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The rolling of Be, W and V foil
NUCLEAR
INSTRUMENTS
AND
METItODS
167 ~1979)
105-166:
©
N()RTtt-ttOLLAND
PUBLISII1N(I
('()
THE ROLLING OF Be, W and V FOIL FRANK J. KARASEK
Micro Foils, .4t;eonne, IH#wis 60429, U.S.A.
Procedures are outlined lor the fabrication of Be, W and V foils. Jacketing and rolling procedures are described in detail, as ure the problems encountered during the processing. A discussion of the effect el "~, arious parameters on the f'abricatinn of the li,ils is also presented.
1. Procedures for the rollin~ of b e r ) l l i u m foil The successful preparation of beryllium foils down to a thickness range of l m g / c m 2 has been most difficult. This is due to the large n u m b e r of factors affecting the fabrication procedure. These factors are: 1) The limited reliability of the metal at room temperature - generally approximately 2% reduction. 2) Plane ductility of the material - which is dependent on previous rolling history. 3) The existence of two ductility ranges at two different temperature ranges. The lower temperature ductility is also dependent upon both the strain rate (rolling speed) and roll pass reduction. Therefore, the normal procedure of pack rolling at room temperature and heat treating was not attempted, as this mode would have been very time consuming due to the very limited cold reliability of the material. An alternative procedure of double pack rolling the foil within the temperature limits of the lower ductility range was utilized. Beryllium lbil 0.05 m m (0.002") thick was inserted in a m o l y b d e n u m pack made from 0.05 m m (0.002") foil. This was then inserted into another pack which was made of stainless steel. This double pack was then heated to 375°C and rolled. Pass reductions were limited to approximately 5% per pass and the rolling speed was on the order of 15 feet per minute. The pack was reheated after every roll pass. Rolling was continued until a total of 50% reduction was achieved. The beryllium foil was then removed from the pack and was v a c u u m heat treated at 7 0 0 - 7 5 0 ° C for 1 h and slowly cooled. The heat treated foil was then reinserted in a new double pack.
The lbil was oriented so that the rolling direction of the foil was perpendicular from the previous rolling direction. Heating, rolling find heat treating were repeated. This process was continued until the final foil thickness was attained. Although the foregoing procedure has been used to prepare about 1000 cm ~ of foil - the procedure is not "'fool proof". Difficulties do occur in the fabrication of this material. Factors, such as starting thickness, previous fabrication history, material purity', as well as the aforementioned characteristics can affect the rerolling procedure. 2. Procedure for the rollin~ of tungsten foil Natural tungsten foils in the range of 5 m g / c m " were produced by the rerolling of commercially available foil stock of approximately 0.05 m m (0.002") thickness. The |'oil was initially cold rolled down to 0.025 m m (0.001") using a stainless steel pack. It was found to be advantageous to heat the rolling pack to 2 0 0 - 3 0 0 ° C during the course of rolling. Although the temperature is low - it is sufficient to enhance the reliability of the foil. The foil was reduced to 50-60
GENERAL
TARGET PREPARATION
ME F t I O D S
166
F . J . KARASEK
temperature. The material can only be reclaimed by rolling at a very high temperature and gradually decreasing this temperature during the course of the rolling so as to redevelop the fibrous structure of the original material. A temperature in the range of 8 0 0 - 9 0 0 ° C was found to be sufficient to decrease the stresses introduced by rolling and did not destroy the developed rolling structure.
3. Procedure for the rolling of vanadium foil Starting materials used in the preparation of the required v a n a d i u m foils were of two types. The first type was annealed foil stock of 0.125 m m (0.005") thickness and an assumed purity of 99.95%. The as received foil was reduced to 0.05 m m (0.002") by direct rolling. This foil was ;then cleaned and heat treated at a 850°C temperature for 30 min in a v a c u u m of 10 `6 torr. The annealed foil exhibited considerable stiffness. To attain the foil thicknesses that were required, it was now necessary to roll the foil using the pack rolling technique, that is, the foil was inserted in a pack made of stainless steel and then this composite was rolled. The foil was reinserted in a new pack after each 5 0 - 6 0 % total reduction. This foil was ultimately reduced to a thickness of 1 m g / c m 2. Further attempts to roll this material down to the range of 2 0 0 - 3 0 0 / l g / c m 2 were not successful. Although the thickness could be attained, the useable areas were m u c h smaller
than specified. Additional intermediate heat treatment was tried, as well as changes in heat treat temperature, but these proved to be of little value, as suitable areas still could not be attained. The second type of vanadium was supplied as random sized crystallites, the purity of which was listed to be 99.9 + %. These crystallites were pelletized and the resulting pellet was arc melted to form a button suitable for rolling. This arc cast button was directly cold rolled down to a foil thickness of 0.05 m m (0.002'9. This rolled foil was cleaned and heat treated at 800°C for 30 min in a vacuum of 10 7torr. After annealing the foil was extremely ductile - that is, it did not exhibit the stiffness that one noted on the previous batch. This foil was then rolled to an ultimate thickness of 300/~g/cm 2 using the aforementioned pack rolling method. The required areas were easily obtained alter the final rolling. It is assumed that the differences in the rollability of the two batches of material was due to interstitial content, namely oxygen and nitrogen. An analysis of the two batches indicated that the metallic impurity levels were approximately the same whereas the oxygen and nitrogen levels were 700 ppm and 1 5 0 p p m , respectively, for the annealed foil stock, whereas the arc cast and rolled material contained only 200 ppm oxygen and 30 ppm nitrogen.
