ORIG. RUSSIAN: EFFECT OF REACTOR IRRAD. ON STRESS-RUPTURE STRENGTH OF AUST. STEELS AND HEAT-RESISTANT MATERIALS

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• Document Release Date: August 12, 2009
• Sequence Number: 38
• Publication Date: May 1, 1964
• Content Type: STUDY

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Third United Nations
International Conference
on the Peaceful Uses
of Atomic Energy
Confidential until official release during Conference
EFFECT OF REACTOR IRRADIATION ON STRESS-RUPTURE
STRENGTH OF AUSTENITIC STEELS AND HEAT-RESISTANT
MATERIALS BASED ON IRON AND NICKEL
A/CONF. 28/P/339a
USSR
May 1964
Original: RUSSIAN
by E.V.Gusev, P.A.Platonov,
N.F.Pravdyuk, N.M.Sklyarov
INTRODUCTION
Until recently it was believed that irradiation of struc-
tural materials at high temperature should not cause consi-
derable changes of their properties, because it was observed
that in most cases irradiated materials restore their ori-
ginal properties when annealed or tested at elevated tempe-
ratures. However it can be seen from the description of some
experiments that follows that a number of materials when ir-
radiated in the high temperature range exhibited most drastic
changes, which is an indication of appearance in a material
structure of the changes that are difficult to remove.
TEST MATERIALS AND TYPES OF TESTS
Investigation was carried out with six alloys which were
so selected that their composition and structure correspon-
ded to the specific features of heat-resistant and stainless
steels and alloys. In addition, commercial nickel (99.95%)
was also tested. he chemical composition 1 of alloys is
presented in Table 1. Amongst the materials so far tested
were both age-hardenable alloys (XH77IOP, X12H22T3MP,
alloy I, steel II) and alloys similar to homogenious solid
solutions (IXI8H9T , XH60B) .
All the alloys, excluding steel type IXI8H9T, were tes-
ted after usual heat treatment. Steel type IXI8H9T. and nickel
were tested in the delivery stage.
The irradiation of specimens was carried out in the
fuel channels of the reactor EFT with the maximum thermal
neutron flux of 8 x 1013 n/cm2. sec ; the fast neutron flux
being 5,x 1013 n/amt. sec ( > I Mev). Hereafter integral
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radiation fluxes will be indicated only for fast neutrons .
The usual irradiation technique was used, which was described
earlier 2 'j. Studies were made of mechanical properties du-
ring tensile tests , as well as of hardness, creep strength
and microstructure.
EFFECT OF IRRADIATIOI' ON TENSILE STRENGTH
Investigation into the effect of irradiation at 150-200?C
on the tensile strength of heat-resistant steels and alloys
has shown that their mechanical properties, as measured at
room temperature after receiving a comparatively high integ-
rated irradiation dose (up to 1020 n/sq. cm) , vary within the
following limits. The yield stress rose by 10-30 per cent,
relative elongation decreased by 30-40 per cent. Somewhat
unexpected was the decrease in the ultimate tensile strength
of the alloys XH77T I0 P, XI2H22T3MP and XIi60B after irradiation.
Such changes in heat-resistant alloys, which are charac-
terized by a large plasticity margin in the initial state,,
cause no serious fear when they are used at room temperature.
At elevated temperatures, however, quite substantial changes
in their mechanical properties are revealed.
Figs.1 and 2 depict the results of mechanical tests of
irradiated and tinirradiated specimens of the alloys
XH77I1D? and XH6OB as a function of the test temperature. The
irradiation was carried out at 150-200?C, the integrated do-
se being approximately (1-3)x1020 n/sq.cm. It can be seen
that beginning with 500-550?C the mechanical properties of
irradiated materials undergo drastic changes. Particularly
conspicuous is the change in relative elongation which redu-
ced practically to zero in the alloy XH77T 1O P and to 5-7 per
cent in the alloy XH60B.
Tests of irradiated specimens at elevated temperatures
were run with other alloys, too. Thus, for steel X12H22T3MP
we also studied the effect of irradiation on mechanical cha-
racteristics at temperatures up to 750?O (i.e. in the tem-
perature range where alloys of this type are used). There-
suits of tensile tests of irradiated and unirradiated spe-
cimens of steel X12H22T3MP at 20 and 750?C have shown that
at ?50?C plasticity drops abruptly from 16 to 3 per cent
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The ultimate strength and the yield stress of irradiated
specimens decreased much less than those of unirradiated
specimens (by 3-5 and 7-10 per cent, respectively).
