US3544691A - Apparatus for degassing metallic melts by sonic vibrations - Google Patents

Description

United States Patent 7 APPARATUS FOR DEGASSING METALLIC MELTS BY SONIC VIBRATIONS 11 Claims, 10 DrawingFigs.

[1.8. CI 13/26 Int. Cl. H05b 5/00 Field ofSearch 13/26, 31,

l;266/34(A), 34(V) References Cited UNITED STATES PATENTS 3,061,298 l0/l962 Yamazoe ..266/34 v Ux 3,100,237 8/1963 Rydingeretal 13/26 .3,l80,633 4/1965 Taylor l3/26X 3,212,767 10/1965 Muller 266/34(V)UX Primary Examiner-G. Harris Assistant Examiner-Roy N. Envall, Jr. AtlorneyMcGlew and Town ABSTRACT: Apparatus for degassing metallic melts by sonic vibrations uses the ponderomotive effect produced by an alternating current supplied from a source at commercial frequencies, the alternating currents being distorted to increase their harmonics. The distortion can be effected by abrupt limitations of amplitude to produce a pulse-type current, The apparatus may include a supply transformer which can be loaded beyond the saturation range, or by providing special laminations or by magnetically preloading the transformer. Alternatively, the distortion of the alternating current is effected electronically.

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APPARATUS FOR DEGASSING METALLIC MEL'IS BY SONIC VIBRATIONS CROSS REFERENCE TO RELATED APPLICATION This application is a division of copending U.S. Pat. application Ser. No. 417,667, filed Dec. 11, 1964, for METHOD AND APPARATUS FOR DEGASSING METALLlC MELTS BY SONIC VIBRATIONS, now U .S. Pat No. 3,434,823, issued Mar. 25, 1969.

BACKGROUND OF THE INVENTION As a consequence of the constantly increasing requirements regarding the quality of cast metal products, and the concomitant increasing requirements with respect to the molten metal used for casting, the gas content of metallic melts has become a very important problem in metallurgy. Consequently, many experts have studied the problem of degassing in an effort to find more economical solutions.

Thus, it is known to utilize a combination of forces, including those produced by the current flowing through the melt and by electromagnetic forces, and those produced by influences other than those normally resulting from the current flow, in order to separate slags from molten metal. To produce an additional magnetic field, solenoids or magnetic shields" are used, and these are arranged outside the container containing the metal to be melted. It has also been tried to separate metal and slags or other impurities, by electromagnetic fields which have certain orientations relative to each other.

From U.S. Pat. Nos. 2,415,974 and 2,381,523, it is known that slags and impurities can be separated from the metallic melt, in submerged resistor-type induction furnaces, by arranging at least two melting channels, so that-the direction of flow of the liquid conductor, which is the molten metal, is changed abruptly when passing from one channel to the other. In one melting channel, which is readily accessible for cleaning, such as a vertical melting channel, the electromagnetic pressure gradient is increased, by suitable selection of the cross section of this channel and of the electromagnetic field induced in the molten metal by a coil or winding arranged exteriorly of the channel, to such an extent less conductive components contained in the melt are pressed toward the wall of For removal of nonconductiveor less conductive impurities, it is known, from Hoke U.S. Pat. No. 2,013,653 to treat a conductive metal in such a way that an electric current is conducted through the material which is in the liquid state, thereby inducing, electromagnetic forces which are unidirectional or substantially unidirectional with respect to the current. The forces acting in different directions, which are normally produced during the flow of the current, cancel each other either completely or practically completely. It is thus necessary to produce at least one additional magnetic field to attain the necessary orientation of the forces, and at least one additional current source is also required.

It has also been assumed, up to the present, that frequencies used for degassing melts by the ponderomotive effect have to be in excess of 500 cycles. Fumaces operating at these frequencies, which are the so-called medium frequency furnaces", are costly, however, as well as being sensitive, as far as their maintenance is concerned. In addition, and as a matter of practice their output is limited, with the limit being well below the furnace outputs most required.

Another expedient is suggested in Rydinger, U.S. Pat. No. 3,100,237, which is the application of a deep-seated induction heater.

