US2744495A - Salt bath furnace - Google Patents

Description

May 8, 1956 A. L. BOEGEHOLD SALT BATH FURNACE Filed Jan. 19, 1952 Q4212 E g/ 11% ATTORNEYS \\IW 4 N W 3 k H H f w f E5: W .QYM 2 0,, I n a j 5 \w\ United States Patent SALT BATH FURNACE Alfred L. Boegehold, Detroit, Mich, assiguor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application January 19, 1952, Serial No. 267,259

13 Claims. (Cl. 118-429) The present invention relates to furnaces and, more particularly, to furnaces adapted to dipping processes for cleaning and for coating metals with aluminum or other metals.

As is well known in the art, in order to obtain a good bond between a ferrous or other base metal and a coating metal such as aluminum, it is essential that the surface of the base metal be absolutely clean at the time the coating metal is applied. This is advantageously accomplished by cleaning the base metal in a suitable salt bath. In order to preclude oxidation of the surface of the base metal prior to the application of the coating material, it is desirable to utilize a dipping process wherein the cleaning treatment and the coating are accomplished in the same bath so that the once cleaned metal is not subjected to air in between the cleaning and the coating steps. For such a process a bath having a lower layer of molten aluminum and an upper layer of molten salt may be used so that the metal article to be coated can be passed through the salt layer for cleaning and for heating and then into the molten aluminum layer for coating. As is disclosed in U. S. Patents 2,544,670, 2,544,671 and 2,569,097, a salt bath which has been found to be particularly adaptable to the cleaning of ferrous and other base metals prior to metal coating is one containing a mixture of metal fluorides, together with other halides and maintained in contact with molten aluminum for activation of the salt.

While such a molten fluoride containing salt in contact with molten aluminum has many advantages as a preheating and cleaning agent for metal coating processes, it is disadvantageous in that it is extremely reactive and will Within a relatively short period corrode the usual furnace linings. Another difliculty which is encountered in coating processes utilizing a bath having contacting fluoride containing molten salt and molten aluminum layers is that the proper temperature control of the salt and aluminum layers is diiiicult.

It is an object of this invention to provide a furnace having increased resistance to corrosion and particularly adapted to metal coating and cleaning processes wherein a fluoride containing salt in contact with molten aluminum is used. Another object is to provide improved heating and heat control means for furnaces used in metal coating and cleaning processes.

These objects are carried out in accordance with this invention by the provision of a silicon carbide lining at least on those portions of the furnace which are subjected to the corrosive action of the metal fluoride salts and by the provision of a resistance heating means in the molten salt layer with or without additional induction heating means in the molten aluminum layer.

Other objects and advantages of my invention will be apparent from the following description and from the drawings in which Figure 1 is a vertical section of one embodiment of my invention; Figure 2 is a view of the embodiment shown in Figure 1 looking in the direction of the arrows 22 of Figure 1, with parts broken away and in section; Figure 3 is a vertical sectional View, in reduced scale, of another embodiment of my invention, and Figure 4 is a vertical sectional view of still another embodiment of my invention.

In the embodiment shown in Figures 1 and 2 the furnace consists of a tank 5 of rectangular cross-section and having a liner 6 surrounded by a heat insulating material 7 such as brick of some suitable ceramic backed up by packed asbestos. This entire structure rests on a support such as the metal beam structure 8.

When in operation the tank 5 contains an upper molten salt layer 9 which consists of a mixture of metal fluorides and other halides, and a lower molten aluminum layer 10. By the use of the term aluminum herein is means not only pure aluminum but also aluminum base alloys.

The dimensions of the tank will of course depend upon the depth of salt and metal layers to be used and on the size of the metal articles to be aluminized, etc. The upper portion 11 of the tank liner 6 consists of a material which is highly resistant to the corrosive action of molten metal fluorides and other halides. Silicon carbide is particularly suitable for this purpose because of its high resistance to attack by the molten fluorides and other halides. The bottom portion 12 in contact with the molten aluminum layer is made of any suitable refractory such as alumina or graphite. Monolithic linings of aluminum silicate bonded by sodium silicate or of silicate bonded silica are, for example, satisfactory. it will be noted that in this embodiment the upper or silicon carbide portion 11 of the liner consists of an insert or surface layer set into the backing portion of the liner 6, the backing portion being of the same material as the lower portion 12. It is understood, of courst, that the entire tank lining could be of silicon carbide. However, in the preferred embodiment a silicon carbide liner is used only on those portions which are subjected to the molten salt, chiefly because of the savings in cost. Also, it has been found that a monolithic alumina or silica liner is more durable than is silicon carbide in those portions contacted by the molten alumina. It will be noted that the silicon carbide portion 11 of the tank lining extends slightly below the depth of the salt layer. This struc ture is desirable because of the slight differences in the heights of the aluminum layer which are encountered in the operation of the furnace. The extension 13 of the silicon carbide liner below the interface of the two layers Will further preclude the possibility of the molten salt coming into contact with the lower liner portion 12 with consequent erosion.

