US3481762A - Metal lubrication process - Google Patents

Description

United States Patent 3,481,762 METAL LUBRICATION PROCESS Michael A. Streicher, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Mar. 10, 1966, Ser. No. 533,225 Int. Cl. B44d 1/02, 1/14 U.S. Cl. 117-49 13 Claims ABSTRACT OF THE DISCLOSURE Metals to be deformed, e.g., by extrusion, drawing, etc., are lubricated by depositing a water insoluble metal oxalate coating on the surface of the metal and/or on the surface of the workpiece which is to deform the metal, impregnating the oxalate coating with a suspension of graphite in lubricating oil, and then decomposing the resulting graphite and oil-impregnated oxalate coating. The oxalate coating preferably comprises manganous oxalate deposited from an aqueous solution of manganese and oxalate ions at a concentration equivalent to more than 20 grams/liter manganous oxalate.

In primary working operations such as extrusion, drawing, rolling and forging, there is extensive deformation and working of the metal workpiece being shaped. Because of the great pressures and extensive deformation which occurs, there are stringent requirements for lubrication between the workpiece and the die or other shaping means in order to reduce power requirements as well as surface scoring, marring, and rupture and seizing between the workpiece and shaping means. This invention provides such a lubricant. Although the applications of techniques described herein can be applied to metals generally, this invention is particularly adapted for use with metals of Groups IVB to VIIB and VIII of the Periodic Table of Elements, as shown on pages 58 to 59 of Langes Handbook of Chemistry, 7th edition (1949) and especially the more refractory and difiicultly worked on metals, such as titanium and zirconium for which, in connection with some metal working operations, the-re have been no adequate lubricants.

The metal working lubrication process of this invention comprises:

(a) depositing a water insoluble metal oxalate coating on at least one working surface, i.e., on the shaping means, surface of the workpiece in contact therewith, or both,

(b) impregnating the oxalate coating with a suspension of graphite in hydrocarbon lubricating oil, and' (c) heating the resulting impregnated coating at a temperature above 320 F. (160 C.) to decompose the oil-graphite-oxalate coating without rapid combustion.

The first step in the process of this invention comprises depositing a water insoluble oxalate coating on at least one of each pair of surfaces which are brought into contact during the metal working operation, for example, the die, billet, or both, during extrusion. Although any oxalate which will form a water insoluble coating, for example as described hereinafter, can be employed, oxalates of metals of atomic numbers 24 to 28 are particularly preferred. Of these, manganous and ferrous oxalates are particularly preferred, manganous because it forms particularly thick adherent oxalate coatings in accordance with the unique deposition process described herein and ferrous oxalate because of its low cost and ease of application by the procedures described hereinafter.

The oxalate coatings can be applied by a number of different procedures including in situ direct reaction of an acidic oxalic acid solution with the surface of the workpiece, in situ redox reaction of the surface of the workpiece with dissolved metal salts to yield insoluble oxalates, and a preferred process of the invention described hereinafter comprising immersion of the workpiece in a supersaturated manganous oxalate solution.

As just indicated, a preferred process of this invention comprises immersing the surface to be lubricated in a bath supersaturated with respect to manganese oxalate containing manganous and oxalate ions present in an amount equivalent to at least 20 grams of manganous oxalate per liter. Although the manganous and oxalate ions may be produced in the form of their soluble precursors, for example, alkali metal and ammonium oxalates, manganous acetate, chloride, bromide, or a combination of one or more of the foregoing, preferably oxalate is provided by oxalic acid, and manganese by manganous nitrate or a combination of soluble salts which yields both manganous and nitrate ions, for example, potassium permanganate, oxalic acid and sodium nitrate or nitric acid. In this preferred process of this invention, the major part of the coating is formed by precipitation of manganous oxalate from a supersaturated solution and the deposition does not depend on continued dissolution or in situ reaction of the metal surface being lubricated. As a result, the process is not self-stifling and coatings of unusual thickness can be formed, such thick coating being desirable with particularly high reduction ratios. One unique characteristic of the solutions of the preferred manganous oxalate process is the fact that manganese and oxalate ions are present in quantities greatly in excess of the normal solubility of manganous oxalate. They are indeed unstable but even at the elevated temperature of the coating operation the manganese oxalate continues to precipitate over a period of several hours. This slow precipitation yields uniform, adherent layer formed on the metal. These solutions contain manganese and oxalate ions equivalent to manganese oxalate, MnC O in excess of about 20 gm./liter. Concentration considerably above 20 gm./liter up to 200 gm./liter or even more, are used and represent a considerable degree of supersaturation. It is preferred that nitrate ions are present to enhance the coating action. Typical manganous nitrateoxalic acid coating baths are illustrated hereinafter. On immersion in fresh solution of manganous oxalate, another similar layer can be deposited on top of the first. This is in contrast to other known oxalating baths in which re-immersion in a fresh bath is substantially without effect. To prevent a drop in temperature of the oxalating solution, it is convenient to preheat the billet in boiling water until it is close to the temperature of the oxalating bath. The bath can be operated from room temperature to boiling, and preferably about from -212 F. (37.8-100" C.)

