US2751312A - Lubricant composition containing powdered lithium carbonate and method of its application to metal bodies - Google Patents

Description

United States Patent O LUBRICANT COMPOSITION CONTAINING POW- DERED L CARBONATE AND METHOD OF ITS APPLICATION TO M'ETAL BODIES Harold J. Ness, Montclair, N. 1., assignor to Metallurgical Processes Co., Newark, N. J., a corporation of New Jersey No Drawing. Application August 1, 1950, Serial No. 177,135

6 Claims. (Cl. 117-533) This invention relates to lubricating and oxidation resistant coatings for metals and to a method of producing such coatings. More particularly it relates to the lubrication of metal for hot working and the protection of the metal surface from scale during working, cooling and storage.

In hot metal working operations, such as forging, extruding, expanding and rolling, temperatures may be employed ranging from 1200 F. to 2500 F. In these operations the speeds may be high, the pressures employed extremely heavy, and the elongation or deformation of the metal severe. The need for a lubricant in such forging and steel mill operations is great and the requirements such that heretofore no satisfactory lubricant has been available. Such a lubricant among other characteristics must have stability up to 2500 F., suificient viscosity to withstand pressures of the order of 250,000 pounds per square inch, good lubricity, and must not be corrosive to the metal being worked. High temperature metal base greases have a maximum temperature limit of about 600 F., and solid lubricants, such as graphite, can not be bonded to the metal and, moreover, they are rapidly consumed in the air at such elevated temperatures. As a result, such hot working operations have been, of necessity, carried out without the use of any satisfactory lubrication. This results in excessive wear of the dies, tools, rolls, and other working implements, and necessitates their frequent resurfacing or replacement. It also slows down the working speeds and places undesirable limitations on the amount of deformation or working that may be effected in a single stage.

The principal object of this invention is, therefore, to provide a suitable lubricating medium or film for metals undergoing hot working.

A further object is to provide a lubricating coating which is sufliciently stable to maintain high lubricating qualities over a temperature range of from 1150 F. to 2500 F. or higher, throughout the working and cooling period of the metal to be worked.

Another object is to provide a lubricant which at high temperatures forms a fluid coating having strong adhesive characteristics and high film strength.

Still another object is to provide a high temperature lubricant which is free from corrosive action.

A still further object is to produce a lubricant for metal undergoing hot working which has low interfacial tension with the metal so as to readily form and maintain a continuous lubricating film over the metal surface during mechanical deformation and elongation and extension of the lubricated area, and which retains its lubricating properties throughout several successive Working operations.

Still another object is to produce such a lubricant which in addition will serve to protect the metal to which it is applied from oxidation or scale during working and cooling, and which may serve as a solid lubricant during cold working of the metal.

A further object is to provide a lubricating and protective film of the above nature which may be left on the metal without detriment thereto or which, if desired, may be readily removed therefrom.

Another object is to provide an economical, rapid and effective method of applying such a lubricant to metal bodies.

Other objects and advantages will hereinafter appear.

In accordance with the present invention, a lubricating film is produced on metals which is comprised essentially of a compound of lithium, primarily the carbonate of lithium. At high temperatures a portion of the carbonate may be reduced to the oxide to form a mixture of lithium carbonate and lithium oxide, but on cooling the film is found to consist mostly of the carbonate. Other alkali or alkaline earth carbonates or various types of glasses having suitable melting or flow points may be added in varying amounts to the lithilun carbonate for the purpose of modifying the melting temperature or viscosity of the lubricating film. When lithium carbonate is employed without such additions a lubricating film is formed which is fluid at temperatures above about 1150 F., the melting temperature of lithium carbonate being 1144.4 F., and the fluid film has sufficient stability up to 2500 F. or higher to maintain its strong film characteristics and exhibit a low coeflicient of friction with suflicient viscosity to withstand the heavy working loads encountered in forging and steel mill operations. The lubricating film, in addition, has active interfacial or film forming qualities and adheres tenaciously to metals by what appears to be both a physical and chemical union with the metal. The lubricating film forms not onlyon clean metal but equally well on metal which has been oxidized or scaled. In the scaled condition of the metal the fluid lithium carbonate penetrates through the scale and appears to bond directly to the underneath metal. In the scale it is desirable, of course, to descale the metal in accordance with established practice before applying the lubricating film.