Extending into the salt layer from above the tank 5 and adjacent a side wall are a series of electrodes 14 which are electrically connected by conductors such as bus bars 15 to a source of electrical energy or current generally indicated at 16. The electrodes and electrical connectors can be supported in any suitable manner as, for example, by the electrical insulators 17 which secure the bus bars 15 to which the electrodes 14 are fixed, to the outer furnace wall. An electrode spacer bar 18 made of some heat resistant electrical insulating material may also be used to help maintain the electrodes in position. As can best be seen in Figure 2, the electrodes 14 are positioned adjacent one side of the tank 5 in order to allow for greater working space. With this arrangement metal articles can be dipped without interference from the positioning of the electrodes. It is to be understood, of course, that electrodes could be positioned on more than one side of the tank or for that matter toward the center of the tank if such were necessary in order to supply sufficient heat to a large furnace. The electrodes 14 are of a length suflicient to extend to a depth above the bottom of the salt layer. Since shorting will result if the electrodes contact the metal layer, it is of course necessary to. position the electrodes so that there will be no contact with the bottom layer even after the immersion of the metal work pieces into the metal with a resulting rise of the level'of that layer. The most desirable length of the electrodes to be us-ed will depend, therefore, upon the depth of the salt layer and also upon the volume of the work to be immersed at any one time into the metal layer.

The salt layer 9 is maintained molten by the resistance heating which results when current is passed through the salt by means of the series of electrodes 14. The aluminum layer 10 is maintained molten by contact with the molten salt layer. I have found that aluminum layers up to 12 inches in depth can be melted and maintained molten under a molten metal halide layer.

In order to effect a better heat transfer from the salt layer to the aluminum layer it is desirable to utilize a stirring mechanism to agitate and drive the hot molten salt down to the salt-aluminum interface. The stirring mechanism used in the modification shown by Figure 1 consists of a pair of impellers 19, one impeller being located on each side of the tank and adjacent a'wall. Each impeller consists of a series of impeller blades'2tl mounted on a shaft 21 which is rotated by the chain or belt-driven pulley 22. The chain or belt 23: is in turn driven by an electric motor 24 which is mounted on an outside wall of the furnace away from the heat. The impellers may be secured to the furnace by bolts (not shown) or by any other suitable means.

It will be seen that, by rotating the impellers 19, the hot salt will be agitated and driven down against the aluminum layer, thus resulting in a greater heat exchange from the salt to the aluminum. It is to be understood that any suitable impeller means can be used, the specific example merely serving for purposes of illustration.

Supplementary to the heating and stirring means described above is that which results from the feeding of the work into the furnace. It is, for example, often the practice to preheatthe metal to be coated prior to immersion into the furnace. In this instance the heat thereby supplied will aid in maintaining the temperature of the salt and aluminum layers. Likewise, when work is fed rapidly through the layers, as for example where sheet metal or individual work pieces are driven through the furnace by means of a conveyor system, the agitation which results will supplement that obtained by the impellers. While it is not always essential to use impellers, it is usually desirable to do so, especially in those cases where a deep aluminum layer is used and where the work is not fed at a rate sufficient to cause considerable agitation.

I wish to point out that any suitable conveyor system can he used in conjunction with the furnace of this invention. Wire, sheet metal, or individual work pieces, can be fed into the furnace either by an automatic conveyor or manually.

Generally it is advantageous to accomplish both the metal cleaning and the metal coating steps in the same furnace. However, it is of course possible to effect each operation in a separate furnace. If this latter procedure is used, then the furnaces of this invention will serve well for the cleaning step wherein a molten fluoride containing salt in contact with molten aluminum for activation thereof is used as the flux or cleaning agent. In this instancea shallower layer of aluminum may of course be used than if the furnace is used for both the cleaning and the coating steps.

