US1704086A - Method of cleaning and/or annealing metal - Google Patents

Description

Patented Mar. 5, 1929.

UNITED STATES NOAK VICTOR HYLBINETTE, 01 NEW YORK, N. Y.

METHOD OF CLEANING AND/OR ANNEAI ZINQ METAL.

No Drawing.

This invention relates to.improve1nents in the cleaning of metal surfaces and more particularly the cleaning of metal surfaces such as nickel,'nickel alloys, etc. from insoluble or 5 diificultly solubleoxides.

The invention also relates to improvements in the annealing of such-metals with protection of the annealed metals from surface oxidation during cooling. When the surfaces of the metal' objects to be annealed require cleaning from an oxide film or coating, the

cleaning and annealing operations can ad vantageously be combined in a single operation.

Acid resisting and heat resisting metals, such as nickel and alloys consisting mainly of nickel, have the peculiar property of becoming covered by an insoluble and strongly adhering coating of oxide when heated in an oxidizing atmosphere. This property, valuable in many instances, is a great drawback in the mechanical Working of the metals, particularly in thecase of the working of the metals by rolling.

In rolling mill work, the present practise consists in cleaning the surface of the metal by mechanical means, such as turning, planing, scraping or sand blasting, after the hot rolling has been stopped at a thickness appreciably greater than that of the final article, and in then continuing with cold rolling and intermediate annealing in a non-oxidizing atmosphere (in annealing boxes or otherwise). Such mechanical cleaning of the metal surfaces is expensive and unsatisfactory.

The present invention provides a method for removing such oxide coating in a way that allows the hot Working to be carried considerably further than heretofore, and allows the cleaning and annealing to be done much better and quicker than heretofore.

According to the present invention the metal, preferably in a heated state, is immersed in a molten bath of a slag capable of dissolving the metal oxides, such as nickel oxide, and the metal is left in the molten bath a sufficient length of time at a sufficiently high temperature to dissolve such oxide or oxides. In the case of nickel or nickel alloys, and with hot metal, only a few minutes are required at a temperature of about 800 C. of

the metal and of the slag. The metal is them withdrawn from the molten bath, and it is protected against reoxidizing by a thin coat- Application filed July 29,

that it can be completely removed after cool- 1921. Serial No. 488,468.

ing of the slag which adheres thereto. The metal is finally cooled and the layer of slag is removed;

The term slag,as used in this specification, means any substance or mixture of substances which when fused is capable of dissolving metallic oxides.

Inasmuch as the molten slag is to be used for dlssolving the metal oxide coating or film, it should be of a suitable composition for this purpose, The slag or flux should alsobe of such a composition as to be easily removed from .the metal after cooling, but sufliciently adhering to protect the metal surface after removal from theslag bath and durmg cooling. Some silica may be present in the slag, but if the silica content is too high the slag will stick to the metal like an enamel and may be difficult to remove, and, upon cooling, such slag, if not completely removed, 7 is liable to berolled into the metal. An excess of silica in the slag should therefore 111 most cases be avoided. It is important that the slag should be of such a character ing. It is desirable therefore that the slag should be soluble in water, or at least partly soluble inwater, so that it can be completely removed by dissolving without the use of acids. Borax alone, or a mixture of soda (soda-ash) and borax, can be used, but it is better to use a mixture containing, for example, borax, soda, sodium sulphate, and sodium silicate in about equal proportion. For best results it is desirable that the slag should be so compounded, that it will stick to the metal surface after it has been cleaned and/or annealed and protected from being oxidized while cooling in the air, and that the slag should come offfairly easily after the object has cooled below the point at which the metal will discolor and oxidize in the air. it is evident that if the metal oxides are removed and the metal surfaces cleaned, for example, at a red heat; and if the metal is lifted out of the bath at that temperature and without protection from the atmosphere it will again become oxidized on the surface.

