Note: Descriptions are shown in the official language in which they were submitted.
1 :~ 63:1~0
This invention relates to zirconium alloy tubes
especially Eor use in nuclear power reactors. More parti-
cularly this invention relates to quaternary 3.5% Sn, 1% Mo,
1% Nb, balance Zr alloy tubes which have been extruded, cold
worked and heat treated to lower their dislocation density.
In one preferred embodiment ~he alloys are cold worked less
than 5~ and stress relieved to produce a low dislocation den-
sity and in another embodiment the alloys are cold worked up
to about 50~ and annealed to produce a very low dislocation
density and also small equiaxed a grains.
Conventionally, pressure tubes for CANDU-PHW type
nuclear reactors (Canada-Deuterium-Uranium-Pressurized Heavy
Water) are fabricated by extrusion of Zr-2.5 wt.~Nb billets,
followed by cold working and age hardening. Other Zr alloys
can also be used for tubinq in CANDU-PHW tYPe reactors, such
as Zircaloy-2~ and quaternary alloys containing 3.5~ Sn, 1% Mo,
1~ Nb, balance Zr, which provide high strength, low neutron
capture cross section and reasonable corrosion resistance.
The heat treatment of the quaternary alloys above is described
in the literature, and attention is particularly directed to
U.S. Patent 4,065,32~ to Brian A. Cheadle, issued December 27,
1977 which describes a process for heat treating the quaternary
alloys noted above and hereinafter referred to as EXCEL alloys,
to produce a duplex micro-structure comprising primary a-phase
and a complex acicular grain boundary phase. The object of the
invention described in the aEoresaid U.S. patent is to provide
an alloy having the maximum possible strength which i9 achieved
by cold working to about 25% followed by age hardening but at
the expense of increasing the dislocation density as well.
r~ 1~ 6;~J
- \
7 ~ ~31~0
Although such heat treated -tubes have relatively good out-
of-reactor creep strength, their in-reactor creep strength
is adversely affected by the high dislocation density.
Unless otherwise stated all alloy percentages in this
specification are percentages by weight.
In CANDU reactors it is desirable for the pressure
tubes to have as low axial elongation and diametral expansion
as possible during service. While it is possible to reduce
elongation and expansion levels in conventional 30% cold worked
Zr-2.5~ Nb pressure tubes by lowering their dislocation density
and making their grains more equiaxed, this, however, also
results in a lowering of the tensile strength which would then
necessitate increasing the wall thickness with a consequent
reduction in reactor efficiency. It is, therefore, necessary
to consider the use of one of the alternative alloys referred to
above. EXCEL is a stronger and more creep resistant alloy both
in and out of reactor than Zr-2.5% Nb, and it has been found
that pressure tubes having similar strength to 30% cold worked
Zr-2.5~ Nb tubes can be fabricated with less than 5% cold work
followed by stress relieving at a temperature in the range 650-
800K. Similarly it has been found that low dislocation density
EXCEL alloys can also be produced by cold working up to about
50% followed by annealing at a selected temperature in the range
900-1100K.
Thus it is an object of the present invention to provide
a process -for heat treating and cold working EXCEL alloys, such
that they have a minimum ultimate tensile strength of 479 MPa,
and during service equivalent to 30 years in a CANDU-PHW 600 MW
reactor they have a maximum axial elongation of about l.5%, and
-- 2
~ 3B3~0
a maximum diametral expansion of 2.5%.
It is another object of this invention to provide a
heat treated and cold worked product consisting essentially of
Sn 2.5-4.0~, Mo 0.5-1.5%, Nb 0. 5~1.5~, 0 800-1300 ppm, balance
Zr and incidental impurities, said product having a minimum
ultimate tensile strength of 479 MPa, a maximum axial elonga-
tion less than 1.5~ and a maximum diametral expansion less
than 2.5~ under conditions equivalent to 30 years service ïn
a CANDU-PHW 600 MW reactor.
For the purposes of the present specification a 600 MW
CANDU-PHW reactor is considered to operate at a temperature of
565K, with a peak neutron flux of 3.85 x 1017 n/(m2.s) and at
a mean coolant pressure of 10.6 MPa.
