ELSEVIER
Journal of Nuclear Materials 212-215 (1994) 388-392
Neutron irradiation creep in stainless steel alloys
Wolfgang Schiile ‘, Hermann Hausen 2
Commission of the European Union, Institute for Advanced Materials, I-21020 Ispra (Va), Italy
Abstract
Irradiation creep elongations were measured in the HFR at Petten on AMCR steels, on 316 CE-reference steels,
and on US-316 and US-PCA steels varying the irradiation temperature between 300°C and 500°C and the stress
between 25 and 300 MPa. At the beginning of an irradiation a type of “primaty” creep stage is observed for doses up
to 3-5 dpa after which dose the “secondary” creep stage begins. The “primary” creep strain decreases in
cold-worked steel materials with decreasing stress and decreasing irradiation temperature achieving also negative
creep strains depending also on the pre-treatment
of the materials. These “primary” creep strains are mainly
attributed to volume changes due to the formation of radiation-induced
phases, e.g. to the formation of a-ferrite
below about 400°C and of carbides below about 7OO”C,and not to irradiation creep. The “secondary” creep stage is
found for doses larger than 3 to 5 dpa and is attributed mainly to irradiation creep. The irradiation creep rate is
almost independent of the irradiation temperature (Qirr = 0.132 eV> and linearly dependent on the stress. The total
creep elongations normalized to about 8 dpa are equal for almost every type of steel irradiated in the HFR at Petten
or in ORR or in EBR II. The negative creep elongations are more pronounced in PCA- and in AMCR-steels and for
this reason the total creep elongation is slightly smaller at 8 dpa for these two steels than for the other steels.
1. Introduction
The irradiation creep behaviour of many different
austenitic stainless steel alloys has been investigated
during the last twenty years in many different reactors
all over the world. Irradiation creep studies were mainly
performed on 20% cold-worked 316~type stainless steel
materials [1,2]. One investigation is concerned with
AMCR-type materials [3,4]. The irradiation
doses
achieved were often beyond 100 dpa and a “tertiary”
creep stage was also observed. However, most of the
measurements of the creep elongation were performed
for doses larger than 5 dpa and the creep features of
the “primary” creep stage were not noticed. For more
details see the various proceedings of the International
Symposia on Radiation Induced Microstructure [1,2].
’ Professor, Institut fiir Angewandte Physik der Johann Wolfgang Goethe Universitat Frankfurt, Robert Mayer Strasse
2-4, D-60054 Frankfurt am Main 1, Germany.
* Institute for Advanced Materials, 1755 ZG Petten, The
Netherlands.
We investigated the creep behaviour mainly of
AMCR-type steels in the HFR at Petten and also
included in this study the 316 CE-reference steel and
two American steels, namely US-316 and US-PCA, the
compositions of which are very similar to that of the
CE-steel. The maximum irradiation dose achieved in
this study was 9 dpa for some of these materials.
The formation of radiation-induced
phases mainly
in 316 stainless steel alloys was extensively studied in
the past and many different phases were found to be
only stable during irradiation with high-energy particles, e.g. a-ferrite, and x-, E-, 6-, G-, and Laves-phases
[1,2]. The transformation temperatures for the various
precipitates formed during irradiation with high-energy
particles were also determined and temperatures between 400°C and 600°C were found depending on the
composition and additions. The composition of the
carbides changed during irradiation such that these
carbides became enriched in the undersized wmponent atoms of the basic elements, i.e. in nickel in the
316 stainless steel alloys. Thus the composition of the
y-phase changed so that it would not be stable without
high-energy particle irradiation.
0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved
SSDZ 0022-3115(94)00214-9
W. Schiile, H. Hausen /Journal
of Nuclear Materials 212-215
The phase diagram of the manganese-containing
stainless steel alloys was not known below 600°C ten
years ago. It was assumed that the extension of the
y-phase field in these alloys would be very similar to
that of the nickel-containing
stainless steel alloys.
However, it turned out that the y-phase field is very
narrow and that AMCR-type alloys are near to the
a-ferrite- and the a-manganese-phase
boundaries [5].
