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A complex carbonitride of niobium and vanadium in 9% Cr ferritic steels
Scripta METALLURGICA et M A T E R I A L I A
Vol. 25, pp. 8 7 1 - 8 7 6 , 1991 P r i n t e d in the U . S . A .
P e r g a m o n Press plc All rights r e s e r v e d
A COIg~LEX CARBOHITRIDg OF NIOBIUN AND VAHADIUN I! 9Z Cr FERRITIC STEELS [. Tokuno", I[. H u e d a ' , E. Oemor[", T. Takeda" and ][. I t o h ' " • • Plate, Bar. Shape & mire Rod Lab., R&D Labs-II, Nippon Steel Corp., Sagazihara 229. Japan. • * Material Characterization Res., Lab.. R&D Labs-I, Nippon Steel Corp., ][awasaki 211. Japan. •** Executive Manager. l~&D Labs-If, Nippon Steel Corp.. Sagamihara 229, Japan. (Received
February
5,
1991)
Introduction It
has
been
considered that small additions of Nb and V have striking
effects
on
the
creep
strength of high Cr ferrltlc steels which are used for elevated temperature services such as boilers, steam generators etc. (I}-(3). Although Nb and V are thought to fore complex precipitates which nay act as obstacles for the dislocation glide, the distribution and morphology of the precipitates have not been clarified yet.
Several examples of simple precipitates of V in low alloy
steels were only reported (4)(5). In this study, the morphology of a complex carbonitride of Nb and V in 9% Cr ferritic steels
was
investigated and the role of the carbonltride on the creep strength was discussed. Experimental Procedure Table I shows the chemical composition of 9% Cr s t e e l examined, s p e c i f i e d in the ASTM as A387Gr.91. Rot r o l l e d to 20== t h i c k p l a t e s were heat t r e a t e d at 1333K/lh/AC (normalized) or 1333E/lh/AC +
1033[/2h/AC ( n o r e a l i z e d - a n d - t e l p e r e d ) .
Precise a n a l y s i s was c a r r i e d out f o r e x t r a c t i o n
replicas
s a l p l e d from both the normalized and the normalized-and-tempered s t e e l s by a 400kV a n a l y t i c a l TEH. Creep r u p t u r e t e s t i n g s were c a r r i e d out f o r the normalized-and-tempered specimens. F o i l s of 3B= diameter
were sampled f r o l the gauge p a r t of ruptured specimens and d i s l o c a t i o n s t r u c t u r e s
ruptured specimens were i n v e s t i g a t e d by TEN. Table 1 Chemical composition of s t e e l used
(wt%)
C
Si
Mn
Cr
Ho
Y
Hb
N
0.09
0.28
0.44
8.93
1.02
0.20
0.08
0.0593
871 0 0 3 6 - 9 7 4 8 / 9 1 $3.00 + .00 C o p y r i g h t (c) 1991 P e r g a m o n Press
plc
of
the
872
COMPLEX
CARBONITRIDE
Vol.
25,
No.
4
R e s u l t s and Discussion Figure
1 shows a microstructure of the as-normalized steel. The basic microstructure of the
as-
normalized steel was martensitic and the average width of laths was 0.2 ~m. of
Figure 2(a) shows an extraction replica of the as-normalized steel and Fig. 2(b) a magnified view the enclosed area in Fig. 2(a). Spherical Nb(C,N) and leaf-like cementite, which were
distributed in the intra-lath region, were observed. and
The average diameter of the spherical
average spacing between the neighbouring Nb(C,N) were 52nm and 502nm, respectively.
energy dispersive spectroscopy
(EDS) of the Nb(C.N)
Nb(C,N) An
X-ray
(arrowed in Fig. 2(b}) is shown in Fig. 2(c).
The
small peak of V in the EDS analysis may be from an embryo of V precipitates which would eventually grow as V-nitride during subsequent tempering (encircled in Fig. 2(b)). After tempering of the normalized steels, peculiar plate-like V-nitride was formed adhering to the spherical Nb(C.N), as shown in Flg. 3(a). Fig.3(b), 3(c), 3(d) and 3(e) show a bright field image, a dark field image, a diffraction pattern and an EDS analysis of a plate-like V-nitride (arrowed in Fig. 3(a)), respectively. The dark field image of Fig. 3(c) was taken by placing the selected-area-aperture
at the (222) reflection of V-nitride (indicated by the arrow
in
Fig. 3(d)).