Comparing the results of the tests of the investigated
materials, XH77TO P, X12H22T3MP and X11608 one can see that
at high temperatures the nature of the changes in the pro-
perties dub to irradiation is nearly identical for all the
alloys. Somewhat sharper changes were observed in the alloys
XH'J?T 10 P and X12H22T3VPr, The alloy XH77T b P, for instance ,
does not show any noticeable deformation at the test tem-
perature of ?50?C, whereas in the case of the alloy XH64B.
deformation reaches about 7 per cent.
Very interesting results were obtained also during the
tests of commercial nickel irradiated. with a fast neutron
integrated flux of 1.7 x 1020 n/sq.cm at 150-200?C.
Fig.3 displays the results of tensile tests of nickel
before and after irradiation as a function of the test tem-
perature.
The graph of Yig.3 shows that in the case of irradiated
nickel, as well as alloys based on it (see Figs.1 and 2)
a drastic decrease in plasticity is observed in the vicinity
of 600?C. The difference in the plasticity of irradiated and
unirradiated nickel at temperatures above 600?C is particular-
ly conspicuous because the nickel was investigated in a de-
formed state. It can be seen that, beginning with 600?C , the
plasticity of unirradiated nickel increases considerably,
which fact is attributed to the restoration of the original
properties of the deformed material, whereas irradiated nic-
kel exhibits an abrupt drop in plasticity in this tempera-
ture range.
It should be noted that a deterioration in properties
of alloys after irradiation is revealed not only in tests
at high temperatures , but also at room temperature after
heating by 700-800?C. Thus, the results of a room temperatu-
re tensile test of irradiated specimens of the alloy
XH77T10 P before and after I hr. exposure to a temperature
of 7500C in a vacuum. have. shown that in the case of the ir-
radiated alloy, only the yield stress is restored. Ultimate
tensile st0ength and especially relative elongation decrease
(approximately by 10 and 30 per cent, respectively )
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Apart from tensile tests at high temperatures, hot
hardness measurements were made. In Fig.4 we present the re-
sults of the measurements of hot hardness of the alloy
-NW7T Y) P and nickel. as a function of the test temperature.
The alloy XH77T 10 P was tested for hardness in two states:
quenched and then aged, and just quenched. The hardness of
irradiated specimens of the investigated materials at room
temperature is considerably higher that for unirradiated
ones. As the test temperature increases, the hardness of ir-
radiated materials approximates that of unirradiated ones,
and at 600?C their values become practically identical. Thus,
if during tensile tests the difference in the properties of
irradiated and unirradiated materials at high temperatures
sharply increases, hardness measurements at these temperatu-
res yield almost identical hardness values. This discrepancy
indicates that the behaviour of irradiated heat-resistant
materials depends on the stressed state during deformation ,
and in this case no correlation is observed between the hot
hardness and the strength of the materials. This may be asso-
ciated with the fact that the rupture strength of the mate-
rial changes irreversibly while shear strength restores on
heating .
Thus, tensile tests of the investigated alloys at eleva-
ted temperatures d.;;onstrate that irradiation causes consi-
derable changes iL alloys, which are hardly revealed during
tests at room temperature. For these alloys, the changes are
characterized by the fact that in a certain temperature range
(550-800?C) there is an abrupt drop in plasticity which in
this case appears to be a characteristic most sensitive to
irradiation. For some alloys, the drop in plasticity is ac-
companied also by a substantial decrEase in ultimate strength.
EFFECT OF IRRADIATION ON STRESS-RUPTURE STRENGTH
Stress-rupture strength tests were run with all the ma-
terials considered in the preceding section. Besides, such
materials as IXI8H9T , the alloy I (with and without boron)
and steel II were tested.
Comparative tests of irradiated and unirradiated speci-
mens of the investigated materials in the temperature range
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from 60u to 90000 have demonstrated o; considerable decrease
in stress-rupture strength.For some nickel-and iron-base alloys,
the decrease proved to be absolutely catastrophic.
The lowest radiation resistance was revealed in the alloy
XH77T JO Pti Irradiated specimens of the alloy XH77T D P proved
to be so unstable under constant load at 750?C that even at
a stress of 8 kg/sq.mm the failure time was 0.5 hr at the
most, while unirradiated spedimens failed only after 30-50 hrs
even at a stress of 35 kg/mm2. At such stresses irradiated
specimens failed instantly. It was only at a stress of 5 kg/sq.
mm (55-70 hrs) that the failure time for irradiated specimens
could be compared with the fail$e time of unirradiated spe-
cimens at a stress of 35 kg/sq.mm.
Thus, irradiation with an integral flux of (1-3)x1020n/cm2
leads to a more than six-fold reduction in the rupture stress
for the alloy XH77T JO P at d test temperature of `150?C,
i. e. after irradiation this high-strength nickel alloy beco-
mes less heat-resistant than many of the conventional low-
alloy steels.
