ir te e n e x p e r im e n ts w e re c a r r i e d o u t , in w h ich th e t e m p e r a t u r e o f a n n e a l v a r ie d fro m 750 t o 1,150 d eg . C ., a n d t h e r e s u l ts a r e sh o w n in T a b le I . W h e n th e m e ta l h a d b e en su ffic ie n tly a n n e a le d , a s u r f a c e o f t h e s p e c im e n w as g r o u n d a n d p o lis h e d . I t w as t h e n e x a m in e d u n d e r t h e m ic ro s c o p e a n d p h o to g r a p h e d . T h e r e s u l t i n g m ic r o g r a p h s a r e show n in F ig s . 4 to 14, a n d a r e e x tr e m e ly i n t e r e s t ­ in g w h en c o n s id e re d w ith T a b le I . I n p a s s in g , i t sh o u ld be n o te d t h a t t h e p r e c i p i t a t i o n o f th e te m p e r- c a r b o n is lia b le t o h e m a s k e d , p a r t i c u l a r l y in t h e case o f t h i n c a s tin g s , if t h e ir o n b e h e a t e d in a n o x id is in g a tm o s p h e r e . T h e r e is a lw a y s a te n d e n c y fo r d é c a r b u r is a tio n t o t a k e p la c e . T h is is e n c o u ra g e d in t h e m a n u f a c t u r e o f m a lle a b le c a s tin g s b y t h e E u r o p e a n p ro c ess, b u t n o t i n th e A m e ric a n p ro cess, w h ic h is fin d in g g r e a t e r a p p li c a ­ tio n in t h is c o u n tr y . I n th e f o r m e r p ro c e s s m u c h o f t h e c a rb o n is re m o v e d b y t h e o x id is in g e ffe c ts o f t h e p a c k in g m a te r ia l, so t h a t in v e r y t h i n c a s tin g s t h e r e is o f t e n less t h a n 1 p e r c e n t o f t o t a l c a rb o n . I n t h e B la c k - h e a r t p ro c e ss, 'h o w ev er p r a c tic a lly all t h e c a rb o n is p r e c i p it a te d a s te m p e r- c a rb o n , a n d d é c a r b u r is a tio n is c o n fin e d m ere ly to t h e e d g e o f t h e c a s tin g .

Discussion of Results.

I n th e i n tr o d u c tio n i t w as sh o w n t h a t lo n g flakes of g r a p h it e w ere f a r m o re h a r m f u l t h a n th e ro u n d e d n o d u le s of te m p e r- c a r b o n fo u n d in go o d m a lle a b le iro n , i.e ., t h e p h y sic a l p r o p e r tie s m a y be p r e d ic te d fro m a s tu d y o f t h e g r a p h it e in th e iro n . I t w as m e n tio n e d also t h a t in t h e m a n u f a c tu r e of m a lle a b le c a s tin g s th e lo w e r t h e a n n e a li n g te m p e r a ­ t u r e , th e b e t t e r t h e p r o p e r tie s o f t h e c a s tin g . S u ch 3. f a c t w o u ld be e x p e c te d w h en we c o n sid e r t h e m ic r o - s tr u c tu r e o f th e r e s u ltin g iro n s. T h e p h o to g ra p h s , w h ich a r e a r r a n g e d in o r d e r of a s c e n d in g te m p e r a t u r e , show t h a t th e r e is a g r a d u a l c h a n g e in t h e s h a p e of t h e p r e c ip ita te d c a rb o n fro m t h e c ir c u la r fo rm o f t r u e te m p e r- c a rb o n t o t h e flak y fo rm o f p r im a r y g r a p h it e . T h e a n n e a lin g of N o. 4 a t 750 d e g . is a lto g e th e r in s u f f ic ie n t; in f a c t, p r e c ip ita tio n o f c a rb o n h a s j u s t b e g u n . F ig . 5 a t 860 d e g . show s ty p ic a l n o d u les of te m p e r-c a rb o n , a n d a lth o u g h t h e le n g th of a n n e a lin g w as in su fficien t, t h e te m p e r a t u r e em p lo y ed w ould y ie ld a v e ry s a tis f a c to r y p ro d u c t.

A s th e te m p e r a t u r e a sc en d s t h e in c r e a s in g flak in es s o f t h e g r a p h it e is s h o w n ; F ig . 9 a t 1,100 d e g ., fo r e x a m p le , show s d i s t in c t p r im a r y g r a p h it e flakes, a n d i t m a y s a fe ly b e a ssu m e d t h a t t h e p h y sic al

p r o p e rtie s w ill c o n se q u e n tly be p o o r. In g e n e ra l, th e r e f o r e , t h e p h y sic a l p r o p e rtie s of a n n e a le d c a s

t

-F *

ta i

/

#►

t - f

■ V ; ' 1 V ; - ¥ % * * * '* ■

'jg. 7.— X 200. Ftc. 8.—X 200. Fig. 9.—x 350. Fig. 10.— x 200. (990DegG.)(1,050Deg. 0.)(1,100Deg. C.)(1.100Deg. 0.)