Average length of the majority of the plates was 180nm. The complex carbonitrides of Nb and V are likely to act as strong obstacles against dislocation glide during high temperature deformation. Fig. 4 shows complex carbonitrides of the spherical Nb(C,N) and the plate-like V-nitride in a foil sampled from the normalized-and-tempered steel which was creep tested at 823K by a tensile stress of 225MPa for 6788.1h. The complex carbonitrides with tangled dislocations (A) and simple spherical Nb(C.N) which did not have tangled dislocations (B) were observed. Goto et al. (6) showed that complex-shaped particles could work as more effective obstacles particles the the
against dislocation glide during plastic deformation at high temperatures than spherical because the local climb of dislocations (7) did not occur for complex particles. Since
complex carbonitride of the present study (Fig. 3) has a constricted part at the joint between spherical Nb(C,N) and the plate-like V-nitride. dislocations which encounter the constricted
part
cannot make the local climb even at high temperatures,
Therefore, this
complex
can give a resistant stress which is equivalent to the simple Orowan stress ( = average spacing of particles, G : rigidity modulus, b : Burgers vector}(8).
carbonitride
O.81Gb/L.
L
:
Figures 5(a) and 5(b) show amounts of Nb (Fig. 5(a)) and V (Fig. 5(b)) extracted as precipitates in the normalized-and-tempered and creep tested specimens at various temperatures ranging from 773K to 923K. Changes of the amounts of the extracted-Nb and -V during creep were very small. About 90% of Nb and 70% of V precpitated, respectively. These results show that Nb and V which were added to 9~Cr steel had strong stability during creep deformation. Concluslon In 9% Cr ferritic steel which contained Nb and V, plate-like V-nitrides were formed adhering to spherical Nb(C,N) during tempering. The complex precipitates of the Nb(C,N) and the V-nitride act as effective obstacles against dislocation glide during high temperature deformation such as since the complex precipitates can trap dislocations.
creep,
References 1
V.K.Stkka
:
Proceedings of Topical Conference on F e r r i t t c AlLoys f o r use
Technologies. Snowbird. Utah. June 19-23. 317 (1983}.
in
Nuclear
Energy
Vol.
25, No.
2.
P. B i l l a r d .
3.
Y. Tsuchida,
4
COMPLEX C A R B O N I T R I D E
873
J.R. Donati, D. Guttmaen, N. Guttmann, S. Licheron and J.C. Van Duysen : i b i d . . R. Yamaba.
E. Tokuno,
E. Rashimoto, T. Osawa and
T. Takeda
:
425 (1983)
Proceedings
of
the
Symposium on the New A l l o y s f o r Pressure Vessels and Pipin¢, sponsored by ASME and the M a t e r i a l 4. 5.
P r o p e r t i e s Council. INC., N a s h v i l l e , Tennessee, June 17-21, 105 (1990). N. Tanino. T.~ishida, T. Ooka and K. Yoshikawa : J. Japan I n s t i t u t e of Metals. 29. 734 (1965). R. Singh and S. Banerjee : S c r i p t a M e t a I l u r g i c a et M a t e r i a l i a , 24 1093 (1990).
6. 7.
S. Goto. K. Mori and H. Yoshinasa : i b i d . . 50, 154 (1986). R.S.W.ShewfeXt and L.M. Broen : Phil. Ma8.. 35, 945 (1977).
8.
A.J.E. Foreman and M.J. Makin : i b i d . ,
t4. 911 (1966).
874
COMPLEX
Fig. 1
CARBONITRIDE
Vol,
25,
No.
4
TEN of a microstructure of as-normalized 9% Cr steel with Nb and V.
Q 5Onto
Nb
Nb
vc,.l
¢
Fig. 2
......
