Nitrogen annealing of zirconium and zirconium alloys

A method of continuously nitrogen annealing zirconium and zirconium alloys at temperatures at from 1000.degree. F. to 1600.degree. F. for from 1/2 minute to 10 minutes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a continuous process for annealing zirconium and 
zirconium alloys. More specifically, it deals with the use of a nitrogen 
atmosphere which allows the process to be continuous. 
2. Description of the Prior Art 
The idea of continuous annealing of metals is old in the art. Even the idea 
of continuous annealing in a nitrogen atmosphere has been used in 
annealing steel and certain metals. This shown in U.S. Pat. No. 4,183,773, 
Kawasoko et al, wherein a hydrogen-nitrogen atmosphere is used. 
It has also been known to nitride metals including zirconium for the 
purpose of producing hardness. This hardness, however, is effected at the 
expense of ductility. 
The usual way of annealing zirconium or zirconium alloys is by vacuum 
annealing since these metals are very reactive and considerably more 
reactive than steel. This vacuum annealing is extremely expensive not only 
with regard to equipment, but also with regard to operation. There is a 
need for a continuous process for annealing zirconium and zirconium alloys 
for economic purposes. However, a nitrogen atmosphere which would be an 
inexpensive atmosphere, has been avoided because of the reactivity of 
these metals. This is also recognized by MacEwen et al, U.S. Pat. No. 
4,000,013, where a vacuum atmosphere is stated as preferred over helium or 
argon which have been treated to remove all traces of deleterious 
substances such as oxygen, nitrogen, etc. 
BRIEF SUMMARY OF THE INVENTION 
1. Objects of the Invention 
It is, accordingly, one object of the invention to provide a process for 
continuously annealing zirconium and zirconium alloys. 
A further object of the present invention is to provide a process for 
continuously annealing zirconium and zirconium alloys in a nitrogen 
atmosphere. 
A still further object of the present invention is to set forth a process 
for continuously annealing zirconium and zirconium alloys less expensively 
than vacuum anneal while still producing products having high yield 
strength, ultimate tensile strength and high ductility.

These and other advantages of the present invention will become apparent 
from the following detailed description and examples. 
In accordance with the above objects, it has been found that zirconium and 
zirconium alloys can be continuously annealed. This process is possible 
using a nitrogen atmosphere, thus avoiding the more expensive and slower 
vacuum annealing process used in the past. 
DETAILED DESCRIPTION OF THE INVENTION 
The inventive concept of the present invention is to continuously anneal 
zirconium and zirconium alloys in the presence of a nitrogen atmosphere. 
Although the idea of using a nitrogen atmosphere with such highly reactive 
metals has been unthinkable in the past, it has now been found that it is 
not only possible, but that it produces a product having better properties 
than that produced by vacuum annealing. The reason this is possible is 
because the continuous process is so much faster than the batch vacuum 
annealing process that the metals are exposed to the heat and atmosphere 
for comparatively very short periods of time. Specifically, what takes 
about two hours to vacuum anneal now can be performed in a continuous 
process in less than three minutes. It has further been found that the 
reaction between these metals and nitrogen is slow enough to make this 
nitrogen annealing not only possible, but desirable. 
The nitrogen annealing process of the present invention produces less grain 
growth because of the limited exposure to heat. This finer grain size is 
responsible for increased yield strength and ultimate tensile strength. 
This nitrogen annealing process is also much more economical than vacuum 
annealing in that the product is produced much faster, the apparatus for 
continuous annealing is less expensive than that for vacuum annealing, and 
the production cost for maintaining a nitrogen atmosphere versus a vacuum 
atmosphere is considerably less. 
Although nitrogen annealing has the above benefits, one must be careful not 
to anneal at too high temperatures or for too long a time since this will 
cause increased nitriding which must be removed or the ductility will 
suffer. If there is too heavy a nitride coating, it will become very 
expensive to remove. 