As can be seen from Fig.9, a decrease in the stress-rup-
ture strength of the alloy XH77T U P after irradiation
takes place also at other temperatures (600 and 800?C). The
decrease in stress-rupture strength is so great that after
irradiation the alloy X 77TIO P became much less heat-resis-
tant at 600?C than its uniradiated specimen was at 800?C.
An increase in the irradiation temperature to 700?C prac-
tically does not decrease the irradiation effect.
Unirradiated steel X12H22T31dP is ppptoximately equi-
valent in heat-resistance to the alloy XH77T DP. The failure
time f or this steel at ?50?C and a stress of 35 kg/a~m2 is
about 25-50 hrs. At this stress, irradiated specimens fail
at the moment of loading. After irradiation this steel exhi-
bited a somewhat higher resistance at a stress of 17.5 and
8 kg/mm2 than the alloy XH77Tp P (3 hours and 30-80 hours
respectively) .
The alloy XH60B is less sensitive to neutron irradiation;
at a temperature of 800?C and the failure time of 50 hours
the stress-rupture is decreased by 20-30%.
A specimen of the alloy I was melted for investigating
the role of boron in high-temperature embrittlement of heat-
resistant alloys. Alloy ingots were prepared in two pourings;
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Tensile tests of nickel, during which a very drastic
change in plasticity was revealed in the temperature range
above 500-600?C, gave grounds to expect a marked decrease
in stress-rupture strength too. Although stress-rupture
strength drops noticeably, its decrease is less than for all
the investigated materials. This difference between nickel
and the other tested materials may evidently be explained by
the fact that although the plasticity of nickel in the region
of 600?C falls off sharply, its ultimate strength rumains at
the same level (or even slightly higher) than for unirradiated
nickel (see Figs.6 and 7).
In order to check the possibility of restoring the pro-
perties of the alloys YH77TD P and XI2H22T3MP , irradiated
specimens were exposed to a temperature of ?50?C for 10 hrs.
This temperature is somewhat higher than the temperature of
annealing of rc tion defects in nickel and iron and corres-
ponds to the temperature at which work hardening by precipita-
tion hardening is possible. Such an exposure, however, does
not result in a niticeable restoration of the heat-resistance
of materials, as can be seen from Table II.
An attempt was made to restore the properties of the
alloys X1177T 10 P and XI2H22T3MP by a complete heat treatment
cycle for irradiated specimens. Additional heat treatment in
a vacuum was carried out according to the following schedule:
(a) for the alloy %H7?ThOP : homogenization at 1,080?C for
1 hr followed by aging at ?50?C for 16 hra; (b) for the alloy
X12H22T3MP: homogenization at 1,150?C for 30 min. , exposure
at 7500C for 16 hrs, then decreasing the temperature to 650?C
with a 16-hr exposure. It appears, however, that even additional
heat treatment does not restore the characteristics of the
instantaneous and stress-rupture strength of these alloys.
METALLOGR PHIC INVESTIGATION
usir9radiated. ones.
3
Selective metallographic investigations have been made
with a view to evaluating the effect of irradiation on the
structural characteristics of heat-resistant steels and alloys.
The microstructure of the heads of irradiated and unirradiated
tensile specimens of the investigated materials did not dif-
fer noticeably. It was only noted that the grain boundaries
of irradiated specimens were etched more strongly than in
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During metallographic studies of the working part of spe-
cimens tested for instantaneous and stress-rupture strength
in the temperature range from 600 to 800?C, a substantial
difference in the deformation of grains in irradiated and
unirradiated specimens was noted. The grains of irradiated
specimens are much less deformed in the direction of the
load applied, as compared with unirradiated specimens. The
difference in the nature of grain deformation increases with
test temperature.
Metallographic investigation of specimens subjected to
tests for stress-rupture strength showed that brittle frac-
ture occurs at the grain boundary without any noticeable de-
formation of the grain. The microstructure of a specimen of
the alloy XH77TJ0 P tested for stress-rupture strength is
presented in Fig. 8. The section was prepared from the non-
fractured part specimen. which was under the same stress as
the fractured portion.
DISCUSSION OF RESULTS
It has been established that reactor irradiation causes
substantial in the mechanical properties of heat-resistant
alloys based on iron and nickel and of austenitic chromium-
nickel steels, these changes occurring in the high-temperature
range. The changes characterized by the fact that at high
tests temperatures, beginning with 500-600?C, the plasticity
and strength of the alloys- decrease considerably. In most
cases, the changes are revealed already in short-term tests
at high temperatures.
In long-term tests a decrease in stress-rupture strength
was revealed for all the investigated alloys. For the majori-
ty of heat-resistant alloys the changes in stress-rupture
strength are much greater than those observed as a result
of heat treatment, pressure-working and other technological
processes.
