ISSN 0031_918X, The Physics of Metals and Metallography, 2010, Vol. 110, No. 5, pp. 501–506. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © O.P. Maksimkin, M.N. Gusev, 2010, published in Fizika Metallov i Metallovedenie, 2010, Vol. 110, No. 5, pp. 524–529.
STRENGTH
AND PLASTICITY
Deformation_Plastic Behavior of Highly Irradiated Stainless
Steels 12Kh18N10T and 08Kh16N11M3 at Cryogenic
and Elevated Temperatures
O. P. Maksimkin and M. N. Gusev
Institute of Nuclear Physics, ul. Ibragimova 1, Almaty, 050032 Republic of Kazakhstan
Received February 26, 2010
Abstract—In the temperature range from –115 to +120°C, mechanical tests of samples of austenitic
stainless steels 12Kh18N10T and 08Kh16N11M3—materials for the casings of spent fuel assemblies of
a reactor BN_350 irradiated by neutrons to damaging doses of 11–55 dpa—have been carried out. In the
range of cryo_ genic temperatures, in these highly irradiated steels there was discovered and
investigated a new phenomenon— a “wave of plastic deformation,” which consists in the initiation,
development, and propagation of a deforma_ tion band over the length of the sample, which leads to a
possibility of reaching total relative deformation of 20% and greater, instead of 3–5% usually observed at
given damaging doses. The role of a martensitic γ
α' trans_ formation in the formation of the “wave” and in the
improvement in the mechanical properties of the metastable steel irradiated by neutrons is discussed.
Keywords: stainless steel, neutron irradiation, plasticity, martensitic γ
DOI: 10.1134/S0031918X10110104
INTRODUCTION
At present, there exists a commonly accepted idea
that the irradiation of austenitic chrome–nickel steels
by neutrons under conditions of a nuclear reactor
almost unavoidably leads to an irreversible reduction
in their plasticity—to radiation embrittlement. Indeed,
the practice of mechanical tests of different irradiated
metallic materials (not only steels) confirms this
concept on a large quantity of experimental data [1,
2]. However, in certain cases (see, e.g., [3, 4]) there
are observed anomalously high values of plasticity of
steels irradiated even to high damaging doses. In par_
ticular, we discovered such an effect in [5, 6] in the
course of post_exploitation study of materials of the
reactor core of a fast reactor BN_350, which have
been working in poorly studied ranges of
temperatures, neutron fluxes, and rates of
accumulation of the dam_ aging dose. It was shown
that a comparatively high plasticity of the highly
irradiated (to 55 dpa) stainless steel was connected
with the specific features of the behavior of the
samples at the stage of the localization of
deformation: the resultant macroneck is developed by
the sequential (“relay_type”) formation and devel_
opment of several micronecks, which adjoin the initial
neck.
With increasing tension, the localized_deformation
region is enlarged due to the propagation of deforma_
tion into the undeformed sections; i.e., here, the
mechanism of an increase in plasticity [7] known for
TRIP steels is realized. The development of deforma_
501
α transformation
tion in such a band leads to the nucleation of
marten_ site formations, which are stronger than
the austenitic matrix; this strengthens such local
regions of the sam_ ple, and deformation will be
developed into adjacent regions, strengthening
them, etc. In our case, it was possible to speak of
the origin and displacement of a “wave” of plastic
deformation in whose front there is developed a
martensitic γ
α' transformation, which is characterized
by a kinetics differing from the kinetics of the diffusionless phase
transition in the unirradiated steel [8].
Taking into account the temperature sensitivity of the
martensitic transformation and the fact that with
decreasing testing temperature the quantity of the α'
phase arising during deformation increases, it was of
interest to investigate the possibility of the formation of
the “wave of deformation” and its role in the formation of
the plasticity of metastable steels irradiated to 10 dpa
and greater at cryogenic and elevated temperatures.