The following equation serves to guide one in performing the invention 
while avoiding the above problem of too heavy nitrogen pick up or 
nitriding: 
EQU Time=a 2.7183.sup.(Q/R) 1/Temp .degree.K. 
a=a constant for each alloy in the range of 3.times.10.sup.-10 to 
2.times.10.sup.-13 
Q/R=an activation energy constant which is in the range of 20,000 to 45,000 
The following examples were made and tested for strength and formability, 
the results of which are illustrated in the tables set forth below. A 
Zircaloy-4 strip having the following composition was prepared in the 
following manner: 
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Zircaloy-4 (nominally) 
______________________________________ 
1.5% Sn 
0.2% Fe 
0.1% Cr 
balance 
Zr 
______________________________________ 
This material was produced by hot forging in the beta phase, hot rolling in 
the alpha phase, and cold rolling at least 50% reduction with alpha phase 
intermediate anneals following each 30 to 40% reduction. 
This material was split into two lots, one of which was vacuum annealed and 
one of which was nitrogen annealed, and subsequently tested for yield 
strength, ultimate tensile strength, elongation, ductility or formability, 
and finally for nitrogen and oxygen pickup. The results of these tests are 
set forth in the following tables. 
The above zirconium alloy strip was nitrogen annealed for 3 minutes at 
1300.degree. F., and the strip was then tested both transversely and 
longitudinally for elongation, ultimate tensile strength, and yield 
strength. The results are shown in Table I. 
TABLE I 
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Ultimate 
Tensile Yield 
% Strength Strength 
Example Elongation psi psi 
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1. Zr-4, Trans. 
32 64,700 51,100 
2. Zr-4, Trans. 
31 64,700 50,700 
3. Zr-4, Trans. 
32 63,300 49,300 
4. Zr-4, Long. 
32 63,800 47,500 
5. Zr-4, Long. 
31 63,500 47,900 
6. Zr-4, Long. 
31 63,800 48,600 
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Trans. = transverse testing 
Long. = longitudinal testing 
A comparison between the average of the results in Table I and the same 
alloys treated by vacuum annealing was made. This comparison is shown in 
the following Table II where there is also shown a comparison of grain 
sizes. 
TABLE II 
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Ultimate 
Tensile Yield 
% Strength Strength 
Grain 
Example Elongation psi psi Size 
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7. Zr-4, Long 
34 60.6 48.9 91/2 
8. Zr-4, Trans. 
34 60.5 49.3 
9. Zr-4, Long. 
31 58.3 46.9 10 
10. Zr-4, Trans. 
32 59.5 48.2 
*11. Zr-4, Long. 
31.3 63.7 48.0 101/2 
*12. Zr-4, Trans. 
31.7 64.2 50.4 
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*Nitrogen annealed for 3 minutes at 1300.degree. F. 
In Table II, Examples 7 through 10 were vacuum annealed and can be compared 
to Examples 11 and 12 which have been nitrogen annealed as set forth 
above. 
TABLE III 
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Ultimate 
Tensile Yield 
% Strength 
Strength 
Example Temp. Elongation 
psi psi 
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*13. Zr-4, Trans. 
600.degree. F. 
42 31,100 20,700 
*14. Zr-4, Trans. 
" 43 31,100 20,400 
*15. Zr-4, Trans. 
" 41 31,300 21,000 
*16. Zr-4, Long. 
" 46 35,300 18,300 
*17. Zr-4, Long. 
" 46 35,400 18,600 
*18. Zr-4, Long. 
" 46 35,300 18,200 
19. Zr-4, Trans. 
" 43 26,900 17,500 
20. Zr-4, Trans. 
" 43 27,000 17,500 
21. Zr-4, Trans. 
" 44 27,200 17,600 
22. Zr-4, Long. 
" 51 29,300 16,500 
23. Zr-4, Long. 
" 51 30,100 15,900 
24. Zr-4, Long. 
" 52 28,900 15,700 
*25. Zr-4, Trans. 
R. T. 31 69,400 61,200 
*26. Zr-4, Trans. 
" 31 68,700 61,000 
*27. Zr-4, Trans. 
" 31 69,100 60,600 
*28. Zr-4, Long. 
" 32 72,900 51,200 
*29. Zr-4, Long. 