When comparing the investigation results for various al-
loys, it can be seen that alloys with a more non-uniform struc-
ture show greater changes in stress-rupture strength. For in-
stance, changes in stress-rupture strength are much greater
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for the alloys XE177T JOP and X12H22T3MP which have the harden-
ing phases in their structure, than for the alloy XH60B which
represents an almost uniform solid solution hardened only
with a small amount of cubic carbides. A similar conclusion
concerning the role of disperse phases in radiation brittle-
ness may be made from a comparision of the results of tests
of steel IXI8H9T and steel II. However, the considerable
changes in the properties revealed during nickel tests sug-
gest that structure uniformity is only one of the many factors
affecting the high-temperature brittleness of heat-resistant
materials.
A characteristic feature of changes occurring on irradi-
ation is their irreversibility. Properties are restored neither
on prolonged heating in the range of temperatures correspon-
ding to the annealing of radiation defects in nickel and iron,
nor on additional heat treatment.
Such peculiarities in the behaviour of irradiated mate-
rials at high temperatures indicate that high-temperature
brittleness of heat-resistant alloys and austenitic steels
is due to complex processes which include both the effects
of radiation damage and the effects caused by nuclear trans-
formations and, possibly, some physico-chemical processes.
It could be assumed that the drastic change in properties
is associated with the possibility of phase transformations
due to irradiation. Since the first data on the changes in
stress-rupture strength were obtained during the study of
complex-composition alloys (XH77T10 P and others), such an as-
sumption could be justified. It could-* however, be supposed
that additional heat treatment should bring the alloy back
to the initial state and eliminate all the phase transforma-
tions which might have occurred in the course of irradiation;
nevertheless, complete heat treatment of alloys which inclu-
ded homogenization, hardening and aging did not result in
a noticebly restoration of the properties. Besides, tests
of nickel specimens prove convincingly enough that phase
transformations could not be the, principal cause of the dras-
tic deterioration in the properties of irradiated materials
at high temperature's.
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It seems most probable that the cause of such irreversible
changes lies in radiation alloying due to the presence of
new impurities appearing as a result of nuclear reactions.
Reactions of the type (n, r ) cannot give rise to se-
rious changes since they lead to the formation of atoms whose
properties differ little from the original ones, and their
amounts are too small to cause noticeable changes. Therefore
the most probable cause of changes in properties at high
temperatures is the development of such nuclear reactions
which lead to the formation of elements widely differing
from the initial ones in their properties.
The most dangerous, from the point of view of a possible
deteroration of properties, are those reactions which may
result in the formation of a gaseous phase in metals. The
most probable sources of the formation of a gaseous phase
appear to be the reaction B10 (n, of ) Li7 and Ni58 (n, p)Co58.
Though the latter reaction produces comparatively small
amount of hydrogen but the fact that so far high-temperature
embrittlement was observed only for nickel-containing alloys
shows that this reaction cannot be neglected.
As -the volume of gasp-filled cavities is determined by
the energy of free surface formation and since the surface
energy has the minimum value at the grain boundaries, the for-
mation of the largest gas volumes may be expected just at the
grain boundaries or at the phase interfaces. Owing to this
the grain boundaries may be greatly weakened, and this is
most pronounced at the temperature range above equicohesive
one, where cohesion at the grain boundaries is below grain
strength. The irradiation leads apparently not only to the
formation of gaseous products as a result of nuclear reactions
but it should also substantionally change the conditions for
the formation of gas volumes, due to the formation of zones
with an increased concentration of vacancies that may act
as nucleles of pores. The major part of the mentioned data
is in agreement with the supposition that the formation of
gas volumes at the grain boundaries is the most probable cause
of high temperature embrittlement of alloys.
In conclusion, the authors deem it their duty to thank
A.D,,Amayev who helped them to carry out the tests, as well
as V.G.Dorofeyev, V.A.Nikolaev and A.N.Lapin who took part
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in investigating stainless steels.
REFERENCES
1. Physical metallurgy and heat treatment (manual),
v.II, 1278 , Metallurgizdat, Moscow 1962.
2. N.F.Pravdyuk, V.A.Nikolaenko, V.I. Karpukhin. Effect of
nuclear radiations on materials, 184, USSR Academy of
Sciences Press, Moscow 1962.
3. V.F.Pravdyuk et al. Effect of nuclear radiations on
materials, 34, USSR Acddemy of Sciences Press, Moscow
1962.
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w
b
0
Table 1
---------------------------------------------------------------------------
No. Steel or alloy
grade
Content of elements, % (hominal )
---------- -------- ---------- -- -- ---- -- ------------------------- -------
C
Si
Mn
Cr
Ni
Ti Al
MQ Fe B Miscella-
neous ele-
ments
I
XH77T RJP
L 0.06
L 0.6
Z- 0.40
19.0-
base
2.30- 0.55-
- 4 4.0:~0.01
Ze(