Manufacturing method of delayed hydride cracking resistant seamless pressure tube made of zirconium (Zr) alloy

A method for manufacturing a delayed hydride cracking resistant zirconium ahoy (Zircaloy-2, Zircaloy-4, Zr-2.5% Nb, pure Zr, etc.) pressure tube includes the steps of making a seamless pressure tube having a diameter smaller than the final size by extrusion or drawing, and then expanding the tube at a temperature below 600.degree. C. by cross rolling.

FIELD OF THE INVENTION 
The present invention relates to a method for manufacturing zirconium alloy 
seamless pressure tube which is used for a CANDU reactor, and which has a 
texture having an improved fracture toughness and having a resistance 
against the crack propagation due to the delayed hydride cracking (to be 
called DHC below) mechanism (the texture indicates the structure in which 
a C-axis of a hexagonal close packed structure is concentrated in the 
direction of the diameter of the seamless pressure tube, i.e., the 
structure in which crystalline grains having c and d orientation the 
seamless pressure tube of FIG. 2 are profuse). 
BACKGROUND OF THE INVENTION 
In the conventional manufacturing method for the seamless pressure tube for 
the CANDU nuclear reactor, a billet having a hole is made to undergo a hot 
extrusion, and then, a cold drawing is carried out, thereby forming a 
zirconium alloy (Zircaloy-2, Zr-2.5% Nb or the like) seamless pressure 
tube. However, the zirconium alloy seamless pressure tube has a special 
texture which is formed during the manufacturing process, and which is a 
micro-structure in which the orientation of the crystalline grains within 
the material is preponderantly distributed in a particular direction. 
Therefore, it is very susceptible to the delayed hydride cracking, and 
therefore, it is liable to be damaged during reactor operation of the 
reactor. 
When the cause of the damage of the pressure tube was investigated, it was 
found that the delayed hydride cracking was the most serious factor for 
impeding the safety. Around the year 1980, studies were made on the 
mechanism of the delayed hydride cracking, the influence of the texture on 
the delayed hydride cracking, and the formation of the texture in the 
material of the pressure tube. 
For example, C. E. Coleman and S. Sagat of the Canadian Nuclear Power 
Corporation (Canada AECL-CRL) manufactured the existing Zr-2.5% Nb alloy 
plate in a different orientation, and, investigated into the influence of 
the texture on the delayed hydride cracking. Consequently, they confirmed 
that the texture gives a great influence to the delayed hydride cracking. 
R. A. Holt et al performed an experiment by adjusting the extrusion ratio 
during the manufacturing process, thereby making a study on the influence 
of the extrusion ratio on the alteration of the texture of the material of 
the pressure tube. Consequently, they could confirm that the extrusion 
ratio does not give any influence to the texture of the pressure tube. In 
Korea, Kim Sung-Soo et all used a Zr-2.5% Nb plate to make a study on the 
influence of the texture on the delayed hydride cracking. Consequently 
they confirmed that the resistance against the delayed hydride cracking 
can be improved through the variation of the texture. 
Thus there have been carried out studies on the influence of the texture on 
the delayed hydride cracking in Canada in Korea. Consequently, a 
conclusion has been derived that the texture of seamless pressure tube 
made of zirconium alloy has to be modified in order to improve DEC 
resistance. However, there has not been developed a method which can 
modify the texture of seamless pressure tube through modification of the 
fabrication process. 
SUMMARY OF THE INVENTION 
The present invention is intended to overcome the disadvantages of the 
conventional technique described above. 
Therefore it is the object of the present invention to provide a zirconium 
alloy seamless pressure tube in which the texture of the zirconium alloy 
seamless pressure tube is modified in order to improve the reactor safety 
and the operation rate of CANDU reactor. 
