Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
N.Shinya et al.
SELF HEALING OF CREEP DAMAGE THROUGH
AUTONOMOUS BORON SEGREGATION AND
BORON NITRIDE PRECIPITATION DURING HIGH
TEMPERATURE USE OF AUSTENITIC STAINLESS
STEELS
N. Shinya1*, J. Kyono1, K. Laha2 and C. Masuda3
1
National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan
2
Indira Gandhi Centre for Atomic Research, Kalpakkam – 603 102, Tamil Nadu, India
3
Waseda University, 2-8-26, Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan
*Phone: +81-29-859-2424,
Fax: +81-29-859-2401,
e-mail: SHINYA.Norio@nims.go.jp
Long time high temperature use of heat resisting steels leads to the premature and low ductility creep fracture by
cavitation. The premature and low ductility creep fracture is caused by the nucleation, growth and coalescence of
creep cavities on grain boundaries. The long time creep fracture properties of heat resisting steels depend mainly
on the growth of creep cavities. As creep cavities are thought to grow by diffusive transport of matter from the
cavity surface to the grain boundaries, the physical properties of the cavity surface are closely connected to the
cavity growth. In this study, the chemical composition of austenitic stainless steels have been modified with
addition of minute amount of elements such as boron and cerium with the aim to alter the physical properties of
the cavity surface by segregation of elemental boron and precipitation of boron nitride onto the creep cavity
surface. Surface chemistry of creep cavities in creep ruptured specimens was analyzed by Auger electron
spectroscopy. On creep cavity surface in the usual austenitic stainless steels without boron and cerium, extensive
sulfur segregation was observed. It is well known that the sulfur segregation enhances the creep cavity growth
remarkably. In the modified steels, the segregation of boron and precipitation of boron nitride were observed on
the creep cavity surfaces. The boron segregation and boron nitride precipitation were thought to alter the
physical properties of the creep cavity surface and to suppress the surface diffusion of creep cavity, since boron
and boron nitride are very stable at high temperatures, whereas the sulfur segregation accelerates the surface
diffusion remarkably. Cerium acted as a getter for soluble sulfur in the steels by the precipitation of cerium-oxysulfide to facilitate the segregation of boron and precipitation of boron nitride. The segregation of boron and the
precipitation of boron nitride reduced creep cavity growth rate substantially and improved long time creep
rupture strength coupled with long time ductility. The boron segregation and boron nitride precipitation onto the
creep cavity surface are autonomously developed during the high temperature use of the modified austenitic
stainless steels, suppressing creep cavity growth. It was considered that the segregation and the precipitation
provided the steels with the function of autonomous self-healing for the creep damage.
Keywords: self-healing, creep damage, stainless steel, creep cavity, boron segregation, boron
nitride precipitation, rupture properties
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
1
N.Shinya et al.
Self-healing materials research in Japan
In Japan the research works on self-healing materials are becoming active and have attracted
considerable attention. The self-healing paint and the self-healing catalyst have been actually
used for automobiles lately.
Among the many research works of the self-healing materials, self-healing structural
materials are especially expected to be developed and turned to practical use. Typical works
on the autonomous self-healing structural materials1-3) are shown in Table 1.
All of the self-healing structural materials in Table 1 can heal the material damage of atomic
to micro sizes without special treatments in the usual environments of the material use.
Although it may take comparatively long time for the self-healing, the material performance
can be recovered due to the healing of the damage. In this report, our research work on the
self-healing heat resisting steels is introduced.
Table 1 Self-healing materials research in Japan
Material
1)
Polymer
(PPE, PC)
2)
Ceramic
(Si3N4, Mullite)
Metals3)
(321, 347 steels)
Material damage
Healing environment
Chains are recombined by eliminating
two protons from the ends with change
from Cu(II) to
Cu(I) in the polymer
Scission of chain
Room temperature
Oxidation of SiC particles at crack
Surface micro cracks
surface SiC+2O2➙SiO2+CO2
SiO2 heals surface cracks
High temperature
B segregation and BN precipitation at
creep cavity surface suppress cavity
growth
Creep cavities
High temperature
2
Healing reaction
Self-healing heat resisting steels research - creep
cavities leading to creep fracture of heat
resisting steels -
Most of the heat resisting steels exhibit a premature and low ductility creep fracture at
specific range of temperatures and stresses, most of which correspond to a long time rupture
and actual service conditions. 4) The premature and low ductility creep fracture arises from the
formation, growth and interlinking of cavities on grain boundaries. This phenomenon, called
the creep cavitation, is of great technical importance because it often limits either the life or
service conditions of structural components. Figure 1 shows the progress of creep cavitation
during creep in a commercial type 304 stainless steel. The creep rupture life and the ductility
of the steel working at the cavitation range depend on the creep cavity nucleation and growth
rates.
