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Vacuum experimental castings of uranium—Tests of solidification rate control
Vacuum experimental castings of uranium-tests of solidification rate control C l a u d e Guichard and J e a n C l a u d e S o r e t , SocieteIndustrielle de CombustibleNucleaire, Laboratoiresde Veurey,BP1,
38 Veurey-Voroize,/sere, France
Equipment and procedure are described for the vacuum melting and casting of low-alloyed uranium into tubes weighing about 40 kg each. The results of tests are presented and the important design parameters are derived. The method is expected to be applicable to metals having high gas content (eg Cu, Ni) or high vapour pressure (Mn, Cr) which are unsuitable for electron beam melting. Introduction The high chemical affinity of Uranium for most elements, particularly oxygen, necessitates the use of vacuum techniques for melting and casting, moreover, the number of convenient materials that can be used for crucibles, moulds, etc . . . . is fairly small. For these reasons the vacuum bottom pouring technique, after more than ten years of large scale casting has been scarcely improved. For both technical and economic reasons we are experimenting new casting processes. It seems to us that the test described represents new advances in these subjects.
constant speed into a cooling zone. In order to ensure that solidification will take place in the right orientation, the part of the mould still remaining in the coil is kept at Tm.
Trend of the tests These have been made in order to cast low alloyed Uranium tubes (mp approx 1130°C) with diameter 100 × 75 mm, 620 mm long, weight about 40 kg, for which it is required to make castings which are chemically homogeneous, free from major defects and with uniform physical and mechanical properties. The two major objects were:
The vacuum equipment It is fairly compact and includes two mechanical pumps (AIcatel--100 m3.h -t two stages); one Roots pump (SEAVOM --2,000 m~.h-1); a three directional electro-pneumatic valve allows the direct and fast evacuation of the chamber; it is possible to isolate the chamber by an electro-pneumatic sliding-
The equipment Beyond the vacuum equipment and the induction heating power supply, the equipment includes three parts, with a common vertical axis (Figure 1): (a) a stationary chamber, connected with the vacuum equipment (b) a semi-stationary furnace, with its two coils (c) a movable cooling chamber.
(a) To avoid shrinkage defects: it is necessary to bring isotherm and isobar lines as close as possible. (b) To control the rate of solidification: it is necessary to control the rate of cooling; in fact during solidification the nucleation rate is approx proportional to the difference AT between the solid-liquid interface temperature and the equilibrium temperature.
Description of tests To start with, the alloyed bar is remelted in the medium frequency induction vacuum furnace. This bar has been produced in a standard uranium furnace (induction heated quartz type furnace, outside coil, and bottom poured with moulds positioned beneath a graphite crucible). The bar is designed to permit the production of several tubes. The liquid metal fills up an induction pre-heated graphite mould. The bar is suspended vertically and is introduced into a short coil, so that only its end is induction heated. This end is left stationary during melting and so the effective coil impedance remains unchanged. The reading on the phasemeter which controls the oscillator circuit should remain constant. Each shift variation will move the bar in order to keep the oscillatorcircuit tuned. In this way the weight of melted metal is always directly proportional to the distance the consumable bar has been lowered. Before melting the mould and its core (made of graphite) are heated and maintained at a temperature Tm which is slightly higher than the melting point of the alloy. As soon as the bar has descended a distance corresponding to the weight of a tube, the melting is stopped; the mould is then gradually lowered at a
Figure 1
Vacuum/volume 19/number 4. PergamonPress LtdlPrintedin GreatBritain
175
Claude Guichard and Jean Claude Sorer: Vacuum experimental castings of uranium--tests of solidification rate control
t,g3_ Scheme of the ctvslin9 process
valve. The control unit involves a discharge tube, a Thermotron gauge and a cold cathode ionization gauge (Leybold). Within a few minutes, the pressure is lowered to 10 -s tort, and then the medium frequency power is switched on.
..... ,, . . . . . . .
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The induction power supply This works in duplex with another plant and includes two 100 kW--600 volts--3,840 Hz rotating generators (AIsthom) connected in parallel. The selected voltage is kept constant by an automatic exciting system (EMA). This voltage feeds two independent self-transformers (EMA), which may be regulated during the operation. The stationary chamber This is connected to the vacuum chamber and carries the outside system which gradually lowers the electrode. This device is driven by a variable speed dc electric motor. The bar is suspended on a rod which crosses a sliding vacuum seal. The furnace It is a quartz type furnace, similar to those worked out by the CEA; the two induction heating coils are outside the quartz tube, in the open air. The mould coil is cylinder shaped, with a constant pitch, except in front of the graphite gating-funnel where the pitch is smaller. The melting coil is funnel shaped, so as to correct the lack of balance in the coupling due to the melting into an end-pencil shaped bar (Figure 2).