Both phases are very brittle and even small amounts of
these phases causes brittleness of the materials. cu-ferrite is formed also by annealing the material at 300°C
after cold-working and is dissolved again after very
long anneals at the same temperature. The formation
and dissolution of the a-phase is connected with a
volume change of the materials, i.e. with shape changes
of the specimens. u-ferrite is also formed if nucleation
sites are provided for the materials, e.g. dislocations or
al-phase which is formed on cooling the specimen
below the MS-temperature.
A further complication of the manganese-containing stainless steel alloys with respect to those containing nickel is the formation of the hexagonal martensitic
e-phase at about ambient temperature. This martensitic phase is formed and dissolved without hysteresis
[5]. It is well known that many more radiation-induced
phases are formed in manganese-containing
steels than
in nickel-containing steels [2].
All these phase changes are connected with volume
changes and thus the specimen elongations measured
after irradiation with high-energy particles must be
analysed with respect to volume changes due to phase
formations and due to irradiation creep.
We developed two types of irradiation creep facilities for uniaxial stresses. In the Trieste facility 49
specimens can be irradiated at a time at various stresses
and temperatures. The creep elongations are measured
out of pile at ambient temperatures in hot cells. In
order to check whether the e-phase which is formed in
the manganese-containing
steels during cooling disturbs the measurements of the creep elongations, Crisp,
a second type of irradiation creep facility was developed. In this facility the creep elongation of only three
specimens can be measured at a time in situ during
irradiation.
(1994) 388-392
389
2. Experiments
The compositions of all investigated steel alloys are
listed in Table 1 and given in Ref. [6]. In addition three
model alloys of the AMCR-type were investigated
which, however, did not contain all additions of the
AMCR-type steels but contained carbon, titanium
and/or silicon instead.
The neutron flux density in the irradiation position
in the HFR at Petten was 2 X 1018 me2 s-l corresponding to a displacement rate of 1.7 X low7 dpa s-l.
After one or more reactor cycles the irradiation rigs of
the Trieste facility were taken off and after a cooling
down period of about two months the lengths of the
specimens were measured in a hot cell by means of a
photoelectric incremental linear measuring system. Dimensional changes as small as f 1.5 pm could be
measured on specimens having an actual length of 0.05
pm. This precision is much larger than that obtained in
the Crisp facility in which the elongation changes were
obtained in situ at the temperature during irradiation.
The irradiation temperature of each measuring point
could be determined with a precision of f 10°C and
the applied stresses with a precision of f 15 MPa. This
means each measuring point shown in the figures is
obtained with these uncertainties.
3. Results and discussion
In the following a small selection of the many data
obtained during the last ten years is presented. The
irradiation creep elongations found are usually and
fortunately very small except in a very few cases which
are presented because they can contribute to the understanding of the microstructural changes introduced
on irradiating with high-energy particles. We irradiated
materials in the solution annealed state, after coldworking, and after annealing at various temperatures.
To interpret the irradiation data we are making use of
the results of studies concerning the phase diagrams of
the various steel alloys and of the finding according to
which e.g. a-ferrite phases are formed during irradiation with high-energy particles or according to which
Table 1
The main compositions of steels (wt%)
Designation
C
Mn
AMCR-0033
AMCR-7758
AMCR-7763
CE-316
US-316
US-PCA
0.105
0.062
0.10
0.024
0.06
0.06
17.50
28.6
19.4
1.81
2.0
1.5-2.25
Ni
Cr
Si
12.32
10-14
15-17
10.12
10.0
10.2
17.44
16-18
13-15
0.555
0.87
0.94
0.46
1.0
0.4-0.6
Ti
0.87
0.85
0.2-0.4
390
W Schiile, H. Hausen /Journal
of Nuclear Materials 212-215
(1994) 388-392
1.00
2l
0.80
ZOOMPa,
s
k?
n
5
0.60
, T-
L
‘.
z
A
1=3LO’C
150
O.&O
3LO’C
MPa,T=330’C
looMPa,T~
340-z
0.20
0.05
0.00
3
1
N:“lron3
DoseL
[dpa\
6
1
0
I
2
Neutron
7
Fig. 1. The length changes of specimens of AMCR-7763 are
plotted versus the neutron dose. The specimens were solution
annealed prior to irradiation. The irradiation temperatures
3
Dose
I
5
L
[dpa]
Fig. 2. The length changes of specimens of AMCR-7758
annealed at 700°C for one hour after a solution anneal at
1100°C are plotted versus the neutron dose.
and the applied stresses are listed in the figure.
carbides are dissolved after a solution anneal at 1100°C
and reformed annealing below 800°C with and without
high-energy particle irradiation.