in g s o f g iv e n c o m p o s itio n w ill v a r y fro m th o s e of good m a lle a b le t o th o s e o f a g re y ir o n c a s tin g , a c c o rd in g to t h e a n n e a lin g t e m p e r a t u r e . I f it w ere p ossible, d u r i n g t h e s o lid ific a tio n o f g r e y c a s t iro n , to c a u s e th e g r a p h it e t o o c c u r in t h e n o d u la r fo rm as i t d oes in good m a lle a b le c a s t iro n , th e r e is e v e ry r e a s o n to s u p p o s e t h a t t h e iro n w o u ld be as s tr o n g a n d a s d u c tile . M o s t w h ite iro n s u se d in t h e E u r o p e a n p ro c e ss h a v e f a ir l y h ig h c o n te n t of s u lp h u r , w h ich h a p p e n s t o b e o n e o f t h e e le m e n ts w h ich te n d t o p r e v e n t t h e g r a p h it i s a ti o n o f c e m e n tite . A s i t is k n o w n t h a t t h e r a t e o f g r a p h i ­ t is a t i o n in c re a s e s w ith in c re a s e of t e m p e r a t u r e , m a n u f a c tu r e r s o f w h ite h e a r t ir o n h a v e t o e m p lo y a s o m e w h a t h ig h e r a n n e a lin g t e m p e r a t u r e t o h a s te n t h e p ro cess of m a lle a b le is in g . F r o m w h a t h a s b e en s a id b e fo re , i t is n a t u r a l to su p p o s e t h a t if m a n u f a c tu r e r s s t i p u la t e d f o r a n iro n c o n ta in in g less s u lp h u r , lo w e r a n n e a lin g t e m p e r a t u r e s c o u ld h e a d o p te d w ith o u t in c r e a s in g t h e tim e of a n n e a l, a n d c o n s e q u e n tly a s u p e r io r p r o d u c t w o u ld b e o b ta in e d d u e to th e im p ro v e d fo rm o f te m p e r - c a r b o n p r o ­ d u ced a t t h e low er t e m p e r a t u r e s .

Conclusions.

F i'o m P a r t I o f t h e P a p e r t h e fo llo w in g c o n c lu ­ sio n s m a y b e d r a w n : —

(1) T h e e u te c tic t e m p e r a t u r e f o r t h e p a r t i c u l a r w h ite iro n u n d e r re v ie w is in t h e n e ig h b o u r h o o d o f 1,145 d eg . C. ; sa m p le s h e a t e d ab o v e t h is t e m p e r a ­

t u r e show n o sig n s o f g r a p h i t i s a t i o n . I t th e r e f o r e a p p e a r s lik e ly , a t le a s t in t h e c a s e o f h y p o - e u te c tic alloys, t h a t g r a p h it e is n o t t h e c o n s t i t u e n t s e p a r a t in g fro m t h e m e lt.

(2) I n t h e c a s e of s a m p le s h e a t e d w ell a b o v e 1,000 d eg . C ., t h e c a rb o n w h ic h is p r e c i p it a te d te n d s t o a s s u m e t h e fo rm o f p rim a iry g r a p h it e , i.e ., t h in fla k e s w h ic h b r e a k u p t h e c o n tin u ity o f t h e m e ta llic m a tr ix , t h u s r e d u c in g c o n s id e ra b ly t h e s t r e n g t h o f t h e iro n .

(3) T h e ty p ic a l ro u n d e d fo rm o f te m p e r- c a r b o n is p ro d u c e d a t t e m p e r a t u r e s w ell below 1,000 d e g . C ., t h e lo w er t h e t e m p e r a t u r e ( w ith in lim its ) , t h e m o re c o m p a c t t h e c a rb o n , a n d c o n s e q u e n tly t h e g r e a t e r t h e m a lle a b ility of t h e c a s tin g .

(4) T h e r e a p p e a r s t o b e a d e fin ite c h a n g e b e tw e e n s p e c im e n s N os. 4 t o 7 a n d N o s. 8 t o 12, i .e ., a

c r itic a l t e m p e r a t u r e b e tw e e n 990 a n d 1,050 d eg . C ., a t w h ich th e c a r b o n c h a n g e s i t s fo rm fro m s p h e ric a l to flak y . I t is s u g g e s te d t h a t t h is is r e la te d to th e th e r m a l p o in ts fo u n d by C a r p e n te r a n d K e e lin g 1, b e tw e e n 1,000 a n d 1,100 d e g . C.

(5) I n a w h ite iro n , t h e h ig h e r t h e a n n e a lin g t e m p e r a t u r e (a s lo n g a s 1,145 deg . is n o t ex c e ed e d ), th e m o re r a p id is t h e f o r m a tio n o f g r a p h it e . T h is is t h e c a u s e o f t h e h ig h a n n e a lin g te m p e r a t u r e s e m p lo y e d in t h e w h ite - h e a r t p ro cess, n e c e s s ita te d by t h e u n b a la n c e d c o n te n t o f th e e le m e n ts p r e s e n t.

I t h a s b e e n s u g g e s te d t h a t a c o m p o s itio n w h ich p e r m itte d lo w er a n n e a lin g t e m p e r a t u r e s t o be e m p lo y ed w ou ld g iv e a s u p e r io r a n d m o re u n ifo rm p ro d u c t.

(6) C o n v e rse ly , t h e r e is a t e m p e r a t u r e , d e p e n d ­ in g u p o n t h e c o m p o s itio n o f t h e w h ite iro n , below w h ich t h e a m o u n t o f g r a p h it e w h ich s e p a r a te s is n e g lig ib le .

(7) B y e m p lo y in g a v e ry o x id is in g p a c k in g m a te r ia l a n d a h ig h t e m p e r a t u r e , i t is p ossible t o e lim in a te c o m p le te ly a ll e le m e n ta r y c a rb o n , le a v in g o n ly f e r r i t e a n d p e a r lite . U ltim a te ly also t h e c a rb o n in t h e p e a r l it e is o x id is e d a n d only f e r r i t e r e m a in s .

