JPH08100998A - Cold-wall melting furnace and melting method capable of melting under ultra-high vacuum condition - Google Patents

Abstract

PURPOSE: To enable a more high degree vacuum state to be realized by a method wherein an induction heating coil of a cold-wall vacuum induction melting furnace and an inter-furnace wiring are formed of a water-cooled copper pipe not covered with an insulation cover and further a composing element such as a vacuum tank is processed to have a smooth surface by an electrolytic grinding and a chemical grinding or the like. CONSTITUTION: An induction heating coil 8 of vacuum pipe arranged at an outer circumference of a main body 1 of a cold-wall vacuum melting crucible stored in a vacuum tank and a vacuum conduit 4 for electrically connecting between the induction heating coil 8 and a power supply and performing a cooling of the induction heating coil 8 ar formed of a bare copper pipe having no insulating coverage so as to eliminate a gas discharged out of an insulation covering layer. An inner surface of the vacuum tank and/or an entire surface of the main body 1 of the cold-wall vacuum melting crucible is smoothed through an electrolytic grinding plus complex grinding, and a glass-bead blast processing. With such an arrangement as above, gas discharged out of the surface of the main body 1 is reduced and a vacuum-reaching pressure is further reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is housed in a vacuum chamber.
A cold wall vacuum induction melting furnace that can be operated under ultra-high vacuum at a pressure lower than ^-6 Torr, and using such a melting furnace,
Dissolve at a pressure lower than 10 ^-4 Torr by evacuation, or introduce an inert gas after reaching such a vacuum degree,
The present invention relates to a cold wall vacuum induction melting furnace and a melting method for melting a high-purity, high-melting point metal, an active metal, or an alloy of these metals by induction heating.

[0002]

2. Description of the Related Art As a cold wall vacuum induction melting furnace for melting a metal or an alloy (hereinafter simply referred to as a metal) by electromagnetic induction heating, the ultimate pressure in the furnace charging state is 10 ^-4 To.
It was in the 10 ^-5 Torr range from rr. Prior to examining the problem of increasing the vacuum degree of the cold wall induction melting furnace, which is the subject of the present invention, the overall structure and operation of the cold wall induction melting furnace will be described in order to facilitate understanding. The cold wall melting furnace is conceptually shown in FIG. 1 as a schematic side sectional view, as a partial sectional front view in FIG. 2, and as a schematic configuration in FIG. 3 as a schematic side sectional view. Referring to FIGS. 1 and 2, a crucible body 1 of a cold wall induction melting furnace is formed of a metal such as copper having good conductivity and thermal conductivity into a hollow cylindrical shape with a bottom, and a side wall portion or a side wall thereof. From the upper end to a part of the bottom wall are divided into a plurality of circumferentially divided segments 3 by a plurality of narrow slits 2 which are vertically opened and have a width of about several mm, and each segment 3 is hollow. A passage for cooling water is provided inside and is cooled by water flowing inside. An induction heating coil 8 is arranged on the outer periphery of the crucible body 1, and the charged metal is melted to form a molten metal 9, but the upper part thereof rises by electromagnetic induction action as shown at 9c to contact the furnace wall. Is prevented.

A hollow portion 11 for introducing cooling water is opened in each of the plurality of segments 3, and a cooling water pipe 11a is inserted into the inside of the hollow portion 11 if necessary to form a double pipe structure and to form a furnace wall. The bottom wall is water-cooled, and the side wall of the segment 3 and the furnace bottom are cooled. As a result, the molten metal 9 is prevented from being welded to the furnace wall, while a thin-skinned skull 5 in which the charged metal is solidified is formed at the bottom of the furnace, and a container for holding the molten metal 9 thereon is formed. Serves as the bottom.
For this reason, since the conventional furnace body was made of refractory material and had a structure that could not be water-cooled, pollution occurs due to direct contact between the furnace body and molten metal at high temperatures, whereas in a cold wall melting furnace Since there is no such concern, it has come to be widely used as a melting furnace suitable for melting high purity and high melting point metals and alloys. FIG. 3 shows a cooling water inlet 4a and an outlet 4 for cooling the induction heating coil 8 made of a hollow tube arranged on the outer periphery of the crucible body 1 whose schematic structure is shown in FIGS.
hollow conduit 4 including b, the side wall of the segment 3 and the bottom 7
It is a front view which shows the structure of the vacuum induction melting furnace 10 provided with the cooling water inlet 9a and the outlet 9b which cool.