2
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
N.Shinya et al.
Figure 1: Progress of creep cavitation with increasing life fraction consumed for commercial austenitic stainless
steel (SUS304H) at 750℃ and 37MPa
Among the two processes of creep cavitation, cavity growth is the most important because the
nucleation of creep cavities is believed to occur almost completely during the loading of creep
specimens. Figure 2 shows the creep cavity growth mechanism, which is assumed to act in the
usual cavitation range, especially at lower applied stresses and high temperature service. The
cavity growth rate is expected to be controlled by the slower process of either grain boundary
diffusion or the creep cavity surface diffusion. 5)
Figure 2: Illustration of creep cavity growth mechanism wherein atomic transport occurs along the creep cavity
surface and then down the grain boundary.
3
Concept of self-healing of creep cavities
It is known that the creep cavity surface diffusion is influenced by the segregation of trace
elements on its surface. Holt and Wallace6) classified the most common trace elements
according to whether they have detrimental or beneficial effects on the creep rupture strength.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
N.Shinya et al.
Among them, O and S can cause severe embrittlement during creep even at low ppm levels,
whereas B and Zr can cause a beneficial effect. Some trace elements diffuse to grain boundary
and creep cavity surface, and segregate there during high temperature services.
It is known that the soluble S segregates on the creep cavity surface very easily, and the cavity
growth rate increases with the increase in surface diffusion rate. 5)
Because of the low melting point of S (112.8 ○C), the creep cavity surface contaminated with
S becomes very active at high temperatures, and then the surface diffusion rate increases by
several orders of magnitude. 7) When the soluble S is removed almost completely, other
elements such as B and N can segregate onto the creep cavity surface. Segregation of B may
suppresses the surface diffusion at creep cavity surface and retards the cavity growth rate
because of high melting point of B (2080 ○C), and co-segregation of B and N may forms BN
at creep cavity surface. As BN is very stable at high temperatures (melting point: 3000 ○C),
the BN precipitation on the creep cavity surface is expected to suppress the surface diffusion
completely and retard the cavity growth remarkably. The B segregation and the BN
precipitation on the creep cavity surface occur continuously during high temperature use of
the heat resisting steels and heal the creep cavity autonomously. Figure 3 shows the S
segregation in usual steels, and B segregation and BN precipitation in self-healing heat
resisting steels.
Figure 3: Illustration of S an B segregation and BN precipitation on creep cavity surface
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
N.Shinya et al.
4
Trial development of self-healing austenitic
stainless steels
4.1
Self-healing of creep cavities by B segregation in modified 347
austenitic stainless steel
Two 347 austenitic stainless steels of chemical compositions shown in Table 2 were melted in
vacuum arc furnace.
The chemical composition of the standard 347 austenitic stainless steel has been modified
with the addition of 0.07 wt% of B and 0.016 wt% of Ce. Cerium has a strong affinity to S
and O, and removes them by formation of Ce2O2S. The steels were given a solution heat
treatment at 1200 ○C for 20min, followed by water quenching before creep test.
Alloy
C
Si
Mn
P
B-free
steel
0.080
0.59
1.68
0.001
B-added
steel
Table 2: Chemical composition of the melted 347 steels (wt%)
0.078
0.68
1.67
0.001
S
Cr
Ni
Nb
N
B
Ce
0.002 17.96
12.04
0.41
0.077
-
-
0.002 18.15
11.90
0.38
0.072
0.069 0.016
Creep rupture tests were carried out at 750℃. The variation of creep rupture life and ductility
of both steels with applied stress are shown in Figs. 4 and 5.
B-free steel
B-added steel
Applied stress, MPa
100
90
80
70
60
50
40
100
1000
10000
Rupture life, hours
Figure 4: Variation of creep rupture life of the steels with applied stress, tested at 750℃
5
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
N.Shinya et al.
Addition of minute amount of B and Ce had remarkable effects on the creep rupture strength
and ductility of the steel. Creep strength and ductility of the steel increased with addition of B
and Ce, effects of which were more pronounced on longer creep exposure.
35
Elongation, %
30
25
20
15
10
B-free steel
B-added steel
5
10
100
1000
10000
Rupture Life, hours
Figure 5: Variation of creep rupture elongation % with rupture life of the steels, tested at 750
Cavity growth rate, μm / hour
Interrupted creep tests at 78MPa, 750 ○C were carried out on the steels in argon atmosphere to
measure the growth rate of the cavities on the specimen surface by SEM. Figure 6 compares
the growth rate of the creep cavities. Measurements were carried out on several cavities until
they coalescence with each other. Boron addition in the steel decreased the cavity growth rate
almost by an order of magnitude.