PART n —THE FORMATION OF PRIMARY GRAPHITE IN A TYPICAL GREY CAS T 1ECN P r e v io u s w o rk e rs h a v e f o r t h e m o st p a r t co n ­ fin ed t h e i r a t t e n ti o n to t h e t o ta l a m o u n t of g r a p h it e f o rm e d 5, a lth o u g h W u s t6 a n d H o n d a7 q u e n ch e d sm a ll sa m p le s o f w h ite ir o n j u s t below th e e u te c tic p o i n t a n d s tu d ie d t h e fo r m a tio n o f g r a p h it e w ith t h e m icro sco p e.

Annealing Experiments.

E x p e r im e n ts w e re c a r r ie d o u t in ordeir t o tr a c e , if possible, t h e m o d e o r m e th o d o f fo r m a tio n of p r im a r y g r a p h it e in a n o r d in a r y c a s tin g . T he p ig -iro n e m p lo y e d a n a ly s e d a s fo llo w s: —

T o ta l C 3 .2 ;' G r a p h itic C 2 .5 ; Si 1 .3 ; S 0 .1 4 ; P 0.04, a n d M n 0.77 p e r c e n t.

4. “ J o u rn a l Iron a n d S teel I n s t it u t e ,” 1904, I, 224.

5. “ S ta h l u . E is e n ,” 1922, 42, 148. See a ls o H a tfield ,

" J o u r n a l Iron a n d S teel I n s t it u t e ," 1907. 2, 79; a n d B row n ,

“ P r o ce ed in g s S ta ffs. Iro n a n d S teel I n s t it u t e ,” 28, 127.

6. “ M e ta lln r g ie ,” 1909.

7. “ J o u rn a l Iron a n d S teel I n s t it u t e ,” 1920, 2, 287.

Fig. 15. —X 200. Fig. 16.— x 350.Fig. 17.— x 200. Gl (1,240Deg. C.). Gl (1,240Deg. C.Ï G2 (1,145Dfg. C.).

T a b l e II.

Quench. T.,

B ar N um ber. in deg. C. G rap h ite p e r cent.

A1 940 1.5

A2 900 1.7

A3 850 2.09

A4 800 2.4

A5 750 2.5

A 6 700 ^2.6

T a b l e I I I . Quench T.,

B ar N um ber. in deg. C. G raphite per cent.

B1 1,150 1.84

B2 1,100 1.9

B3 1,050 1.94

B4 ^{1,000 2.1}

B5 950 2.29

B6 900 2.42

T h e g e n e ra l m e th o d o f p r o c e d u r e w a s a s follow s : —■

T he p ig -iro n w as c a s t in to s e v e ra l b a r s (6 in . lo n g

X 1 in . d ia .) i n o n e s a n d b ox. S o o n a f t e r s o lid i­

fic a tio n a n d w h ils t t h e ir o n w as r e d h o t , t h e b a r s w ere q u ic k ly t r a n s f e r r e d t o a la r g e g a s m u ffle, h e a te d to a p r e -d e te r m in e d t e m p e r a t u r e . A f t e r a n in te r v a l o f five m in u te s t h e f ir s t b a r w as t a k e n o u t a n d q u e n c h e d in cold w a te r . T h e t e m p e r a t u r e of t h e m uffle w as lo w e re d a n d t h e r e m a in i n g b a r s q u e n ch e d a t in te r v a ls o f 50 d e g . C. S a m p le s w e re

Ta b l e I V .

B ar N um ber.

Quench. T.

in deg. C. G ra p h ite p e r c en t.

F I 1,140 1.5

F2 1,050 1.6

F3 950 1.7

F4 900 1-9

F 5 850 2.05

F6 750 2.15

F7 650 _2.2

t a k e n fro m t h e b a r s fo r c h e m ic a l a n d m ic ro sc o p ic a l a n a ly s is . T h e r e s u lts o f t h e f ir s t tw o e x p e r im e n ts a r e sh o w n in T a b le s I I a n d I I I , a n d a lso in G ra p h I .

I n t h e t h i r d e x p e r im e n t t h e q u e n c h in g te m p e r a ­

t u r e iran g e c o v e re d th o se o f th e firs t tw o e x p e r i­

m e n ts, a n d t h e r e s u lts a r e show n in T a b le IV a n d G r a p h I I .

A s i t a p p e a r e d d e s ira b le to a s c e r ta in t h e tim e r e q u ir e d in th e f u r n a c e f o r th e b a r s to a t t a i n a s t a te o f e q u ilib r iu m fo r a d e fin ite te m p e r a t u r e , a f u r t h e r s e t o f 'b a rs w as c a s t as b e fo re a n d t r a n s ­ f e rr e d to th e m uffle a t 1,000 d eg . C.- T h e firs t b a r w as t a k e n o u t a n d q u e n c h e d a f t e r five m in u te s in

t h e fu r n a c e , a n d t h e o th e r b a rs w e re q u e n c h e d a t in te r v a ls o f five m in u te s . T a b le V show s th e r e s u lts of th is e x p e r im e n t, a n d i t w ill b e seen t h a t fo r a te m p e r a t u r e o f 1,000 deg . a t le a s t t e n m in u te s a r e r e q u ir e d in t h e f u r n a c e fo r e q u ilib r iu m co n ­ d itio n s t o occu r.

T a b l e V.

B ar No.

Q uench T.

in deg. C.

Time M inutes.

G raphite per cent.

Cl 1,000 5 2.03

C2 1,000 10 2.2

C3 1,000 15 2.22

C4 1,000 20 2.2

C45 1,000 25 2.25

C6 1,000 30 2.22

D iscussion of Results.