The action of these manganese coating baths is enhanced, at least initially by the addition thereto of an anionic wetting agent such as the sodium salts of organic derivatives of sulfonic acids, e.g., sodium alkyl aryl sulfonates such as sodium dodecylbenzenesulfonate, sodium decylbenzenesulfonate and sodium octylbenzenesulfonate. Those marketed under the trade mark Alkanol WXN are preferred. Other anionic wetting agents are illustrated in 3 Detergents and Emulsifiers-Up to Date, 1963, John W. McCutcheon, Inc.

Another form of this novel bath can be prepared by using potassium permanganate as the source of manganese. Although oxalic acid and potassium permanganate are known to react quantitatively the reaction is delayed until a certain concentration of manganous ions have slowly developed which catalyze the reaction. By mixing potassium permanganate and oxalic acid in concentrations giving the quivalent of over 20 gm./ liter MnC O the reaction is delayed so that the metal to be coated can be immersed and the solution heated to the 170-212" F. (76.7-100 C.) range before much reaction occurs. In a few minutes reaction does occur and manganous oxalate is precipitated in the solution. A large portion of it precipitates on the metal surface as an adherent coating. Presumably the reaction involved is The presence of nitrate, added as NaNO or other convenient salt, or as HNO improves the coating action. Presumably the mechanism of coating is the same as when Mn(NO is used; the KMnO merely serves as a cheaper source of manganese ions which are then precipitated as MnC O Solid manganous compounds or concentrated aqueous solutions thereof can be added continuously during the coating operation to maintain the desired concentration i.e., keep the solution in the unstable or supersaturated state with respect to the manganous oxalate.

A pre-coat of ferrous oxalate can be used to promote the growth and adhesion of the manganese oxalate on the metal surface. In the case of coating iron or steel, iron oxalate is also present in the coating even though the bath is initially iron free. Presumably the oxalic acid first reacts with the iron to dissolve it whereupon ferrous oxalate precipitates as a conversion coating on the surface.

The manganese oxalate coatings, which can also contain iron oxalate or other similar oxalates, are admirably suited to the subsequent treatment with graphite and oil for producing the novel lubricant of this invention.

Another process for the deposition of oxalate coatings involves immersing the piece to be lubricated in an aqueous solution of a soluble metal oxalate which by reaction with the surface being lubricated is reduced to a cognate insoluble oxalate. This process can be used either alone or in combination with the direct reaction of acidic solutions of oxalic acid with the surface being lubricated. For example, when iron or low alloy steels are exposed to oxalic acid, ferrous ions are formed at anodic sites which are electrochemically equivalent to the amount of hydrogen evolved at cathodic sites on the surface. The ferrous ions react with oxalate ions to form insoluble ferrous oxalate, which then precipitates on the surface of the dissolving metal to form an adherent layer of crystals. Formation of this coating gradually reduces the dissolution of the metal and, therefore, only relatively thin coatings can be formed by this process. The rate of formation of this coating can be increased by adding another source of ferrous ions to the solution in the form of ferric oxalate, which is soluble in oxalic acid. The result is that at cathodic sites not only hydrogen ions are reduced to hydrogen atoms but also soluble ferric ions to insoluble ferrous ions. In such solutions the rate of the coating process is increased by (1) having two sources of ferrous ions in the dissolution of iron and reduction of ferric ions and (2) by increasing the rate of the cathodic reaction via reduction of ferric ions which, in turn, increases the rate of the anodic dissolution reaction. Another advantage of adding a ferric salt to oxalic acid solutions is that this addition makes it possible to form ferrous oxalate coatings on non-ferrous metals. For example, to form iron oxalate coatings on titanium the metal is exposed to a solution of oxalic acid, ferric .4 oxalate and a fluoride. The fluoride is added to increase the rate of attack of oxalic acid on titanium. The dissolution of titanium metal to titanium ions at anodic sites makes possible the reduction of ferric to insoluble, ferrous ions and consequent precipitation of a coating of ferrous oxalate at cathodic sites on the surface of the titanium. Preferably, in the deposition of ferrous oxalate coatings the concentration of iron and oxalate ions in solution are equivalent to about 5 to 20% by Weight of ferric oxalate.

Other oxalate deposition procedures which can be used in accordance with this invention are described, for example, in US Patent 2,273,234, as Well as in US. Patents 2,577,887; 2,669,532; 2,774,696; 2,800,421; 2,813,816; 2,935,431; and 2,987,427, which are incorporated herein by reference.

With the variety of metal oxalates available and the numerous metal surfaces to be lubricated, it is not surprising that considerably variation occurs in the degree and quality of the coating obtained. Some of the oxalating solutions used will coat some of the metals far better than others. However, since either the workpiece or the tool surface can be coated, there is always an operable combination available. In addition there are two ancillary steps which can be taken to enhance the oxalate coating step. These include, in addition to normal cleaning and pickling procedures, electroplating with another metal which oxalates better, preseating the metal, activating by etching in a mixture of sulfuric, hydrochloric and hydrofluoric acids, and abrading.