EXPERIMENTAL
We investigated samples of stainless austenitic steels
12Kh18N10T
(12Cr18N10Ti)
and
08Kh16N11M3
(08Cr16Ni11Mo3) that were cut out of the hexahedral
casings of the spent fuel assemblies of a BN_350
reactor; their elemental compositions are given in Table
1. The flat samples of the steels for the mechanical tests
with dimensions of 20 × 2 × 0.3 mm (with a length of the
gage part equal to 10 mm) were made in the “hot”
chamber, by cutting out fragments of the wall of a
502
MAKSIMKIN, GUSEV
Table 1. Chemical composition of stainless steels under investigation (wt %)
Element
Steel
C
08Kh16N11M3
12Kh18N10T
Ni
0.08 11–12
0.12 10.7
Cr
Mn
15–17
17.0
1–2
1.7
Mo
Ti
Si
–
0.5
2.5–3
–
0.5–1
0.34
B
0.005
–
S
P
Fe
0.02
0.01
0.01
0.03
For balance
For balance
Table 2. Parameters of the irradiation of the investigated samples of steels 12Kh18N10T and 08Kh16N11M3
Assembly
Steel
Mark (distance from the center
of the core), mm
Irradiation temperature, °C
Damaging dose, dpa
–300
290
13.2
TsTs_19
''
+500
423
26
TsTs_19
''
–160
330
55
N_214(1)
''
0
337
17
–500
302
11
12
N_42
12Kh18N10T
V_300
08Kh16N11M3
V_337
''
–500
305
N_214(2)
''
–900
281
,σ MPa
hexahedral casing from different “marks”—sites located
at different distances from the center of the core. In a
number of cases, samples with a length of the gage part
of 6–7 mm were also used. The parameters of the
irradiation of the samples investigated are given in Table
2. Before the irradiation, the steels were sub_ jected to a
thermomechanical treatment, which con_ sisted in cold
deformation to 15–20% with a subse_ quent annealing at
800°С for 1 h.
1100
1000
900
800
700
600
500
400
300
200
100
0
1
3
4
2
1.27
The mechanical tests of the irradiated and unirra_
diated samples (for the uniaxial tension) at a strain
rate of V = 8.4 × 10–4 s–1 (0.5 mm/min) were carried out
on an Instron_1195 machine additionally equipped
with pneumatic grips and a temperature chamber,
which makes it possible to carry out experiments in
the range of temperatures from –120 to +300°С. For
studying
the
specific
features
of
the
deformation_plastic behav_ ior of the highly irradiated
steel, the method of “digital marker extensometry” was
employed [9]. The use of this method, which includes
a digital filming of a min_ iature continuously deformed
irradiated sample, makes it possible to trace the
development of the local_ ized deformation and to
calculate true_stress–true_ strain dependences.
After the termination of tension, the content of
the magnetic α' phase was measured in the sample
with the aid of a ferroprobe and the distribution of
this phase along the working length was analyzed.
ini
RESULTS AND DISCUSSION
10
20
30
40
ε, %
50
60
Fig. 1. Engineering diagrams for the unirradiated and
irra_ diated samples of steels 12Kh18N10T and
08Kh16N11M3: (ini) unirradiated steel 12Kh18N10T
after austenitizing (1050°С); (1) steel 08Kh16N11M3,
V_337 (mark, “–500”; dose, 12 dpa); (2) steel
08Kh16N11M3, N_214(2) (mark, “–900”; dose, 1.27
dpa); (3) steel 12Kh18N10T, TsTs_19 (mark, “–160”;
dose, 55 dpa); (4) steel 12Kh18N10T, TsTs_19
(mark, “+500”; dose, 26 dpa).
Phenomenon of the “Wave of Deformation”
in Highly Irradiated Steels
Figure 1 displays engineering diagrams obtained
in uniaxial tensile tests of the unirradiated and
irradiated samples. As follows from the figure, the
unirradiated stainless steel is characterized by a
comparatively high plasticity and a significant
capability for strain harden_ ing: its ultimate
strength 2.5–3 times exceeds the yield stress.
Upon irradiation, the yield stress grows with
increasing damaging dose, and the plasticity is
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DEFORMATION_PLASTIC BEHAVIOR
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σ, dσ/dε
2.200
5
2.000
1.800
4
1.600
1
.
4
0
0
1
.