We distinguish between “primary” and “secondary”
creep to characterise phenomena which are known
from creep without irradiation while recognising that
our nomenclature may not be entirely correct.
3.1. Primary creep
We plotted the length changes of five specimens of
a titanium-modified
AIvICR-7763 alloy which were solution annealed prior to irradiation versus the neutron
irradiation dose in Fig. 1. The various stresses and
irradiation temperatures are indicated in the figure. A
huge increase of the length increasing with increasing
applied stress is found for an irradiation dose of 0.25
dpa. Beyond this dose a further increase of the length
with irradiation dose is observed, the rate of which
increases again with increasing applied stress.
The length changes of solution annealed specimens
aged subsequently at 700” are plotted versus the neutron dose in Fig. 2. Only a very small linear increase of
the lengths with irradiation dose is observed.
We attribute the huge length change observed in
the primary creep stage (Fig. 1) to the volume change
due to titanium carbide formation during irradiation.
We believe that the linear increase of the creep elongation with increasing applied stress is due to the
formation of stress oriented carbides.
If the solution annealed material is annealed at
700°C prior to irradiation titanium carbides are formed
and seemingly no “primary” irradiation creep is observed (Fig. 2). The irradiation creep rates are as large
as those measured for the “secondary” creep stage in
the solution annealed materials (Fig. 1).
It is known that the formation of titanium carbides
in steels is connected with large volume changes [2].
AMCR-type steels containing titanium carbides could
not be cold-worked without introducing cracks. We
learned from these results that titanium-modified manganese-containing
steels should not be applied in fusion reactors and that in general only aged steel materials should be used.
The length changes of specimens of AMCR-0033
annealed at 400°C prior to irradiation are plotted versus the neutron dose in Figs. 3 and 4. The total length
changes of the materials annealed at 400°C after the
solution anneal are much smaller than those shown in
Fig. 1. We believe that (i) the solution annealed state
could not be maintained on cooling the large bars of
the AMCR-0033 alloys from which the specimens were
z
o’20
I
0.15
130MPa.
In
:
0
1s 35O’C
9
t
I
6
2
Ncu:ron
0:se
7
[d’pa]
Fig. 3. The length changes of specimens of AMCR-0033
annealed at 400°C are plotted versus the neutron dose.
W. Schiile, H. Hausen /Journal
IJl
O.lOL
of Nuclear Materials 212-215
I
0
1
~e~!Ton
L
D~?se [dpa]
5
6
Fig. 4. The length changes of specimens of AMCR-0033
annealed at 400°C are plotted versus the neutron dose.
cut and that (ii) the volume change due to carbide
formation in AMCR-0033 which does not contain titanium is much smaller than that in AMCR-7763. It is
furthermore very strange that the length changes decrease with increasing irradiation temperature (Figs. 3
and 4). We believe that this feature is connected with
a-ferrite formation, the formation rate of which increases with increasing irradiation temperature.
We further recognize that the creep elongation and
also the creep rate is smaller for 75 MPa than for 50
MPa at the lower irradiation temperature (Fig. 3). This
phenomenon has been frequently observed - we will
come back to this point - and is attributed to stress-assisted formation of a-ferrite. We found that a-ferrite is
formed during irradiation but also without irradiation
if nucleation sites are provided [5].
The length changes of 20% cold-worked AMCR0033 is plotted versus the neutron dose in Fig. 5. We
notice that the magnitude of the “primary” creep stage
O.‘O
0.20
ZSOMPa,T=350’C
0.10
0.00
0.10
0
I
2
Fig. 5. The length changes of specimens of 20% cold-worked
AMCR-0033 are plotted versus the neutron dose.