In many cases the use of a thin plating of iron or other readily oxalated metal will provide a thicker oxalate coating. Metals such as titanium, columbium, and zirconium, which either do not react rapidly with oxalating solutions, and which do not form insoluble oxalates, can be iron plated to provide a substrate which will acquire a heavy oxalate coating. Plating is especially useful when using the preferred manganese nitrate-oxalic acid solution. Any method of producing a thin adherent iron deposit can be used. However, electrodeposition has been found very satisfactory. The deposit usually ranges from 1 mil to 10 mils (0.0254 to 0.254 cm.) in thickness. Thicker layers can be used, but since their subsequent removal by pickling is usually contemplated the thinner plates are preferred. The preferred method for preparing the surface for plating is sand blasting. The iron plate can be used advantageously on a work piece made by compaction of metal powders. In plating a workpiece or die with iron any electrolysis procedure which gives a reasonably uniform deposit can be used. A good bath is a simple 15% solution of FeSO in water. The preferred solutions are those adjusted to a pH ranging from 1.6 to 2.6 in which the iron content may vary quite widely. The plating is carried out above 160 F. (71.1 C.), and preferably between 200 F. (93.3 C.) and boiling to assure a ductile, adherent deposit. At pH values below 1.6 and a current density of 10 arnps./dm. the current efficiency is low due to re-solution of the iron which can lead to discontinuities in the plate. At pH value above 2.6, the deposit is non-uniform due to tree formation. A current density range of from 5 to 20 amperes per square decimeter has been found satisfactory.

It has also been discovered that the process of coating metals with oxalate by immersion in any oxalating bath is improved by preheating the metal to temperatures up to about 212 F. C.), and preferably 100 to 212 F. (37.8 to 100 C.). The improvement lies chiefly in obtaining a thicker coating. To illustrate this the following two solutions were used in coating a low carbon steel.

(a) 850 ml. water, 99 g. oxalic acid, 99 g. ferric oxalate. (b) 900 ml. water, 45 g. oxalic acid, 118.6 g. 50% soln.

of Mn(NO Both solutions were used at 210 F. (99 C.) with immersion for 30 min. In one case the .02 cm. thick steel was not preheated, in the other it was preheated in plain water to 210 F. (99 C.) with the following results.

Wt. of coating This effect is present with other metals and metallic compositions.

Metal surfaces can be activated toward these oxalating baths especially those containing manganese oxalate by roughening the surface with an abrasive of 80 to 120 grit size. This treatment is especially effective on nickel and the stainless steels. This action is not though to be merely one of cleaning the metal surface since the grit size is quite critical. Rather it is believed that the degree of roughness produced is such that effective nucleation sites for initiating growth of the oxalate crystals are produced.

In other instances nickel or nickel alloys may be treated with a mixture of sulfuric, hydrochloric, and hydrofluoric acids to activate the surface for coating formation in the oxalic acid-maganese nitrate solution in which these alloys dissolve with difiiculty. This pickling treatment enhances the deposition of adherent crystalline oxalate coatings by forming a very thin seed bed of insoluble nickel oxalate for the subsequent deposition of manganese oxalate.

The amount of oxalate coating varies depending on whether one or both working surfaces are lubricated and the extent of deformation in the metal working operation. In general, it is greater if only one surface is lubricated and increases with the reduction ratio. Usually, the oxalate coatings vary from 0.01 to 4 g./dm. and preferably .1 to 3 g./dm. After the adherent oxalate coatings have been formed, they are usually rinsed in water and allowed to dry. Drying can be accelerated by heating in an oven, preferably below 100 C.

Having deposited an adherent oxalate coating on the surface of the metal to be lubricated, the next step in the process of this invention comprises impregnating the oxalate coating with a dispersion of graphite in a hydrocarbon lubricating oil, for example, by dipping the oxalate-coated metal in the suspension or by brushing, spraying or otherwise applying the suspension to the oxalate coating. Although the amount of suspension applied can vary widely, in general sufficient is infused into the oxalate coating to yield 5 to 200% of graphite based on the weight of oxalate coating. The graphite-oil suspension, which, depending on the particular materials employed, can vary from a fluid suspension to a paste or grease-like composition, usually contains 5 to 50%, and preferably 5 to 25% by weight of graphite based on the total composition. The graphite, which can be natural or synthetic, is preferably of colloidal size. Graphites having a number average particle size of less than microns and especially less than 1 micron are particularly preferred. The oil preferably has a flash point above 300 F. (149 C.), and preferably is an asphaltic-base hydrocarbon lubricating oil. Examples of such oils include asphalt-base oils such as Texaco 738-738M as well as the various motor oils, transmission oils and heavy oils and greases meeting the foregoing criteria. Graphite in oil dispersions such as those described in MILL-3572, Aug. 21, 1951, are particularly convenient graphite-in-oil dispersions of the type useful in this invention.