F ro m a s tu d y of G ra p h I I i t w ill be seen t h a t a b a r w hich c o n ta in s 1.5 p e r c e n t, g r a p h it e a t 1,140 d e g ., on c o o lin g to 650 d eg . in c re a s e s its g r a p h it e c o n te n t to 2.2 p e r c e n t. T h is in c re a s e , h o w ev er,

is n o t c o n s t a n t , b u t th e r a t e o f p r e c i p it a ti o n of g r a p h it e f ir s t in c re a s e s a n d t h e n f a lls off a g a in , so t h a t a t a b o u t 900 deg . t h e r e is a m a x im u m r a t e of p r e c i p it a ti o n . T h e s h a p e o f t h e c u r v e is con­

s id e re d t o h e d u e t o t h e c o m b in e d in flu e n c e o f th e te n d e n c y f o r t h e f o r m a tio n of g r a p h i t e a n d th e in c r e a s in g r i g id i t y o f t h e m e ta l. I t m a y sa fe ly be a ssu m e d t h a t in t h e so lid s t a t e g r a p h i t e is t h e s ta b le fo rm o f c a rb o n , a n d n o t c e m e n tite . I n t h a t c a s e , t h e lo w er t h e t e m p e r a t u r e fro m t h e m e ltin g p o in t, t h e g r e a t e r th e te n d e n c y o f t h e u n s ta b le c e m e n tite to decom pose i n to g r a p h i t e a n d ir o n . I t is, h o w e v e r, a re co g n is e d f a c t t h a t d e c o m p o s itio n o f c e m e n tite c a u s e s a n e x p a n s io n in t h e c a s t i n g ; t h a t is w hy w h ite ir o n s c o n t r a c t m u c h m o re t h a n g re y iro n s . "With lo w e rin g o f t e m p e r a t u r e , t h e r i g id i t y o r s t r e n g t h of t h e ir o n in c r e a s e s c o n s id e r ­ ab ly , as w ould b e e x p e e te d , a n d t h is r e s i s t s t h e i n te r n a l e x p a n s io n c a u s e d 'by t h e d e c o m p o s in g c e m e n tite .

I n t h e fo re g o in g e x p e r im e n ts t h e m in im u m a m o u n t o f g r a p h it i c c a r b o n w a s 1.5 p e r c e n t, o u t o f a t o t a l c a rb o n c o n te n t o f o v e r 3 p e r c e n t. In o th e r w o rd s, n e a r ly h a lf t h e g r a p h i t e h a d b e e n p r e c ip ita te d b e fo re t h e a c t u a l i n v e s ti g a ti o n s h a d co m m en ced . F u r t h e r w o rk w as n e c e s s a r y , t h e r e ­ fo re , to a c c o u n t f o r t h e f ir s t q u a n ti t ie s o f g r a p h i t e fo rm e d , a n d i t w as re c o g n is e d t h a t a d if f e r e n t m o d u s o p e r a n d i w as c a lle d f o r .

Quenching Experiments.

T h e ir o n u s e d w as a n o r d i n a r y g r e y ir o n o f th e fo llo w in g c o m p o s itio n :

—-T o ta l C 3 .4 ; G r a p h itic C 2 .7 ; S i 1 .4 ; S 0 .0 9 ; P 0.07, a n d M n 0 .9 p e r c e n t., a n d t h e m e th o d fin ally a d o p te d w as a s f o llo w s : —

U s in g a sm all M o rg a n c r u c ib le a s a p a t t e r n , five c a s t-iro n m o u ld s w e re p r e p a r e d . O n e w a s h e a t e d to a b r i g h t r e d h e a t a n d in to i t w a s p o u r e d a sm a ll q u a n ti t y ( a p p ro x im a te ly 1 lb .) o f m o lte n iro n , a n d t h e w hole q u e n c h e d in c o ld w a t e r b e fo re t h e iro n h a d so lid ifie d , t h e t e m p e r a t u r e b e in g d e te r m in e d w ith a n ic k e l-c h ro m e th e rm o -c o u p le . A n o th e r i n g o t w as p o u r e d , a s b e fo re , h u t t h i s tim e i t w as q u e n c h e d a t t h e s o lid if y in g p o in t. T h r e e m o re in g o ts w e re s im ila r ly p o u r e d a n d q u e n c h e d a t d iffe re n t t e m p e r a t u r e s below t h e s o lid if y in g

tern-p e r a t u r e of t h e iro n . S a m p le s f o r c h e m ic a l a n d m ic ro sc o p ica l a n a ly s is w e re o b ta in e d fro m t h e c e n tr e o f e a c h sp e c im e n . T h e r e s u lts a r e g iv e n in T a b le V I , ill u s t r a te d in G r a p h I I I , a n d m ic ro ­ sco p ically i n F ig s . 15 t o 20.

G ra p h I I is t h e m o st i m p o r t a n t o f t h e c u rv e s show n. I t w ill b e s e e n t h a t b e tw e e n t h e e u te c tic p o in t 1,145 d eg . a n d 1,100 d eg . t h e r e is a c r itic a l

F i r . . 2 1 . — x 8 0 0 .