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Figure 3 Preparation The bar is let down into the melting coil, but the two lowest coil turns are left empty. Heating of mould and core This is achieved with a maximum power input of 50 or 60 kW, the core being principally heated by radiation. A uniform temperature about 1,150°C is kept therefore with 30 kW. Bar melting It is carried out with 75 or 80 kW, the electrode being lowered at a speed of 0.7 mm s-L
CASTING P R O C E S S (see Figure 3).
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Downward pull of the mould The downward pull is accomplished at a constant speed (2.5 mm.s-t). The phase shift of the mould coil circuit, produced by the lowering of the mould, causes an automatic fall of the active power induced in the mould. Results The different rates of cooling which have been obtained are: --31 to 37°C mn -1, about 1,000°C - - 2 4 to 27°C mn -1, about 800°C - - 1 3 to 14°C mn -1, about 700°C The phase transformations temperatures are observed at: --750°C for the phase transformation 7 --* fl --650°C for the phase transformation fl -+ ct A good surface quality is obtained if the mould temperature has been maintained above 1,150°C and under 1,300--1,350°C. Further, within these conditions the gammagraphic control shows few internal defects. The results of chemical analysis are identical to those of basis alloy, except with hydrogen content, reduced from l to 0.5 ppm, following the second vacuum casting and the excellent degassing produced on metal streams of small diameters. Conclusions In order to melt the core, direct induction melting requires a suitably shaped coil, and a frequency specially adjusted to the characteristics of the consumable bar Reference depth: d # electrode diameter
Figure 2 176
5
Claude Guichard and Jean Claude Sorer." Vacuum experimental castings of uranium--tests of solidification rate control The system used gives fairly good control of the melt weight (-"- 3 ~o in the present case). On the other hand it does not permit superheating of the liquid metal. The mould heating also requires a carefully selected frequency, d < mould thickness, in order to ensure uniform distribution of power in the mould during filling. Although it is necessary to start from an elaborate alloy, the method may be interesting when applied as a double vacuum melting technique: degassing is excellent, even with a high
melting rate (necessary to reduce the chemical contamination of the liquid metal by the mould material) and sound castings may be obtained by this fairly well orientated solidification. We expect this process could be applied to the remelting under reduced pressure of metals or alloys whose high gas contents (Copper, Nickel) or high vapour pressures (Manganese, Chromium) are unsuitable for melting in electron beam furnaces.
177
38 Veurey-Voroize,/sere, France
Equipment and procedure are described for the vacuum melting and casting of low-alloyed uranium into tubes weighing about 40 kg each. The results of tests are presented and the important design parameters are derived. The method is expected to be applicable to metals having high gas content (eg Cu, Ni) or high vapour pressure (Mn, Cr) which are unsuitable for electron beam melting. Introduction The high chemical affinity of Uranium for most elements, particularly oxygen, necessitates the use of vacuum techniques for melting and casting, moreover, the number of convenient materials that can be used for crucibles, moulds, etc . . . . is fairly small. For these reasons the vacuum bottom pouring technique, after more than ten years of large scale casting has been scarcely improved. For both technical and economic reasons we are experimenting new casting processes. It seems to us that the test described represents new advances in these subjects.
constant speed into a cooling zone. In order to ensure that solidification will take place in the right orientation, the part of the mould still remaining in the coil is kept at Tm.
Trend of the tests These have been made in order to cast low alloyed Uranium tubes (mp approx 1130°C) with diameter 100 × 75 mm, 620 mm long, weight about 40 kg, for which it is required to make castings which are chemically homogeneous, free from major defects and with uniform physical and mechanical properties. The two major objects were:
The vacuum equipment It is fairly compact and includes two mechanical pumps (AIcatel--100 m3.h -t two stages); one Roots pump (SEAVOM --2,000 m~.h-1); a three directional electro-pneumatic valve allows the direct and fast evacuation of the chamber; it is possible to isolate the chamber by an electro-pneumatic sliding-
The equipment Beyond the vacuum equipment and the induction heating power supply, the equipment includes three parts, with a common vertical axis (Figure 1): (a) a stationary chamber, connected with the vacuum equipment (b) a semi-stationary furnace, with its two coils (c) a movable cooling chamber.
(a) To avoid shrinkage defects: it is necessary to bring isotherm and isobar lines as close as possible. (b) To control the rate of solidification: it is necessary to control the rate of cooling; in fact during solidification the nucleation rate is approx proportional to the difference AT between the solid-liquid interface temperature and the equilibrium temperature.