In Yamazoe, U.S. Pat. No. 3,061,298, there is disclosed an arrangement for heat treating a subsequent degassing of magnetic melts. A pair of substantially parallel melt discharge streams formed, in a first example, a short-circuited secondary conductor, in a second example, a closed ferric circuit with eddy currents, and an induction heating system in a third example. The melt discharge strands are heated to compensate for heat losses in refilling, and a degassing operation in vacuum is carried out subsequently.

SUMMARY OF THE INVENTION This invention relates to the degassing of metallic melts and, more particularly, to an improved and simplified apparatus utilizing alternating current at low frequencies, preferably with distortion of the wave form of the alternating current to increase the harmonic content thereof.

In accordance with the invention, the disadvantages of the prior art are avoided by using low frequency AC potential sources, including commercial sources at 50 cycles or 60 cycles. In the case of using commercially available frequencies, the necessity of providing a separate generator is eliminated, which results in a very substantial advantage. As long as sufficiently large supply mains are provided, any desired output can be attained without an undue increase in cost for a separate source. The use of frequencies less than 50 cycles is, in any event, desirable only in a few cases. Also, it will be necessary to use frequencies above 50 cycles in only a few cases, as the supply frequency, and even then these can be obtained from commercial frequencies by static frequency multiplication without having to use a separate generator.

The apparatus of the invention degasses metallic melts by sonic vibrations produced in the melts by ponderomotive effects, and with these effects being caused by alternating currents flowing through the melt. The apparatus is also advantageously useful to produce sonic vibrations in the melt by ponderomotive effects caused by alternating currents flowing in the melt, and using, as the alternating current, the alternating current normally used to heat the melt, with the heating alternating current being temporarily or constantly distorted, as mentioned above, for degassing the melt. In such a case, the use of separate currents and fields for degassification in the melt is unnecessary.

The distortion of the alternating current in the sense of the invention can be effected by abrupt limitation of its amplitudes before they reach a periodic maximum amplitude in at least one direction of flow, so that a pulse-type current flow is produced. The thus limited or clipped alternating current half waves are induced in a known manner into the melt, for example, by at least one similarly limited or clipped alternating magnetic field. An increase of the ponderomotive effect can be obtained if a stationary magnetic field is superposed, in a known manner, on the melt. In cooperation with the alternating current flowing through the melt, this even further increases the ponderomotive effect.

The simplest apparatus for achieving this effect of the invention comprises a supply transformer which can be loaded beyond the saturation range of the magnetic induction. The same effect can be achieved, however, if the transformer core is magnetically preloaded, preferably by providing at least one DC winding thereon. Thereby, the primary current in at least one direction of flow effects a saturation of the magnetic induction in the iron core even before its peak values are obtained. The resulting stationary magnetic field is superposed in a known manner on the DC field, so that the ponderomotive effect in the melt is also increased by the inductive action of the two fields on the melt.

Transformers of this type are particularly suitable for use as transformers for induction furnaces, where the secondary circuit is formed by the molten material flowing in a closed melting channel of the furnace. In particular, such a transformer can be used in a heat retaining or reheating furnace which can be correspondingly dimensioned, with the furnace channel opening into the crucible at the bottom surface thereof with the channel forming part of a reheating furnace designed as a low-pressure founding or casting furnace.

A feature of the invention, as applied to such a low-pressure founding or casting furnace, is that the filling tube extending into the storage crucible extends substantially to an opening of the furnace channel and protrudes into this opening, which latter is preferably funnel-shaped. The entire furnace transformer, together with the furnace channel, preferably is so inclined that its end remote from the storage crucible is lower than the bottom surface of the storage crucible, so that the released gas bubbles can rise into the storage crucible in bypassing relation with the filling tube.

In further accordance with the invention, the distortion of the alternating current wave forms can be effected in another manner, wherein at least one rectifier, or a controllable electronic valve, can be connected in the supply circuit so as to transform the alternating current at least partially into a pulsating direct current. If necessary, this can be effected by applying an initial DC bias to the rectifier or to the controllable electronic valves to control the ratio of the DC portion of the supply current in relation to the AC portion thereof.

As a further feature, the electric current can be introduced into the melt by means of electrodes such as used in so-called resistance-heated furnaces. Additionally, the frequency of the alternating current derived from a commercial frequency source can be increased, if necessary, up to about seven times by using so-called static frequency converters.