10
-1
10
-2
10
-3
B-free steel
B-added steel
100
1000
Time, hours
Figure 6: Variation of cavity growth rate with creep exposure at 78MPa, 750℃ for both the steels
The chemistry of the creep cavity surface in both the steels was examined by Auger electron
spectroscopy (AES). The crept specimens of the steels were fractured by impact loading at
liquid nitrogen temperature in the AES chamber to expose the creep cavity surface. Figures 7
and 8 shows the fracture surfaces with creep cavities on grain boundaries and the Auger
spectra obtained from the cavity surfaces of B-free and B-added steels.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
N.Shinya et al.
Figure 7: SEM micrograph showing creep cavity surface, exposed by breaking at liquid nitrogen
temperature under impact loading, of 347 stainless steel ruptured for 998h at 750 ○C and 69MPa, and Auger
spectrum obtained from the creep cavity surface
Figure 8: SEM micrograph showing creep cavity surface, exposed by breaking at liquid nitrogen
temperature¥ under impact loading, of Mod.347 stainless steel ruptured for 5461h at 750○C and 69MPa, and
Auger spectrum obtained from the creep cavity surface.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
N.Shinya et al.
Presence of S segregation was observed on the cavity surface of B, Ce-free steel (Fig.7)
whereas Auger peak of elemental B, instead of S, was observed in B, Ce-added steel.
The soluble S in the B, Ce-added steel may be removed almost completely by formation of
Ce2O2S. In B-added steel, in the absence of S contamination, most of the nucleated cavity
surfaces were covered with a filmy segregation of elemental B. Boron segregation is expected
to decrease the diffusivity along the cavity surface because of its relatively high melting point
of around 2080 ○C. The suppression of creep cavitation in the B-added steel was observed,
and this is thought to be caused by the segregation of B on the cavity surface. The suppression
may have increased the creep strength associated with increase in creep ductility.
As the segregation of B on creep cavity surface occurs autonomously during high temperature
use of the steel, the segregation provides the steel with self-healing function for creep
damage.
4.2
Self-healing of creep cavity by BN precipitation in modified 321
austenitic stainless steel
A 321 austenitic stainless steel was modified with addition of 0.07 wt% of B, 0.063 wt% of N
and 0.008 wt% of Ce. The results of creep rupture tests carried out at 750 ○C showed that the
addition of minute amount of B and Ce increased creep rupture strength coupled with
ductility, which was more pronounced on longer creep exposure. The creep cavities in the
ruptured specimens of the modified 321 steel are few and fine, compared to those of the usual
304 and 321 austenitic stainless steels.
The crept specimens were fractured by impact loading at liquid nitrogen temperature in the
AES chamber to expose the creep cavity surface. Figure 9 shows the fracture surface of the
modified 321 steel with creep cavities and the Auger spectrum obtained from the cavity
surface. The presence of B and N on the cavity surface was observed, whereas the presence of
S was not observed. The energy positions and the shapes of B and N peaks indicate that the
segregated B and N form a stable compound of BN. X-ray diffraction pattern obtained from
the precipitates of the modified 321 steel after creep rupture test showed the presences of
Ti4C2S2 and Ce2O2S.
Figure 9: SEM
micrograph showing
creep cavity surface,
exposed by breaking
at liquid nitrogen
temperature under
impact loading, of
Mod.321 steel crept
for 10200h
(t/tr=0.86) at 750○C
63MPa, and Auger
spectrum obtained
from the creep cavity
surface
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
N.Shinya et al.
The co-addition of Ce and Ti is highly effective to remove the soluble S in the steel and to
prevent from its depositing on the cavity surface. In the absence of S segregation, the
precipitation of BN film on the cavity surface decreases the cavity growth rate remarkably
and provides the steel with the property of the self-healing effect of creep cavitation. The selfhealing of creep cavitation by BN precipitation increases the creep rupture strength and
ductility of the steel, particularly for long rupture time region.
REFERENCES
(1) K. Takeda, M. Tanahashi and H. Unno, Science and Technology of Advanced Materials, 4(2003), p.435
(2) K. Ando, K. Houjyou, M.C. Chu, S. Kakeshita, K. Takahashi, S. Sakamoto and S. Sato, Journal of the
European Ceramic Society, 22(2002), p.1339
(3) N. Shinya, J. Kyono and K. Laha, Journal of Intelligent Material Systems and Structures, 17(2006), p. 1127
(4) N. Shinya, J. Kyono and H. Kushima, ISIJ International, 46(2006), p. 1516
(5) W.D. Nix, K.S. Yu and J.S. Wang, Metall. Trans. A, 14A(1983), p.563
(6) R.T. Holt and W. Wallace, Int. Met. Rev., 21(1976), p.1
(7) G.E. Rhead, Surf. Sci. 47(1975), p.207
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© Springer 2007