[0004]

In the conventional cold wall melting furnace, the ultimate pressure in the charging state is 10 ^-4 as described above.
The reason for operating at a vacuum degree of 10 ^-5 Torr from Torr at all is that there is little demand for ultra-high-purity metal produced using this type of melting furnace, so if 1) up to 10 ^-4 Torr For example, it is not necessary to apply any special treatment to the inner surface of the vacuum tank. 2) Since the amount of gas released from the cold wall induction melting furnace itself was quite large, no special treatment was required up to 10 ^-4 Torr. However, in order to reduce the pressure below 10 ^-4 Torr, it was necessary to perform a special surface smoothing treatment to reduce the gas released from the induction melting furnace body. The gas released from the body of the induction melting furnace includes copper that constitutes the body of the cold wall melting furnace, the stainless steel that constitutes the vacuum chamber that houses the furnace body, and the insulating portion of the electromagnetic induction heating coil, and the heating coil. Gas emission from the insulating layer such as rubber or other organic material on the surface of the water-cooled cable that supplies electric power to the can be mentioned as the material of this insulating layer, such as taping with silicon rubber or glass tape, or special insulation. Has been done. The reason why the insulation treatment is required is that the discharge phenomenon occurs because the voltage applied to the heating coil is 600 to 1000V.

This discharge phenomenon is shown in Paschen's law.
As you can see, the numbers Torr to 10^-1Corona discharge from Torr stand
Insufficient insulation is required in order to cause insulation breakdown.
Was. Paschen's law above is explained as follows:
You. The gap d (m between the parallel plate electrodes to which a predetermined voltage is applied
m) is kept constant and the pressure P is lowered, the spark voltage becomes
Gradually decreases. This is because the mean free path becomes smaller
In addition, the acceleration of electrons due to the electric field is large, and the impact ionization is active.
This is because The spark voltage is a value that has a product Pd of P and d
Shows the minimum value at the point where
To increase. This is because the mean free path becomes too small
Since the number of collisions of electrons between the electrodes decreases,
This is because the separation becomes inactive. As an example, the minimum spark voltage
And the value of Pd that gives it is shown below.atmosphere Voltage (V) Pd (Torr)  Air 330 5.67 N₂ 250 6.70 The discharge gas amount Q of the vacuum melting apparatus material is shown below. Material name 1)Stainless steel Time after starting exhaust Amount of released gas Q (Pa m ³ / s ・ m ² ) Raw material for 1 hr 3.7x10^-Five Mechanical polishing 1hr 2.8x10^-6 2)copper Raw material for 1 hr 5.3x10^-Five Mechanical polishing 1hr 4.7x10^-6 The stainless steel of 1) above is mainly used for vacuum tanks.
2) copper is used to make cold
As a material for crucibles and induction coils that are water-cooled in a melting furnace
Used.

To achieve a high vacuum, the amount of gas released from the material exposed to the vacuum becomes a problem, which is related to the ultimate pressure of the vacuum tank. If the ultimate pressure is P, P
Is given by the following equation. P = Q / S + P ₀ where Q is the total amount of released gas (Torr · l / s) S is the effective pumping speed of the vacuum pump (l / s) P ₀ is the ultimate pressure of the vacuum pump (Torr) Thus, it can be seen that it is effective to reduce the total amount Q of the gas released from the materials forming the furnace body and the coil in order to reduce the ultimate pressure. In order to lower P ₀ , it is natural to use a vacuum pump with a low ultimate pressure. To meet this requirement, a jet flow is created by vapor of oil or mercury and gas molecules that have jumped in by diffusion are caught and carried. An oil diffusion pump (also called a diffusion pump or diffusion pump) that leaves, a liquid nitrogen trap (used in combination with other vacuum pumps) and liquid that condenses and captures water vapor and organic vapors (excluding methane) at low temperature obtained by liquid nitrogen A cryopump for condensing a gas on a solid surface at the temperature of hydrogen or liquid helium can be used.