INSTRUMENTS
AND
METItODS
167 ~1979)
105-166:
©
N()RTtt-ttOLLAND
PUBLISII1N(I
('()
THE ROLLING OF Be, W and V FOIL FRANK J. KARASEK
Micro Foils, .4t;eonne, IH#wis 60429, U.S.A.
Procedures are outlined lor the fabrication of Be, W and V foils. Jacketing and rolling procedures are described in detail, as ure the problems encountered during the processing. A discussion of the effect el "~, arious parameters on the f'abricatinn of the li,ils is also presented.
1. Procedures for the rollin~ of b e r ) l l i u m foil The successful preparation of beryllium foils down to a thickness range of l m g / c m 2 has been most difficult. This is due to the large n u m b e r of factors affecting the fabrication procedure. These factors are: 1) The limited reliability of the metal at room temperature - generally approximately 2% reduction. 2) Plane ductility of the material - which is dependent on previous rolling history. 3) The existence of two ductility ranges at two different temperature ranges. The lower temperature ductility is also dependent upon both the strain rate (rolling speed) and roll pass reduction. Therefore, the normal procedure of pack rolling at room temperature and heat treating was not attempted, as this mode would have been very time consuming due to the very limited cold reliability of the material. An alternative procedure of double pack rolling the foil within the temperature limits of the lower ductility range was utilized. Beryllium lbil 0.05 m m (0.002") thick was inserted in a m o l y b d e n u m pack made from 0.05 m m (0.002") foil. This was then inserted into another pack which was made of stainless steel. This double pack was then heated to 375°C and rolled. Pass reductions were limited to approximately 5% per pass and the rolling speed was on the order of 15 feet per minute. The pack was reheated after every roll pass. Rolling was continued until a total of 50% reduction was achieved. The beryllium foil was then removed from the pack and was v a c u u m heat treated at 7 0 0 - 7 5 0 ° C for 1 h and slowly cooled. The heat treated foil was then reinserted in a new double pack.
The lbil was oriented so that the rolling direction of the foil was perpendicular from the previous rolling direction. Heating, rolling find heat treating were repeated. This process was continued until the final foil thickness was attained. Although the foregoing procedure has been used to prepare about 1000 cm ~ of foil - the procedure is not "'fool proof". Difficulties do occur in the fabrication of this material. Factors, such as starting thickness, previous fabrication history, material purity', as well as the aforementioned characteristics can affect the rerolling procedure. 2. Procedure for the rollin~ of tungsten foil Natural tungsten foils in the range of 5 m g / c m " were produced by the rerolling of commercially available foil stock of approximately 0.05 m m (0.002") thickness. The |'oil was initially cold rolled down to 0.025 m m (0.001") using a stainless steel pack. It was found to be advantageous to heat the rolling pack to 2 0 0 - 3 0 0 ° C during the course of rolling. Although the temperature is low - it is sufficient to enhance the reliability of the foil. The foil was reduced to 50-60
GENERAL
TARGET PREPARATION
ME F t I O D S
166
F . J . KARASEK
temperature. The material can only be reclaimed by rolling at a very high temperature and gradually decreasing this temperature during the course of the rolling so as to redevelop the fibrous structure of the original material. A temperature in the range of 8 0 0 - 9 0 0 ° C was found to be sufficient to decrease the stresses introduced by rolling and did not destroy the developed rolling structure.
3. Procedure for the rolling of vanadium foil Starting materials used in the preparation of the required v a n a d i u m foils were of two types. The first type was annealed foil stock of 0.125 m m (0.005") thickness and an assumed purity of 99.95%. The as received foil was reduced to 0.05 m m (0.002") by direct rolling. This foil was ;then cleaned and heat treated at a 850°C temperature for 30 min in a v a c u u m of 10 `6 torr. The annealed foil exhibited considerable stiffness. To attain the foil thicknesses that were required, it was now necessary to roll the foil using the pack rolling technique, that is, the foil was inserted in a pack made of stainless steel and then this composite was rolled. The foil was reinserted in a new pack after each 5 0 - 6 0 % total reduction. This foil was ultimately reduced to a thickness of 1 m g / c m 2. Further attempts to roll this material down to the range of 2 0 0 - 3 0 0 / l g / c m 2 were not successful. Although the thickness could be attained, the useable areas were m u c h smaller
than specified. Additional intermediate heat treatment was tried, as well as changes in heat treat temperature, but these proved to be of little value, as suitable areas still could not be attained. The second type of vanadium was supplied as random sized crystallites, the purity of which was listed to be 99.9 + %. These crystallites were pelletized and the resulting pellet was arc melted to form a button suitable for rolling. This arc cast button was directly cold rolled down to a foil thickness of 0.05 m m (0.002'9. This rolled foil was cleaned and heat treated at 800°C for 30 min in a vacuum of 10 7torr. After annealing the foil was extremely ductile - that is, it did not exhibit the stiffness that one noted on the previous batch. This foil was then rolled to an ultimate thickness of 300/~g/cm 2 using the aforementioned pack rolling method. The required areas were easily obtained alter the final rolling. It is assumed that the differences in the rollability of the two batches of material was due to interstitial content, namely oxygen and nitrogen. An analysis of the two batches indicated that the metallic impurity levels were approximately the same whereas the oxygen and nitrogen levels were 700 ppm and 1 5 0 p p m , respectively, for the annealed foil stock, whereas the arc cast and rolled material contained only 200 ppm oxygen and 30 ppm nitrogen.