Thus, by one aspect of this invention there is provided
a method of fabricating an extruded product from an alloy con-
sisting essentially of Sn 2.5-4%, Mo 0.5-1.5%, Nb 0.5-1.5%,
0 800-1300 ppm balance Zr and incidental impurities wherein a
billet of said alloy is preheated in the temperature range gO0-
~ 1200K and extruded into said produc-t, and said extruded
product is cold worked, by an amount up to about 50%, and heat
treated at a selected temperature in the range 650-1100K, so
as to have a dislocation density of less than about 5 x 1014 m 2
a minimum U.T.S. of 479 MPa, a maxlmum axial elongation less
than 1.5~ and a maximum diametral expansion less than 2.5% under
conditions equivalent to 30 years service in a CANDU-P~IW 600 M~
reactor.
By another aspect of this invention there is provided
a heat treated and cold worked alloy for use in nuclear
, `t~' ':'
,~
3~2~
reactor tubes and other extruded products and consisting
essentially of Sn 2.5-4.0~/ Mo 0.5-1.5%, Nb 0.5-1.5% 0 ~00-
1300 ppm, balance Zr and incidental impurities, having a
minimum ultimate tensile strength of 479 MPa, a maximum in-
service axial elongation of 1.5% and preferably in the range
0.5-0.8%, a maximum in-service diametral expansion of 2.5~ and
preferably in the range 1.1 to 1.4~ and an equiaxed grain
structure.
The invention will be described in more detail herein-
after with reference to the accompanying drawings in which:
Figure 1 is a flow chart of a general fabrication
route for alloys of the present invention;
Figure 2 is a flow chart of a specific fabrication
route for alloys according to one aspect of the present
invention;
Figure 3ta) is a transmission electron micrograph at
11,500X of extruded tubes cold worked less than 5% and stress
relieved at 700K, of the present invention;
Figure 3(b) is a transmission electron micrograph at
11,500X of tubes cold worked greater than 5% and annealed at
1075 K, of the present invention;
Figure 4 is an average (0002) pole figure for seven
tubes of the present invention; and
Figure 5 is a ~eries of optical micrographs showing
the effect of stress on the orientation of zirconium hydrides
in E~CEL and Zr-2.5 wt.~ Nb tubes.
In power reactors that use internally pressurized tubes
two important mechanical property requirements are tensile
strength and dimensional stability during service. Dimensional
4 -
~ :~ 63 1 2~)
stability is a function of both creep and growth (dimensiona]
change during irradiation without an applied stress). In zircon-
ium tubes the ratio of creep in the axial and circumferential
directions is a function of their crys-tallographic texture and
the ratio of their growth in the axial and circumferential
directions i5 a function of both crys-tallographic texture and the
shape of the a grains. The crystallographic -texture of extruded
and cold drawn tubes is largely a function of the extrusion
conditions - temperature, die shape, strain rate, billet micro-
structure and extrusion ratio. It has been found that the ratio
of di`ametral expansion to axial elongation of a tube duriny
service in a power reactor can be controlled by selecting the
appropriate extrusion conditions.
The longitudinal tensile strength of 30% cold worked
Zr-2.5 weight % Nb pressure tubes is due to their combination
of high dislocation density, very small a grain thickness (0.3
x 10 3mm) and a duplex microstructure of a grains and grain
boundary network of ~-phase. However, the in-reactor creep of
of these tubes is adversely affected by their dislocation density
and their in-reactor axial elongation due -to growth is adversely
-affected by both their dislocatlon densit~and their very long
elongated a grains (0.3 x 10 3mm thick x lOmm long)~ EXCEL is
a stronger material than Zr-2.5 wt.% Nb. There~ore EXCEL tubes
can be fabricated that are as strong or stronger than 30% cold
worked Zr-2.5 wt.% Nb tubes, but have lower dislocation densities
and/or more equiaxed ~ grains. These tubes have considerably
better dimensional stability during service in pGwer reactors.
The tensile strength of these EXCEL tubes is largely
a function of their dislocation density and grain size. Tubes
~ ~3~O
cold worked a minimum after extrusion and stress relieved will
have thin elonyated a grains (Figure 3a). Their longitudinal
tensile strengths can be up to 600 MPa at 575K depending on
the stress relieving temperature. If the tubes are annealed
after cold working to produce equiaxed recrystallized a grains
(Figure 3b) then the size of the grains depends on -the amount
of cold work and the annealing heat treatment.
