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CONCLUSIONS
i. Being an austeni te-form ing element, nitrogen (up to 0.36%) hardens austenite into
austeni tic-fer ritic steels, but significan tly reduces the amount of ferrite - from 65 to 35%.
The overall effect of this influence of nitrogen is to reduce the strength properties of the
steels under investigation.
2. A reduction in the amount of a-phase in the structure of dual-phase steels with the
introduction of nitrogen is accompan ied by pronounced enrichment of the ferrite with ferrite-
forming elements, primar ily with chromium and molybdenum, and its impoveris hment with austenite-
forming elements. This contributes to the formation of o-phase, which leads to embrittlemen t
of the steel.
LITERATURE CITED
i. N. V. Korolev, V. V. Ryukhin, and S. A. Gorbunov, Spect ral Emis sion Micro anal ysis [in
Russian], Mashinostroenie, Leningrad (1971).
2. N. V. Korolev, G. G. Kolchin, and A. N. Podust, Appara tus for elec tri cal- dis char ge spectral
microan alysis and its use, in: Machine Buil ding - A Progress ive Technolo gy and High Com-
ponent Qual ity [in RussianJ , Tol' yatti (1983), pp. 16-17.
3. R. Scherer, G. Riedrich, and H. Kessner, Die wirkung von stickstoff in austeniti schen
und austenit isch - ferritisc hen chrom- nickel-s tahlen, Stahl Eisen, 62, No. 17, 347-352
(1942).
4. A. G. Alton, An Fe-C r-Mo -Ni si gma phase, J. Met., 6, No. 8, 904-905 (1954).
5. H. T. Shirley, Micr ostr uctu ral char acte rist ics of acid corro sion in 18% Cr, 8-14% Ni,
3% Mo steels , J. Iron Steel Inst., 174, No. 3, 242-249 (1953).
6. J. J. Gilman, Harde ning of hig h-c hro miu m steels by sigm a-ph ase formation , Trans. Am.
Soc. Met., 43, 161-192 (1951).
7. I. Ya. Sokol, Dual-P has e Steels [in Russian], M etall urgiy a, M osc ow (1974).
SOLUBILITY OF TITANIUM AND NIOBIUM CARBIDES IN
HIGH-CHROMIUM FERRITE
N. A. Gorokhova, V. I. Sarrak,
and S. O. Suvorova
UDC 668.15'26'71:539.67:661.665
Steels alloyed with ch romium and aluminum are widely used for high-temperatu re operations.
The stability of these steels depends heavi ly on their carbon content [1-3], and deteriorates
as it increases. Alloyi ng of type Khl5Yu5 steels with such carbide-for ming elements as
titanium and niobiu m leads to the formation of diffic ult-to -dissol ve carbides and nitrides
and to a reduction in the carbon content in the solid solution. Data on carbon content in the
solid solution of these steels are limited.
The purpose of the present study was to investigate the solubility of titanium and
niobiu m carbides in type Khl5Yu5 steels. The chemical compositi on of the steels investigated
is presented in Table i.
The method of investigating the temperatur e depen dence of internal friction (TDIF) was
used to de te rm in e the carbon content in the solid solution. For this purpose, we employed a
reverse torsion pendul um-typ e relax ation unit and wire specimens 0.8 mm in diameter; the
vibrati on frequency was 1H z. All specimens were prequench ed in water from 1250°C (holding
for 1 h) to produce a similar grain size, and then quenched repeatedly from different temper-
ature s -- from 1350°C to 550°C (after each 100°C). The TDIF curves of the steels that we in-
vestigated, which were prequenched from 1350°C in water, are presented in Fig. i. In con-
formity with the results of a theoretica l investiga tion conducted earlier [4], the maximu m
observed on these TDIF curves at 260...380°C is the Snoek peak, and is governed by
I. P. Bardin Central Scientific -Rese arch Institute of Ferrous Metallurgy. Translate d
from Metall oveden ie i Termich eskaya Obrabotka Metallov, No. 4, pp. 38-40, April, 1986.
276 0026-0673/86/0304-0276512.50 © 1986 Plenum Publishing Corporation
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TABLE i.
Element content,
Melt
Nb
C Cr AI [ Ti
0 03 15,1 4,9 [
0,06 14,6 4,6 0~-40
0 03 17 2 5 5 [ 0 28
0,04 15,5 5,5
0,03 14,8 5,5 0736
0,24
Q-7.IO4 ,,
8o
I
i \
20
~
0 100 200 300 ~QO~c,
Fig. i. Tempera ture dependen ce
of internal friction, obtained
after quenching specimens from
1350°C. Melt numbers are indi-
cated near curves
the presence of carbon atoms in the solid solution (high-chr omium ferrite); in this case, the
height of this maxim um is proporti onal to the carbon content in the solid solution. The
presence of nitrogen in the steels investigated did not affect the shape of the TDIF curves,
since virtually all the nitrogen is bonded in di fficul t-to-d issolve alumin um nitrides or titani
or niobiu m carbides, and its content in the solid solution is negligible. Proceeding from th
solu bili ty p rodu ct of alumi num nit ride [5] for a total A1 content of 5% in the steel, it is
possible to calculate the amount of nitrogen in the solid solution using the following
equation:
7500
log
~ |AI] [NIl ----T--- 1 , 4 8 , I
wher e L is the solubi lity product, and T is the temper atur e in °K.