In order to vary the texture which is formed in the zirconium alloy 
seamless pressure tube which is manufactured by a hot extrusion process, 
the extrusion and drawing are not used in varying the seamless pressure 
tube, but a cross rolling or an other method for expanding a tube under a 
planar deformation condition is used, thereby improving the texture of the 
final seamless pressure tube product.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The delayed hydride cracking resistant zirconium (Zr) alloy seamless 
pressure tube according to the present invention is manufactured by 
applying a cross rolling on an extruded zirconium alloy (such as 
Zircaloy-2, Zircaloy-4, Zr-2.5% Nb, Zr-1% Nb, pure Zr or the like) so as 
to expand the tube, thereby raising the basal pole component in the radial 
direction of the tube. 
The method for manufacturing the zirconium alloy (Zircaloy-2, Zircaloy-4, 
Zr-2.5% Nb, pure Zr etc.) according to the present invention includes the 
steps of: making a seamless pressure tube having a diameter smaller than 
the final size by applying a high temperature extrusion expanding the tube 
at a temperature below 600.degree. C. without causing a significant phase 
transformation and without causing a deformation of the deforming 
mechanism; and applying a tube expansion method such as a rotary rolling, 
a rotary piercing or the like so as to expand the tube through a cross 
rolling during the process, and so as to improve the texture of the 
seamless pressure tube. 
Further, during the tube expansion process, a pressure (hydraulic pressure 
or explosion) is applied, thereby improving the texture of the seamless 
pressure tube. Of the above methods for expanding the tube, at least two 
methods or more are applied so as to improve the texture of the seamless 
pressure tube. Further, one or more tube expanding methods, an 
intermediate annealing and a drawing are applied to improve the texture of 
the seamless pressure tube. 
In other words, a hot extruded seamless pressure tube having a smaller 
diameter and a thicker wall thickness than the final product is expanded 
by applying a cross rolling method such as a rotary rolling (if the tube 
is expanded in planar expanding method in the orientation a and b of FIG. 
2, the wall thickness becomes thinner). Thus (1012)&lt;1011&gt; and (1121)&lt;1126&gt; 
twin plane deformation is made to occur within the material of the 
hexagonal cross packed structure, so that a slip mechanism should act on 
the deformed crystal grains. Thus the C-axes of the crystal grains are 
made to be preponderantly distributed to the radial direction of the tube 
(the d and c directions in FIG. 2). FIG. 1 illustrates the twin crystal 
planes which act on the hexagonal close packed structure in the 
deformation mechanism. FIG. 3 illustrates the shape of the twin plane 
deformation. FIG. 4 is a flow diagram showing a comparison of the 
conventional manufacturing process with the manufacturing process 
according to the present invention. FIG. 5 schematically illustrates the 
process of manufacturing the pressure tube from a billet. 
According to a modified embodiment of the present invention, the phase 
constituting the greater part of Zircaloy-2, Zircaloy-4, Zr-2.5% Nb, Zr-1% 
Nb and the pure zirconium is an .alpha.-Zr of the hexagonal close packed 
structure. The problems of the texture of these alloys are related to the 
concentration of the basal pole components of the hexagonal close packed 
structure. Therefore, in these alloys, the sensitivity to the delayed 
hydride cracking is common, and the present invention improves the 
resistance of these alloys against the delayed hydride cracking. 
In the process of expanding the seamless pressure tube (the crystal grains 
having a and b orientation in which the c axes of the crystal grain are 
concentrated, there appears a variation of the texture. This phenomenon 
improves the resistance against the DHC crack propagation. Therefore, even 
the tube expansion under a planar deformation condition can improve the 
texture. 
Further, the proportions of the crystal grains having the c and d 
orientation of FIG. 2 can be increased by increasing the deformation 
amount during the cross rolling. 
Therefore, the texture of the seamless pressure tube can be improved by 
applying a cross rolling by means of the rotary piercing mill of FIG. 9 
which can fabricate through a planar deformation similarly to the rotary 
rolling mill of FIG. 8. 
Further, a tube expanding method through explosion or hydraulic pressure 
can be carried out within a casing of a limited size, thereby improving 
the texture of the seamless pressure tube. For the explosion method, gas, 
explosive and electro-magnetic force can be utilized, while, for the 
hydraulic pressure, water, silicon oil and other hydraulic fluid can be 
used. 