Fabrication of Experimental Tubes
A double arc melted ingot of EXCEL alloy was forged to
215mm diameter bar and machined to form seven hollow billets
numbered 248-254. The billets were clad in steel and copper
and preheated to about 1130K for approximately 5 hours and
then extruded into tubes at a ratio of 13.5:1. The cladding was
removed by dissolution in ni-tric acid, the inside of the tubes
were sand blasted and the outside centerless ground. One end
of each of the tubes was flame annealed, air cooled and pushed
onto a die to point the end. A conversion coating was then
applied and the tubes cold drawn between 2 and 5% as shown in
Table 2. The chemical composition of the tubes is recorded in
Table 1. The cold worked tubes were then sand blasted inside
and centerless ground on the outside.
-- 6
~ ~ ~3~ ~
TABLE 1: The Chemical Analysis of -the EXCEL Tubes
._ _
__ .. _____ _ . I
Tube Element
Number Sn wt% Mo wt% Nb wt% O ppm H ppm
.. _ _~ ._
248 F 3.32 0.81 0~83 1157 34
248 B 3.08 0.77 0.79 1203 48
249 F 3.31 0.81 0.80 1142 30
249 B 3.29 0.82 0.81 1089 26
250 F 3.23 0.79 0.82 1142 36
250 B 3.31 0.82 0.82 1131 26
251 F 3.32 0.79 0.83 1149 , 34
251 B 3.42 0.80 0.81 1134 28
252 F 3.46 0.83 0.80 1142 29
252 B 3.29 0.75 0.79 1119 25
253 E' 3.39 0.78 0.84 1149 32
253 B 3.31 0.80 0.70 1116 18
254 F 3.38 0.78 0.82 1118 54
254 B 3.47 0.81 0.80 1115 34
.. _._ _ _ _
MEAN 3.33 0.80 0 80 1136 34
F is the front end of the tube and comes out of the
extrusion press first.
B is the back end of the tube and comes out of the
extrusion press ]as-t.
-- 7 --
~ 3 63 1 ~
TA~LE 2: Extrusi~ C~ld Dr~wing Data
for the EXCEL Pressure Tubes
.. _.. _ ..._, ,_ . . .
Billet Total Furnace Pressure I.ength of % Cold
Number Preheat Time to Start Tube Extruded Draw
ps i m
__ . _ . _ ._ . . . _ .. .__ ~
248 5 hours 52 minutes 1800 7.5 2.83
249 5 hours 56 minutes 2000 5.8 3.71
250 6 hours 3 minutes 1750 7~3 3.16
251 7 hours 22 minutes 1700 7.5 2.77
10 252 7 hours 17 minutes 1800 7.4 4.36
253 6 hours 48 minutes 2300 4.3 2.90
254 7 hours 10 minutes 160- .. _.. __ ~.. .. .__. 2.89
Two tubes, 249 and 251 were annealed in a vertical
vacuum furnace for 30 minutes at 1023K to produce an equiaxed
alpha grain s-tructure. An equiaxed alpha grain structure should
produce a lower ln-reactor axial elongation rate at the expense
. . - of a slightly lower tensile strength.
. ~ -
- Sections o~ tube 248:were cold worked up to 40% and :~
-
then annealed for 30 minutes at a selected temperature in the ::
range 1025-1075K.
All the tubes were finally stress relieved in an auto-
clave for 24 hours at 675K.
The general fabrication route is shown in Figure 1
and the particular steps for these seven tubes are shown in
Figure 2.
3~ 2~
TABLE 3: a Grain Size and Dislocation Density
-
of the EXCEL Pressure ~ubes
Tube % Cold Grain Size mmxlO 3 Disloca-
Number Drawn Front eod Back end Aver. D n~i
250 3.7 0.15 0.48 0.62 8.4 x
252 3.2 0.81 0.46 0.6
253 2.8 0.76 0.39 0.5
254 4.4 0.70 0.5~ 0.62
Mean 0.76 0.51 0.64
_ __
24~ 2.9 0.80 1.4 x
251 2.9 0.74 l~l~
Cold worked 5-9 x
Zr-2.5% Nb 0.4 0.2 0.3 1~14
tubes - _
PROPERTIES OF THE EXPERIMENTAL TUBES
- Microstructure and Texture
.