r a n g e in w h ich m o st o f t h e g r a p h it e is p r e c ip i­

t a t e d , i.e .. m o st o f t h e g r a p h it e fo rm s j u s t below t h e e u te c tic t e m p e r a t u r e . I t sh o u ld ’ be s t a t e d h e re t h a t t h e a c tu a l g r a p h it e v a lu e s g iv e n in T a b le V I do n o t r e p r e s e n t e q u ilib riu m c o n d itio n s , b u t a re a p p lic a b le so lely t o t h e p a r t i c u l a r c a s e u n d e r re v ie w , w ith co o lin g r a te s , lik e th o se in a c tu a l p ra c tio e , to o r a p id f o r e q u ilib riu m to o c cu r. I t w ill n o t be d iffic u lt t o u n d e r s ta n d , h o w e v er, t h a t th e g e n e r a l fo rm o f th e c u rv e , w ith in m a n y cases d if f e r e n t g r a p h it e v a lu e s, m a y b e a p p lie d to m o st g re y iro n s a n d in d ic a te s t h e q u a n t i t a t i v e f o r m a ­ tio n o f g r a p h it e in g re y iro n c a s tin g s . F ro m T ab le V I i t w ill b e see n t h a t t h e c e n t r e o f th e firs t sp ecim en G I c o n ta in s 0 .27 p e r c e n t, of g r a p h ite , a lth o u g h q u e n c h in g to o k p la c e fro m t h e m o lte n s ta te . O ne m u s t t a k e in to a c c o u n t, h o w ev er, th e fa c t t h a t t h e q u e n c h e d s a m p le w as c o m p a ra tiv e ly la rg e a n d w eig h ed n e a r ly 1 lb. w ith o u t t h e m o u ld , so t h a t t h e q u e n c h in g w as n o t in s ta n ta n e o u s . T h e

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tim e f a c to r m u s t b e c o n sid ere c h a n d i t is m o st p ro b a b le t h a t t h e g r a p h i t e in G I w as f o rm e d ju s below t h e e u te c tic t e m p e r a t u r e . T h is v iew is co n ­ firm e d b y t b e m ic ro sc o p ic e x a m in a tio n (see F ig ^ Ivl»

w h e re i t is se e n t h a t p a r t o f t h e g r a p h it e is show n in t h e a u s te n o id a re a s .

G r a p h I I I .

Graphite Content of Molten Metal.

T o d e c id e w h e th e r g r a p h i t e is p r e s e n t a s s u c h in t h e p a r ti c u l a r iro n u n d e r d is c u s s io n w h e n m o lte n , a s a m p le of t h e c a s t ir o n as u s e d in t h e p r e v io u s e x p e r im e n t w as m e lte d in a S a la m a n d e r c r u c ib le a n d p o u re d , fro m j u s t ab o v e t h e m e ltin g p o i n t, in a s t h i n a s tr e a m a s p o ss ib le in to c o ld w a te r , in o r d e r t o o b t a i n t h e m a x im u m q u e n c h in g e ffe c t. A p iec e o f t h e r e s u l ti n g m e ta l w a s c le a n e d a n d t h e n d isso lv ed in n i t r i c a c id o f 1.2 0 s p . g r . a n d t h e s o lu tio n filte re d . N o t r a c e o f g r a p h i t e c o u ld b e o b se rv ed , s h o w in g t h a t in m o lte n ir o n g r a p h i t e d o e s n o t o c c u r in i t s e le m e n ta r y f o r m , b u t a s a c o m p o u n d w ith iro n , as iro n c a r b id e . A n o t h e r s p e c im en of t h e q u e n c h e d ir o n w as s e c tio n e d a n d p o lis h ed f o r m ic ro sc o p ic e x a m in a tio n . F i g . 21 'show s t h e m ic r o g r a p h ( x 8 0 0 ) , a n d i t w ill b e s e e n t h a t t h e q u e n c h in g h a s b e e n s u ffic ie n tly s e v e re t o r e t a i n c r y s ta ls of a u s t e n i t e , c o n ta in in g n e e d le s o f m a r te n s ite , b u t n o g r a p h i t e c a n b e o b s e rv e d .

Equilibrium Graphite Content.

I n o r d e r to d e te r m in e t h e m a x im u m g r a p h it e c o n te n t a t d if f e r e n t t e m p e r a t u r e s , s ix b a r s w ere e a s t a n d t r e a t e d as in t h e f ir s t e x p e r im e n ts , e x c e p t

t h a t e a c h b a r w as h e ld a t its q u e n c h in g t e m p e r a ­ t u r e fo r t h i r t y m in u te s b e fo re q u e n c h in g . T h e re s u lts a r e g iv e n in Ta'ble V I I .

T h e c o m p o s itio n o f t h e iro n w as a s fo llo w s: — T o ta l C 3 .4 ; G r a p h i ti c C 2 .7 ; S i 1 .4 ; S 0 .0 9 ; P 1.07, a n d M n 0.9 p e r c e n t.

I n G ra p h I V t h e c a rb o n s o lu b ility c u rv e h a s b e e n p lo tte d on t h e ir o n - c a r b o n e q u ilib riu m

T a b l e VI.

Specimen.

Q uench T.

in deg. C.

G raphite

p er cent. Micro.

G1 1,240 0.27 Figs. 15 & 16

G2 1,145 0.87 Fig. 17

G3 1,083 1.9 Fig. 18

G4 980 2.3 Fig. 19

Go 915 2.4 Fig. 20

d ia g ra m , a n d w ill ¡be se e n to c o n fo rm f a ir ly closely w ith t h e lin e u s u a lly re p re s e n te d a s A g r, a lth o u g h th e iro n h a s t h e u s u a l m in o r e le m e n ts o r im p u r itie s as fo u n d in c o m m e rc ial iro n s. T h e d o tte d lin e show s a t a n y r e q u ir e d t e m p e r a t u r e how m u ch c a rb o n is d isso lv e d i n t h e ir o n as c e m e n tite o r iro n c a r b id e ; t h e d iffe re n ce b e tw e e n th e s e v a lu e s a n d th e t o ta l c a rb o n c o n te n t is t h e a m o u n t of c a rb o n p r e s e n t a s g r a p h ite .

T a b l e V II.

B ar N um ber.

Quench T.

in deg. C. G raphite p e r cent.