_A_
ENERGY
OF
A
Mo
0
20.48 X-RAY.
keV
TEMs of extraction replica (Fig. 2(a) and 2(b)) and an EDS analysis of a spherical precipitate (Fig.2(c)} of as-normalized steel. Fig. 2(b) and 2(c) are a magnified view of the enclosed area in Fig. 2(a) and EDS analysis of the carbonitride indicated by the arrow, respectively.
Vol.
25, No.
4
COMPLEX C A R B O N I T R I D E
875
V
O J
O
e
Fig3
ENERGY OF X - R A Y . keV
20.48
TEMs of an e x t r a c t i o n r e p l i c a (Fig. 3(a), 3(b), 3(c) and 3(d)) and an EDS analysis of a p l a t e l i k e c a r b o n i t r i d e (Fig. 3(e)) of the normalized-and-tempered steel. Fig.3(b), 3(c), 3(d) and 3(e)
are a BF image, a DF image, a d i f f r a c t i o n pattern and EDS analysis of
the
precipitate
indicated by the arrow in Fig. 3(a), r e s p e c t i v e l y .
Fig. 4
TEN
of
complex c a r b o n i t r i d e s in the s t e e l , creep tested at 823K by
225MPa for
6788.1h.
Complex c a r b o n i t r i d e s with tangled
c a r b o n i t r i d e (B) are observed.
a
dislocations
tensile (A)
and
stress
of
spherical
876
COMPLEX
CARBONITRIDE
Vol.
25, No.
4
I00 ~, .m.
, "~
80 L!
8z3x
"~
i
8Z3K
,
40 :-!
~---C*O
~
-
20 ~ ¢.~ L~
=
80
.
.
.
.
CI 0 ~
F
I
1
10
]
!
I00 RUPTURE TIME
I, 000
10,000
(h)
773K
, "~
80
823K 8 73K
BO
40 -[ c~
II
-1 j
2O rI 0
i" 1
~
L
I0
I
~
/00 RUPTURE TIME
Fig. 5
!
J
/, 000
/0, 000
(h)
Amounts of Nb (Fig.5(a)) and V (Fig.5(b)) extracted as precipitates tested at various temperatures rangln~ from 773K to 923K.
in
specimens,
creep
Vol. 25, pp. 8 7 1 - 8 7 6 , 1991 P r i n t e d in the U . S . A .
P e r g a m o n Press plc All rights r e s e r v e d
A COIg~LEX CARBOHITRIDg OF NIOBIUN AND VAHADIUN I! 9Z Cr FERRITIC STEELS [. Tokuno", I[. H u e d a ' , E. Oemor[", T. Takeda" and ][. I t o h ' " • • Plate, Bar. Shape & mire Rod Lab., R&D Labs-II, Nippon Steel Corp., Sagazihara 229. Japan. • * Material Characterization Res., Lab.. R&D Labs-I, Nippon Steel Corp., ][awasaki 211. Japan. •** Executive Manager. l~&D Labs-If, Nippon Steel Corp.. Sagamihara 229, Japan. (Received
February
5,
1991)
Introduction It
has
been
considered that small additions of Nb and V have striking
effects
on
the
creep
strength of high Cr ferrltlc steels which are used for elevated temperature services such as boilers, steam generators etc. (I}-(3). Although Nb and V are thought to fore complex precipitates which nay act as obstacles for the dislocation glide, the distribution and morphology of the precipitates have not been clarified yet.
Several examples of simple precipitates of V in low alloy
steels were only reported (4)(5). In this study, the morphology of a complex carbonitride of Nb and V in 9% Cr ferritic steels
was
investigated and the role of the carbonltride on the creep strength was discussed. Experimental Procedure Table I shows the chemical composition of 9% Cr s t e e l examined, s p e c i f i e d in the ASTM as A387Gr.91. Rot r o l l e d to 20== t h i c k p l a t e s were heat t r e a t e d at 1333K/lh/AC (normalized) or 1333E/lh/AC +
1033[/2h/AC ( n o r e a l i z e d - a n d - t e l p e r e d ) .