Description of tests To start with, the alloyed bar is remelted in the medium frequency induction vacuum furnace. This bar has been produced in a standard uranium furnace (induction heated quartz type furnace, outside coil, and bottom poured with moulds positioned beneath a graphite crucible). The bar is designed to permit the production of several tubes. The liquid metal fills up an induction pre-heated graphite mould. The bar is suspended vertically and is introduced into a short coil, so that only its end is induction heated. This end is left stationary during melting and so the effective coil impedance remains unchanged. The reading on the phasemeter which controls the oscillator circuit should remain constant. Each shift variation will move the bar in order to keep the oscillatorcircuit tuned. In this way the weight of melted metal is always directly proportional to the distance the consumable bar has been lowered. Before melting the mould and its core (made of graphite) are heated and maintained at a temperature Tm which is slightly higher than the melting point of the alloy. As soon as the bar has descended a distance corresponding to the weight of a tube, the melting is stopped; the mould is then gradually lowered at a
Figure 1
Vacuum/volume 19/number 4. PergamonPress LtdlPrintedin GreatBritain
175
Claude Guichard and Jean Claude Sorer: Vacuum experimental castings of uranium--tests of solidification rate control
t,g3_ Scheme of the ctvslin9 process
valve. The control unit involves a discharge tube, a Thermotron gauge and a cold cathode ionization gauge (Leybold). Within a few minutes, the pressure is lowered to 10 -s tort, and then the medium frequency power is switched on.
..... ,, . . . . . . .
o
~eoph,!*
The induction power supply This works in duplex with another plant and includes two 100 kW--600 volts--3,840 Hz rotating generators (AIsthom) connected in parallel. The selected voltage is kept constant by an automatic exciting system (EMA). This voltage feeds two independent self-transformers (EMA), which may be regulated during the operation. The stationary chamber This is connected to the vacuum chamber and carries the outside system which gradually lowers the electrode. This device is driven by a variable speed dc electric motor. The bar is suspended on a rod which crosses a sliding vacuum seal. The furnace It is a quartz type furnace, similar to those worked out by the CEA; the two induction heating coils are outside the quartz tube, in the open air. The mould coil is cylinder shaped, with a constant pitch, except in front of the graphite gating-funnel where the pitch is smaller. The melting coil is funnel shaped, so as to correct the lack of balance in the coupling due to the melting into an end-pencil shaped bar (Figure 2).
..,
•
•.,.~.t~.
:
o
o~oulff_ coil
o
Gcophile_mo~ild O[ll
II o
0 0
0 0
~¢~aph,le
Core
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° I-'_~ o
~rer
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jacket
STATE. I preheotmg
STATE_2 M e l h n g o~d fd/in s o f the m o u l d
STATE _ 3 .~olidihcatio~
Figure 3 Preparation The bar is let down into the melting coil, but the two lowest coil turns are left empty. Heating of mould and core This is achieved with a maximum power input of 50 or 60 kW, the core being principally heated by radiation. A uniform temperature about 1,150°C is kept therefore with 30 kW. Bar melting It is carried out with 75 or 80 kW, the electrode being lowered at a speed of 0.7 mm s-L
CASTING P R O C E S S (see Figure 3).
r
o
Downward pull of the mould The downward pull is accomplished at a constant speed (2.5 mm.s-t). The phase shift of the mould coil circuit, produced by the lowering of the mould, causes an automatic fall of the active power induced in the mould. Results The different rates of cooling which have been obtained are: --31 to 37°C mn -1, about 1,000°C - - 2 4 to 27°C mn -1, about 800°C - - 1 3 to 14°C mn -1, about 700°C The phase transformations temperatures are observed at: --750°C for the phase transformation 7 --* fl --650°C for the phase transformation fl -+ ct A good surface quality is obtained if the mould temperature has been maintained above 1,150°C and under 1,300--1,350°C. Further, within these conditions the gammagraphic control shows few internal defects. The results of chemical analysis are identical to those of basis alloy, except with hydrogen content, reduced from l to 0.5 ppm, following the second vacuum casting and the excellent degassing produced on metal streams of small diameters. Conclusions In order to melt the core, direct induction melting requires a suitably shaped coil, and a frequency specially adjusted to the characteristics of the consumable bar Reference depth: d # electrode diameter
Figure 2 176
5
Claude Guichard and Jean Claude Sorer." Vacuum experimental castings of uranium--tests of solidification rate control The system used gives fairly good control of the melt weight (-"- 3 ~o in the present case). On the other hand it does not permit superheating of the liquid metal. The mould heating also requires a carefully selected frequency, d < mould thickness, in order to ensure uniform distribution of power in the mould during filling. Although it is necessary to start from an elaborate alloy, the method may be interesting when applied as a double vacuum melting technique: degassing is excellent, even with a high
melting rate (necessary to reduce the chemical contamination of the liquid metal by the mould material) and sound castings may be obtained by this fairly well orientated solidification. We expect this process could be applied to the remelting under reduced pressure of metals or alloys whose high gas contents (Copper, Nickel) or high vapour pressures (Manganese, Chromium) are unsuitable for melting in electron beam furnaces.
177