The final step of the basic process of this invention comprises heating the graphite-oil-oxalate combination at a temperature of at least 320 F. (160 C.) to initiate slow decomposition of the coating, i.e., decomposition without rapid burning. The heat treatment is usually complete when visual decomposition, i.e., smoking, stops. The heat treatment is preferably carried out at a temperature of from 400950 F. (204.4510 C.) for a period of about from 3 minutes to 3 hours. Although the heat treatment can conveniently be carried out in air, particularly if the lower temperatures are employed, in order to insure against rapid decomposition particularly at higher heat-treatment temperatures, the heating can be done in inert or unreactive gases such as argon, nitrogen, carbon dioxide, or steam.

The heat treatment of the oxalate-oil-graphite coating is an essential part of the invention, and the marked improvement gained thereby obtains only when all constituents thereof are heated simultaneously. During this treatment, the oxalates decompose to oxides. Usually there is evidence of charring of the oil and this contributes to the ultimate composition of the lubricating film by forming a pitch which binds the various solid phases and serves to maintain separation of the opposing working surfaces and hold any additional oil or other lubricant subsequently added. The solid phases which contribute to the lubrication are the graphite and the residual oxide decomposition products of the metal oxalates formed in the presence of decomposing oil. These oxides are stable at the higher working temperatures and serve, inter alia, to separate the working surfaces thus preventing seizing. In addition, oxides have been observed to exhibit plastic flow under the pressures encountered in metal working operations. This action also contributes to the lubrication. The amount of lubrication separately contributed by either the graphite, the oil and its residues or the oxalate residues is quite minor compared to the effect of the combination and this synergistic effect results only from the step of decomposing the oxalate in the presence of the oil and graphite.

The process of this invention can be used for improving any of the primary metal working operations including extrusion, drawing, rolling, forging, etc., wherein extensive deformation and working of the metal is encountered. As previously indicated, either the shaping means and/or the workpiece can be lubricated. The process of this invention finds particular utility in extrusion especially of the more difliculty worked refractory metals, such as titanium, columbium and zirconium for which wear, seizing, extrusion pressure and surface finish problems hecome acute with known methods for extrusion which are often solved only by canning, i.e., enclosing object to be formed in a steel or copper container followed by forming of the object, removal of the can and refinishing.

The lubricant of this invention may be used over a wide range of temperatures depending on the nature of the metal being worked and the degree of reduction desired. Temperatures up to 2600 F. (1427 C.) have been successfully used. In most cases lower temperatures are desired for metallurgical reasons. Extrusions of titanium and drawing of steel rifle barrels at room temperature has been improved by this method.

The following examples describe specific applications of this invention without implying any limitation thereof. Parts and percentages are by weight unless otherwise indicated.

EXAMPLE 1 Ductile titanium powder 30+200 mesh is hydrostatically compacted to of theoretical density and shaped to fit a tube extrusion press having a liner of 2.15 inches (5.46- cm.) ID. and a chrome steel 90 cone die contoured at the throat. The billet is axially drilled to receive a 21/32-inch (1.67 cm.) mandrel mounted on the follower block. The die port is 1 -inch (3.18 cm.) diameter at the land. The working surfaces of the tools (die and mandrel) are cleaned, heated to C. in an oven, and immersed for 30 minutes at 98 C. in a solution containing 45 g. oxalic acid, 75 ml. of a solution of 50% Mn(NO in water, and 1000 ml. of water (63 g./liter MnC O to yield an oxalate coating weighing 0.6 to 0.7 g./dm. Tools are rinsed and air dried. The billet is oxalate coated by preheating to about 100 C. and then immersing at 98 C. for 30 minutes in a solution containing 53 g. ferric oxalate, g. oxalic acid, 10 g. NaF and 1000 g. water, then rinsing and drying in air. Both the tools and the billet are coated with graphite in oil and heated to decompose the oxalate and oil. The graphite particles have a number average particle size of less than 1 micron, are substantially all less than 4 microns in diameter, and amount to 10% of the total of oil plus graphite. The oil is an asphalt base, lubricating oil having a flash point of 325 F. (162.8 C.), a pour point of F. (9.44 C.), and a viscosity of 110 to 135 universal Saybolt seconds at 210 F. (99 C.). The weight ratio of graphite to oxalate after impregnation is about 0.5: 1. The tools are oven heated in heated in argon to 450 C. for about 5 minutes. The tools are assembled in the press with the liner temperature at about 390 C. and given a final coat of the graphite dispersion. The billet is removed from the induction furnace dipped again quickly in the graphite preparation and extruded at a ram speed of inches (50.8 cm.) per minute. The resulting extruded %-inch (1.9 cm.) tube of -inch (.16 cm.) wall thickness is about 4 ft. (122 cm.) long is smooth having a mirror-like surface both inside and out, and is only slightly curved. The metal has theoretical density except for the first 5 or 6 inches (12.7 or 15.2 cm.) which sometimes are not worked as much as the rest. The tube is easily straightened, annealed and is ductile enough to be further reduced by drawing. The dark bluish residue on the surface can be removed easily with acid.

