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S o l u b i lit y P r o d u c t f o r N i o b i u m C a r b i d e in u s t e n i t e

V . K . L A K S H M A N A N a n d J . S . K I R K A L D Y

The so lub i l i ty p rodu c t fo r NbC0.87 has been exp e r im en ta l ly de te rm ined be twe en 950 ~ and 1250 ~ by

gas equ i l ib ra t ion and ex t rac t ion m e thod s . The new re la t ion log l0[pc t Nb] [pc t C] ~ = 3 . 4 - 79 20 /T

on a we igh t pe rcen t bas i s has been eva lua ted f rom the da ta . Th i s i s in good agreem ent wi th a

c o m p e n d i u m o f m e a s u r e m e n t s f r o m t h e l i te r a tu r e . T h e d a t a d o n o t y e t j u s t i f y a n a n a l y s is w h i c h t a k e s

a c c o u n t o f a v a r i a b l e s t o i c h i o m e t r y .

I . I N T R O D U C T I O N

A S p a rt o f a c o m p r e h e n s iv e t h e rm o d y n a m i c e x a m i n a ti o n

o f t h e e q u i l i b r i u m o f n i o b i u m c a r b o n i t r i d e s w i t h a l l o y e d

aus ten i te I we h ave e xpe r im enta l ly de te rm ined the so lub i l i ty

produ c t fo r NbC0. 87 in una l lo yed aus ten i t e b e twe en 95 0

and 1250 ~ This expands , and to a l a rge ex ten t b r idges

and conf i rm s , the ex tens ive da ta s e t a l ready ava i l ab le in

t h e l i t e r a t u r e . 2-~1 T h e p r i n c i p a l m e t h o d o l o g y i n v o l v e d

m u t u a l e q u i l i b r a t i o n o f e n c a p s u l a t e d F e - C a n d F e - C - N b

a l loy s am ples in hydrogen . S o lub i l i ty p roduc t de te rm ina -

t ions and prec ip i t a te iden t i f i ca t ions were a l so ca r r i ed ou t

o n t w o h i g h p u r i t y v a c u u m - m e l t e d s t e el s u s i n g c h e m i c a l a n d

e lec t ro -chem ica l m e thods to i so la te the p rec ip i t a ted phases .

I I. T H E G A S E Q U I L I B R I U M

E X P E R I M E N T A L M E T H O D

T h e s e a le d c a p s u l e t e c h n i q u e , e m p l o y e d b y K / i ll s tr o m a n d

Om sen, ~2 H eck ler and W inch el l , ~3 Zu pp and Stev enso n, ~4

Nish izawa , ~5 and Uhren ius and Harv ig ~6 was chosen fo r the

presen t s tudy . Th i s cons i s t s o f a s e r i e s o f d i f fus ion- i so la ted

F e-Nb a l loys and F e -C a l loys enc losed in an evacua ted , H2

back- f i l l ed , th ick-wa l led qua r tz tube , annea led a t the des i red

tem pera ture . C arbon i s s e lec t ive ly t rans fe r red be tween the

F e-C and the F e -N b a l loys which a re no t in phys ica l con tac t .

The ca rbon ac t iv i ty ob ta in ing in the capsu le i s de te rm ined

f r o m t h e f i n a l c a r b o n c o n t e n t o f t h e b i n a r y F e - C a l l o y

samples

v i a

t h e r e s u lt s o f B a n - y a

e t a l . 1 7

T h e m a i n e x p e r i m e n t a l c o n s i d e r a t i o n s i n t h i s m e t h o d

were :

1 ) H i g h p u r i t y F e r r o v a c e l e c t r o l y t ic i ro n a n d f e r r o - n i o b i u m

w i t h a n o x y g e n c o n t e n t le s s t h an 0 . 0 0 1 2 W p c t h a s b e e n u s e d

in th i s s tudy Table I ) . S am ples w ere p repa red by m e l t ing

f e r r o - n i o b iu m a n d e l e c t r o l y t i c i r o n u n d e r a n a r g o n a t m o -

s p h e re . H i g h p u r i t y H2 g a s w i t h a v e r y l o w H 2 0 c o n t e n t

has been used in the equ i l ib ra t ion . We the re fore conc lude

tha t the oxy gen con tam ina t ion of the NbC087 prec ip i t a te s

is negl igible .