The nitroge n content in the solid solution is 2.5 10-4% at 1200°C. This amount of nitro
gen does not affect the shape of the TDIF maximum.
The results of phase analysis conducted on type Khl5Yu5 steels after holding at 780°C
for 20 min (cooling in water) are presented below.
Steel
Khl5Yu5
KhI5Yu5T
KhI5Yu5TB
Excess phases
(Fe, Cr)TC s
(Fe, Cr)TCa; TiC, TiN
(Fe, Cr)TCs; (Ti, NB) (C, N)
Carbon-solubility curves, which are plotted for the type Khl5Yu5 steels alloyed with
titanium and niobium and niobiu m jointly with titanium, are presented in Fig. 2. The intro-
duction of up to 0.47% and 0.28% Ti in the steel with 0.06 and 0.03% C, respectively, leads
to a reduction in carbon solubility as compared with its solubility in the unalloyed steel
to 0.012...0. 015% at I150°C. For a combined alloying of 0.36% Ti and 0.24 % Nb and
a total carbon content of 0.03% in the steel, the carbon solubility is reduced to 0.03% at
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. / /
, . ; p t / .
71
~ i
i
i
3
~
7
6 ~
0 o,07 o ,02 o,03 ,o,o~ o,05 Cso.s
Fig. 2.
log L
-I
-2
-
I
/ e
7,0 6,f 70 /T, o K_ 1
Fig. 3.
Fig. 2. Carbon content in solid solution of steels with
different alloying additives. Melt numbers are indicated
near curves.
Fig. 3. Temper ature depend ence of solubility product of
titaniu m and niobi um carbides: i, 3) data from Tomilin
and Shor [6]; 2, 4) data obtain ed by authors.
I150°C. It must be considered that according to phase-ana lysis data, complex titanium and
niobium carbonitri des, the thermody namic properties of which differ from similar properties
of simple titanium carbides and nitrides, form in steel containing titanium and niobium.
The lowest carbon solubility (approxima tely 0.0015%) at I150°C is characteris tic for the
0.03% C steel alloyed with niobium. It is apparent from the carbon- solubil ity curves obtained
(Fig. 2) that a reduc tion in the overall carbo n cont ent in tbe steel from 0.06 to 0.03% and
also a certain increase in titaniu m content shift the solubility curves into the region of
lower carbo n conc entr atio ns (Fig. 2).
Let us calculate the solubility product of the titanium and niobium carbides contained
in Cr-A1 steels. It is possible to determin e the carbon content from the solubilit y curve
(Fig. 2), and then calculate the titanium and niobiu m concent ration in the solid solution at
1150, 1250, and 1350°C. The temperatu re de pendence of the solubility pr oduct of the titanium
and niobium carbides for steels 2 and 4 is presented in Fig. 3.
For convenience of calculat ion and comparison of the carbide solubility, the content
of elements is expressed in atom percents. Data on the solubility of titanium and niobium
carbides in austenit e [6] are presented here, however, since there are no data on the solu-
bility of similar carbides in ferrite in the literature.
It is poss ible on the basis of the data obta ined (Fig. 3) to deriv e equat ions descr ibin g
the temperatu re depende nce of the solubilit y products. For the titanium carbide contained
in steel KhI5Yu5T (melt 2) with 0.06% C, for example, the temperature dependence of the
solubility product is as follows:
825O
l p g L [ T i ] [ c ] = - - T 4,11, 2 )
For the niobium carbide contained in steel KhlSYu5B (melt 5) with 0.03% C, this relationship
is described by the equation
95O0
log LlN bl [C| ---- - ~ + 3, 4, 3 )
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A c c o r d i n g t o N a r i t a [ 7] a n d G o l d s c h m i d t [ 8 ] , t h e s o l u b i l i t y p r o d u c t s o f t i t a n i u m a n d
n i o b i u m c a r b i d e s i n a u s t e n i t e c a n b e d e s c r i b e d b y t h e f o l l o w i n g e q u a t i o n s :
I 0 4 7 5
l o g L I T i ] I t ] = - - ~ + 5 , 3 3 4 )
( f o r s t e e l w i t h 0 . 1 % C a n d 0 . 0 3 . . . 0 . 1 0 % T i ) , a n d
7 7 0 0
l o g L IN bl [C ] ---- - - ~ + 3 ,1 8 5 )
( f o r s t e e l c o n t a i n i n g u p t o 0 . 1 % C a n d u p t o 0 . 8 % N b) .