Accordingly, an object of the present invention is to provide an apparatus for degassing metallic melts by sonic vibrations produced in the melt by ponderomotive effects caused by alternating currents flowing through the melt.

Another object of the invention is to provide such an apparatus in which the alternating currents are subjected to a distortion of their periodic course in order to increase their harmonics and thus enhance the degassification, the supply current being derived from a low frequencyAC source, such as a commercial frequency source.

A further object of the invention is to provide apparatus for degassing metallic melts by sonic vibrations produced in the melts by ponderomotive effects caused by alternating currents in the melts and to increase the ponderomotive effects by superposing a stationary magnetic field on the alternating current flowing through the melts.

For an understanding of the principles of the invention, reference is made to the following description of a typical embodiment thereof as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a transverse sectional view of a furnace transformer embodying the invention illustrating the iron core thereof in its initial state, with the ceramic body containing the furnace channel and the primary winding being sectioned along the center of the transformer core;

FIG. 2 is a top planview of the transformer shown in FIG. 1, with a portion of the crucible shown in section;

FIG. 3 is a central vertical sectional view through a low pressure casting furnace embodying the invention, with the transformer, such as shown in FIGS. 1 and 2, illustrated in side elevation;

FIG. 4 is a front elevation view, partly in section, of another embodiment of a melting furnace in accordance with the invention;

' a FIG. 5 is a partial top plan view of the melting furnace shown in FIG. 4;

FIG. 6 is a schematic wiring diagram, with a schematically illustrated furnace crucible, illustrating another arrangement embodying the invention;

FIG. 7 is a schematic wiring diagram illustrating the principle of an electronic circuit for distorting the wave forms of the alternating current;

FIG. 8 is a schematic representation of an actual wiring diagram embodying the principle shown in FIG. 7;

FIG. 9 is a graphic representation of the voltage and current curves obtained with the circuitry of FIG. 8; and

FIG. 10 is a schematic wiring diagram of a static frequency converter which may be used to produce a third harmonic from an AC supply at a commercial frequency.

4 DETAILED DESCRIPTION OF THE PRE EMBODIMENTS Referring to FIGS. 1 and 2, the transformer has losed iron core 10 comprising three stacks of laminations mounted in juxtaposition. The center stack is displaced laterally with respect to the two outer stacks, so that its right leg 10a protrudes from the right hand side of the transformer core, whereas the left legs 10a of the two outer stacks protrude from the left hand side of the transformer core. The left leg of the center stack forms, with the right legs of the two outer stacks, the center leg 10b of the transformer core which is embraced by the primary winding 11. Transformer core 10 embraces a ceramic body 12 in which there is formed a heating channel 13 extending around center leg 10b. A part of channel 13, extending parallel to the plane of core 10, is closed by means of plugs 14 which can be removed for cleaning of the channel, as best seen in FIG. 2.

Referring to FIGS. 2 and 3, on the side opposite the plugs 14, channel 13 opens into a funnel-shaped mouth 15 formed in the storage crucible 16 of the low pressure founding furnace. Mouth 15 opens into the bottom surface 17 of storage crucible 16.

A filling tube 18 is positioned inside storage crucible 16 and extends along a wall thereof from the top of the crucible. The lower end of tube 18 projects into the mouth 15 of heating channel 13. The open top of crucible 16 is closed by a swivel cover 19 having a pressure gas pipe 20 extending therethrough. Filling tube 18, which is disposed laterally of the cover 19, is closed by means of a stopper 21 of the valve type, and communicates with a trough 22. The furnace transformer is suspended on supporting elements 23 and 24.

If the melt is to be degassed during operation of the founding furnace, at least one of the legs 10a of transformer core 10 is removed. In this manner, if the normal magnetic field in the iron core 10 is 12,000 Gauss, the field is increased to about 18,000 Gauss so that the transformer is working in the saturation range. Thus, the wave form of the current in the melt forming the secondary conductor and in the heating channel 13 is distorted without any increase in current input. Naturally, in normal operation with afield of about 12,000 Gauss, there is a certain degassing effect, since the energy density in the cross section of the heating conductor formed by the melting channel 13 is rather high.