H I 1,100 2.2

H2 1,000 2.3

H3 900 2.45

H4 800 2.65

H5 700 2.8

H6 600 2.9

Effects of Casting Temperature.

I t is now w ell k n o w n t h a t c a s tin g t e m p e r a t u r e h a s a b e a r in g on t h e p h y sic a l p r o p e r tie s o f t h e iro n . L o n g m u ir p o in te d t h is o u t tw e n ty y e a r s ag o . O th e r w o rk e rs h a v e sin ce c a r r ie d o u t in v e s tig a tio n s o n t h e s a m e s u b je c t. I t a p p e a r e d d e s ira b le to a s c e r ta in t h e e ffe c t of p o u r in g te m p e r a t u r e o n g r a p h ite f o r m a tio n , b e a r in g in m in d , a s m e n tio n e d

p re v io u s ly , t h e in flu e n c e o f t h e size a n d s h a p e o f t h e g r a p h it e o n t h e p r o p e r tie s o f c a s t iro n , t h e fro m 1,410 t o 1,210 d eg . C. T h e b a r s w e re b ro k e n t h e p re v io u s e x p e r im e n t. F iv e b a r s w ere p o u re fro m t h e sa m e c ru c ib le , a t t e m p e r a t u r e s r a n g in g fro m 1410 t o 1210 d e g . C. T h e b a r s w e re b ro k e n a n d s a m p le s o b ta in e d f o r m iciroscopical a n d c h e m i­

c al a n a ly s is . H a r d n e s s t e s t s w e re c a r r i e d o u t o n t h e m ic ro -sp e c im e n s , u s in g a 10-m m . b a ll f o r t h e c e n t r e a n d 1 m m . b a ll f o r t h e e d g e of t h e s p e c i­

m en , o n a s u r f a c e a t r ig h t- a n g le s t o t h e l e n g th o t h e b a r . T h e f u ll r e s u lts a r e g iv e n in T a b le a n d show n in G r a p h V. I t m ig h t.b e e x p e c te d t h a t a d e c re a s e in g r a p h i t e c o n te n t w o u ld te n d to in c re a s e t h e h a rd n e s s , o n a c c o u n t o f t h e im p lie d in c re a s e of t h e h a r d c e m e n tite c o n s t i t u e n t in t h e m ass. T h is s u p p o s itio n in g e n e r a l is b o r n e o u t b y th e c u rv e s sh o w n in G r a p h V . I n a d d itio n , i t w ill be n o tic e d t h a t a lth o u g h t h e g r a p h i t e c o n te n t o f th e c e n tr e o f t h e b a r s d o e s n o t v a r y c o n s id e r a b ly , th e r e is a c o n tin u a l d e c re a s e in t h e g r a p h i t e on th e ed g e.

R e f e r r in g no w t o m ic ro sc o p ic a l e v id e n c e : m ic ro ­ g r a p h s w ere t a k e n o f e a c h sp e c im e n a t t h e c e n t r e a n d n e a r th e e d g e, a t a m a g n if ic a tio n o f xlO O d ia .

a n d u n e tc h e d (see F ig s . 23 t o 32). T h e c o m p a r a ­ tiv e ly la r g e s iz e o f g r a p h it e fla k e s i n t h e c e n tr e s of th e f irs t t h r e e s p e c im e n s c o n tr a s t s la r g e ly w ith t h e siz e on t h e o u ts id e of t h e l a s t t h r e e s p e c im e n s . T h e r e is a d i s t in c t c o n n e c tio n a ls o b e tw e e n t h e

h a rd n e s s a n d size o f g r a p h it e flak es, i t b e in g pos­

g r a p h i t e c o n te n t , i t h a s t h e lo w e r h a ird n e ss n u m b e r, d u e to l a r g e r size o f i ts g r a p h i t e flak es.

Conclusions.

T h e fo llo w in g c o n c lu sio n s m a y b e d r a w n fro m th e w o rk d e s c rib e d in P a r t I I o f t h is P a p e r :

(1) F r e e g r a p h i t e is n o t p r e s e n t in m o lte n c a s t iro n , b u t t h e g r a p h i t e w h ic h is p r e s e n t in a c a s tin g r e s u lts fro m t h e d e c o m p o sitio n o f c e m e n tite in th e

so lid s t a t e . . .

(2) O n c o o lin g fro m t h e liq u id c o n d itio n , t h e i r ° n p r e c i p it a te s m o st o f its g r a p h i t e j u s t 'below t h e s o lid ify in g p o in t, i.e ., b e tw e e n 1,145 a n d 1,000 d e g ., i n t h e p a r t i c u l a r iro n u n d e r re v ie w .

(3) S m a lle r q u a n ti t ie s o f g r a p h i t e a r e p r o d u c e d below 1,000 deg. C ., d u e t o t h e d e c o m p o s itio n o f t h e p r o - e u te c to id c e m e n tite .

(4) I n o n e s a m p le of c a s t ir o n i t is sh o w n t h a t w h ils t a f te r a n n e a lin g a t 1,10 0 d e g . t h e g r a p h i t e c o n te n t is 2 .2 p e r c e n t ., o n slow ly c o o lin g t o 600 d eg . a n d a n n e a lin g a t t h a t t e m p e r a t u r e t h e g r a p h it e is in c re a s e d t o 2.9 p e r c e n t.

(5) A t 1,000 d eg . e q u ilib r iu m of g r a p h i t e c o n te n t is o b ta in e d a f t e r a v e ry s h o r t a n n e a l.

(6) F o r a n y p a r t i c u l a r ir o n t h e r e is a d e f in ite g r a p h it i c c a rb o n c o n te n t u n d e r c o n d itio n s o f e q u ili- bru-m f o r a n y g iv e n t e m p e r a t u r e .