2
0
0
1
1
2
3
4
5
6
7
Fig. 2. Films taken during the deformation of the sample
of steel 12Kh18N10T irradiated to 55 dpa (TsTs_19,
“ 160 mm”). White rounded elements, markers (drop_
lets of a coloring substance), which facilitate the analysis
of images and conducting calculations [9]. Arrows near
photos 5–7 note the second (immobile) neck near one of
the grips. The images were subjected to a digital processing
for increasing contrast.
2
1.000
800
3
600
400
200
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
ε
Fig. 3. True_stress–true_strain (σ–ε) diagrams for
reduced, being approximately 3–7% at 5–15 dpa [10].
This fact is illustrated by curves 1 and 2 in Fig. 1. It is
also seen from the figure that the highly irradiated
(to 12 dpa) steels 12Kh18N10T and 08Kh16N11M3
have low plasticity; deformation occurs locally and,
according to the data of videofilming, almost
immedi_
ately after the yield stress is reached, there is developed
a neck [9, 10]. This result agrees with the data of other
authors, who investigated the effects of radiation on
analogous steels [11]. Taking into account these data,
we expected that the plasticity for steel 12Kh18N10T
irradiated to doses 26–55 dpa will be no more than a
few percent and that in the highly irradiated sample a
region of localized deformation (neck) will be formed
immediately after the yield stress is reached. However,
the experiments showed a considerably larger
plastic_
steel
12Kh18N10T irradiated to (1) 55 dpa, (2) 26 dpa and
(3) unirradiated. The dσ/dε [MPa]–ε [rel. units] curves for
the samples irradiated to (4) 55 dpa and (5) 26 dpa are also
shown.
It is obvious that to “decelerate” the development
of a neck and to prevent the premature fracture or, as a
minimum, the translation of deformation into an
adjacent, undeformed volume, it is necessary that the
law of strain hardening be changed, i.e., that after a
certain degree of deformation the relationship (1)
would cease to be fulfilled. For the unirradiated metals
and pure metals irradiated by neutrons, this, as a rule,
does not occur; the value of dσ/dε diminishes with
increasing ε and the material fails rapidly. To increase
plasticity, it is necessary that a certain process occurr,
which can change the structural_phase state of
ity: to 20% and more, and the engineering diagrams the
material and would ensure additional local
(Fig. 1, curves 3, 4) had long yield plateaus.
strengthen_
An analysis of the photographs that were made ing. Such a process in stainless metastable steel is a
sequentially in the course of experiment (Fig. 2) deformation_induced martensitic γ
α transforma_
showed that in the steel sample irradiated to high
dam_
tion, where the resultant α' phase is somewhat stronger
aging doses (55 dpa) there is observed a “deformation than the γ matrix.
wave” (running neck), which, moving along the
sam_
Figure 3 displays stress–strain curves obtained
ple, passes more than half its gage part. Upon the
using the method presented in [9] (the curves are
pas_
cal_
sage of the “wave,” the local deformation almost culated in the approximation of a uniaxial state of
jumpwise increases (by the value of the “wave
stress). Graphs of derivatives dσ/dε, which character_
ampli_
tude”) and after this—outside of the wave—the
mate_
ize the intensity of the strain hardening, are also
rial practically is not deformed.
shown.
The condition for the appearance and development
It is seen from Fig. 3 that for steel 12Kh18N10T
of localized deformation is, as is known, determined
by an inequality [12]
irradiated to 26 and 55 dpa, almost immediately after
the yield point the magnitude of dσ/dε is reduced to
dσ/dε ≤ σ.
(1)
values less than the operating stresses σ, which, taking
In accordance with condition (1), the localization into account expression (1), leads to the development
of deformation begins when the local strain hardening of a neck. However, at the degree of local deformation
does not compensate the “geometric softening,” of ~20–25% in the σ–ε curve there is observed a
which occurs as a result of the reduction in the area of break, after which dσ/dε grows and its value soon
the sample section upon tension.