2 ) A ccura te t em pera tu re con t ro l and e s tab l i shm ent o f a un i -

f o r m t e m p e r a t u r e z o n e + 4 ~ f o r T < 1 1 0 0 ~ + -6 ~ f o r

T > 1100 ~

3) P reven t ion of l eakage of H2 f rom the th ick-w a l led qua r tz

tube

v i a

a 1 a tm ex te rna l H2 ba th .

v .K. LAKSHMANAN s a Research Engineerwith Tata Iron and Steel

Company, Jamshedpur, India. J. S. K IRKALDY s Professor,Department

of Metallurgy and Materials Scien ce, McMaster Un iversity, Ham ilton,

Ontario, Canada, L8S 4K1.

Manu script subm itted Septe mb er 1, 1982.

4) Es tab l i shm ent and a s ses sm en t o f equ i l ib r ium wi th re -

s p e c t t o c a r b o n c o n t e n t 0 . 5 m m s h e e t a n n e a l e d f o r t i m e s u p

to 120 hours) .

5 ) I n v e st i g a ti o n o f t h e p o s s i b il i ty o f t r a n s f e r o f n i o b i u m

b e t w e e n t h e s a m p l e s o r o f s i l ic o n c o n t a m i n a t i o n f r o m t h e

quar tz capsu le .

6 ) C a r b o n a n a l y s i s L e c o c o n d u c t o m e t r i c a p p ar a t u s f o r

low concen t ra t ions ) .

F u l l de ta i l s a re to be found in Refe rences 1 and 12 to 16 .

I I I. S T U D I E S O N P R E C I P I T A T E S

E X T R A C T E D

F R O M N I O B I U M B E A R I N G S T E E L S

Effec t iv e e lec t roche m ica l m eans fo r i so la ting p rec ip i t a te s

h a v e b e e n d i s c u ss e d b y B l i c k w e d e a n d C o h e n , ~8 A n d r e w s

and Hug hes , 19 W alz and Bloo m , 2~ and Gu rry

e t a l . 2t

T h e

pr inc ip le in a l l these t echn iques i s the s am e ,

v i z .

a n o d i e

d i s so lu t ion o f the hea t trea ted s t ee l spec im en under su i t ab le

condi t ions . M andry and D orne las z2 have succes s fu l ly used a

c i t ra te ce l l fo r ex t rac t ing ca rboni t r ides o f n iob ium . The ad-

van tage o f an e lec t rochem ica l t echn ique i s tha t i t i s condu-

c ive to the ex t rac t ion o f l a rge am ounts o f p rec ip i t a te s in a

shor t t im e .

T h e s m a l l a m o u n t s o f n i o b i u m n o r m a l l y f o u n d i n s t ee l s

a n d t h e n e e d f o r a n e v a l u a ti o n o f t h e f i x e d a n d f r e e n i o b i u m

to ca lcu la te the so lub i l i ty l im i t s o f the in te rs t i ti a l phases has

l e d t o t h e d e v e l o p m e n t o f c h e m i c a l d i s s o l u ti o n t e c h n i q u e s

for i so la t ing these phases and accura te p rocedures fo r ana -

lyz ing the re s idues f l3-26

M o i l

e t a1 .1~

h a v e m a d e e x t e n s i v e s t u d i e s o n n i o b i u m

com pounds p rec ip i t a ted in s t ee l us ing a s im ple chem ica l

d i s so lu t ion m e tho d to ex t rac t the p rec ip i t a te s quan t i t a t ive ly .

T h e c h e m i c a l a n a l y s i s i s b y t h e K a w a m u r a s p e c t r o p h o t o -

m e t r i c t ech n ique . 23 Num erous o the r ch em ica l p roced ures

a r e a v a i l a b l e a n d h a v e b e e n r e v i e w e d b y B h a r g a v a a n d

Donovan .28 T hese au thors have a l so g iven new and accura te

procedures fo r n iob ium ana lys i s .