Figure 3 shows a modification wherein a lesser amount of aluminum is utilized to obtain the same layer depth, and consists of a tank 26 having a silicon carbide liner 27 surrounded by insulating material 28 such as asbestos and/or ceramic brick and supported by metal beams or other suitable structure 29. A second and smaller tank 30 having silicon carbide walls 31 is submerged within the tank 26. The submerged tank 30 is constructed with a large base portion 32 for purposes of strength and stability. In this construction the molten aluminum 33 is contained in the submerged tank 3t and is maintained molten by heat transfer from the molten salt 34 in the tank 26. In the particular embodiment shown the submerged tank 31 is positioned in the center lengthwise of the tank 26 and toward one side; however, it may also be positioned in the exact center of the tank or, alternatively, it may be positioned against the side of tank 26 so that it has one wall in common with the larger tank. In the embodiment shown the small tank 30 is completely separate from the large tank 26 in which it is submerged and thus can be removed for maintenance without disturbing the large tank.

The walls 31 of the submerged tank 30 may be made entirely of silicon carbide or else may be made of silicon carbide only in those positions subjected to molten salt and of another refractory such as aluminia, silica or graphite in those portions in contact with the molten aluminum.

Electrodes 35 extend into the salt layer 34 and thereby maintain the salt molten by way of the resistance heat ing which results when current is passed. Suitable electrical connectors and a source of electrical energy the same or similar to those shown in Figure 2 are of course used in the embodiment shown in Figure 3. Impellers such as those shown in Figures 1 and 2 may be used in the furnace shown in Figure 3; however, in this particular embodiment there is less need for agitation of the salt layer because of the larger volume of the salt in comparison to that of the aluminum and because of the fact that the molten salt more nearly surrounds the aluminum layer. Also, the excellent heat conductivity of the silicon carbide walls 31 effect efficient heat transfer from the salt to the aluminum.

When the aluminum is heated by heat transfer from the salt layer, the aluminum is of course always colder than the salt. With a 12-inch deep layer of aluminum and the salt at 1300 F., there may be, for example, a 40 to 60 F. difference in the temperature between the salt and aluminum layers even when the salt is circulated by impellers. As the required aluminum temperature becomes greater, so also the temperature differential between the salt and aluminum layers increases. To maintain the aluminum at 1300 F., for example, a salt layer temperature of 1400 F. is often required.

In order to attain better temperature control of the salt and aluminum layers, as well as other advantages, in the preferred embodiment as shown by Figure 4 separate heating means for the aluminum and salt layers are used. With reference to Figure 4 the preferred embodiment consists of a tank 38 having a lining 39 backed up by a heat insulating material 40 and adapted to hold a molten aluminum layer 41 and a molten salt layer 42. The upper portion 43 of the lining 39, or that portion in contact with the molten salt, consists of silicon carbide while the lower portion 44 consists of any suitable refractory such as graphite or silicate bonded alumina or silica. As in the embodiment shown in Figures 1 and 2, the silicon carbide section of the liner extends slightly below the salt aluminum interface as shown at 45 in order to prevent possible continued contact of the molten salt with the lower portion 44 of the liner.

The particular furnace shown in Figure 4, it will be noted, has an L-shaped cross-section in order to allow a larger aluminum layer depth for the amount of aluminum used. This shape furnace having a deep portion 46 and a shallow portion 47, the cross-sectional area of the deep portion being less than that of the shallow portion, is particularly useful where bulky articles are to be coated or where, for example, a precoating heat treatment requiring a large length of salt bath, through which the work piece is run before coating, is necessary or desirable. In Figure 4 the aluminum layer is contained Within the deep portion 46 of the tank; however, it is often desirable to have the aluminum layer extend high enough to furnish at least a thin layer on the bottom of the shallow portion 47 of the tank in order to provide additional aluminum layer surface for salt activation.

Electrodes 48 for resistance heating of the salt layer and impeller 54 extend into the shallow portion 47 of the tank 38 from above. The electrodes are, of course, suitably connected to a source of electrical energy.

To afford independent heating means for the aluminum layer an induction heater 49, suitably connected to a source of electrical energy, is positioned at the bottom and adjacent the floor of the deep portion 46 of the tank 38. This induction heater 49 is capable of providing suflicient heat to maintain the aluminum layer molten and at temperatures up to 1800 F. independently of the heat transferred from the molten salt. While only one induction heater has been shown, it is understood that a plurality of heaters could be used depending upon the size of the furnace and heater and upon the amount of heat required.