(1994) 388-392
391
decreases with decreasing applied stress achieving also
negative creep strains. Such observations were observed recently in various AMCR-type alloys [3,4]. The
negative creep strain is a common phenomenon found
in many different materials even without irradiation
with high-energy particles namely when structural
changes develop which are connected with negative
volume changes. It is furthermore found that the
amount of the negative creep strain increases with
decreasing irradiation temperature and with decreasing stress. The negative creep strain is assumed to be
due to a-ferrite formation and the positive creep strains
to the formation of carbides and/or to stress-assisted
alignment of already existing carbides and other compounds or precipitates and last but not least to rearrangement of dislocations. Thus there is a superposition of two (or more) structural changes causing positive and negative volume changes of the materials at a
time. No “primary” creep stage is seemingly found for
irradiation temperatures of 305 and 350°C and for the
respective stresses of 100 and 130 MPa (Fig. 5). We
believe that for these irradiation conditions a more or
less complete compensation of the volume changes due
to the structural changes occurring during irradiation is
obtained. Only a linear increase of the creep elongations is observed with increasing irradiation dose and
we must conclude that the linear increase of the creep
elongation, i.e. the constant creep rate, is still not due
to irradiation creep but due to a superposition of
volume changes introduced by radiation-induced
structural changes.
It seems that the “primary” creep stage ends at
about 0.3 dpa (Figs. 1, 3-5). However, if we check the
curves more carefully we recognize that the “primary”
creep stage often ends at irradiation doses much larger
than 0.3 dpa. If we determine the stress exponent of
the “secondary” creep rate then the stress exponent
1.0 is often only obtained after irradiation doses larger
than 3-5 dpa depending on the treatment of the materials prior to irradiation, on the irradiation temperature, and on the applied stress. We conclude that the
“primary” creep stage is extended up to about 3-5 dpa
and that all creep elongation changes up to this dose
are mainly due to volume changes due to structural
changes introduced on irradiating with high-energy
particles and that the “secondary” creep stage begins
after doses larger than 3-5 dpa.
It is very clear that the positive elongation changes,
i.e. the real primary creep stages, which are observed
without irradiation in pure materials and alloys due to
irradiation hardening is also present in these materials.
However, it is assumed that these changes are small
compared to the volume changes due to structural
changes introduced during irradiation. It is not evident
that the “secondary” creep rates observed during irradiation after doses larger than 3-5 dpa are only due to
392
W. Schiile, H. Hausen /Journal
of Nuclear Materials 212-215
3.2. Secondary
0.20
7
015
IL
:
F
0.10
n
SOblPa.
1=360-c
E
f
005
Gl
:
-I
0.00
0.05 I
0
1
I
2
3
Nculron
L
Dose
5
6
7
0
[dpa]
Fig. 6. The length changes of specimens of AMCR-0033 are
plotted versus the neutron dose. The specimens were irradiated in the as-received state.
irradiation creep alone and that structural changes due
to the formation of precipitates and to the alignment
of structures to the applied stress do not contribute to
the observed creep rates.
In the present work we have shown only results
obtained on AMCR - type alloys. All the other alloys
investigated namely the European and the American
316~type steels, show the same features as the AMCR
alloys. However, there are slight differences from steel
to steel. The negative creep elongation found for the
US-PCA steels is as large as that of the AMCR-type
steels. All the other steels, namely the US-316 and the
CE-reference steels show slightly smaller negative creep
elongations than the AMCR-type alloys.
There is still a further result, which has often been
reproduced and which we have already mentioned
(Fig. 3). In contrast to the usual primary” and “secondary” creep features the creep elongations decrease
with increasing applied stress. An example of this
feature is shown in Fig. 6. We believe that superimposed on a-ferrite formation during irradiation and to
the alignment of precipitates under stress, nucleation
sites for a-ferrite formation are introduced by the
applied stress itself. The number of nucleation sites
increases with increasing stress.
(1994) 388-392
creep
The stress exponent of all materials investigated is
n = 1.0 regardless of the treatments to which the materials were subjected prior to irradiation after an irradiation dose of 3-5 dpa. The same stress exponent is also
found in all previous investigations of stainless steel
alloys [1,61.
We found that the “secondary” creep rates are
almost independent on the irradiation temperature, i.e.
namely an activation energy of 0.132 eV is obtained for
irradiation doses equal to or larger than 5 dpa.
The total creep elongations for a neutron dose of 8
dpa are smaller in US-PCA and in AMCR-type alloys
than in all the other investigated alloys because the
negative creep elongations are larger in these two
alloys than in all the other investigated materials.
The creep elongations measured in the HFR at
Petten or in ORR at Oak Ridge or in the EBR II at
Idaho are approximately the same for the same dpa
number.
References
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1046, STP 955, etc.
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