In the p resen t inves t iga t ion , a few so lub i l i ty p roduc t de -

te rm ina t ions and prec ip i t a te iden t i f i ca t ions have been m ade

T a b le I. C o m p o s i t io n s o f t h e F e N b a n d

F e C A l l o y s fo r G a s E q u i l i b r i u m S t u d i e s

Fe-Nb Wt Pct)

0.012 -+ 0.0010

0.032 -+ 0.0010

0.056 + 0.0010

0.073 -+ 0.0024

0.091 + 0.0030

0.108 -+ 0.0040

0.315 -+ 0.0050

0.402 -- 0.0060

0.510 +- 0.0070

0.605 -+ 0.0080

Fe-C: 0.1 to 0 .4 pct C

METALLURGICAL RANSACTIONSA VOLUME 15A, MARCH 1984--541

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T a b l e I I C o m p o s i t i o n o f S t e el s U s e d

f o r P r e c i p i t a t e I s o l a t i o n S t u d i e s

Steel Com position, Wt Pct

No. Nb C N S O

S1 0.180 0.030 - - 0.004 0.0010

2 0 .026 0 .175 0 .0026 -0 . 01 -0 .0 018

u s in g ch emica l an d e lec t r o ch emica l me th o d s to i so la te th e

p r ec ip i ta ted p h ases . Tw o h ig h p u r i ty v acu u m m el ted s tee l s ,

S1 an d 2 , w h o se co mp o s i t io n s a r e g iv en in Tab le I I , w er e

u sed f o r th e s tu d y . Th ey w er e an a ly zed p a r t i cu la r ly f o r

n i t r og e n , o x y g e n , a n d s u l fu r . A b o u t 2 0 0 g m s p e c i m e n s

w er e u sed f o r each e lec t r o ly s i s to y ie ld ad eq u a te q u an t i t ie s

o f th e p r ec ip i ta ted ca r b id es a f te r i so la t io n . Th e samp les in

th e f o r m o f th in sh ee t s o f 4 cms • 2 .5 cms • 0 .3 cms

we re f ir s t so lu t io n treat ed at 1200 ~ and 1300 ~ re-

sp ec t iv e ly , an d q u en ch ed in to b r in e . I so th e r mal h ea t t r ea t -

men ts w er e th en ca rr ied o u t a t 1 0 00 ~ f o r mo r e th an tw o

w eek s . Th e sp ec imen s w er e a l l s ea led in h ig h ly ev acu a ted

an d a r g o n - b ack - f i l l ed q u ar tz cap su les , an d th e a r g o n g as a t

an a tmo sp h er ic p r es su r e w as p assed co n t in u o u s ly th r o u g h

th e f u r n ace tu b e , a r o u n d th e sea led q u ar tz tu b es . Th e h ea t

t r ea tmen ts w er e th en in te r r u p ted b y q u en ch in g in b r in e .

Th e sp ec imen s a lw ay s h ad a c lean su r f ace a t th e en d o f

th e h ea t t r ea tmen ts . Th ey w er e th en c lean ed in 6 N H C1

f o r a f ew min u tes an d w er e th en r ead y f o r e lec t r o ly s i s o r

ch emica l d i s so lu t io n .

A f u l l acco u n t o f th e ch emica l an d e lec t r o ch em ica l t rea t -

men ts i s g iv en in Ref e r en ce 1 .

X - r ay d i f f r ac t io n o f th e r es id u es f r o m s tee l s S 1 an d 2

w as ca r r ied o u t in a D eb y e - Sch e r r e r 1 14 mm cy l in d r ica l

camer a u s in g a Cu - ta r g e t f o r th e X - r ay so u r ce an d a N i f i l te r .

H e l iu m w as f lu sh ed th r o u g h th e camer a th r o u g h o u t th e ex -

p o su r e p e r io d . Becau se o f th e lo n g co ar sen in g t r ea tmen ts ,

th e p r ec ip i ta tes w er e ad eq u a te ly s ized to y ie ld th e d es i r ed

h ig h an g le r eso lu t io n . S tan d ar d ex t r ap o la t io n p r o ced u r es

w e r e u s e d t o m e a s u r e l a t t i c e p a r a m e t e r s t o b e t t e r t h a n

0 .0 0 1 /~ accu r acy , th i s b e in g ad eq u a te to id en t i f y th e n o n -

s to ich io met r ic co mp o u n d s .

IV . E X P E R I M E N T A L R E S U L T S

Fig u r es 1 an d 2 g iv e ex amp les o f th e r aw g as eq u i l ib r a -

t io n d a ta f r o m w h ich th e so lu b i l i ty l imi t s w er e ex t r ac ted .

E a c h p o i n t r e p re s e n t s t h e a v e r a g e o f t w o i n d e p e n d e n t

eq u i l ib ra t io n s an d a t l eas t 8 ca r b o n d e te r m in a t io n s o n p u r e

i r o n samp les u sed to ch eck f o r eq u i l ib r a t io n ( eq u a l to th e

f in a l ca r b o n co n ten ts o f th e Fe- C samp les ) . Po in t s o n each

iso ac t iv ity l in e co r r esp o n d to th e a l lo y co mp o s i t io n s w i th in

th e same cap su le .