The induction heating unit is positioned in a recessed portion 50 of the tank 38 to provide the necessary channeling of the aluminum for induction heating. The width of the recessed portion 50 is suflicient to provide the channels 51 and 52. The center of the heater 49 furnishes another channel 53.

While I have shown the induction heating unit positioned on the bottom of the tank, it is understood of course that the heater, or heaters, could also be located on the sides of the tank in the aluminum layer.

The separate heating means for the aluminum layer is advantageous in that it allows independent temperature control of the aluminum and salt layers, which is not obtainable when the aluminum is heated only by contact with the salt. Higher aluminum temperatures can be attained without raising the salt layer temperature to a point where it is undesirable. It also allows for the use of a shallower salt layer than that required when the aluminum is heated by heat transfer from the salt. This is desirable for the control of the rinsing action of the salt following immersion of the work in the aluminum. The depth of the molten aluminum which can be used below the salt layer is independent of the temperature of the salt layer. Also, it is not necessary to restrict furnace design to that which affords suflicient contact area between the salt and aluminum to assure sufiicient heat transfer. It has also been found that the continued agitation of the aluminum and salt interface due to the pumping action of the aluminum induction heater increases the activating action of the aluminum on the fluoride containing salt layer. Another advantage of the use of independent aluminum layer heating means is that a failure of power supply to either heating circuit will not result in a freezing of the salt and aluminum to a point where operation cannot be resumed.

The exact furnace design will of course depend upon the nature of the coating process to be practiced. In many processes the salt layer and aluminum layer temperatures and depths required are such that no separate heating means for the aluminum layer is required, while in others precoating heat and cleaning treatment or postcoating rinsing will make desirable the use of separate heaters for the aluminum. It is to be understood that the various features described in conjunction with a particular embodiment, for purposes of illustration, could be utilized in other modifications, some of which are shown. Thus, the various described features may be interchanged in the several modifications without departing from the scope of the invention.

The furnaces of this invention provide eficient means for the practice of metal coating and cleaning processes utilizing fluoride containing molten salt as a treating agent. The cleaning and coating of steel or other metals with aluminum can be accomplished in one furnace.

It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

I claim:

1. A furnace for treating metals by dipping into a layer of fluoride containing molten salt in contact with molten aluminum comprising a tank adapted to hold a lower layer of molten aluminum and an upper layer of molten salt, and said tank having a lining of silicon carbide at least in those portions contacted by said molten salt, and a plurality of electrodes extending into said tank for resistance heating of said salt layer.

2. A furnace for treating metals by dipping into a layer of fluoride containing molten salt in contact with a layer of molten aluminum comprising a tank, adapted to hold a lower layer of molten aluminum and an upper layer of molten salt, a plurality of electrodes extending into said tank for resistance heating of said salt layer, and an impeller in said tank for agitating said salt layer, said tank having a lining of silicon carbide at least in those portions contacted by said salt layer.

3. A furnace for treating metals by dipping into a layer of fluoride containing molten salt in contact with a layer of molten aluminum, comprising a tank adapted to hold a lower layer of molten aluminum and an upper layer of molten salt, electrodes extending into said tank from above and adjacent a wall of said tank for resistance heating of said salt layer, an impeller in said tank rotated by driving means positioned outside said tank for agitating said salt layer, said tank having a lining of silicon carbide at least in those portions contacted by said salt layer and of a refractory other than silicon carbide in portions in contact with said aluminum layer.

4. A furnace for coating metal with aluminum by dipping into a fluoride containing molten salt layer and then into a molten aluminum layer comprising a tank having a silicon carbide lining and adapted to hold a molten salt layer, a second tank submerged in said first mentioned tank for holding molten aluminum, said second tank having silicon carbide walls, and a series of electrodes extending into said first mentioned tank from above for resistance heating of the salt layer.

5. A furnace for coating metal with aluminum by dipping into a fluoride containing molten salt layer and then into a molten aluminum layer comprising a tank having a portion adjacent one side wall thereof deeper than the other portions of said tank, said deeper portion containing a molten aluminum layer and the shallower portion of said tank containing a molten salt salt layer, said tank having a lining of silicon carbide at least in those portions contacted by said molten salt layer, and a series of electrodes extending into said tank along one side thereof and from above for resistance heating of said salt layer.