(7) T h e g r a p h i t e a t t h e t im e o f f o r m a tio n a ssu m e s t h e c u rv e y fla k y s h a p e s im ila r t o t h a t fo u n d in t h e fin al c a s tin g .

(8) D e c re a s in g t h e p o u r in g t e m p e r a t u r e lo w e rs t h e g r a p h it e c o n te n t, p a r ti c u l a r ly o n t h e o u t s i d e o f t h e b a rs .

(9) I n t h e c a s e u n d e r re v ie w t h e h i g h e s t p o u r in g t e m p e r a t u r e y ie ld s a n ir o n w ith m o re g r a p h i t e on t h e o u ts id e t h a n in t h e c e n tr e .

(10) G r a p h ite size is g r e a t e r a t t h e c e n t r e t h a n a t t h e e d g e.

(11) I n c r e a s e in siz e of g r a p h i t e te n d s t o d e c re a s e t h e h a rd n e s s .

(12) I n g e n e r a l t h e less t h e g r a p h it e , t h e h a r d e r t h e iro n .

F in a lly , th e a u th o r w ish es p a r ti c u l a r ly to t h a n k P ro fe s s o r T. T u r n e r , u n d e r w hose d ir e c tio n th e w o rk w as c a r r i e d o u t , f o r t h e m a n y u s e f u l s u g ­ g e s tio n s a n d c r itic is m s -made d u r i n g t h e c o u rs e o f th is re s e a rc h .

DISCUSSION.

Heat Treatment and the Production of Pearlitic Structure.

M r . J . Sh a w c o n g r a tu la te d t h e a u t h o r u p o n th e w ork h e h a d c a r r i e d o u t a n d t h e r e s u lts o b ta in e d . D isc u ssin g t h e f ir s t p a r t of t h e P a p e r , h e a s k e d w h a t w as t h e s e c tio n o f t h e a n n e a lin g sp e cim en s.

W o u ld th e a n n e a lin g effec t o n c o m p a ra tiv e ly la rg e , se c tio n s— say 3 in . sq .— be t h e sa m e a ll th r o u g h as sa m p le 8, T ab le I . I n th is case t h e t e m p e r a t u r e w as 1.050 d eg . C ., t h e a n n e a lin g tim e w as 17^

h o u rs , a n d t h e m ic ro -c o n s titu e n ts w ere g iv e n as g r a p h ite , p e a r lite a n d f e r r i t e . I t w ou ld b e i n ­ te r e s tin g to le a r n w h e th e r th e a u th o r h a d a n n e a le d c o m p a ra tiv e ly la r g e se c tio n s o f w h ite c a s t iro n a n d o b ta in e d a p e a r litic s t r u c tu r e a ll t h r o u g h t h e piece.

Precipitation of Graphite After Attaining Equilibrium.

I n r e g a r d t o P a r t I I of th e P a p e r . F r a n k l y h e could n e t re co n c ile t h e r e s u lts o b ta in e d in o n e ta b le w ith th o se g iv e n i n t h e o t h e r ta b le s . T h ey w ere to ld in o n e case t h a t e q u ilib r iu m w as a tt a in e d a t t h e e n d of 10 m in s ., a n d w h en th e h e a t t r e a tm e n t w as c o n tin u e d f o r 30 m in s. p r a c t i ­ cally n o f u r t h e r g r a p h it e w as o b ta in e d . W h e re th e y t o u n d e r s ta n d fro m t h i s t h a t if a c a s tin g w as h e ld a t a g iv e n te m p e r a t u r e , ab o v e t h e c h a n g e p o in t, a f te r a tim e n o f u r t h e r g r a p h ite w as fo rm e d . C a s tin g te m p e r a tu r e p la y s a p a r t in th e a m o u n t of g r a p h it e f o rm e d , y e t n o c a s tin g t e m ­ p e r a t u r e is g iv e n in T ab les I I , I I I o r IV . M r.

S h aw a ls o p o in te d o u t t h a t t h e tim e t a k e n to re d u c e t h e te m p e r a t u r e in th e m uffle fro m 940 to 700 deg. C . in T ab le I I is n o t g iv e n . T h e sam e f a c t a p p lie s t o T ab les I I I a n d IV , so i t is d iffic u lt to fo rm co n clu sio n s. B u t B a r s A2, B l , a n d F 4 all q u e n ch e d a t 900 d eg . C ., y e t 1.7, 2.42 a n d 1.90 p e r c e n t, g r a p h it e re sp e c tiv e ly a r e o b ta in e d . A g a in , B 3 a n d F 2 . I n t h e f ir s t c ase 1.94 p e r c e n t, g r a p h ite w as th ro w n o u t, w h ile in t h e l a t t e r o n ly 1.6 p e r c e n t, w as o b ta in e d , y e t b o th t h e s e l a t t e r m u s t h a v e b e en h e ld o v er t h e 10 m in s .—

m e n tio n e d as t h e tim e to o b ta in e q u ilib riu m a t 1.000 deg. C

As to t h e v ex ed q u e s tio n o f w h e th e r g r a p h it e co u ld e x is t in t h e f r e e s t a te in t h e m o lte n m e ta l, th e f a c t p o in te d o u t b y M r. N o r t h c o tt t h a t in he q u e n c h e d sp e c im e n , G l , in T a b le V I , i t w as p o s­