Each a l lo y samp le w as meta l lo g r ap h ica l ly ch eck ed f o r

t h e p r e s e n c e o f p r e c i p i t a t e d c a r b i d e s , a n d t h e s a m p l e s

in th e s in g le p h ase an d tw o p h ase r eg io n s co u ld c lea r ly

b e d e l in ea ted .

Th e p o in t a t w h ich each i so ac t iv i ty l in e ch an g es s lo p e

co r r esp o n d s to th e Y /7 + N b Cx eq u i l ib r iu m an d r ep r esen ts

t h e m a x i m u m a m o u n t s o f n i o b i u m a n d c a r b o n t h a t c a n r e -

main in so lu t io n in au s ten i te a t th a t ca r b o n ac t iv i ty b e f o r e

p r ec ip i ta t io n o f N b Cx b eg in s . Th e u n cer ta in t ie s in b o th th e

n io b iu m an d ca r b o n co n ten ts h av e b een u sed to es t ima te th e

e r r o r s in th e t i e l in es . A t each ca r b o n ac t iv i ty , th e p o ss ib le

ch an g es in s lo p es o f th e s t r a ig h t l in es th a t r ep r esen t th e b es t

l in ea r leas t sq u ar es f i t o f th e p o in t s in th e tw o p h ase an d th e

s in g le p h ase r eg io n s h av e b e en tak en in to ac co u n t in a r r iv in g

at the so lubil i ty produ cts ind ic ated in Table I I I . In som e

c a s e s , h o w e v e r , h o r i z o n t a l l i n e s h a v e n e c e s s a r i l y b e e n

d r aw n in th e s in g le p h ase r eg io n c o r r esp o n d in g to th e b in a r y

car b o n lev e l . Th e u n cer ta in ty r eco r d ed f o r each o f th e so lu -

b i l i ty l imi t s r ep r esen ts th e max imu m d ev ia t io n an d th u s

co r r esp o n d s ap p r o x im ate ly to a 9 5 p c t co n f id en ce l imi t .

Th e r esu l t s o f th e ch emica l an a ly s i s o f ca r b id es ex t r ac ted

f r o m s tee l S 1 a re sh o w n in Tab le I V . Th e ca r b id e p r esen t in

th is s teel a t 1000 ~ was ide ntif ied as NbC0.9. Th e so luble

n io b iu m an d ca r b o n w er e ca lcu la ted f r o m th e to ta l amo u n ts

o f n io b iu m an d ca r b o n in th e s tee l S 1 an d th e mean v a lu e o f

n io b iu m p r esen t a s ca r b id e .

z

o

E l

0.24

0.16

0.08

SOLUBILITY LIMITS

-- ISOACTIVITY LINES %NB %C

0 CARSON ACTIVITY = 0.04 38 0.014 + 0.00 5 0.118 + 0.001

V = " = 0.012 7 0.0 34 + 0.0 06 0.0 36 .-1- 0.001

[ ] = 9 = 0.0065 0.052 • 0.008 0.017 • 0.0007

= = = 0.0 04 0.073 + 0.0 09 0.012 +_ 0.0 007

X .

i . . . _ O - - - - - -

----------O-

._._O----------

..---------V-'-'---

I I I I I I I I i I I I I

o . o 8 0 . 1 6 0 . 2 4 o . 3 2 o .1 1 o o . 4 8 0 . 5 6

WT . NIO IUM

Fig. 1--Ca rbon content of Fe -Nb alloys, equilibrated at 1000 ~ Standard state of carbon activity referred

to graphite.