becomes more than the working stress σ. Note that an
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MAKSIMKIN, GUSEV
Table 3. Mechanical properties of samples of steels 12Kh18N10T and 08Kh16N11M3 deformed at different temperatures
Steel, assembly, mark
12Kh18N10T N_42, –300 mm
12Kh18N10T N_214(1), 0 mm
12Kh18N10T TsTs_19, +500
mm
12Kh18N10T TsTs_19, –160
mm
08Kh16N11M3 V_300, –500
mm
08Kh16N11M3 V_300, –500
mm
08Kh16N11M3 V_300, –500
mm
08Kh16N11M3 V_337, –500
mm
Dose,
dpa
13
Testing
tempera_
ture, °C
σ
,
0.2
MPa σu, MPa
ε ð, %
Presence of a
“deforma_
ε, %
tion wave”
20
830
1020
<2
5
–40
1030
1110
~1
53
20
970
1100
<2
5
No
–50
980
1120
23
Yes
60
740
850
40
43
Yes*
20
780
930
18
18.5
Yes
120
940
980
~1
<4
No
20
960
1070
20
22
Yes
11
20
970
1100
1.6
4
No
11
–40
1200
1270
1.5
12
No
11
–80
1110
1270
1
7
No
12
–115
1130
1370
27
28
17
26
55
2.1
No
Yes*
Yes*
* Samples with two "deformation waves” are observed see Fig. 4.
can be called a “macrowave” in stainless chrome–
nickel steel irradiated to a high damaging dose.
analogous increase in the rate of strain hardening was
observed in unirradiated steels of this class in the
course of deformation at cryogenic temperatures [12].
Furthermore, a similar phenomenon—local strength_
ening and the formation of a “running neck”—was also
noted for the manganese steels such as Hadfield steel
and for TRIP steels [7]. It is possible to assume that
an increase in strengthening (dσ/dε) reflects an
increase in the rate of the γ
α' phase transition and
leads to a displacement of the deformation zone into the adjacent undeformed
volume.
Let us emphasize a difference of the “wave of
defor_ mation” that is considered in this work for
highly irradi_ ated steels, from similar phenomena
described earlier. This difference is in the fact that
the γ
α' transfor_ mation, which takes place in the wave front, in
highly irradiated steel is characterized by a high intensity of the formation
of the α' phase. This is connected with a developed structure of radiation
defects and, in turn, it influences the velocity of the displacement of the
wave front.
Another important difference is the circumstance
that the amplitude of deformation in the “wave” is
extremely high—in our case the local deformation
reaches 30–35%. For comparison, in the Chernov–
Luders bands (waves) the deformation amplitude on
the yield plateau during the deformation, for example,
of Armco iron is on average no more than 4–6%, and
the waves of deformation in the unirradiated polycrys_
tals (see, e.g., [13]) have an amplitude of deformation
on the order of a fraction of percent or less. This
means that the phenomenon considered in our work
Effect of the Temperature of Testing
on the “Waves of Plastic Deformation”
As is known [14], the occurrence of the γ
α'
transformation is strongly affected by the testing tem_
perature. The lower the temperature, the greater the
amount of α' martensite formed during the deformation in
the metastable chrome–nickel steel. The tempera_ ture
equal to 100°С is maximum for the unirradiated steel
12Kh18N10T; at higher temperatures, no marten_ sitic
transformation occurs during deformation. Taking into
account these facts, we performed a series of exper_
iments with irradiated steel samples in the range of tem_
peratures of tensile tests from–115 to +120°С. The data
obtained are given in Table 3, from which it is seen that
the variation of the damaging dose and temperature of
testing qualitatively changes the nature of plastic defor_
mation and the value of plasticity.
Thus, for steel 12Kh18N10T irradiated to doses more
than 10 dpa, the passage from room temperature of
testing to –40 and –50°С led to a sharp increase in the
plasticity of the material from a few percent (Fig. 5, curve
1) to 20 and 50% (Fig. 5, curves 2 and 3). The data of
the videofilming of the surface of the sam_ ples deformed
in a cryogenic chamber make it possible to assert that in
this case there appears and propagates along the
sample a “wave of deformation” which was absent at 20°С.
A single “wave” appears near the grip and passes
approximately 2/3 of the sample length. Sometimes
(approximately in 20% of cases) after the first wave there
arises a second wave, which starts from the second grip.
In this case, no undeformed space remains in the sample
(see Fig. 4).