FIG. 4 illustrates several fabrication examples, and tube expansion and 
drawing can be combined to improve the texture. When the intermediate 
annealing is applied to eliminate work hardening effect during the 
processing at a temperature below the recrystallization level, the 
processing deformation amount is increased. 
As an actual example, an annealed plate which has a texture similar to that 
of the pressure tube was subjected to a 30% cold rolling in the initial 
rolling direction like when the cold drawing (with the deformation amount 
being 25-30%) is applied to the pressure tube. Consequently, there was 
obtained a plate having the texture of the conventional seamless pressure 
tube. 
Further, the above described annealed plate was subjected to a 30% cold 
rolling in the direction perpendicular to the initial rolling direction, 
with the result that there was obtained a texture which was similar to 
that of the seamless pressure tube of the present invention. 
These two plates were used to form a subsize CT specimen (W=17 mm, t=3.3 
mm), and then, was hydrogenized. Then the test piece was stress-relieved 
at a temperature of 367.degree. C., and subjected to a hydrogen 
homogenization. Then tests were carried out such as a DHC crack 
propagation rate measuring test, a critical stress intensity factor 
measuring test, and a cracking factor measuring test. FIG. 6 illustrates 
the variation of the texture of the plates, and Table 1 shows the 
variation of the basal pole component. When the deformation amount of the 
cross rolling is 30%, the basal pole component of the plates in the 
transverse direction is as shown in Table 2. 
The crack propagation speed due to the delayed hydride cracking mechanism 
was lowered to one half by the improvement of the texture as shown in 
Table 3. The critical stress expansion factor which is required for 
causing the delayed hydride cracking through the improvement of the 
texture rose to a double as shown in Table 4. 
TABLE 1 
______________________________________ 
Variation of basal pole component 
versus cold rolling amount 
Deformation rate 
Basal pole component 
Condition (%) FN* FT* FL* 
______________________________________ 
As received 
0 0.31 0.63 0.06 
Direct rolled 
23.7 0.30 0.52 0.10 
Cross rolled 
3.7 0.3 0.6 0.1 
8.4 0.32 0.57 0.11 
13.0 0.37 0.52 0.11 
23.1 0.47 0.39 0.14 
______________________________________ 
*FN: Radial direction of the tube or the perpendicular direction to the 
plate. 
FT: Transverse direction of the plate or circumferential direction of the 
tube. 
FL: Basal pole component in the lengthwise direction of the tube or the 
rolling direction of the plate. 
FN + FT + FL = 1 
TABLE 2 
______________________________________ 
Basal pole component of the plates 
subjected to test for crack propagation speed 
caused by delayed hydride cracking mechanism. 
Deformation rate 
Basal pole component 
Condition (%) FN* FT* FL* 
______________________________________ 
As received 
0 0.42 0.60 0.04 
Direct rolled 
30 0.41 0.53 0.06 
(existing) 
Cross rolled 
30 0.54 0.39 0.07 
(improved) 
______________________________________ 
*: Same as Table 1. 
TABLE 3 
______________________________________ 
Comparison of delayed hydride cracking 
propagation speeds for different rolling methods 
DHC propagating speeds for diffrnt temp (m/sec) 
Condition 170.degree. C. 
200.degree. C. 
230.degree. C. 
______________________________________ 
Direct rolled 
1.1 .times. 10.sup.-8 
2.3 .times. 10.sup.-8 
4.2 .times. 10.sup.-8 
(existing) 
Cross rolled 
5.0 .times. 10.sup.-9 
1.1 .times. 10.sup.-8 
2.0 .times. 10.sup.-8 
(improved) 
______________________________________ 
TABLE 4 
______________________________________ 
Variation of critical stress expansion 
factor through improvement of texture 
Critical stress expansion factor 
Condition (MPa m) 
______________________________________ 
Existing texture 
4.5-6 
Improved texture 
11 
______________________________________