The low interfacial tension of the lubricant with the metal not only permits uniform thin adhesive films to be readily produced over an extensive surface of metal during the original application thereof, but also causes the lubricant to follow deformation or extensions of the surface to several times its original area during mechanical working and to maintain its lubricating values with successive working steps or passes of the metal.

The liquid lithium carbonate film, upon cooling of the metal, solidifies below 1144 F., to form a thin, impervious coating of low density and of a soft, non-vitreous and nonabrasive nature which, when cold working is desired, will serve as an effective solid lubricant. This coating may be left on the metal since it is non-corrosive and, in fact, protective in nature, and due to its thinness, strong adherence and substantially neutral nature, it serves as a good base for various metal finishes. It also may easily be removed by a short dip in a weak acid, such as acetic or dilute hydrochloric acid.

The required lubricating film may be formed on the metal by the direct application of lithium carbonate, either alone or mixed with other ingredients in sufiicient volume, and while the metal is at a sufliciently high temperature to cause the lubricating material to rapidly form a fused coating or film of the required thickness on the metal. It has been found that when the film forming conditions are controlled so as to produce the required weight of coating in a relatively short period of time, that the operation may be carried out in the air without detriment to the lubricating characteristics of the film although, if desired, the film may be formed in a similar manner in a neutral gas or in ordinary combustion gases, as for instance, during the furnace heating of the metal for mechanical working. Lithium carbonate, as stated, melts at -l144 F. and it is necessary, therefore, that the metal be brought at least up to this temperature before the fused coating will be produced, but lubricating films may be produced at metal temperatures up to 2500 F. or higher, and may be maintained on the metal throughout this range of temperatures during the hot working cycle of the metal. This is possible because of the low vapor pressure of the molten coating material.

While lithium carbonate starts to decompose into lithium oxide and carbon dioxide at these elevated temperatures, the rate of decomposition in air is slow and the compound sufiiciently stable to maintain a substantial layer of the carbonate, possibly mixed with small amounts of the oxide, on the metal surface for a time far beyond the cooling period of even large masses of metal, so that the required lubrication is provided during the entire hot working cycle. If desired, when applying the lubricating material to metal at its upper temperature limits, that is, at 2300 F. to 2500 R, an excess of material may be deposited thereon to compensate for possible loss by vaporization or reduction to the oxide.

The lithium oxide which is formed by such decomposition rapidly sublimes so that the composition of the lubricating film remains primarily the carbonate through the entire hot working range and during subsequent cool- The bond obtained between the lubricating film and the metal appears to be due in large part to chemical union of the carbonate with the metal resulting, in the case of steel, from the formation of a lithium-iron compound at the surface of the metal. There also appears to be a strong physical bond created by penetration of the film into the metal. This strong union is believed to account largely for the film strength which enables this material to maintain separation of the working tools or rolls from the work even at the highest temperatures specified and at the enormous pressures employed in such steel mill operations as rolling, expanding, and the like.

As heretofore stated, the lithium carbonate not only has no corrosive elfects upon the hot metal but in fact serves as a protective medium to prevent or retard oxidation of the surface to which it is bonded. This feature is of extreme importance, particularly in rolling or expanding operations where fresh surfaces are continuously produced. In the absence of the protective film of the present invention, such new surfaces rapidly become oxidized, but due to the protection aiforded by the lubricating film which rapidly forms on this fresh surface, scaling thereof is prevented or greatly retarded. As a consequence, even though the metal on removal from the furnace may be highly oxidized, after the first working operation it will be substantially free from scale and will be maintained in such condition throughout the successive working stages.

The fused lubricating film may be produced by bringing the coating material into contact with the heated metal in various ways, as by spraying the powdered material directly onto the hot metal, by dusting it onto the metal, or by creating a fog or suspension of the material about the hot metal. The lubricating film may also be applied as a paste or other viscous composition to cold metal and the metal brought up to the fusion temperature of the compound. The essential feature of all of these methods is the bringing of a sufficient quantity of the film forming material into contact with the metal while the latter is at a temperature above the melting point thereof, so as to form a molten film of the required thickness on the metal in a short interval of time, measurable in seconds.