6. A furnace for coating metals by dipping into a layer of fluoride containing molten salt and then into a layer of molten aluminum comprising a tank adapted to contain a layer of molten aluminum and a layer of molten salt, said tank having a lining of silicon carbide at least in those portions contacted by said molten salt, an induction heater in said tank for heating said aluminum layer and means in said tank, independent of said induction heater for heating said salt layer.

7. A furnace for coating metal with aluminum by dipping into a fluoride containing salt layer and then into a molten aluminum layer comprising a tank adapted to hold a lower layer of molten aluminum and an upper layer of molten salt, said tank having a lining of silicon carbide at least in those portions contacted by said molten salt layer, a series of electrodes extending into the upper portion of and fromabove said tank for resistance heating of said salt layer and an induction heating unit in the lower portion of said tank for heating said aluminum layer.

8. A furnace for coating metal with aluminum comprising a tank having a'deep portion adapted to hold molten aluminum and a shallow portion adapted to hold molten salt, said deep portion having a smaller crosssectional area than said shallow portion, a silicon carbide liner in said tank at least in those portions contacted by said molten salt, a series of electrodes extending into the shallow portion of said tank for resistance heating said salt layer, and an induction heating unit in the deep portion of said tank for heating said aluminum layer.

9. A furnace for coating metal with aluminum by dipping into a fluoride containing salt layer and then into a molten aluminum layer comprising a tank adapted to hold a lower layer of molten aluminum and an upper layer of molten salt, said tank having a lining of silicon carbide at least in those portions contacted by said molten salt layer, a series of electrodes extending into the upper portion of and from above said tank for resistance heating of said salt layer, an induction heating unit in the lower portion of said tank for heating said aluminum layer, and an impeller in said tank rotated by driving means positioned outside said tank for agitating said salt layer.

10. A furnace for treating metals by dipping into a layer of fluoride containing molten salt in contact with molten aluminum comprising a tank adapted to hold a layer of molten aluminum and a layer of molten salt, said tank having a lining of silicon carbide in those portions contacted by said salt layer and a lining of a refractory other than silicon carbide in those portions contacted by said aluminum layer, and means in said furnace for heating said salt layer.

11. A furnace for treating metals by dipping into a layer of fluoride containing molten salt in contact with a layer of molten aluminum comprising a tank adapted to hold a layer of molten aluminum and a layer of molten salt, means in said furnace for heating said salt layer, and an impeller in said tank for agitating said salt layer, said tank having a lining of silicon carbide at least in those portions contacted by said salt layer.

12. A furnace for treating metals by dipping into a layer of fluoride containing molten salt in contact with a layer of molten aluminum, comprising a tank adapted to hold a lower layer of molten aluminum and an upper layer of molten salt, electrodes extending into said tank from above and adjacent a wall of said tank for resistance heating of said salt layer, an impeller in said tank rotated by driving means positioned outside said tank for agitating said salt layer, said tank having a lining resistant to corrosion by molten fluoride in those portions contacted by said salt layer and refractory lining resistant to deterioration by molten aluminum in those portions contacted by said aluminum layer.

13. A furnace for treating metals by dipping into a layer of fluoride containing molten salt in contact with molten aluminum comprising a tank adapted to hold a lower layer of molten aluminum and an upper layer of molten fluoride salt, said tank having a lining of silicon carbide at least in those portions contacted by said molten salt, and heating means in said tank for maintaining said layers in a molten state.

References Cited in the file of this patent UNITED STATES PATENTS 880,743 Van Kugelgen et al Mar. 3, 1908 1,637,486 Kelleher Aug. 2, 1927 1,740,081 Finkbone Dec. 17, 1929 2,315,725 Moller Apr. 6,. 1943 2,539,215 Weil et al Jan. 23, 1951

Claims (1)

13. A FURNACE FOR TREATING METALS BY DIPPING INTO A LAYER OF FLUORIDE CONTAINING MOLTEN SALT IN CONTACT WITH MOLTEN ALUMINUM COMPRISING A TANK ADAPTED TO HOLD A LOWER LAYER OF MOLTEN ALUMINUM AND AN UPPER LAYER OF MOLTEN FLUORIDE SALT, SAID TANK HAVING A LINING OF SILICON CARBIDE AT LEAST IN THOSE PORTIONS CONTACTED BY SAID MOLTEN SALT, AND HEATING MEANS IN SAID TANK FOR MAINTAINING SAID LAYERS IN A MOLTEN STATE.

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