- - Grain sizè and shape are important parameters in -the
.
tensile strength and in-reactor dimensional stability of
zirconium alloy pressure tubes. The microstructures were
examined by thin film electron microscopy. The results,
Figure 3a and Table 3, show that the microstructure of the
cold worked tubes consists of elonga-ted a grains, a thin
grain boundary network of ~-phase, and a few localized areas
of martensitic a'. The a grain thicknesses were larger than
typical cold worked ~r-2.5% Nb pressure tubes, Table 3. The
two annealed tubes~ 249 and 251 had larger relatively equiaxed
a grains, Figure 3b, with the ~phase concentrated a-t grain
~ ~i33~
corners. The five cold worked and stress relieved tubes had
much higher average dislocation density than the annealed
tubes, as seen in Table 3. The texture of the annealed and
cold worked tubes was similar and an average (0002) pole ~igure
for the seven tubes is shown in Figure 4.
The effect of varying amounts of cold work and anneal-
ing temperature on the a grain thickness of an extruded tube is
shown in Table 4 (below). The smallest grain thickness was
obtained with 30% cold work followed by annealing for 30 minutes
10at 1025K.
TABLE 4: The Effect of Cold Work and Annealing Heat
Treatment on the Grain slze of Extruded
EXCEL Tube 248
Thlckness of a Grain, mm x 10 ~
% Cold Work 30 minu-tes at30 minutes at
1025K 1075K
...
0 0.80 0.80
0.79 1.08
0.72 _
0.59 0.98
0.53 0.97
1.11 1.72
; 20Tensile Strength
The longitudinal and transverse tensile strengths of
the tubes are shown in Table 5. The cold-worked and stress
relieved tubes were considerably stronger than the annealed
tubes due to their smaller grain thickness and higher disloca-
tion densityO The annealed tubes met -the minimum specifications
: for 30~ cold-worked Zr-2.5 wt~ Nb pressure tubes.
Hydride Orientation
As fabricated the hydrides were oriented in -the radial-
axial plane. The effect of hoop stress on the orientation of
-- 10 --
.
3 ~ 2 ~
the hydrides that precipitate during cooling from 575K is
shown in Fiyure S. To precipi-tate hydrides in the radial-
axial plane required a hoop stress of 827 MPa.
TABLE 5: Tensile Properties of the EXCEL Pressure Tubes and
Typical Tens.ile Properties of 30% Cold-Worked
-
Zr-2._~ Nb Pressure Tubes
Alloy Tube Test Test 0.2~ Yield UTS
Condi- Tempera- Direc- Stress MPa MPa Elonga-
tion ture K tion _ tion
575 L 525 580 12
5% T 620 645 13
cold
drawn 300 L 736 845 12
EXCEL _
L 385 500 19
an- 575 T 490 555 13
_ nealed 300 T 615 745 17
Zr- co%ld 575 L 580 56200o 152
Nb drawn 3 0 0 T 640 810 15
L is longitudinal T is transverse
COMPARISON WITH ST~NDARD Zr-2.5% Nb ALLOY ~RES5VRE TUBES
Tensile Strength
Cold worked Zr-2.5% Nb is the reference pressure tube
material for CANDU-PHW reactors. EXCEL alloys having chemical
compositions in the range 2.5-4.0% Sn, 0.5-1.5% Mo, 0.5-1.5% Nb,
800-1300 ppm O, balance Zr plus incidental impurities, have been
found to have higher strengthG than the Zr-2.5% Nb alloys and
good in-reactor creep resistance.
In all metallurgical conditions EXCEL alloys are
stronger than Zr-2.5% Nb but when heat treated to produce the
~, - 11 -
~ t ~3~2~
required high strengths for use in a reactor the ductility is
relatively low as shown in Table 6.
TABLE 6: T} Tensile Properties of Zr-2.5% Nb
and EXCEL alloy at 575K
Alloy Conditlon 0.2% YS UTS Total
MPa K psi MPa K psi Elongation
_
Zr-2.5% Nb Annealed 207 30 28040 30
EXCEL Annealed 338 40 46065 20
Zr-2.5% Nb 20% cold
worked 365 53 40659 11
EXCEL 20% cold
worked 517 75 57984 11
Zr-2.5% Nb Heat
treated 579 84 644935 15
EXCEL Heat
_ _ __ treated 620 115 860130 1_ _ ___ _ _
Typical tensile properties of cold worked Zr-2.5%
Nb pressure tubes and EXCEL pressure tubes in the extruded
condition and also cold drawn about 3%, 10%, and 15% are
shown below in Table 7.