I prefer to spray the dry coating material in finely powdered form directly onto the hot metal in the open atmosphere. The carbonate by itself, however, tends to pack in the supply container and in the conduits of the spray system and, therefore, I find it desirable to mix it with a lubricant such as lithium stearate, and I have found that approximately 4% of this material is sufficient to overcome the packing tendency. However, other stearates or metallic soaps may be employed in varying quantities. The carbonate and stearate are preferably milled for a period of one-half hour or until the carbonate particles are thoroughly coated with the stearate. I have also found that the finer carbonate powders are more susceptible to spraying and I prefer to use a compound ground to pass a 300 mesh screen.

The amount of lithium carbonate required to produce the desired film on the metal is very small, one-fiftieth of an ounce being sufficient to produce a fused lubricating film of a thickness of a few microns per square foot of surface area. This requires an extremely thin or sparse distribution of the carbonate over the surface of the metal, and such distribution can most readily be produced by mixing the carbonate and stearate mixture with a dilutent, thereby enabling the spray equipment to operate with larger quantities of the lubricating material, and thus permitting more accurate control both of the quantity and the distribution of the material to be obtained. The dilutent may be composed of a material which will be readily consumed or vaporized by contact with the hot metal without leaving any appreciable residue so that the carbonate fused on the metal will be uncontaminated thereby. Lithium carbonate is an extremely light fluffy material having insufiicient density to permit it to be readily projected through the air and the dilutent acts additionally as a carrier for the film forming material. The dilutent should, therefore, be a material to which the carbonate will adhere and it should have good spraying properties. It should also have free flowing properties so that it may be readily forced from the storage container to the spray nozzle. Materials I have found suitable are wood flour, various starches, and finely ground cereals, or mixtures of these materials. The flow qualities of the mixture may be tested by passing the same through a smooth bore tube of one-quarter inch diameter. Materials which freely flow therethrough will ordinarily work satisfactorily in commercial powder spraying equipment.

The amount of dilutent employed is not critical and is governed by the nature of the spray apparatus and particularly by the minimum amount of material which can be passed through the apparatus while still obtaining a uniform spray pattern of a size which will cover a considerable area of the work, so as to permit the spraying process to be quickly effected. It is often desirable to apply the coating to the metal while it is in transit from the heating furnace or from the descaling equipment adjacent thereto, and in such cases the area and volume of spray should be adjusted so as to give the required coating while the metal is on the fly. In practice I have obtained satisfactory coatings with a mixture containing from 25% to 75% of stearate coated lithium carbonate mixed with wood flour.

As stated, however, other alkali carbonates, such as sodium carbonate or alkaline earth carbonates, as barium carbonate, may be added to the lithium carbonate, but I prefer to employ a preponderance of lithium carbonate in order to retain the lubricating properties thereof in the molten film. Various glasses may also be added, such as ordinary sodium-calcium-magnesium glass, in order to increase the melting temperature and viscosity of the lubricating film. Other glasses, such as lead glass and borax glass, may also be added. I have satisfactorily employed a mixture of equal parts of lithium carbonate and glass for hot lubrication. However, the addition of glass destroys the cold lubrication of the film by imparting a hard vitreous character to the solidified film. Glass coatings unmixed with the carbonate are extremely difficult to remove from the cold metal but in the lithium carbonate mixture may be readily removed by a Weak acid dip.

Various types of dry spray apparatus are commercially available, one form being that employed in the spraying of dry fire extinguishing chemicals. In such apparatus the powder is forced at considerable velocity by a compressed gas from a storage container through a spray nozzle or horn. Where a gas is used as the propellant for the powdered material, and particularly where there is direct impingement of the gases on the work I prefer to employ a gas which is not strongly oxidizing in order to prevent any scaling of the metal before the protective film has had an opportunity to form. Such gases may be dry carbon dioxide, dry hydrogen, or the dried products of combustion of a suitable fuel.

The thickness of the fused coating will depend upon the quantity of compound sprayed onto the surface and may vary from a few microns to a few hundredths of an inch. The thickness required for satisfactory lubrication is not critical but in general I have found that a lubricating film of from several microns to .002 inches is satisfactory for most hot working operations. It is not essential, moreover, to obtain any particular exactitude in the uniformity of the sprayed coating since by virtue of the low interfacial tension of the fused compound with the metal, it will evenly spread over the surface in a substantially uniform film.

The dry compounds may be applied in other ways, as by dusting onto the work or by creating a fog or suspension of the coating particles through which the hot metal is passed, or by electrical precipitation. Due to the lightness of the carbonate powder, it will remain suspended in the air for a considerable period, particularly if carried out under a hood or other chamber through which the heated metal may be passed.