EXAMPLE 2 Ductile titanium powder 30+200 mesh is hydrostatically compacted to rough billets of approximately 95% of the theoretical density. Three such compacts are machined to extrusion billets 2 inches (5 .08 cm.) in diameter with a 90 cone chamber on one end. An extrusion press is fitted with a heated container, 2.05 inch (5.20 cm.) I.D. linerand a 90 cone die of steel. A mild steel follower block is used and the press set for a ram speed of 16 in./min. (40.6 cm.). Prior to each extrusion the die is cleaned, assembled in the press, heated to 375 C. and

- coated with a colloidal 10% dispersion of submicron graphite particles dispersed in petroleum oil just before use. The oil is an asphalt base lubricating oil having a flash point of 325 F. (162.8 C.) and a viscosity at 210 F. (99 C.) of 110 to 135 universal Saybolt seconds.

The three machined billets are preheated to about 212 F. (100 C.) by 20 minute immersion in boiling water. Each warm billet is then immersed for 30 minutes at 98 C. in a solution containing 53 g. ferric oxalate hexahydrate, 10 g. oxalic acid dihydrate, 10 g. sodium fluoride and 1000 g. water to form the oxalate coating having a weight of about 0.2 to 0.3 g./dm. The billets are then air dried and cooled to room temperature.

Billet A.The coated billet is dipped in the graphite dispersion and extruded into a /2-inch (1.27 cm.) rod at room temperature. The surface of the extruded rod is rough and torn.

Billet B.-The coated billet is heated by induction in argon to 450 0., held 5 minutes to decompose the oxalate, and cooled in argon to room temperature. The cooled billet is dipped in the graphite dispersion and extruded into a rod of the same dimensions as those of A. The extruded surface is rough and scored.

Billet C.The coated billet is first dipped in the graphite dispersion (oxalate graphite ratio 0.5 :1) and then put through the same oxalate decomposition cycle as Billet B, cooled to room temperature and extruded into a rod of the same dimensions as those of A and B. The extruded surface is smooth and substantially free of evidence of galling and illustrates this invention by showing the marked advantage resulting from thermally decomposing the oxalate while in contact with the graphite-in-oil lubricant. Furthermore, the improved lubrication is evident even under the extreme conditions encountered during room temperature extrusion of titanium.

EXAMPLE 3 A titanium powder billet 4 inches (10.2 cm.) long is compacted and shaped to fit a 2.15-inch (5.46 cm.) diameter liner and a cone die. The working surface of the die is coated by contacting it with an oxalating solution at 96 C. for 20 minutes. The solution contained 99 g. oxalic acid, 99 g. ferric oxalate and 850 g. water. The oxalate coating on the die (2 g./dm. was dried, swabbed with a viscous 10% dispersion of colloidal graphite in a heavy petroleum oil (e.g., Grade C, MIL-3572, Aug. 21,

l951graphite/oxalate 0.5 :1), and heated to 350 C. in air for 10 minutes. After covering the bare billet with colloidal graphite in petroleum oil it is heated in argon to 450 C. The press is assembled, all working surfaces given a final graphite dispersion coating and the billet extruded to a /2-inch (1.27 cm.) rod. The extruded surface is smooth.

Improved lubrication is also obtained if an equal weight of ceric oxalate is substituted for ferric oxalatein the lubrication procedure described above.

EXAMPLE 4 A cast 8-inch (20.4 cm.) long ingot of a columbiumbase alloy containing 1% zirconium is machined to a billet 4 inches (10.2 cm.) in diameter and 8 inches (20.4 cm.) long having a 90 chamber at one end. A 1.5-inch (3.80 cm.) diameter axial hole is drilled through it. The billet is iron plated, then oxalated as described in Example 2. The working surfaces of an extrusion press, having a steel cone die with a 2-inch (5.08 cm.) round opening, and a 1.49-inch (3.78 cm.) diameter mandrel mounted on a follower are coated by immersion for a half hour at 98 C. in a solution containing 45 g. oxalic acid, 50 ml. of a 50% manganous nitrate solution and 1000 g. water. The coating is rinsed with water and dried to yield a coating weighing 2 g./dm. A 10% viscous dispersion of graphite in a petroleum oil similar to that in Example 3 is spread over the oxalte coating on both die and mandrel (graphite/oxalate .5 :1) which are then placed in an oven at 375 C. for one hour. The uncoated billet is heated to 585 C. in argon by induction. The press is assembled, the container heated to 475 C. and the tool surfaces swabbed with a conventional commercial metal working lubricant. The heated billet is transferred quickly from the argon furnace atmosphere to the press and extruded to a smooth surfaced tube 2 inches (5.08 cm.) in diameter and 5 feet (152 cm.) long having a wall Mz-inch (.635-cm.) wall thickness by use of a break through pressure of 132,000 p.s.i. (9,300,000 g./ cm?) calculated on billet cross section.