" 28 73,700 50,900 
*30. Zr-4, Long. 
" 29 74,100 51,200 
31. Zr-4, Trans. 
" 30 65,500 56,200 
32. Zr-4, Trans. 
" 31 65,400 56,000 
33. Zr-4, Trans. 
" 31 65,100 56,500 
34. Zr-4, Long. 
" 32 69,600 49,400 
35. Zr-4, Long. 
" 31 69,300 49,400 
36. Zr-4, Long. 
" 30 70,200 50,600 
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*Nitrogen annealed for 3 minutes at 1300.degree. F. 
R. T. = Room Temperature 
Table III further illustrates comparatively properties of Zircaloy-4 metal 
which has been nitrogen annealed versus the same Zircaloy-4 metal which 
has been vaccum annealed. 
Two strips of Zircaloy-4 were separately treated by nitrogen annealing and 
vacuum annealing and then tested for ductility and formability. The result 
of this test is represented in Table IV where 2T and 1.6T represent the 
bending of the metal around a mandrel having a radius two times and 1.6 
times the thickness of the material, respectively. 
TABLE IV 
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Example 2T 1.6T 
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*37. Zr-4, Trans. 
no cracks no cracks 
*38. Zr-4, Trans. 
no cracks no cracks 
39. Zr-4, Trans. 
slight orange peel 
slight orange peel 
40. Zr-4, Trans. 
slight orange peel 
slight orange peel 
*41. Zr-4, Long. 
no cracks no cracks 
*42. Zr-4, Long. 
no cracks no cracks 
*43. Zr-4, Long. 
no cracks no cracks 
44. Zr-4, Long. 
slight orange peel 
slight orange peel 
45. Zr-4, Long. 
slight orange peel 
slight orange peel 
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*Nitrogen annealed at 1300.degree. F. for 3 minutes 
Trans. = transverse testing 
Long. = longitudinal testing 
In an attempt to determine the depth and amount of oxygen and nitrogen 
pickup from the annealing process, an Auger analysis was performed on 
Zircaloy-4, the results of which are shown in Table V. 
EXAMPLE V 
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Ex- 
am- Posi- Composition of Nitride Layer, Weight Percent 
ple tion C O N S Fe Sn Zr F Si 
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I AR 22.1 5.9 .55 .72 .63 -- 69.0 1.1 -- 
Base 1.94 .35 -- -- .19 .83 95.8 -- .88 
(200.ANG.) 
II AR 9.2 12.5 1.7 .42 1.1 -- 71.5 -- 3.2 
100.ANG. 
11.0 2.2 3.65 -- -- .77 82.2 -- -- 
Base 1.3 .28 -- -- .27 .93 96.3 -- .82 
(500.ANG.) 
III AR 8.7 15.4 .37 .27 .95 .47 70.9 -- 2.8 
100.ANG. 
5.9 12.6 .51 -- .66 .32 79.1 -- .83 
7000.ANG. 
3.3 2.8 -- -- .24 .92 91.7 -- .96 
IV AR 17.5 7.47 
.40 -- 1.3 .28 69.6 -- 2.9 
700.ANG. 
5.5 11.9 .36 -- 1.5 .34 78.6 -- 1.8 
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AR = As Received 
Example I represents unannealed material in the as-received condition. 
Example II was annealed for 10 minutes at 1250.degree. F. in pure 
nitrogen. Examples III and IV were annealed for 5 minutes 1250.degree. F. 
in nitrogen; however, it was discovered that the furnace leaked during 
these examples and, therefore, there was a considerable amount of air in 
the furnace during the annealing. 
Some tests made at different times and temperatures on Zircaloy-4 gave the 
result shown in Table VI. 
TABLE VI 
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Transverse Longitudinal 
Heat Treatment 
YS UTS El YS UTS El 
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3 Min. 704.degree. C. 