What I claim is:

1. The method of producing a high temperature lubricating film on the surface of a metal body which comprises applying powdered lithium carbonate to the surface of the metal body in the open air while the latter is at a temperature of from 1150 F. to 2500 F., in a quantity which will rapidly produce a fused film of said carbonate on the surface of the metal body of a thickness of from several microns to several hundredths of an inch.

2. The method of producing a high temperature lubricating film on the surface of a metal body which comprises distributing solid material selected from the group consisting of the alkali and alkaline earth carbonates and including at least 50% of lithium carbonate over the surface of said metal body while the latter is in the open air and at a temperature of from about 1150 F. to 2500 F. and above the fusion temperature of said material, in suflicient quantity to form a continuous fused layer of said material over said surface and being equal at least to 0.02 ounces per square foot of the surface to which said fusible material is applied.

3. The method of producing a high temperature lubricating film on the surface of a metal body which comprises applying a solid mixture of a glass and lithium carbonate, in the proportion of at least fifty percent of the latter, to the surface of said metal body while the body is in the open air and at a temperature of from about 1150 F. to 2500 F. and above the fusion point of said mixture, said mixture being applied in sufficient quantity to form a fused layer over said surface of at least several microns in thickness.

4. The method of producing a high temperature lubricating film on the surface of a metal body which comprises mixing from 25% to of powdered lithium carbonate with from 75% to 25% of a powdered organic dilutent which has a density greater than that of lithium carbonate and is substantially completely combustible at temperatures below 1150 F., distributing said mixture in said powdered form over said surface while said surface is in the open air at a temperature of from about 1150 F. to 2500 F., in such quantity as to produce a fused lithium carbonate film on said surface of a thickness in excess of a few microns.

5. The method of producing a high temperature lubricating film on the surface of a metal body which com prises heating the body to a temperature of from about 1150 F. to 2500 F. and spraying a material consisting essentially of powdered lithium carbonate onto said surface while the latter is in the open air to form a continuous fused coating thereof on said surface.

6. A lubricant for the hot working of metal bodies composed of a powdered mixture of glass and lithium carbonate, each of said ingredients being present in sufficient amount to modify substantially the viscosity of the other when said mixture is in the molten state and said lithium carbonate comprising at least 50% of said mixture.

References Cited in the file of this patent UNITED STATES PATENTS 862,285 Schmidt Aug. 6, 1907 1,076,590 Liebmann Oct. 21, 1913 1,683,437 Batie Sept. 4, 1928 2,120,496 Hauger June 14, 1938 2,181,093 Ness Nov. 21, 1939 2,430,083 Sherman Nov. 4, 1947 2,469,473 Ovozco et a1. May 10, 1949 2,479,979 Spence et al. Aug. 23, 1949 2,535,284 Harris Dec. 26, 1950 2,538,917 Sejournet et al. Ian. 23, 1951 FOREIGN PATENTS 16,883 Great Britain 1903 OTHER REFERENCES Ceramic Ind. 32 (2), 57-59 (1939). The Merck Index, 5th Ed., Merck & Co., Rahway, N. 1., page 320, published 1940.

Claims (2)

1. 5. THE METHOD OF PRODUCING A HIGH TEMPERATURE LUBRICATING FILM ON THE SURFACE OF A METAL BODY WHICH COMPRISES HEATING THE BODY TO A TEMPERATURE OF FROM ABOUT 1150* F. TO 2500* F. AND SPRAYING A MATERIAL CONSISTING ESSENTIALLY OF POWDERED LITHIUM CARBONATE ONTO SAID SURFACE WHILE THE LATTER IS IN THE OPEN AIR TO FORM A CONTINUOUS FUSED COATING THEREOF ON SAID SURFACE.
2. 6. A LUBRICANT FOR THE HOT WORKING OF METAL BODIES COMPOSED OF A POWDERED MIXTURE OF GLASS AND LITHIUM CARBONATE, EACH OF SAID INGREDIENTS BEING PRESENT IN SUFFICIENT AMOUNT TO MODIFY SUBSTANTIALLY THE VISCOSITY OF THE OTHER WHEN SAID MIXTURE IS IN THE MOLTEN STATE AND SAID LITHIUM CARBONATE COMPRISING AT LEAST 50% OF SAID MIXTURE.