[0007]

As is apparent from the above description of the problem to be solved, it is necessary to reduce the amount of gas discharged from the vacuum chamber and the induction coil to a certain level or less in order to achieve a high vacuum. In order to change from a vacuum of 10 ^-4 Torr level to a vacuum of 10 ^-8 Torr or 10 ^-9 Torr level, 1) the induction coil and the part of the connecting conductor from the induction coil to the power source in the middle vacuum chamber are: since at present not found commercial products of insulating coating material to be low satisfaction of released gas quantity commercially for the conductor portion between the induction coil is used without isolation, the applied voltage is more than 600V, 10 ^{- Four}
Melting is performed under a high vacuum at a pressure lower than Torr, or after reaching the above-mentioned high vacuum, an inert gas is introduced and the melting is performed in a region where discharge is not generated under a pressure of 200 Torr or more. 2) As the surface treatment of copper members such as water-cooled crucibles and induction coils, the above-mentioned blast treatment of glass beads (hereinafter referred to as GB
B treatment), chemical polishing, electrolytic polishing, electrolytic polishing and composite polishing (hereinafter referred to as electrolytic polishing + composite polishing), etc. are selected and processed. 3) As a method for treating the inner surface of the vacuum chamber, when the material is SUS304, which is a typical steel type of 18-8 stainless steel, any of electrolytic polishing, electrolytic polishing + composite polishing, GBB treatment, mirror finishing, etc. The processing is selected and performed according to the value of the ultimate pressure. In any case, the above process is performed as a measure to reduce the released gas.

[0008]

In the cold wall vacuum melting furnace of the present invention, the inner surface of the stainless steel vacuum chamber, the furnace body of the cold wall melting furnace, and the induction heating coil are electrolytically polished, electrolytically polished + compositely polished, chemically polished, GBB. Since the surface is treated extremely smooth by the treatment, the gas molecules adsorbed on the surface are reduced, and the induction heating coil and the wiring part in the vacuum chamber use bare conductors without insulation coating. ing.
Therefore, the amount of gas released from these parts into the vacuum chamber is extremely small, and the vacuum chamber can be evacuated from 10 ^-4 Torr to a lower vacuum of 10 ^-8 or 10 ^-9 units. Under high vacuum, or after reaching such a vacuum state, an inert gas such as argon gas or N ₂ gas is introduced and melting is performed under a pressure of 200 Torr or more to dissolve ultra-high purity metals and alloys. Enable Further, under such a high vacuum, the partial pressure of oxygen in the vacuum chamber can be further reduced, so that a slight oxidation reaction of the active metal or the like can be prevented.

[0009]

【Example】

Example 1 An uncoated copper pipe is used between the induction coil of the cold wall melting furnace and a portion from the induction coil to the power source introduction part of the vacuum chamber without using a water cooling cable coated with an organic insulating material such as silicon rubber. did. As a result, the released gas Q is reduced to 1
It can be set to 0 ^-4 Torr.l / s and the degree of vacuum is 10 ^-6 Torr.
It was possible to carry out the dissolution while keeping it lower. Example 2 In addition to the induction coil and the copper tube from the induction coil to the power source introduction portion of the vacuum chamber, the copper bus bar, and the copper wire being uncoated, the SUS304 (18-8 stainless steel) forming the vacuum chamber was used. On the inner surface, the amount of released gas Q was set to 8 × 10 ⁻⁶ Pa m ³ / s · m ² one hour after the start of exhaustion by GBB treatment. As a result, it was possible to carry out the melting in a state where the degree of vacuum was kept lower than 10 ⁻⁷ Torr.

Example 3 As in Example 1, an uncoated copper pipe was used between the induction coil of the cold wall melting furnace and the power supply introduction section of the vacuum chamber. The inner surface of SUS304 that constitutes the vacuum chamber is
The amount Q of released gas is 6 after 1 hour from the start of evacuation by electrolytic polishing.
× 10 ⁻⁶ Pa m ³ / s · m ² On the other hand, regarding the copper that constitutes the furnace body of the cold wall melting furnace, the amount of released gas Q was 1 × 10 ^-5 Pa m ³ by GBB treatment one hour after the start of exhaust.
/ S · m ² By these means, the total amount Qt of all the released gas in the vacuum chamber could be set to 9 × 10 ⁻⁵ Torr · l / s, and the degree of vacuum could be kept lower than 10 ⁻⁶ Torr. After reaching this vacuum degree, argon gas was introduced and dissolution was performed at a pressure of 200 Torr.