Precise a n a l y s i s was c a r r i e d out f o r e x t r a c t i o n
replicas
s a l p l e d from both the normalized and the normalized-and-tempered s t e e l s by a 400kV a n a l y t i c a l TEH. Creep r u p t u r e t e s t i n g s were c a r r i e d out f o r the normalized-and-tempered specimens. F o i l s of 3B= diameter
were sampled f r o l the gauge p a r t of ruptured specimens and d i s l o c a t i o n s t r u c t u r e s
ruptured specimens were i n v e s t i g a t e d by TEN. Table 1 Chemical composition of s t e e l used
(wt%)
C
Si
Mn
Cr
Ho
Y
Hb
N
0.09
0.28
0.44
8.93
1.02
0.20
0.08
0.0593
871 0 0 3 6 - 9 7 4 8 / 9 1 $3.00 + .00 C o p y r i g h t (c) 1991 P e r g a m o n Press
plc
of
the
872
COMPLEX
CARBONITRIDE
Vol.
25,
No.
4
R e s u l t s and Discussion Figure
1 shows a microstructure of the as-normalized steel. The basic microstructure of the
as-
normalized steel was martensitic and the average width of laths was 0.2 ~m. of
Figure 2(a) shows an extraction replica of the as-normalized steel and Fig. 2(b) a magnified view the enclosed area in Fig. 2(a). Spherical Nb(C,N) and leaf-like cementite, which were
distributed in the intra-lath region, were observed. and
The average diameter of the spherical
average spacing between the neighbouring Nb(C,N) were 52nm and 502nm, respectively.
energy dispersive spectroscopy
(EDS) of the Nb(C.N)
Nb(C,N) An
X-ray
(arrowed in Fig. 2(b}) is shown in Fig. 2(c).
The
small peak of V in the EDS analysis may be from an embryo of V precipitates which would eventually grow as V-nitride during subsequent tempering (encircled in Fig. 2(b)). After tempering of the normalized steels, peculiar plate-like V-nitride was formed adhering to the spherical Nb(C.N), as shown in Flg. 3(a). Fig.3(b), 3(c), 3(d) and 3(e) show a bright field image, a dark field image, a diffraction pattern and an EDS analysis of a plate-like V-nitride (arrowed in Fig. 3(a)), respectively. The dark field image of Fig. 3(c) was taken by placing the selected-area-aperture
at the (222) reflection of V-nitride (indicated by the arrow
in
Fig. 3(d)).
Average length of the majority of the plates was 180nm. The complex carbonitrides of Nb and V are likely to act as strong obstacles against dislocation glide during high temperature deformation. Fig. 4 shows complex carbonitrides of the spherical Nb(C,N) and the plate-like V-nitride in a foil sampled from the normalized-and-tempered steel which was creep tested at 823K by a tensile stress of 225MPa for 6788.1h. The complex carbonitrides with tangled dislocations (A) and simple spherical Nb(C.N) which did not have tangled dislocations (B) were observed. Goto et al. (6) showed that complex-shaped particles could work as more effective obstacles particles the the
against dislocation glide during plastic deformation at high temperatures than spherical because the local climb of dislocations (7) did not occur for complex particles. Since
complex carbonitride of the present study (Fig. 3) has a constricted part at the joint between spherical Nb(C,N) and the plate-like V-nitride. dislocations which encounter the constricted
part
cannot make the local climb even at high temperatures,
Therefore, this
complex
can give a resistant stress which is equivalent to the simple Orowan stress ( = average spacing of particles, G : rigidity modulus, b : Burgers vector}(8).
carbonitride
O.81Gb/L.
L
:
Figures 5(a) and 5(b) show amounts of Nb (Fig. 5(a)) and V (Fig. 5(b)) extracted as precipitates in the normalized-and-tempered and creep tested specimens at various temperatures ranging from 773K to 923K. Changes of the amounts of the extracted-Nb and -V during creep were very small. About 90% of Nb and 70% of V precpitated, respectively. These results show that Nb and V which were added to 9~Cr steel had strong stability during creep deformation. Concluslon In 9% Cr ferritic steel which contained Nb and V, plate-like V-nitrides were formed adhering to spherical Nb(C,N) during tempering. The complex precipitates of the Nb(C,N) and the V-nitride act as effective obstacles against dislocation glide during high temperature deformation such as since the complex precipitates can trap dislocations.
creep,
References 1
V.K.Stkka
:
Proceedings of Topical Conference on F e r r i t t c AlLoys f o r use
Technologies. Snowbird. Utah. June 19-23. 317 (1983}.
in
Nuclear
Energy
Vol.