When molten material is to be removed from the furnace, valve stopper21 is lifted, after which the melt will rise in filling tube 18 under the effect of pressure gas fed to pipe 20 and will flow off through trough 22. As filling tube 18 projects into the mouth 15 of channel 13, in which latter the degassing occurs and is brought about relatively rapidly, it is possible to remove continuously quantities of degassed molten material without first having to degas the entire contents of storage crucible 16. It is thus possible to operate with relatively low energy inputs, such as those merely sufficient to keep the melt warm or molten and to degas the material that, at any given moment, is in channel 13. With a frequency of 50 cycles, the depth of penetration of the induction in iron is about 7.4 cm., and in aluminum about 3.5 cm. It will be appreciated that other embodiments of reheating and founding furnaces come within the scope of the invention as previously mentioned.

FIGS. 4 and 5 show a melting furnace in which, in accordance with the invention, the magnetic core of the transformer is provided with an initial DC bias or precharging. For this purpose, a pair of DC windings 25 are mounted on the core 10 as illustrated in FIGS. 4 and 5. Coils 25 preferably are polarized in such a manner that the unidirectional magnetic fluxes resulting therefrom extend in opposite directions through the two halves of the magnetic core 10 of the transformer, especially through their outer legs 10c, this core in cluding a central leg 10d through which the DC magnetic fluxes extend in the same direction. The strength of the steady magnetic fields produced by windings 25 are determined in such a manner, through the selection of the value of the DC current flowing in windings 25, that the AC current in primary winding 11 causes, in one direction of flow, a saturation of the magnetic induction of the core before the AC current attains its peak values. Thereby, the ponderometriceffect in the melt is increased.

FIG. 6 is a schematic wiring diagram of another arrangement for superposing a stationary magnetic field on the transformer core. Referring to FIG. 6, DC current from a DC source 26 flows through a choke 27 to the induction coil 28 surrounding a melting crucible 29. This induction coil is also supplied with alternating current from an AC source 30 through a condenser 31. Choke 27, in combination with filter condenser 32, prevents flow of alternating current to the DC source 26. The degassing effect which the distorted alternat ing field has on the melt is substantially increased by the superposed DC magnetic field. Furthermore, the steady magnetic field may be disconnected at will, and could also be produced in a winding separate from the AC induction winding 28.

FIG. 7 illustrates the principle of current limitation using controllable electronic valves, and which current limitation can be used with advantage, in accordance with the invention, to increase the degassing effect. Referring to FIG. 7, the control electrode of an electronic valve 33 has a sinusoidal AC voltage applied thereto through a series resistance 34. Since .the voltage applied to the control electrode through a working resistance 35, and through a resistance 36 connected in parallel with the anode and the control electrode, has a tendency to maintain a zero potential, a narrow strip is cut off of the AC wave near the zero line. The amplified output voltage corresponding to this strip, and which can be tapped from the working resistance 35 through the coupling condenser 37, has a substantially rectangular wave form and is very rich in harmonies, which enhance the degassing effect.

FIG. 8 illustrates a practical circuit in which the current limitation principle can be applied. The circuit of FIG. 8 is supplied with a sinusoidal AC voltage through transformer 38 which has a center tapped secondary winding dividing the secondary winding into two halves, with the center tap having a zero potential. Two leads connect the ends of the secondary winding of the transformer 38 to the induction winding 40 of the furnace, and each of these leads has disposed therein an electronic valve 39. In exactly the same manner as the secondary winding of transformer 38, furnace winding 40 is center tapped with its center tap at zero potential. The voltage applied to the control electrodes of the valve 39 is composed of several'components.

FIG. 9 illustrates one complete cycle of the voltage of the secondary winding of transformer 38 at a commercial frequency. When the AC voltage is very low, as represented in section a of the curve U shown in FIG. 9, a negative potential is predominant at the control electrodes of valves 39. This negative potential is derived from a rectifier 41 connected to another secondary winding of transformer 38, and the rectified potential is applied to a potentiometer 42 having several taps. Two of these taps are connected through resistances 43 with the control electrodes of valves 39. Under these conditions, valves 39 remain substantially nonconductive, or have a highresistance, so that the current I in induction coil 40 is very small in that portion of the voltage wave of FIG. 9, represented at a.