~ 12 -
J :~ 63:~2~
TABLE 7: Typical Te_sile Proper-ties of Cold Worked
Zr 2.5% Nb and EXCEL Alloy Pressure Tubes
at 57SK
.... _
Alloy Condition Test 0.2%
. Direc- Yield
tion Stress UTS
Kpsi MPa Kpsi MPa %EL %RA
......... _ __
Zr-2.5% ex-truded and
: Nb cold drawn L 50 379 71 48918 50
28% T 79 544 88 60612 75
extruded L 5~ 400 75 51715 47
extruded and
cold drawn L 60 413 83 57214 48
EXCEL ~ 3% T _ 99 682 _ 60
Alloy ex-truded and
cold drawn L 73 503 87 59915 46
~10% T _ 90 620 _ 59
extruded and
cold drawn L 75 517 90 62013 40
15~ T 96 661 _ 58
L is longitudinal
T is transverse
; 20 ~ CEL alloy tubes in the extruded condition are shown
to be stronger than conventional 30% cold drawn Zr-2.5% Nb tubes
:~ but cold drawing of the EXCEL tubes 15% does not increase their
strength very much.
Pressu_e Tube Safety
The design stress of reactor pressure tubes is only one
third of the minimum ultimate tensile strength in the unirradiated
condïtion at the design -temperature so that it is inconceivable
for failure to occur by tensile rupture, in view of the
pressure warning and relief systems in a power reactor. IE
- 13 -
~ ~ ~3 ~ 2~
the pressure tube should sustain a deEect o~ sufficient
severity, however, its rup-ture strength will be reduced to the
level of the design or operating stress, and the tube would
break. The most severe defect is a sharp longitudinal -through
wall crack, because the maximum (hoop) tensile stress acts to
open and extend the crack. An important parameter in the ability
oE tubes to tolerate longitudinal defects is the presence o~
zirconium hydrides. The tolerance of pressure tubes to such
deEects depends on such factors as neutron irradiation, test
tempera-ture and hydrogen concentration. Test results show both
Zr-2.5~ Nb and EXCEL tubes have similar tolerances with respect
to neutron irradiation, test temperature, and hydrogen concen-
tration although the effects of hydrogen will be described in
more detail hereinafter. Normally it is expected that pressure
tube alloys will fracture in a completely ductile manner with
large local plasticity and that a tube will leak coolant before
it actually breaks.
CANDU PHW reactors are normally operated with a reduc-
ing coolant chemistry which is maintained by adding hydrogen to
the water. During service the pressure tubes corrode in the
heavy water coolant and some of the deuterium is picked up by
the tube. Hydrogen and deuterium have a very low solubility in
zirconium alloys and form zirconium hydride or zirconium deuteride
platelets which are brittle. As-fabricated pressure tubes only
contain 10-15 ppm hydrogen and no hydride platelets are present
at reactor operating temperatures ~530-575K). However towards
the end of their service life (`15 years) they are predicted to
contain 30-50 ppm hydrogen (60-100 ppm deuterium~ and hydride
p]atelets could be present at the operating temperatures. 1'he
~.
_ 14 -
~.~., .
~ ~ ~312~
orientation of the hydride platelets is a function of crystallo-
graphic texture and stress. Although EXCEL alloys tend to corrode
marginally faster under these conditions than do Zr-2.5% Nb alloys,
the hydrogen pick-up (hydriding) rate is about the same.
Hydrogen pick-up is particularly significant because
it is known that failures, due to delayed hydrogen cracking, can
occur at stresses below the ultimate tensile strength of the
alloy if such stresses are present for long periods of time as
would be the case in-reactor. Crack propagation i5 quite slow
and the fracture surfaces are characterized by areas of flat
cleavage compared to the dimpled surface of a ductile fracture.