5 4 2 - - V O L U M E 1 5 A M A R C H 1 9 84 M E T A L L U R G I C A L T R A N S A C T I O N S A

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Z

o . 1 6

~ a

SOLUBILITY L IMITS

I S O A C T IV I T Y L I N E S % N B % C

0 . 2 4 O C A R B O N A C T I V I T Y = 0 . 0 2 5 9 0 . 0 6 3 + 0 . 0 0 9 0 . 1 2 2 + 0 . 0 0 2

V '= 9 = 0 . 0 1 9 6 0 . 0 8 7 + O . 01 1 0 . 0 9 7 + 0 . 0 0 2

[ ] 9 9 = 0 . 0 1 5 3 0 . 0 9 8 + 0 . 0 1 3 0 . 0 7 3 + 0 . 0 0 1 4

A - 9 = 0 . 0 0 6 3 O . 2 1 0 + 0 . 0 1 9 0 . 0 3 0 + 0 . 0 0 0 8

~

0 0 8 1 [ ] ~

I i I I I I i

0 0 08 o 16 0 24 0 32 0 40 0 48 0 56

W T . % N I O B I U M

Fig. 2--Carbon content of Fe-Nb alloys, equilibrated at 1200 ~ Standard state of carbon activity referred

to graphite.

T a b l e I I I. S o l u b i l it y P r o d u c t s f r o m E q u i l i b r a t i o n S t u d i e s

Solubi lity Limits Log~o

T~ Pet Nb Pet C [Pct Nb] [Pct C] ~

1000 0.014 0.118 -2 .6 6 • 0.19

0.034 0.036 -2 .7 2 • 0.12

0.052 0.015 -2. 85 • 0.07

0.073 0.010 -2 .8 8 • 0.06

1050 0.057 0.031 -2 .5 6 • 0.06

1100 0.031 0.125 -2 .2 9 • 0.09

0.047 0.064 -2 .3 7 • 0.08

0.091 0.032 -2 .3 4 • 0.05

0.110 0.024 -2 .3 5 • 0.06

1150 0.110 0.035 -2 .2 3 • 0.05

0.185 0.020 -2 .21 • 0.06

1200 0.063 0.122 -2 .0 0 • 0.07

0.087 0.097 -1 .9 4 • 0.06

0.098 0.073 -2 .0 0 • 0.06

0.210 0.030 -2 .0 0 • 0.05

1250 0.180 0.065 -1 .7 8 • 0.05

The results of X-ray analysis of the precipitates extracted

from steels S 1 and 2, held at 1000 ~ are giv en in Table V.

The lattice parameters of the precipitates are also given

along with their respective uncertainties.

The lattice pa rameter o f the precipitate in steel S 1 was

compared with the results of Kempter et al. 29 and identified

as NbC0.9. The data of Brauer and Lesser 3~ have bee n uti -

lize d to identi fy the carbonit ride in steel 2 as NbC09~No 04.

T a b l e V . X - R a y A n a l y s i s o f P r e c e p i t a t e s

Extrapolated

Steel Lattice Parameter Compound

S1 4.465 -- 0.001 ,~ NbCo.9

2 4.469 • 0.001 A NbCo.glNoo4

Z

O

E l

tw

<

o

t i I I I I I I I I

0 . 1 6 T E M P E RA T U R E 1 0 0 0 0 C

q / - - - A U S T / A U S T . + N I O B I U M C A RB ID E

PH SE BOUND RY C LCUL TED

FROM THERMOCHEMIC L D T

0 12

I / 1 ~

EXPERIMENT L

9 T H I S S T U D Y ( E Q U IL I B R A T IO N )

[ ] J O H A N S E N E T A L ( E Q U I L IB R A T I O N )

Z ~ S M I T H ( E Q U I L I B R A T I O N )

9 T H I S S T U D Y ( C H E M I C A L )

M O R I E T A L ( C H E M I C A L )

0 . 0 8 ~ / \ V R E IS T A D E T A L ' ( C H E M I C A L 1

\ ~ O D E K A Z I N C Z Y ( H A R D N E S S )

i \ \ E T A L

\ ~ \ \ \ T + N b C x

0 . o 4 \ *'~'G~X

k

0 . 0 4 o . o 8 o . z 2 o . z 6

W T . % N IO B I U M

Fig.

3 - - 7 / 7

NbCx phase boundary at 1000 ~

T a b l e I V . T h e D e t a i ls o f R e s i d u u m A n a l y s i s fo r S t e e l S l

Temp. of austenitization, ~

Holding time (hours)

Wt pet Nb in solution in austenite (total Nb-fixed Nb)

Wt pet C in solution (total pet C-fixed Nb

0.87 12.01/92.91)

Log[pet Nb] [pet C ] 0 8 7

950 1000 1050

120 100 94

0.045 0.065 0.085

0.015 0.017 0.019

-2.93 -2.73 -2.57

6900

-- - • 0.12

T

1Oglo K = 2.7

1100 1150 1200

48 48 12

0.128 0.158 0.180

0.024 0.028 - -

--2.30

--2.15

METALLURGICALTRANSACTIONSA VOLUME 15A, MARCH 1984--543

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0 . 1 6

g 0 , 12

0 . 08

~ 1 ~ . . . . . .