However, when the AC potential applied through resistances 44 to the control electrodes increase, the valve 39 to which the positive half wave is then being applied becomes rapidly more conductive over the section b of the voltage curve U of FIG. 9. This is due to the fact that the potential at the respective control electrode is then within the control range, even after division of the voltage.

The current in the induction coil 40 thus rises rapidly in section I; of current curve I. As soon as a certain positive AC potential U has been attained, as in section 0, the corresponding control valve 45, which valves are positioned ahead of valves 39 in cascade fashion and which, up to then, has been nonconductive because of the voltage division of its negative control potential tapped through resistances 46 and 47, begins to carry current. The potential at the base of resistance 44, acting as a working resistance, being fed to the control electrode of the valve 39, the current droops again. Consequently, the current I passing through the valve also droops in section c of FIG. 9.

Since the sections 0, b, a, repeat themselves in reverse order with a drooping potential U, and the valves 39 and 45, respectively, function in the same manner on the other side of the circuit in the following half wave, the other half of the induction coil 40, working in the manner of a push-pull transformer, and carrying the oppositely directed current I, results in the production of a pronounced third harmonic of the fundamental wave corresponding to the voltage curve U. The degree to which this third harmonic is simulated depends, to a large extent, on the selection of the resistances 43, 44, 46 and 47 and on the selection of the grid potential on the potentiometer 42. Of course, it is possible to use transistor devices instead of the electronic valves 39 and 45, represented in FIG. 8 as power vacuum tubes, and in such case the voltages must be reversed.

FIG. 10 illustrates a so-called static frequency converter" which can be used to produce a third harmonic from an AC source at commercial frequency. The primary windings 56 of a three-phase transformer are connected, through a switch 54 and through chokes 55 to the phases R, S, T of the three-phase AC source at commercial frequency. Chokes 55 serve to block the harmonics from entering the supply mains. Primary windings 57 are connected in star fonnation and are charged into the saturation range so that they form, together with the respective phase condensers 57, inter alia, resonant circuits for the third harmonic produced by chokes 55.

The secondary windings 58, which are connected in delta, now have only this third harmonic induced therein. These secondary windings are connected in series with condensers 59, which can be regulated in steps. Thus, the secondary windings 58 are tuned to resonance for the third harmonic. By means of the step connection of condensers 59, and by means of a transducer 60, this resonance can be more or less mistuned in the sense of an idle power variation, so that the output voltage appearing at the terminals U and V can be regulated to a great extent with the three-fold frequency of the fundamental wave, as affects its amplitude.

Although specific embodiments of the invention have been shown and described in detail to illustrate the application of the-principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

Iclaim:

1. Apparatus for degassing metallic melts by sonic vibrations produced in the melts by ponderomotive effects produced by alternating currents traversing the melts, said apparatus comprising, in combination, an induction furnace including a container for a molten metallic melt; and a transformer operatively associated with said container and operable to supply alternating currents to traverse a molten metallic melt in said container; said transformer having a primary winding arranged for connection to a source of low frequency alternating current at substantially commercial frequencies; said transformer having an iron core on which said primary winding is arranged, and said core having a saturating magnetic flux density very substantially in excess of the magnetic flux density corresponding to normal induction heating of molten metallic melt in said container; said transformer being constructed for continuous operation under loading of said core beyond the saturation range of themagnetic induction of said core; whereby the low frequency alternating currents supplied to traverse a molten metallic melt in said container are distorted to increase their harmonic contents to enhance degassification of a melt by ponderomotive effects.

2. Apparatus for degassing metallic melts by sonic vibrations produced in the melts by ponderomotive effects produced by alternating currents traversing the melts, said apparatus comprising, in combination, a container for a molten of a melt by ponderomotiveeffects; said transformer including an iron core comprising plural. stacks of laminations selectively removable from the core; said transformer having a primary winding with a normal current rating such that, with said core fully assembled, the magnetic induction thereof does not exceed the saturation point whereby, when at leastone of said stacks is removed, a primary current of the normal rating will effect saturation of the magnetic induction of the iron core at less than the peak amplitude of half waves of alternating current.