[0011]

EFFECTS OF THE INVENTION In the conventional cold wall vacuum induction melting furnace, the ultimate pressure in the charged state of the furnace was from 10 ^-4 Torr to 10 ^-5 Torr at most, and the induction heating coil and the in-tank wiring were insulated. The water-cooled copper pipe that is not applied, while maintaining the structure of the components such as the vacuum chamber and the furnace body, or further applying electrolytic polishing, chemical polishing, GBB treatment, etc., to a smooth surface before assembling these components. By performing the treatment, the amount of gas released from the surfaces of these members is extremely reduced with only a small increase in the treatment cost. As a result, the vacuum chamber can be evacuated from 10 ^-4 Torr to a lower vacuum of 10 ^-8 or 10 ^-9 units, and it becomes possible to melt ultra-high purity metals, active metals and alloys, which is industrially possible. It is great to contribute to.

[Brief description of drawings]

FIG. 1 is a schematic side sectional view showing an outline of a conventional cold wall melting furnace.

FIG. 2 is a partial sectional front view of the cold wall melting furnace shown in FIG.

FIG. 3 is a schematic side sectional view showing a structure of a conventional cold wall melting furnace.

[Explanation of symbols]

 1 Crucible body 2 Slit 3 Segment 4 Hollow conduit 8 Induction heating coil

Claims (7)

[Claims] 1. A vacuum chamber maintained under vacuum and a vacuum induction melting furnace installed in the vacuum chamber for melting refractory metals, active metals, etc., separated by vertically extending slits and adjacent to each other. , A crucible body having a bottomed cylindrical structure having a side wall of the crucible each of which is composed of a plurality of water-cooled segments, and a water-cooled crucible bottom plate integrally formed at a lower end of the side wall, and an outer periphery of the crucible body. A cold wall vacuum induction melting furnace comprising an induction heating coil arranged in: a superheater, characterized in that said induction heating coil is uninsulated and is capable of reaching a vacuum below 10 ^-6 Torr. Cold wall melting furnace that can melt under high vacuum. 2. The cold wall vacuum induction melting furnace according to claim 1, wherein the in-vessel wiring from the induction heating coil to the power source introduction section of the vacuum vessel is not covered with an insulating coating, and is less than 10 ⁻⁷ Torr. Cold wall melting furnace that can melt under ultra-high vacuum that can reach low vacuum. 3. The cold wall vacuum induction melting furnace according to claim 1 or 2, wherein the surface of the induction heating coil is blasted with glass beads, chemical polishing, electrolytic polishing, electrolytic polishing and complex polishing, or the like. A cold wall melting furnace that has been processed and is capable of reaching vacuums below 10 ^-8 Torr and capable of melting under ultra high vacuum. 4. The cold wall vacuum induction melting furnace according to claim 1 or 2, wherein the vacuum chamber is made of stainless steel, and the inner surface thereof is electrolytically polished, electrolytically polished and combinedly polished, blasted with glass beads, and mirror finished. A cold wall melting furnace that can be melted under ultra-high vacuum by being treated by any of the treatments, and can reach a low vacuum of up to 10 ^-10 Torr. 5. The cold wall vacuum induction melting furnace according to claim 1, wherein the crucible is made of copper, and the surface of the crucible is blasted with glass beads, chemical polishing, electrolytic polishing, and electrolytic polishing. Cold wall melting furnace that can be melted under ultra-high vacuum by being processed by any one of compound polishing etc. and capable of reaching a low vacuum of up to 10 ^-10 Torr level. 6. A melting method using the cold wall induction melting furnace according to any one of claims 1 to 5, which is high in a predetermined ultra-high vacuum degree determined by the kind of metal or alloy to be melted. Pure and ultra high purity metals,
Cold wall melting method for melting alloy. 7. The cold wall vacuum induction melting method according to claim 6, wherein after reaching a predetermined ultra-high vacuum degree determined by the kind of metal or alloy to be melted, an inert gas is introduced to discharge 200 Torr or more. A cold wall melting method in which a high-purity and ultra-high-purity metal, alloy, or active metal is melted in a region where it does not occur.