It is impractical to cool the metal surface while still immersed since this would involve cooling the molten bath each timeit is used. Accordingto the present invention, the slag is of such a character that it covers the metal surface and protects it at elevated temperatures so that the metal can be cooled without oxidizing. The slag should therefore be noncracking, and it may even be of a plastic or sticky consistency, at the elevated temperatures atwhich the metal would'oxidize if not protected. If the slag has too high a melting point and begins to crack off at too high a temperature, the metal will be exposed and oxidize. If the protecting layer of slag does not crack of]? at all, even at low temperatures,

below those at which the metal requires prooff fairly. completely, so that the bulk of the slag will be obtained in a solid state such that it can be remelted without further manipulation and so that only a smallamount has to be removed from the metal by water or steam or weak acid. It is fairly easy to obtain a slag of this character and a wide range in its composition is permissible.

It is desirable that the slag used shall contain nothing of a corrosive nature which will attack the metallic surface. Such fluxes as chloride-s, fluorides and bi-sulphates, accordingly, are not to be recommended, and, when used should be present only in relatively small quantities such that they have no objectionable corroding action. If any considerable corrosion takes place, there is an unnecessary loss of metal, and-the operation may become objectionable due to gas evolution, etc., while the resulting metallic surface tends to become etched or corroded and therefore less desirable, for many purposes than when the oxide film alone is removed without such corrosion. Small quantities of the above mentioned ingredients may, however, be used, although generally they should not be used in objectionable amount. i

In thespecific composition of the slag above mentioned, the addition of soda to the borax increases the-toughness and decreases the fiuidity, since borax alone is too fluid at temperatures around 700 C. The addition of silica, in the form of sodium silicate, aids in regulating the brittleness and cracking qualities of the slag. The sodium sulphate serves to dilute the other ingredients and increases the water solubility of the slag. If the amountof silica is increased, for example, if too much silica is inadvertently added, the slag may adhere so firmly to the metal and be so difficultly soluble in water that dilute sulphuric acid may be reuired to remove the slag from the metal sur- 'ace. Other additions such as fluorspar, etc. are alsopossible; in fact, the composltlon of the slagcan be varied between rather wide limits.

Inasmuch as the slags, which are separated .fIOIIl the cool metal surfaces will contain the oxides dissolved from the metal surfaces, and

inasmuch as the amount of such oxides will increase with repeated use of the slag, it may become necessary to remove such oxides in order to prevent the impairing of the dissolving qualities of the slag and prevent an undesirable increase in its melting point or in its degree of insolubility, since an increase in the content of such oxides will increase the w insolubility and also the melting point of the slag. The slag can be regenerated or freed from such dissolved oxides, when necessary, by cooling the slag and dissolving it in water,

precipitating the oxides, filtering the result ing solution and evaporating the solution to obtain the slag ingredients for reuse. molten bath can also be freed from oxides or regenerated by subjecting the fused bath to electrolysis, the oxides being dissociated and the metal recovered on the cathode much as in the electrolytic production of metallic aluminum from a fused bath. If, however, copper is present in the nickel alloy to be cleaned and in the oxide to be removed, then part of the copper will cement out on the.

metallic surface and will have to be removed by other means. This process therefore does not readily lend itself to the cleaning of nickel alloys containing much copper.

The process of the present invention is of more or less general application, not only to nickel and nickel alloys, but also for the cleaning of other metals, although it is of particular value for cleaning surfaces having oxides which are substantially insoluble in acid pickling liquors. The process can thus be used for cleaning metal surfacesprior to electroplating. It can also be advantageously employed for annealing purposes, both when the metal surface does not require cleaning from oxides, and when the metal is to be both cleaned and annealed. In the latter case the process enables the cleaning and annealing to be affected in one process. This can be accomplished by maintaining the bath at the annealing temperature and permitting The the metal object to remain therein a sufficient period of time to bring about the annealing subjected to rolling and while it'is still at an elevated temperature.

The apparatus required for carrying out the present process is simple in character. It may consist merely of a container of some refractory material provided with suitable heating means either for heating the container from the outside or for heating the slag from above, or for heating the' bath in both of these ways. The receptacle may be a .shallow pan or receptacle similar to a reverberatory furnaceand provided with heat ing means for heating the slag from above; or the receptacle may be deeper and be ex.- ternally heated. The necess ary heat may be generated by direct coal or oil furnaces or by electric heating, preferably through resistance wires or resistors submerged in the slag.