I t s h o u l d b e n o t e d t h a t a c c o r d i n g t o E q s . ( 2 ) - ( 5 ) , s t r a i g h t l i n e s e x i s t i n t he r e g i o n
o f d i f f i c u l t - t o - d i s s o l v e c a r b i d e s i n c o n f o r m i t y w i t h t h e c l a s s i f i c a t i o n p r e s e n t e d b y T o m i l i n
a n d S h o r [ 6 ] .
C O N C L U S I O N S
I . T h e i n t r o d u c t i o n o f n i o b i u m ( 0 . 7 3 % ) in 0 . 0 3 % C s t e e l l e a d s t o a r e d u c t i o n f r o m
0 . 0 3 4 t o 0 . 0 0 3 % i n c a r b o n s o l u b i l i t y a t I 1 5 0 ° C , w h e r e a s u p t o 1 . 1 3 % C m a y b e c o n t a i n e d i n t h e
s t e e l a t t h e t e m p e r a t u r e i n d i c a t e d w i t h o u t a l l o y i n g a d di t i v e s . W i t h t h e i n t r o d u c t i o n o f
t i t a n i u m , t h e c a r b o n s o l u b i l i t y a l s o d e c r e a s e s a n d a m o u n t s t o 0 . 0 1 5 % a t I 5 0° C w i t h a t o t a l
c a r b o n c o n t e n t o f 0 . 0 6 % i n t h e s t e e l .
2 . T h e s o l u b i l i t y c u r v e i s s h i f t ed i n t o t h e r e g i o n of h i g h c a r b o n c o n c e n t r a t i o n s w i t h
i n c r e a s i n g t o t a l c a r b o n c o n t e n t i n t h e s t e e l a l l o y e d w i t h t i t a n i u m o r n i o b i u m .
3 . A c c o r d i n g t o r e s u l t s of t he c o m p u t a t i o n o f t h e s o l u b i l i t y p r o d u c t s o f t i t a n i u m a n d
n i o b i u m c a r b i d e s i n h i g h - c h r o m i u m f e r r i t e i n t h e I 1 5 0 . . . 1 3 5 0 ° C i n t e r va l , n i o b i u m c a r b i d e i s
m o r e s t a b l e t h a n t i t a n i u m c a r b i d e .
L I T E R A T U R E C I T E D
i . I . I . K o r n i l o v , I r o n A l l o y s [ i n R u s s i a n ] , V o l . i , I zd . A k a d . N a u k S S S R , M o s c o w ( 1 9 4 5 ) .
2 . E . G u d r e m o n , S p e c i a l S t e e l s [ in R u s s i a n ] , V o l. I , G o s u d a r s t v e n n o e N a u c h n o - T e k h n i c h e s k o e
I z d. L i t e r a t u r y p o C h e r n o i i T s v e t n o i M e t a l l u r g i i , M o s c o w ( 1 9 5 9 ) , p p . 8 8 9 - 8 9 9 .
3 . A . V. R y a b c h e n k o , A . I . M a k s i m o v , a n d B. I . B e k e t o v , " E f f e c t o f c a r b o n o n t h e h e a t r e s i s -
t a n c e o f s t e e l K h I 3 Y u S , " Z a s h c h . M e t . , 1 2, No . 4 , 4 6 5 - 4 6 9 ( 1 9 7 6 ) .
4 . I . A . T o m i l i n , V . I. S a r r a k , N . A . G o r o k h o v a , e t a l ., " N o n u n i f o r m d i s t r i b u t i o n o f c a r b o n
a t o m s i n i r o n - c h r o m i u m a l l o y s , " F i z . M e t . M e t a l l o v e d . , 5 6 , N o . 3 , 5 0 1 - 5 0 6 ( 1 9 8 3 ) .
5 . T . G l a g m a n a n d F . B . P i c k e r i n g , " G r a i n - c o a r s e n i n g of a u s t e n i t e , " I r o n S t e e l I n s t . , 2 0 5 ,
6 5 3 - 6 6 4 ( 1 9 6 7 ) .
6 . I . A . T o m i l i n a n d F . I . S h o r , " C a r b i d e a n d n i t r i d e s o l u b i l i t y o f t r a n s i t i o n m e t a l s i n
i r o n a l l o y s , " i n: P r o b l e m s o f M e t a l S c i e n c e a n d t h e P h y s i c s o f M e t a l s [ i n R u s s i a n ] ,
M e t a l l u r g i y a , M o s c o w . ( 1 97 2 ) , p p . 9 9 - 1 0 6 .
7 . K . N a r i t a , " S t u d i e s o n e l e m e n t s o f s m a l l q u a n t i t i e s i n i r o n a n d s te e l . V I I. O n t i t a n i u m
c a r b i d e i n i r o n a n d s t e e l , " J . C h e m . S o c. J p n . , 8 0 , 2 6 6 - 2 6 9 ( 1 9 5 9 ) .
8 . H . J . G o l d s c h m i d t , I n t e r s t i t i a l A l l o y s , B u t t e r w o r t h s (1 9 6 7 ) .
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