Method of manufacturing tubes of zirconium alloys with improved corrosion resistance for thermal nuclear reactors

The invention relates to a method of making tubes of Zirconium alloys containing 1-5 percent by weight of alloying elements such as Sn, Fe, Cr and Ni. According to the invention an improved corrosion resistance can be reached by means of annealing after extrusion and between cold rollings within a well defined temperature range in the .alpha.-phase zone during considerably longer times than standardized for the purpose of reaching equilibrium between secondary phase particles and Zirconium matrix and in that way a minimum content of Fe in solid solution.

The present invention relates to a method of making tubes of Zirconium 
alloys containing 1-5 % by weight of alloying elements such as Sn, Fe, Cr 
and Ni, and the rest essentially Zr, for the purpose of improving and 
corrosion resistance of the tubes in media such as water and steam at high 
pressure and high temperature. Among commercial alloys having a 
composition within the mentioned range there are the Zirconium alloys 
"Zircaloys" which are essentially used as cladding tubes in water cooled 
thermal boiling water and pressurized water nuclear reactors. The alloys, 
"Zircaloys" combine a small cross section for neutron absorption with 
excellent corrosion resistance and good mechanical properties. A typical 
composition of such alloys is, in percent by weight, 1-2 Sn, 0.05-0.25 Fe, 
0.03-0.20 Cr, at the most 0.1 Ni and the rest essentially Zr. 
The most frequently used Zircaloy alloys so far are Zircaloy-2 and 
Zircaloy-4. These alloys have the following composition: 
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Percentage by weight 
Element Zircaloy-2 
Zircaloy-4 
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Sn 1.2-1.7 1.2-1.7 
Fe 0.07-0.20 0.18-0.24 
Cr 0.05-0.15 0.07-0.13 
Ni 0.03-0.08 -- 
Zr Rest Rest 
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So far, these two alloys have generally shown sufficient resistance to 
corrosion under the working conditions existent in a nuclear reactor. The 
development goes, however, towards greater utilization of the fuel which 
means longer working times of the fuel elements. Therefore the cladding 
material will be exposed to the corrosive water during a longer period of 
time than normal at the present, which means an increased risk of 
corrosion damages. The constructors of the fuel elements therefore wish 
increased corrosion properties of the used Zircaloy alloys retaining 
sufficient mechanical strength and ductility. 
During the development of suitable cladding materials for thermal reactors 
studies were made of the influence of various alloying elements on the 
corrosion resistance of Zirconium in water and steam. These studies, which 
were performed several decades ago, resulted in two corrosion resistant 
alloys, Zircaloy-2 and Zircaloy-4. These alloys are today prescribed in 
all valid specifications for cladding tubes. The alloying with the 
elements Fe, Cr and Ni (only Zircaloy-2) results in precipitation of 
intermetallic phases containing also Zirconium besides said elements. The 
alloying element Sn dissolves in the matrix of Zr and contributes to an 
increase of the strength by a so-called solution hardening effect. 
At the corrosion of Zircaloy in reactor environment two main types of 
corrosion mechanisms can be distinguished, namely general corrosion which 
is predominant in pressurized water reactors and so-called accelerated 
nodular corrosion which is predominant in boiling water reactors. It has 
been known for a long time that the corrosion resistance of cladding tubes 
of Zircaloy against so-called accelerated nodular corrosion in water and 
steam of high pressure and high temperature is markedly improved by a 
.beta.-phase transformation, socalled .beta.-quenching of the material at 
an early stage of the manufacturing. This .beta.-quenching is performed 
according to valid specifications after forging of the ingot to bar. The 
same favourable effect is also obtained at .beta.-quenching of the tube 
billet before the last cold rolling(s), see U.S. Pat. Nos. 3,865,635, 
4,450,016 and 4,450,020, respectively. The exact reason for the improved 
resistance to accelerated nodular corrosion in water and steam of high 
pressure and high temperature has not yet been fully explained. It seem, 
however, as if the improvement of the corrosion resistance is related to 
the size to the intermetallic phases and their dispersion in the material. 
These phases, so-called secondary phases, are present in the form of 
particles. In conventionally manufactured tubes, i.e. tubes made with a 
.beta.-quenching at the bar stage, the size of said particles lies in the 
interval 0.1-0.6 .mu.m and with a mean particle size of about 0.3 .mu.m. 
In tubes, which in addition are manufactured with a .beta.-quenching of the 
tube billet before the cold rolling(s), on the other hand, a considerable 
decrease of the size of the secondary phase particle is obtained. The 
refined particle size contributes to the desired improvement of the 
resistance to nodular corrosion. 
While the connections between the structure of the Zircaloy tubes and the 
resistance to nodular corrosion are relatively well known and documented, 
the relations between manufacturing procedure, structure and resistance to 
general corrosion in pressurized water reactor environment are not at all 
just as well known. It has been found, however, that the standardized 
manufacture does not result in a cladding tube with optimum corrosion 
properties in pressurized water reactor environment, which means an 
increased risk for corrosion damages at a prolongation of the running time 
of the fuel. The alloying element in Zircaloy, which essentially seems to 
influence the corrosion resistance in PWR water environment, is Fe. As 
earlier indicated, Fe is bound in so-called secondary phase particles. In 
conventionally manufactured tubes, however, all the added iron is not 
