Solubility Product for Niobium Carbide in ustenite V.K. LAKSHMANAN and J. S. KIRKALDY The solubility product for NbC0.87 has been experimentally determined between 950 ~ and 1250 ~ by gas equilibration and extraction methods. The new relation logl0[pct Nb] [pct C] ~ = 3.4 - 7920/T on a weight percent basis has been evaluated from the data. This is in good agreement with a compendium of measurements from the literature. The data do not yet justify an analysis which takes account of a variable stoichiometry. I. INTRODUCTION AS part of a comprehensive thermodynamic examination of the equilibrium of niobium carbonitrides with alloyed austenite I we have experim entally determined the solubility product for NbC0.87 in unalloyed austenite between 950 and 1250 ~ This expands, and to a large extent bridges and confirms, the extensive data set already available in the literature. 2 ~1 The principal methodology involved mutual equilibration of encapsulated Fe-C and Fe-C-Nb alloy samples in hydrogen. Solubility product determina- tions and precipitate identifications were also carried out on two high purity vacuum-melted steels using chemical and electro-chemical methods to isolate the precipitated phases. II. THE GAS EQUILIBRIUM EXPERIMENTAL METHOD The sealed capsule technique, emp loyed by K/illstrom and Om sen, ~2 Heckler and Winchell, ~3 Zupp and Stevenson, ~4 Nishizawa, ~ 5 and Uhrenius and Harvig ~6 was chosen for the present study. This consists of a series of diffusion-isolated Fe-Nb alloys and Fe-C alloys enclosed in an evacuated, H2 back-filled, thick-walled quartz tube, annealed at the desired temperature. Carbon is selectively transferred between the Fe-C and the Fe-Nb alloys which are not in physical contact. The carbon activity obtaining in the capsule is determined from the final carbon content of the binary Fe-C alloy samples via the results of Ban-ya et al.17 The main experimental considerations in this method were: 1) High purity Ferrovac electrolytic iron and ferro-niobium with an oxygen content less than 0.0012W pct has been used in this study Table I). Samples were prepared by melting ferro-niobium and electrolytic iron under an argon atmo- sphere. High purity H2 gas with a very low H20 content has been used in the equilibration. We therefore conclude that the oxy gen contamination of the NbC087 precipitates is negligible. 2) A ccurate temperature control and establishment of a uni- form temperature zone +4 ~ for T < 1100 ~ +-6 ~ for T > 1100 ~ 3) Prevention of leakage of H2 from the thick-walled quartz tube via a 1 atm external H2 bath. v .K. LAKSHMANAN s a Research Engineer with 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) Establishment and assessment of equilibrium with re- spect to carbon content 0.5 mm sheet annealed for times up to 120 hours). 5) Investigation of the possibility of transfer of niobium between the samples or of silicon contamination from the quartz capsule. 6) Carbon analysis Leco conductometric apparatus for low concentrations). Full details are to be found in References 1 and 12 to 16. III. STUDIES ON PRECIPITATES EXTRACTED FROM NIOBIUM BEARING STEELS Effective electrochemical means for isolating precipitates have been discussed by Blickwede and Cohen, ~8 Andrews and Hughes, 19 Walz and Bloom, 2~ and Gurry et al. 2t The principle in all these techniques is the same, viz. anodie dissolution of the heat treated steel specimen under suitable conditions. Mandry and D ornelas z2 have successfully used a citrate cell for extracting carbonitrides of niobium. The ad- vantage of an electrochemical technique is that it is condu- cive to the extraction of large amounts of precipitates in a short time. The small amounts of niobium normally found in steels and the need for an evaluation of the fixed and free niobium to calculate the solubility limits of the interstitial phases has led to the development of chemical dissolution techniques for isolating these phases and accurate procedures for ana- lyzing the residuesfl 3-26 Moil et a1.1~ have made extensive studies on niobium compounds precipitated in steel using a simple chemical dissolution method to extract the precipitates quantitatively. The chemical analysis is by the Kawamura spectrophoto- are available and have been reviewed by Bhargava and Donovan.28 These authors have also given new and accurate procedures for niobium analysis. In the present investigation, a few solubility product de- terminations and precipitate identifications have been made Table I. Compositions of the Fe Nb and Fe C Alloys for Gas Equilibrium Studies 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 METALLURGI CAL RANSACTIONS A VOLU ME15A,MARCH1984--541
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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