59.00 68.00 31.0 47.05 73.20 32.0 
6 Min. 704.degree. C. 
60.90 70.20 29.67 48.60 74.00 31.67 
6 Min. 732.degree. C. 
59.67 69.70 30.0 46.40 74.73 31.67 
4 Min. 760.degree. C. 
59.73 64.43 31.0 47.87 74.63 32.0 
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Annealing trials performed on zicronium-2.5% columbium alloy strip at 
diferent times and temperatures produced the results shown in Table VII. 
TABLE VII 
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Room Temperature Tensile Test Results 
Heat 
Treat- 
Transverse Longitudinal 
ment YS, ksi UTS, ksi El % YS, ksi 
UTS, ksi 
El % 
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2 Min. 
87.0 91.3 17.0 66.4 91.1 26.0 
732.degree. C. 
4 Min. 
88.5 95.2 16.3 64.6 93.7 21.3 
732.degree. C. 
8 Min. 
89.6 97.1 17.3 62.1 92.2 20.3 
732.degree. C. 
1 Min. 
86.6 95.1 17.7 65.4 93.7 25.0 
760.degree. C. 
2 Min. 
85.7 96.8 17.3 66.5 94.5 21.7 
760.degree. C. 
4 Min. 
85.2 95.5 18.0 68.3 98.8 22.0 
760.degree. C. 
8 Min. 
84.9 95.5 18.3 66.2 96.8 20.7 
760.degree. C. 
1 Min. 
85.4 95.4 19.3 65.2 96.5 20.0 
815.degree. C. 
2 Min. 
90.2 102.3 17.3 64.7 96.3 20.3 
815.degree. C. 
4 Min. 
85.5 98.0 15.7 67.3 96.6 15.3 
815.degree. C. 
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Two additional batches of zirconium-2.5% columbium alloy strip were 
annealed at 760.degree. C. for four minutes at temperature. Tensile test 
results for these two batches are shown in Table VIII. 
TABLE VIII 
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Transverse Longitudinal 
Batch YS UTS El YS UTS El 
______________________________________ 
840392 85.7 89.3 18.7 64.0 88.2 25.0 
840510 106.8 109.5 18.5 71.0 97.6 18.5 
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Annealing trials performed on pure zirconium strip at different times and 
temperatures produced the results shown in Table IX. 
TABLE IX 
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Heat Treatment 
Vickers Hardness, HV10 
% Recrystal 
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3 Min. 538.degree. C. 
173 0 
5 Min. 538.degree. C. 
167 0 
7 Min. 538.degree. C. 
178 0 
2 Min. 566.degree. C. 
171 0 
3 Min. 566.degree. C. 
158 25 
4 Min. 566.degree. C. 
156 50 
6 Min. 566.degree. C. 
150 80 
2 Min. 593.degree. C. 
146 100 
3 Min. 593.degree. C. 
142 100 
4 Min. 593.degree. C. 
143 100 
6 Min. 593.degree. C. 
142 100 
2 Min. 621.degree. C. 
139 100 
3 Min. 621.degree. C. 
139 100 
4 Min. 621.degree. C. 
138 100 
6 Min. 621.degree. C. 
137 100 
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Although most of the above nitrogen annealing performed on Zircaloy strip 
was performed at 1300.degree. F. for 3 minutes, the nitrogen annealing can 
be performed at lower and higher temperatures inversely proportional to 
the residence time of the material in the furnace. Therefore, it is 
possible to produce an acceptable product at temperatures from 
1000.degree. to 1600.degree. F. and times of treatment can be from 1/2 
minute to 10 minutes. The parameters can, therefore, vary from 1 minute at 
1250.degree. F. to 5 minutes at 1200.degree. F. to 10 minutes at 
1150.degree. F. The important thing is that the temperature and time 
coincide for a time sufficient to cause stress relief (recovery before 
recrystallization) but no longer than complete recrystallization. In this 
regard, the formula stated above applies. 
As this invention may be embodied in several forms without departing from 
the spirit or essential characteristics thereof, the present embodiment 
is, therefore, illustrative and not restrictive, since the scope of the 
invention is defined by the appended claims rather than by the description 
preceding them, and all changes that fall within the mete and bounds of 
the claims or that form their functional as well as conjointly cooperative 
equivalent are, therefore, intended to be embraced by those claims.