I | 1 o

I ~ TEMPERATURE 1100 C

I ~ [ AUST. /AUST.+NIOBIUM CARBIDE

| ~ ~ - - - PHASE BOUNDARYCALCULATED

l \ ~ - - - FRO M THERM O CHEM ICALDATA

i / ~ EXPERIMENTAL

i ~ 9 T H I SSTUDY ( EQ UI L I BRATI O N)

! \

~ D

K O Y A M A

T I

(EQUILIBRATION)

~ o ~ ~ S M , T H E O U , L ,B . A T , O N )

\ \ \ I I TH I S S T U D Y ( C H E M I C A L )

\ \ O M O RIE T A L ( C H E M I C A L )

\ \ . V~ \ \ '~7 NI LSSO N T AL ( CHEM I CAL)

C ~ \ , O D E K A Z I N C Z Y ( H A R D N E S S )

\ 9 \ ET AL

\ . t _ ~ . ~ N x

\ \ 0~ \ \ ~ + N b C x

I

0.0/+ 0.08 0.12 0.16 0.2

W T . % N I OBI U M

Fi g. 4 - -y / 7 N b C ~ p h a se b o u n d a ry a t 1 1 0 0 ~

0 , 2 4

0 . 2 0

0 . ~ 6

Z

0

El

n

O o.za

o , o 8

O.O~,

i I i 1 I I i I I i I

T E M P E R A T U R E 1 2 0 0 ~

L ~ A U S T . / A U S T . + N I O B IU M C A R B I D E

~ - - - P H A S E B O U N D A R Y C A L C U L A TE D

\ ~ - - - F R O M T H E R M O C H E M I C A L D A T A

\ \ EXPER I M ENTAL

\ \ \ 9 T H I S STUDY EQUILIBRATION)

I \ \ / ~ S M I T H ( E Q U I L I B R A T I O N )

\ \ \ ~ , M O R I E T A L ( C H E M I C A L )

\ \ \ , [ ] W IL S O N T A L C H E M ,C A L )

\ ~ 1 ~

\ O

D E K A Z I N C Z Y ( H A R D N E S S )

\ O \ \ \ ET AL

k \

\ \ \ ~ " ~ \ \ , " ~ + N b C x

\ \ \ \

N ~ . ~

}

o o . o ~ 0 . o 8 o . z a o . z 6 o . z o o . 2 ~

W T / , N I O B I U M

Fi g. 5 - - y / y N b C x p h a se b o u n d a ry a t 1 2 0 0 ~

A se lec tion o f y /y + NbCx phase boundar ies ca lcu la ted

from thermochemical data I along with the present experi-

mental information including that of Nillso n e t a l . 3~ and

the reported solubi l i ty data are summarized in Figures 3

through 5.

The average solubi l i ty produc t generated by the results o f

the prese nt equilibration exp erim ents Table III) is

7 9 2 0

logl0[pct Nb] [pct C ] 0 B y = 3.4 - ~ [1]

This is to be compared with an empirical and thermodynam i-

cal ly self-consis tent formula developed by Sharma

e t a l . 32

7019 .5

loglo[pCt Nb ] [pct C] ~ = 2.81 T [2]

The latter, which includes the data leading to [1] is recom-

mended as a rel iable summary formula.

A C K N O W L E D G M E N T S

The generosi ty of Dr. J . Lapointe of the Univers i ty of

Sherbrooke and Dr. T. M. Hoo gendor n of the Research and

Work Labora to r ies, Hoogov ens J jmuiden B . V. , H o l land i s

acknowledged for providing some of the materials used in

this s tudy. Thanks are due to Mr. B. Jam es of the De-

par tmen t o f B iochemis t ry , M cMas ter U n ivers ity fo r p rov id-

ing the photometer faci l i ty and Mr. H. Hodds of the Steel

Com pany of Canada R esearch Centre fo r assis ting in the

chem ical analysis. W e are grateful to Dr. R. C. Sharm a o f

IIT Kanp ur for helpful d iscussions and ass is tance w ith part

of the analysis.

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Trans. A, 1984, vol. 15A, p. 545.

544--VOLUM E 15A, MARC H 1984 METALLURGICAL TRANSACTIONS A