EXAMPLE 5 A 10% thoria dispersion modified nickel billet is machined to a 2-inch (5.08 cm.) diameter billet, 4 inches (10.2 cm.) long with a 90 cone end. The billet surface is etched by 5 minutes immersion at 90 C. in a solution containing 600 ml. of 50% H SO ml. of 37% HCl and 25 ml. of 48% HF, then quickly transferred without rinsing to a coating bath at 98 C. containing 950 g. water, 45 g. oxalic acid and 40 g. of Mn(NO (about 63 g./liter MnC O for half an hour. The oxalate coated billet is rinsed and air dried, coated with a graphite-oil dispersion similar to that of Example 1, and heated to 400 C. for about one hour in air (graphite/oxalate 0.5 :1). The die to be used is cleaned, polished, coated with the oil-graphite and heated to 350 C., then the press is assembled and the die and liner are swabbed with the dispersion. The billet is heated to 925 C. by induction and quickly transferred and extruded at a ram speed of 25 inches (63.5 cm.) per minute to a A-inch (1.9 cm.)

smooth rod about 30 inches long. A silimar extrusion using only the graphite-oil suspension results in a scored extrusion and evidence of die wear.

EXAMPLE 6 The metal used in this experiment is nickel containing 2% of finely dispersed thoria. A rod billet of this metal 4-inch (10.2 cm.) diameter by 8 inches (20.4 cm.) long is prepared having a conical end to fit the die used. The billet is electroplated with iron in a FeSO solution with the acidity adjusted to pH 3. Three grams of iron per square decimeter were deposited. This plated billet is coated with manganese oxalate by immersion at 100 C. for 15 minutes in an oxalating bath similar to that of Example 5, rinsed, air dried and coated with oil-graphite suspension as described in that example. The extrusion press is assembled and the /z-inch (1.27 cm.) steel rod die heated to 200 C. and coated with the suspension. The billet is then heated in argon from room temperature to 1700 F. (927 C.) and extruded to a smooth /z-inch 1.27 cm.) rod. In a control test, in which no oxalate coating is used, but other conditions are the same, a smooth rod results but the breakthrough pressure is considerably higher. Removal of the iron plate by hydrochloric acid leaves a smooth nickel rod with a finish much closer to a polish than is obtained when extrusions are made with the billet enclosed in a steel can.

EXAMPLE 7 Nine rod billets 2 inches (5.08 cm.) in diameter and ranging from 1%a-inch (4.75 cm.) to 3 /2-inch (8.9 cm.) long are prepared from various batches of columbium base alloy nominally containing 10% Ti, 6% Mo and W. The billets are machined with 60 conical front ends to fit 60 tool steel cone dies. The billets are all electroplated in ferrous sulfate solution to give them approximately a 3 g./dm. codating of iron. These billets are then coated with oxalate by immersion for 20 minutes in a solution containing 50 g./liter oxalic acid, and 40 g./liter Mn(NO at 180 F. (82 C.). This coating is found to be chiefly manganese oxalate with some ferrous oxalate in it. The latter results from the action of the oxalic acid on the iron plate. The extrusion press is a high speed device known as a Dynapac. In this device the billet is forced through the die by a ram in the usual manner except that the ram is activated by the sudden release of gas pressure. The plated and coated billets are prepared for extrusion into 1 inch rods by dipping them in a dispersion of graphite in lubricating oil, draining and heating in argon to various temperatures ranging from 2370 F. (1299 C.) to 2500 F. (1371 C.) (oxalate 2 g./drn. graphite/oxalate 0.5:1). The extrusion times vary from 0.008 to .015 seconds depending apparently on the composition. The extrusion pressures range from 150,000 to 210,000 p.s.i. (10,500,000 to 14,800,000 g./cm. on the billet cross section. Straight rods having good surfaces are produced. The best surfaces are obtained at the lower temperatures. Clean separation of the butt from the die is obtained in all cases. In control tests using graphite-in-oil in conjunction with glass as a lubricant, the rod surfaces were rought, cracked, or pitted indicating seizure between die and extrusion. Furthermore, the dies had to be discarded or recovered by machining out the butts.

EXAMPLE 8 Small samples of a chrome die steel about 5% Cr, 0.35% C, 0.12% V, 0.5% M0, 0.9% Si, are degreased with acetone, rinsed with water, dried, weighed and immersed for 30 minutes at about 98 C. in coating baths having the compositions shown in the following table. At the end of 30 minutes the test pieces are withdrawn, rinsed with water and dried at room temperature. The coating weights in the table are determined by first weighing the coated specimens then subtracting the weight of Coating Tool Steel Composition, percent Wt. 01 Coating,

Mn(NO (00 OH) 1211 0 mgJdmJ The coatings obtained are uniform in thickness and not easily removed. The outer part can be scraped oif with a sharp probe but considerable efiYect is required to'scrape the metal bare. The crystalline coating is shown by X-ray to be manganous oxalate. The lubrication process of this invention is completed by impregnating the oxalate coatings with graphite-oil suspension and heating the resulting products as described in the preceding examples.