Taking into account the results of magnetometry
and the conclusions of the work [4], we can suppose
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DEFORMATION_PLASTIC BEHAVIOR
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Fig. 4. Diagrams of the displacement of the “wave of deformation” in the case of a single wave (to the left) and a
double wave (to the right) in the irradiated samples deformed at cryogenic temperatures (the deformed part is noted
by dark gray; the undeformed part, by white. The directions of the wave displacements are indicated by arrows).
that the reason for the effect noted is the occurrence
of a martensitic γ
α' transformation in the deformed
steel. Its kinetics and degree of influence on the strain hardening have been
studied insufficiently for the highly irradiated material, but nevertheless it is
obvi_ ous that they can differ significantly from those observed in the
unirradiated or weakly irradiated steel. The results obtained attest to the fact
that the low_ temperature stimulation of a “wave of deformation” makes it
possible to remove or to considerably decrease the effect of radiation
embrittlement.
the stimulation of a “wave” is substantially lower (–115°С),
0
,σ MPa
For steel 08Kh16N11M3 (Table 3), the tempera_ ture of
1100
1050
1000
950
900
850
800
750
700
650
600
550
2
1
4
10
20
α' transformation in comparison with 12Kh18N10T.
For the samples irradiated to 55 dpa, with increas_
ing testing temperature to above 20°С, the “wave” (and the
related high plasticity) still is retained at +60°С, but
disappears at +120°С (Fig. 5, curve 4). The total plasticity at
120°С is no more than 4–6%. The fact of the suppression
of the “wave of deformation” at temperatures exceeding
100°С, which is close to the Md temperature for steel
12Kh18N10T, can indicate the key role of the martensitic
γ
α transformation in the development of this phenomenon.
As follows from the above results, the “wave of
deformation” can be both stimulated (by decreasing
the temperature to below 20°С) and suppressed (by
increasing the temperature to above 100°С). Let us
emphasize that the effect of the “wave of deformation”
was observed in steel samples cut out from the walls
of casings of five different fuel assemblies, which have
been exploited in the reactor BN_350 in different
times. Accordingly, it cannot be related to the occa_
sional events caused, for example, by a deviation
from the steel_grade composition. As a “successful
circum_ stance,” which could favor the manifestation
of the effect, the fact can be indicated that the largest
dam_ aging dose (~55 dpa) was accumulated in the
reactor BN_350 at a relatively low temperature of
irradiation (330°C), when the swelling of the material is
absent.
THE PHYSICS OF METALS AND METALLOGRAPHY
30
40
50
ε, %
which is caused by the larger stability of this steel with
respect to the γ
3
Fig. 5. Engineering diagrams of deformation for highly irradiated
steel 12Kh18N10T: (1) assembly, N_42; “mark, "–300 mm”; 13
dpa; testing at 20°С; (2) N_214(1); mark, “0 mm”; 17 dpa; testing
at –50°С; (3) N_42; mark, “ 300 mm”; 13 dpa; testing at –40°С; and
(4) TsTs_19; mark, “–160 mm”; 55 dpa; testing at +120°С.
CONCLUSIONS
The above results of materials_science studies of
the stainless steels 12Kh18N10T and 08Kh16N11M3
carried out at the stage of decommissioning of the fast
reactor BN_350 indicate that the characteristics of
strength and plasticity of the materials exploited in the
core after prolonged irradiation (55 dpa) are very sen_
sitive not only to the parameters of irradiation (tem_
perature, damaging dose), but also to the testing tem_
perature. As a rule, an increase in the testing
tempera_ ture led to a worsening, and a decrease in
temperature, to an improvement in the plastic
characteristics of highly irradiated steels. It has been
experimentally shown that in this case in the sample
there arises and propagates along its gage part a
deformation band (or several bands)—a “wave,” in the
front of which there occurs a deformation_induced γ
α' phase transfor_ mation.
Apparently, during the deformation of highly irra_
diated metastable stainless steels under consideration,
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MAKSIMKIN, GUSEV
at some damaging doses, temperatures, and tensile
strain rates there is achieved an optimum combination
of the parameters of the defect structure, rate of
nucle_ ation of martensite α' phase at radiation
defects, etc., which in total ensures a strengthening
level that com_ pensates for the geometric softening
in the place of localization of plastic flow.
7.
8.
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