3. Apparatus for degassing metallic meltsby sonic vibra tions produced in the melts by ponderomotive effects produced by alternating currents traversing the melts, said apparatus comprising, in combination, a container for a molten metallic melt; a transformer operatively associated with said container and operable to supply alternating currents to traverse a molten metallic melt in said container; said transformer being constructed and arranged for connection to a source of low frequency alternating current at substantially commercial frequencies; said transformer having an iron core and being constructed and arranged for loading beyond the saturation range of the magnetic induction of said iron core; whereby the low frequency alternating currents supplied to traverse a molten metallic melt in said container are distorted to increase their harmonic contents to enhance degassification of a melt by ponderomotive effects; said transformer including a saturable iron core; means for magnetically preloading said transformer core whereby thetransformer primary current will effect saturation of the magnetic induction of the iron core below peak amplitudes of the half waves of alternating current; and means for superposing the magnetic field in the melt to augment the ponderomotive effect in the melt due to the inductive action of the two fields of the melt.

4. Apparatus for degassing metallic melts by sonic vibrations, as claimed in claim 3, in whichsaid means for magnetically preloading said transformer core comprises a direct current winding on said core in addition to the primary and secondary windings thereon.

5. Apparatus for degassing metallic melts by sonic vibrations, as claimed in claim 1 including a closed loop connected at both ends to said container and surrounding said transformer core, for circulation of molten metallic melt therethrough, said closed loop comprising the secondary winding of said transformer.

6. Apparatus for degassing metallic melts by sonic vibrations, as claimed in claim 5, said induction furnace beinga low-pressure founding furnace; said container having sidewalls and a bottom wall; said loop communicating at each of said magnetic preloading means on the alternating field produced end with the bottom of said container and embracing said iron core, said loop constituting the secondary winding of said transformer when a molten metallic melt is circulating in said loop.

7. Apparatus for degassing metallic melts by sonic vibrations, as claimed in claim 6, including a filling tube extending from the upper end of said container along a sidewall thereof to the base of said container and projecting into one end of said loop opening into said bottom surface.

8. Apparatus for degassing metallic melts by sonic vibrations, as claimed in claim 7, said transformer being supported in inclined orientation on said container with its end remote from said container being lower than that bottom surface of the container so that released gas bubbles can rise in said container upwardly of said filling tube.

9. Apparatus for degassing metallic melts by sonic vibrations produced in the melts by ponderomotive effects produced by alternating currents traversing the melts, said apparatus comprising, in combination, a container for a molten metallic melt; a transformer operatively associated with said container and operable to supply alternating currents to traverse a molten metallic melt in said container; said transformer being'constructed and arranged for connection to a source of low frequencyalternating current at substantially commercial frequencies; said transformer having an iron core and being constructed and arranged for loading beyond the saturation range of the magnetic induction of said iron core; whereby the low frequency alternating currents supplied to traverse a molten metallic melt in said container are distorted to increase their harmonic contents to enhance degassification of a melt by ponderomotive effects; a feed circuit connected to the primary winding of said transformer; and controllable rectifier means in said feed circuit effective to transform the low frequency alternating current into a pulsating direct current.

10. Apparatus for degassing metallic melts by sonic vibrations, as claimed in claim 9, including means for applying an adjustable constant DC bias to said rectifier means to control the ratio of the direct current and alternating current supplied to a melt in said container.

11. Apparatus for degassing metallic melts by sonic vibrations produced in the melts by ponderomotive effects produced by alternating currents traversing the melts, said apparatus comprising, in combination, a container for a molten metallic melt; a transformer operatively associated with said container and operable to supply alternating currents to traverse a molten metallic melt in said container; said transformer being constructed and arranged for connection to a source of low frequency alternating current at substantially commercial frequencies; said transformer having an iron core and being constructed and arranged for loading beyond the saturation range of the magnetic induction of said iron core; whereby the low frequency alternating currents supplied to traverse a molten metallic melt in said container are distorted to increase their harmonic contents to enhance degassification of a melt by ponderomotive effects; a feed circuit for the primary winding of said transformer; and static frequency multipliers included in said feed circuit to increase the frequency of the currents traversing a melt in said container to a multiple of said low frequency supply at a commercial frequency.

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