EXAMPLE 9 A series of oxalating solutions of somewhat different compositions are prepared and used to coat samples of the vPD3 steeLThe compositions and weight of coating formed are shown below. Coating conditions were 30 min. at 98 C.

Composition, g./1.

Coating,

MI1C304 N3(NO )Q H 090 mgJdmfl The coatings are of substantially the same character as those in Example 8. Some oxalate is precipitated in the beaker. In this series the manganese is added to the solution as manganese oxalate. Although the solubility of MnC O is slight, for some reason not clearly understood, enough initially dissolved in the presence of the sodium nitrate and oxalic acid to provide for the desired reprecipitation of it on the metal during the heating period. The coatings are oil-graphite impregnated and heated as described in the preceding examples.

The thickness of coating can be increased in the more difiicult case by preliminary treatment or cleaning of the surface by acid pickling or mechanical abrading as shown in the following example.

EXAMPLE 10 A coating bath is prepared by dissolving 76.5 ml. of a 50% solution of manganous nitrate and 40 g. of hydrated oxalic acid in 900 ml. of water. This solution was heated to 210 F. (99 C.). The samples of metal are all degreased with acetone and some, as indicated in the following table, are pretreated mechanically or chemically and immediately immersed in the bath at temperature for 30 minutes. Fresh solutions are prepared as needed so that the bath is never more than one hour old at the end of the treatment.

Activating Weight Metal Pretreatment (g./sq. dm.)

None 2.6

.............. Mechanical 0.762

A181 446 (25% Cr)-.. Chemical 1. 46 A181 304 (18 Cr-8 Ni) do. 4. 42 AISI 316 (CrNiMo). Mechanical 0. 776 A181 310 (25 Cr2oNi) "do? 1. 28 Carpenter 20 (20 (Jr-29 Ni-2 Mo3 Cu) (10. 1. 52 Nickel None 0.731 Do Mechanical 2. 37

Do Chemical 1.38 Do do.- 0. 962 Nickel2% Thoria Mechanical 0. 996 Inconel (72 Ni-l7 C do. 0. 822 Nionel (80 Ni-20 Cr) do. 4. 20 Nichrome do. l. 54 Do Chemical 2.82 Hastelloy B (65 Ni-30 Mo) Mechanical 0. 965 Hastclloy C (55 Ni-15 Cr-l Mo)- (10. l. 49 Hastelloy F (44 Ni-22 Cr-24 Fe) 2. 12 Copper 2. 14 Brass (70 Cu-30 Zn) 2. 58 antalum "do! l. 42 Columbium .do. 0. 471 Zirconium do. 0. 270

1 Abrading with 80grit emery paper. 1 Immersed in solution oi:

600 ml. 50% sulfuric acid. 100 ml. 37% hydrochloric acid. 25 ml. 48% hydrofluoric acid at 90 C. for 5 min.

3 Immersed in solution of 37% hydrochloric acid at 70 C. for 5 min. If heavier coatings are desired, some more easily coated metal such as iron is first placed on the diificult metal, e.g., titanium, and the manganese oxalate coating then applied. When iron is first plated on the metals above, except for the first two, the adhesion of the oxalate coating is enhanced. The lubrication process of this invention is completed by impregnating the oxalate coatings with graphite-oil suspension such as that used in Example 1 at a graphite-oil ratio of 0.5 :1; then heating the resulting product to about 450 F. (232 C.) for 30 minutes.

EXAMPLE 11 Water. 1, 100 g. 5% oxalic acid. Ml'l(N03)z 50% soln. 94 ml. ferric oxalate. Oxalic acid. 55 g. Water.

The barrels are immersed at 100 C. for 30 min., removed, rinsed and air dried. A 10% graphite dispersion in petroleum oil similar to that of Example 1 is then poured through each barrel to coat the interior. The barrels are then heated to 800 F. for one hour. The blanks are then clamped on the draw bench and the rifling plug of tungsten carbide pushed through at room temperature. The maximum pressures required to force the plugs through are 72 and 82 p.s.i. (5060 and 5760 g./cm. respectively, for the iron oxalate and manganous oxalate coated blanks respectively. In all cases the surface and condition of the rifiing marks are satisfactory.