These flat fracture areas corresponding to failure either through
hydride platele-ts or at the hydride/matrix interface. For de-
layed hydrogen cracking to occur, hydrogen concentration in the
alloy must exceed the terminal solid solubility at the test/
operating temperature. Important parameters for crack initiation
and propagation include (a) stress or stress intensity at a
notch; (b) hydrogen concentration and hydride orientation and
(c) temperature.
Crack initiation at the inside surface of cold worked
Zr-2.5~ Nb pressure tubes has been studied using cantilever
beam specimens. Specimens from the transverse direction were
loaded in cantilever beam test rigs so that the maximum outer
:~ :
fiber tensile stress was imposed on the inside surface of the
tube in the circumferential direction. The -test results, Table
8, show that the probability of crack initiation increases with
stress and at 350K also increases with hydrogen concentration.
Similar tests have been performed on EXCEL alloys and the results,
summarized in Table 9, show that crack initiation by delayed
~ r
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-- 17
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hydrogen cracking is more difficult to initiate in EXCEL
pressure tubes than in Zr-2.5~ Nb pressure tubes.
In cold worked Zr-2.5% Nb and EXCEL alloy pressure
tube materials the hydrides in unstressed material lie in cir-
cumferential planes, and have very little effect on the tolerance
of the tubes to longitudinal defects. Howe~er if the hydrides
precipitate under a hoop stress as during a reactor shut down,
above a critical stress the hydrides precipitate in the radial-
axial plane and severely reduce the tolerance of the tubes to
longitudinal defects. When the Zr-2.5% Nb material is thermally
cycled to 575K under a circumferential tensile stress, then some
of the hydrides become reoriented to the radial plane. As the
zirconium hydrides are less ductile than a zirconium, hydrides
perpendicular to a tensile stress lower the ductility. It will
be noted that even relatively low stress levels of the order of
200 MPa causes reorientation of mos-t of the hydrides into the
radial axial plane. The results of thermally cycling EXCEL
alloys to 575 K at similar stress levels are also shown and it
will be observed that the hydrides in the EXCEL tubes are very
much more resistant to reorienting in the radial direction, which
.,
is a very desirable property. Therefore EXCEL tubes should be
more tolerant to longitudinal defects than Zr-2.5% Nb tubes.
In summary, therefore, the axial elongation and
diametral expansion of current 30% cold worked Zr-2.5% Nb
pressure tubes could be reduced by lowering their dislocation
density and making their grains more equiaxed. This would,
however, also lower the tensile strength below specifications.
EXCEL a]loys are stronger and more creep resistant than Zr-2.5
Nb. This enables EXCEL pressure tubes to be made that have
18 -
.
63~2~
similar strength to 30~ cold worked Zr-2.5% Nb tubes yet only
be cold worked ~5%. This dislocation density of EXCEL alloys
can be further lowered by annealing to produce a more equiaxed
grain structure as shown in Figure 3b. The predicted dimen-
sional changes for EXCEL tubes after 30 years service in a
C~NDU-PHW 600 MW reac-tor are shown in Table 10. The 5% cold-
worked tubes were much s-tronger than the current requirements
for CANDU-PHW reactors (minimum longitudinal UTS at 575K, 479
MPa). If these tubes were stress relieved at a higher tempera-
ture to reduce their longitudinal strength at 575K to 500 ~Pa,
then their dimensional changes would be much less as shown in
Table 10. Similarly, if the extrusion ratio used ~or these
tubes was reduced from 13.5:1 to 11:1 then the texture would be
changed and the axial elongation could be further reduced.
TABI,ElQ: Predicted Dimensional Performance of the EXCEL
Pressure Tubes in 600 MW CANDU-PHW Reactors
.... __ , Dimensional Change for
: Central Channel after
Alloy Tube Type 30 Years
% Axial % Diametral
Elongation Expansion
_ _
extruded 13.5:1
5% cold worked
stress relieved 675K 2.2 1.8
extruded 13.5:1
5% cold worked
stress relieved ~700K 1.4 2.2
extruded 11:1
EXCEL 5% cold worked
stress relieved >700K 1.0 2.0
extruded 13.5:1,
cold worked, annealed 0.8 1.1
extruded at 11:1
cold worked, annealed 0.5 1.4
_ _ . ..... ... ...
Zr-2.5% Nb 30% cold worked 2.5 3.9
-- 19 --