25, No.
2.
P. B i l l a r d .
3.
Y. Tsuchida,
4
COMPLEX C A R B O N I T R I D E
873
J.R. Donati, D. Guttmaen, N. Guttmann, S. Licheron and J.C. Van Duysen : i b i d . . R. Yamaba.
E. Tokuno,
E. Rashimoto, T. Osawa and
T. Takeda
:
425 (1983)
Proceedings
of
the
Symposium on the New A l l o y s f o r Pressure Vessels and Pipin¢, sponsored by ASME and the M a t e r i a l 4. 5.
P r o p e r t i e s Council. INC., N a s h v i l l e , Tennessee, June 17-21, 105 (1990). N. Tanino. T.~ishida, T. Ooka and K. Yoshikawa : J. Japan I n s t i t u t e of Metals. 29. 734 (1965). R. Singh and S. Banerjee : S c r i p t a M e t a I l u r g i c a et M a t e r i a l i a , 24 1093 (1990).
6. 7.
S. Goto. K. Mori and H. Yoshinasa : i b i d . . 50, 154 (1986). R.S.W.ShewfeXt and L.M. Broen : Phil. Ma8.. 35, 945 (1977).
8.
A.J.E. Foreman and M.J. Makin : i b i d . ,
t4. 911 (1966).
874
COMPLEX
Fig. 1
CARBONITRIDE
Vol,
25,
No.
4
TEN of a microstructure of as-normalized 9% Cr steel with Nb and V.
Q 5Onto
Nb
Nb
vc,.l
¢
Fig. 2
......
_A_
ENERGY
OF
A
Mo
0
20.48 X-RAY.
keV
TEMs of extraction replica (Fig. 2(a) and 2(b)) and an EDS analysis of a spherical precipitate (Fig.2(c)} of as-normalized steel. Fig. 2(b) and 2(c) are a magnified view of the enclosed area in Fig. 2(a) and EDS analysis of the carbonitride indicated by the arrow, respectively.
Vol.
25, No.
4
COMPLEX C A R B O N I T R I D E
875
V
O J
O
e
Fig3
ENERGY OF X - R A Y . keV
20.48
TEMs of an e x t r a c t i o n r e p l i c a (Fig. 3(a), 3(b), 3(c) and 3(d)) and an EDS analysis of a p l a t e l i k e c a r b o n i t r i d e (Fig. 3(e)) of the normalized-and-tempered steel. Fig.3(b), 3(c), 3(d) and 3(e)
are a BF image, a DF image, a d i f f r a c t i o n pattern and EDS analysis of
the
precipitate
indicated by the arrow in Fig. 3(a), r e s p e c t i v e l y .
Fig. 4
TEN
of
complex c a r b o n i t r i d e s in the s t e e l , creep tested at 823K by
225MPa for
6788.1h.
Complex c a r b o n i t r i d e s with tangled
c a r b o n i t r i d e (B) are observed.
a
dislocations
tensile (A)
and
stress
of
spherical
876
COMPLEX
CARBONITRIDE
Vol.
25, No.
4
I00 ~, .m.
, "~
80 L!
8z3x
"~
i
8Z3K
,
40 :-!
~---C*O
~
-
20 ~ ¢.~ L~
=
80
.
.
.
.
CI 0 ~
F
I
1
10
]
!
I00 RUPTURE TIME
I, 000
10,000
(h)
773K
, "~
80
823K 8 73K
BO
40 -[ c~
II
-1 j
2O rI 0
i" 1
~
L
I0
I
~
/00 RUPTURE TIME
Fig. 5
!
J
/, 000
/0, 000
(h)
Amounts of Nb (Fig.5(a)) and V (Fig.5(b)) extracted as precipitates tested at various temperatures rangln~ from 773K to 923K.
in
specimens,
creep