sib le to o b ta in 0 .27 g r a p h it e in t h e v e ry s h o r t tim e i t to o k to s o lid ify t h is sp e c im en , g a v e a r e a s o n f o r som e o f th in g s e n c o u n te r e d t h a t w as c o n s id e re d p ro o f t h a t f r e e c a r b o n w as p r e s e n t in m o lte n iro n . A t t h e sa m e tim e , i t w as q u ite p o ss ib le t h a t p o u i- in g a v e ry t h i n s tr e a m o f m o lte n m e ta l i n to w a te r any* f r e e c a rb o n m ig h t be w a sh e d a w a y in t h e w a te r . T h e fig u re s g iv en in T a b le V I I I w e re in c o n tr a d ic tio n w ith th o s e o f H a ils to n e s m h is P a p e r b e fo re t h e I r o n a n d S te e l I n s t i t u t e . H a ils to n e s s t a te d t h a t w h en c a s tin g a t 1,428 d e g . C. w ith t h e m e ta l h e u se d , t h e g r a p h i t e w a s 2.83 d eg . C. H e c a s t fre s h B ars a t in te r v a l s o f 2 m in s . T h e l a s t b a r w as c a s t a t 1,264 d eg . C ., a t w h ic h te m p e r a t u r e h is g r a p h it e c a rb o n c o n te n t w as in c re a s e d to 3.126 p e r c e n t. W h ic h r e s u l t w a s t o be c o n s id e re d c o r re c t?

D écarb u risatio n D anger.

Mr. N o r t h c o t t , r e p ly in g to M r. S h a w , s a id t h a t t h e s p e c im e n s h e h a d a n n e a le d f o r t h e f i r s t p a r t o f t h e P a p e r w ere n o t m o re t h a n 1 cm . s q u a r e . H is c h ie f t r o u b le w ith s e c tio n s o f su c h s m a ll siz e w as t h a t d é c a r b u r is a tio n to o k p la c e o n t h e o u ts id e . T h a t w as w hy th e y h a d b e e n a n n e a le d in a p a r t i a l v a c u u m . O n t h e sm a ll sp e c im e n s , o r, f o r t h a t m a t t e r , o n b ig o n e s, if th e y w e re a n n e a le d f o r a su fficien tly lo n g tim e , p a r ti c u l a r ly in a n o x id is in g a tm o s p h e re , d é c a r b u r is a tio n w ou ld t a k e p la c e . W ith r e g a r d to p e a r l it e , h e c e r ta in ly t h o u g h t in w h ite h e a r t m e ta l w ith h ig h s u lp h u r o r c h r o m iu m th e y w ould g e t a m a t r i x o f p e a r l it e w ith n o d u le s of te m p e r-c a rb o n . As to M r. S h a w ’s r e m a r k s w ith r e g a r d to T a b le s I I a n d I I I , i t m u s t b e u n d e r ­ sto o d t h a t th o s e w ere n o t i n te n d e d t o b e e q u ili­

b r iu m c o n d itio n s . I n T a b le V th e r e w e re in e q u i­

lib riu m c o n d itio n s , a n d i t w as se e n t h a t a f t e r a b o u t 10 m in u te s a t a t e m p e r a t u r e of 1,000 d e g . e q u ili­

b r iu m w as a tt a i n e d . H e c o u ld c e r ta in ly °say t h a t a t a n y t e m p e r a t u r e ab o v e 700 d eg . th e r e w as o n ly o n e c a rb o n c o n te n t w hen t h e m e ta l w as in e q u ilib riu m . I n th is case i t w as a b o u t 2.2 p e r c e n t a t 1,000 deg .

A Sp e a k e r a s k e d how M r . N o r t h c o tt a c c o u n te d fo r t h e d iffe re n c e in g r a p h it i c c o n te n t b e tw ee n sp ecim en s A 2 a n d B6 in T ab les I I a n d I I I ; t h e te m ­ p e r a tu r e w as th e sa m e in eac h case, a n d e ac h w as h e ld fo r 10 m in u te s .

Mr. N o r t h c o t t re p lie d w ith t h e a id o f a sk e tc h o f t h e iro n -c a rb o n e q u ilib riu m d ia g r a m . H is fig u res in T a b le s I I a n d I I I w ere d is tin c tly n o t in e q u ilib riu m . I f th e s e iro n s h a d b een h e ld a t e ac h te m p e r a tu r e fo r a t le a s t 10 m in u te s h e b e lie v e d M r.

S h aw w ould see t h a t th e fig u re s w ould h a v e b e e n e q u al in b o th ta b le s , b u t w ith a d e sc e n d in g te m ­ p e r a t u r e — t h e c o o lin g w as f a ir ly r a p id in a sm all g as m uffle— e q u ilib riu m c o n d itio n s h a d n o t b een o b ta in e d . W ith r e g a r d to m o lte n m e ta l, h e h a d no d o u b t t h a t cairbon in m o lte n iro n d id n o t e x is t as g r a p h ite . H e c o u ld n o t p ro v e t h a t v e ry well a t t h e m o m e n t, b u t if th e y m a d e th e e x p e r im e n t—

n o t a n o r ig in a l e x p e rim e n t— of p o u r in g m o lte n iro n in to a b u c k e t o f w a te r, a n d t h e n t a k i n g a sm all p iece o f i t a n d d isso lv in g u p in a cid , th e r e w ould b e no g r a p h ite th e r e a t all. H e h a d h e a r d

n o t a n o r ig in a l e x p e rim e n t— of p o u r in g m o lte n iro n in to a b u c k e t o f w a te r, a n d t h e n t a k i n g a sm all p iece o f i t a n d d isso lv in g u p in a cid , th e r e w ould b e no g r a p h ite th e r e a t all. H e h a d h e a r d

W dokumencie Proceedings of the Institution of British Foundrymen, Vol. 17 (1923-1924) (Stron 106-138)