EXAMPLE 12 Two tube billets of Zircaloy are prepared from an ingot. The billets machined to 4-inch (10.2 cm.) diameter x 8-inch (20.4 cm.) long with a l /z-inch (3.80 cm.) axial hole for insertion of the mandrel. The end is made conical to fit the steel cone die used. Zircaloy contains 1.45% Sn, 0.125% Fe, Cr, 0.05% Ni with the balance being zirconium. Both billets are electroplated with iron, 3 g./ sq. dm., over the entire surface and coated with manganese oxalate at a coating weight of 2 g./dm. by immersion for 30 minutes at 98-100 C. in a freshly prepared solution containing 1000 g. water, 45 g. oxalic acid, 35 g. Mn(NO 0.25 g. of Alkanol WXN which is an aralkyl sodium sulfonate wetting agent. The die and mandrel for the first billet are also similarly coated with manganese oxalate. Both billets and the die and mandrel for the first billet are immersed 'in an oil-graphite suspension similar to that of Example 1 (graphite/oxalate 0.5:1), drained and heated to 540 C. The tools for the second billet are not oxalate coated. Just prior to each extrusion all working surfaces were swabbed or dipped in oil-graphite suspension containing some dispersed molybdenum disulfide. After extrusion at 540 C. the surfaces of the resulting tubes, five ft. (152 cm.) long with A-inch (.635 cm.) thick walls, are mirror-like and uniform.

EXAMPLE 13 Hollow cylindrical extrusion billets of 55-A titanium (0.25-inch (.635 cm.) thick) on 1020 steel (0.85 inch (2.16 cm.) thick) are electroplated with iron to a weight of 3 g./sq. drn. on outside surfaces and the bore. The billets are then oxalated and coated as shown in Example 11, Solution A. The mandrels are heated to 440 F. (226.7 C.) and the die to 850 F. (454.4 C.). A 2-inch (5.08 cm.) long billet is extruded to a 40-inch (102 cm.) tube at 1700 F. (926.7 C.) at a ram speed of 2 inch/ min. (5.08 cm.). Both interior and exterior surfaces are excellent.

Billets comprised of 55-A titanium on OFHC copper and nickel on OHFC copper are extruded in the same manner with similar results.

I claim:

1. A process for lubricating a metal surface[s] comprising at least one metal selected from Groups IVB to VIIB and VIII of the Periodic Table, which comprises:

(a) depositing on said surface about from 0.01 to 4 g./dm. of water insoluble metal oxalate coating wherein the metal is of atomic number 2A to 28 inclusive;

(b) impregnating said coating with a suspension comprising about from 5 to 25% colloidal graphite, based on the total weight of the suspension, and hydrocarbon lubricating oil having a flash point above 300 F. (149 C.), the amount of graphite thus impregnated in said coating being about 5 to 200% based on the weight of metal oxalate; and

(c) heating the resulting impregnated coating at a temperature above 320 F. C.).

2. A process of claim 1 wherein said oxalate is ferrous oxalate.

3. A process of claim 1 wherein a manganous oxalate coating is deposited on said surface by immersing said surface in an aqueous solution containing the equivalent of at least 20 grams per liter of manganous oxalate.

4. A process of claim 3 wherein prior to deposition said surface is abraded with an abrasive of '80-120 grit size.

5. A process of claim 1 wherein said impregnated coating is heated at a temperature of from about 400 to 950 F. (204.4 to 510 C.).

6. In the process of metal extrusion, the improvement which comprises lubricating at least one of the die and work piece by the process of claim 1.

7. A metal surface lubricated with a coating obtained by the process of claim 1.

8. A process for a coating metal with manganous oxalate which comprises immersing said metal in an aqueous solution consisting essentially of manganese, nitrate and oxalate ions, the manganese and oxalate ions being at a concentration equivalent to more than 20 grams/liter of manganous oxalate.

9. A process of claim 8 wherein the metal is coated 13 with iron oxalate before it is coated with manganous oxalate.

10. A process of claim 8 wherein the metal is stainless steel, carbon steel or die steel, and the temperature of the aqueous solution is about from 180 to 212 F.

11. A process of claim 10 wherein the metal is preheated to about from 180 to 212 F. before immersion in the aqueous solution.

12. A process of claim 8 wherein said metal is a thin coating of iron on a substrate of columbium, nickel, titanium, zirconium or an alloy based on one of these metals, the temperature of the aqueous solution is about from 180 to 212 F., and the metal is preheated to about from 180 to 212 F. before immersion in said solution.

13. A process for lubricating a metal surface that is coated with at least about 0.01 g./dm. of water insoluble metal oxalate wherein the metal is of atomic number 24 to 28 inclusive, said metal surface comprising at least one metal selected from Groups IVB to VIIB and VIII of the Periodic Table, which comprises:

(A) impregnating the coating of metal oxalate with a suspension comprising about from 5 to 25% colloidal graphite, based on the total weight of the suspension, and hydrocarbon lubricating oil having References Cited UNITED STATES PATENTS 4/ 1912 Dempster 72--42 4/ 1941 Eddison 1l7127 XR 4/1943 Lowit 117--49 XR 11/ 1957 Otto.

5/1958 Rausch et al. 1486.14

6/ 1961 Shaw.

FOREIGN PATENTS 2/1961 France.

DAVID KLEIN, Primary Examiner US. Cl. X.R.