A particularly advantageous container for carrying out the process is one made out of.

heat resisting metal such as an alloy of nickel, for example, a nickel-chromium-iron alloy. The furnace construction should preferably be such that the flame from the fuel (when a combustion furnace is used) comes in contact with the sides of the metal container but not with the top of the container or the moltenslag therein, so that the metal to be annealed and/or cleaned does not have to pass through the flame. When such a container is used, the object to be cleaned and/or annealed may be supported from outside the furnace or may be completely immersed in the container.

- r In the case of rods, sheets, wires, etc., the cleaning and/or annealing process can be carried out in a continuous manner by passing the metal through the bath and regulating the time of submergencetherein so that e the desired cleaning and/or annealing will take place. If the metal is still hot from a rolling or other operation it will not materially cool the molten slag bath. If the metal is cold it can advantageously be preheated before it is introduced into the molten bath. Even in the case of individual objects which are to be cleaned and/or annealed, the metal is preferably preheated, by an open fire or in a Inufile, as this saves considerable time in the completion of the operation, and the same tained, both the cleaning and annealing operations are almost instantaneous.

. The cleaning and/or annealing of the metal, for example, ofnickel or nickel alloys, can advantageously be combined with the mechanical working of the metal, so that the further mechanical working of the metal after cleaning and/or annealing will have cleanmetal surfaces provided therefor. Accordingly, the mechanical working of such metals is materially facilitated. For example, malleable nickel slabs or bars can be rolledinto sheets or shapes of substantially the desired thickness ,or dimension, by hot rolling, the resulting object or material, while still hot can then be cleaned from oxide, and annealed if desired, and, after the removal of the slag, can then be further worked, for example, by cold rolling to give the final desired shape or thickness. Nickel or nickel alloy sheets can thus be hot rolled to practically the desired final thickness, the sheets then cleaned in the manner above described,

and finally cold rolled to the final thickness.

The cleaning operation above described leaves the metal surfaces well adapted for further manipulation, so that the finished sheets or shapes of the desired dimensions can readily be obtained by cold working of the cleaned, or cleaned and annealed, mate rial.

Even for'annealing purposes where no removal of scale,'but only protection against gases, is desired, the annealing of the metal in the mannerabove described is more advantageous than box-annealing inasmuch as the surface is made perfectly free from oxide and no objectionable hardening of the surface takes place, while the process is also one which requires only a short periodpf time and can be carried out at moderate expense. Wire, such as nickel wire or chrome-nickel wire, can thus be annealed in the manner above described and with advantages such as those indicated. for instance, by passing the wire through slag baths between the successive dies. I

It will accordingly be seen that the present invention is of particular advantage not only for the cleaning of metal surfaces such as those having difiicultly removable oxide films, but also for the annealing of'metals,

and for the combined cleaning and annealmg of metals; while it can advantageously be combined with the mechanical working of the metals so that the metals can be Worked hot to practically their final shape, then cleaned and/or annealed, and finally finished by cold working.

I claim v 1. The method of cleaning metal surfaces of a slag capable of dissolving such oxides,

.by removing oxides therefrom, which comprises submerging the same in a molten bath withdrawing the hot metal from the slag and protecting the same against oxidation by the adhering layer of the slag, such slag containing only sufiicient silica to impart to the slag a sticky consistency at the temperatures of about 300 to 700 C.

2. The cleaning of metallic nickel, nickel alloys, metals containing chromium and the like from a relatively insoluble coating of oxide, which comprises subjecting the same to the action of a molten slagcapable of dissolving such oxide such slag containing boricacid together with other ingredients which give a slag which is elastic while hot bu brittle When cold. I

3. The cleaning ofmetallic nickeh nickel alloys, metals containing chromium and the.

like, from a relatively insoluble and strongly adhering coating of oxide, which comprises subjecting the same to the action of a molten slag capable of dissolving such oxide, such slag containingboric acid and silica.

In testimony whereof I affix my signature.

NOAK VICTOR HYBlNETTE.