present in these particles but a great share is also dissolved in the 
Zirconium matrix. In this connection, it seems as if Fe dissolved in the 
matrix is detrimental to the corrosion resistance. In accordance with the 
present invention it has now been found possible to reach a considerable 
decrease of the content of dissolved Fe in the Zirconium matrix and in 
that way an increased corrosion resistance of cladding tubes of Zircaloy 
by means of certain modifications of established manufacturing procedure. 
These modifications imply annealing after extrusion and/or annealing(s) 
between the cold rollings within a well defined temperature range in the 
.alpha.-phase zone during considerably longer times than standardized in 
order to obtain equilibrium between secondary phase particles and 
Zirconium matrix and in that way a minimum content of Fe in solid 
solution. More precisely, the modification according to the invention 
contributes to a decrease of the content of dissolved Fe in the Zirconium 
matrix according to the following: 
Annealing after extrusion and between cold rollings. 
In conventional manufacture there is performed annealing of extruded tubes 
and annealing between cold rollings in the .alpha.-phase range at 
625.degree.-790.degree. C. for the purpose of recrystallizing the 
structure before the following cold rolling. The extruded product can be 
subjected to a series of cold rollings with an intermediate anneal at 
625.degree.-700.degree. C. between two successive cold rollings to make 
possible the succeeding cold rolling paths. The cooling of the material 
after each intermediate anneal is carried out at a maximum rate of 
3.degree. C. per minute. At these annealings there is a growth of existing 
secondary phase particles and precipitation of new particles, at which the 
content of dissolved Fe in the Zirconium matrix gradually decreases. This 
precipitation process goes relatively slow within the used annealing 
interval. The total annealing time being used as standard is completely 
unsufficient for the process to reach equilibrium and for obtaining a 
minimum content of Fe dissolved in the matrix. By performing the 
annealings at such a combination of temperature and time which leads to 
essentially complete equilibrium, the share of secondary phase particles 
can be maximized and the content of Fe in solid solution in the Zirconium 
matrix can be minimized. The combination of temperature/time is defined by 
an annealing parameter A, according to the following: 
A=t.multidot.e.sup.-Q/RT where 
t=annealing time in hours 
Q=the activating energy of the process in cal/mole 
T=temperature in .degree.K 
R=general gas constant, cal/mole.multidot. degree. 
By annealing tests the activating energy has been estimated to 65000 
cal/mole. In order to reach equilibrium, A has to exceed a critical value 
A.sub.c. This critical value is: 
EQU A.sub.c =2.3.multidot.10.sup.-14. 
Within the actual annealing interval 625.degree.-790.degree. C. said value 
of A.sub.c means a shortest annealing time according to the following 
table, which gives examples for some annealing temperatures. 
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Annealing temp. 
Shortest annealing time 
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.degree.C. h 
790 0.5 
725 3.9 
675 22.2 
650 56.5 
625 151.3 
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According to a modification within the scope of the invention, there is 
performed a further annealing before the last rolling(s) within a well 
defined low temperature range in the .alpha.-phase zone in order to obtain 
the maximum volume fraction of secondary phase. By performing the 
annealings first at a combination of high temperature, more precisely 
between 650.degree. and 790.degree. C., and long time in the .alpha.-phase 
range which leads to equilibrium between the matrix and the secondary 
phase particles and then at a lower temperature, more precisely between 
540.degree. and 650.degree. C. in the .alpha.-phase range, which gives an 
additional contribution of precipitated secondary phase, the share of 
secondary particles can be maximized and the content of Fe in solid 
solution in the Zirconium matrix be minimized. The time at the later low 
temperature annealing in the =60 -phase range shall exceed 2 hours in the 
mentioned temperature range of 540.degree.-650.degree. C. 
From the following example it is clear that an improved corrosion 
resistance has been obtained in manufacturing according to the invention 
compared to conventional technique.

EXAMPLE 
The corrosion resistance has been surveyed by autoclave testing in steam at 
a pressure of 10.3 MPa and a temperature of 400.degree. C. Testing has 
been accomplished on samples taken out from conventionally manufactured 
tubes as well as from tubes made according to the invention. After a 
testing time of 1344 h the increase in weight was measured having the 
following result: 
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Type of tube Increase in weight mg/dm.sup.2 
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According to the invention 
Less than 50 
According to the invention but with 
Less than 48 
an additional annealing before the 
last cold rolling 
Conventionally manufactured 
52-59 
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As appears, the cladding tubes according to the invention have shown a 
smaller increase in weight than standard tubes, which means that the tubes 
according to the invention are expected to be more resistant to corrosion 
in reactor environment. The difference regarding the increase in weight 
may seem small. The testing time is only 1344 h, however, while normal 
working times in commercial pressurized water reactors are of the size of 
4 years or longer time. An extrapolation of above data with knowledge of 
the kinetics of the oxide growth as function of time indicates, however, 
that the tubes according to the invention obtain a considerably better 
corrosion resistance compared to conventionally manufactured tubes.