US3429158A - Protective cladding and lubricant for mechanically deformable reactive metals - Google Patents

Description

United States Patent 3,429,158 PRUTECTIVE CLADDEJG AND LUBRICANT FOR MECHANTCALLY DEFORMABLE RE- ACTlIVE METALS Robert E. McDonald, George A. Reimann, and Fred H. Patterson, Oak Ridge, Tenn., and Carl F. Leitten, Jr., Indianapolis, Ind, assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Nov. 28, 1966, Ser. No. 597,483 U.S. Cl. 72--47 4 Claims Int. Cl. B21c 23/22, 23/32, 29/00 ABSTRACT OF THE DISCLOSURE The hot-working or mechanical deformation of a reactive metal, i.e., a metal normally oxidizable at an elevated temperature, is facilitated by providing the metal with a coating of molybdenum, tungsten, or certain alloys thereof. These coating materials protect the underlying metal from oxidation and contamination and also provide a lubricant for the mechanical deformation of the metal when the materials are oxidized. The oxides of molybdenum, tungsten, and alloys thereof are volatile above certain temperatures so as to provide a mechanically formed metal structure that is essentially free of surface oxides and virtually free of coating material.

The invention described herein was made in the course of, or under, a contract with the U.S. Atomic Energy Commission. This invention relates generally to the mechanical deformation of reactive metals, and more particularly to the extrusion or other mechanical deformation of metals normally oxidizable or contaminated at elevated temperatures by protecting the metal from oxidation or contamination with a metal coating which also serves as a self-removing extrusion lubricant when in the form of an oxide.

Metals and alloys, particularly of the refractory type, suffer shortcomings which render the fabrication thereof at elevated temperatures somewhat difficult. For example, many metals oxidize or become easily contaminated at elevated temperatures so as to render the metals unsuitable for their intended purposes. Some of the metals found difficult to extrude or otherwise deform from a billet configuration into a product such as a T-shaped body, a hollow tube, round rods, etc., include the refractory metals and alloys niobium, vanadium, tantalum, tungsten-rhenium (W-26Re), molybdenum-zirconium (Mo-lZr), etc. Other metals such as mild steels and stainless steels are also oxidizable at elevated temperatures and suffer an additional problem in that galling and seizing take place during fabrication.

Previous efforts to protect the above and other metals during high temperature fabrication operations included the use of an inert gas atmosphere to prevent oxidation. However, some contamination often occurred due to the presence of small quantities of oxygen and other contaminants in the inert gas. Consequently, efforts to provide some protective cladding for these metals during deformation thereof proved to be unsuccessful. This previously known cladding was usually in the form of a can or sheath made up of a suitable ductile metal, such as mild steel, into which the reactive metal can be placed. The problems with this type protective cladding often resulted from working the metals at temperatures greater than that which the cladding material can withstand. Further, this cladding is usually of such a thickness that the size of the reactive metal being worked is substantially smaller than that which could be worked in a single operation.

3,4Z9,l58 Patented Feb. 25, 1969 'mally require that the extrusion or other mechanical deformation be carried out at relatively high temperatures in the presence of some lubricant. Lubricants, such as graphite, graphite in combination with other metals, glass, and highly ductile metals, e.g., mild steel, have not been found to be successful with these metals which require high temperatures for the fabrication thereof. This is due in some instances to the high extrusion temperatures required which inhibits the adherence of the lubricant to the underlying metal and in other instances to the poor boundary layer lubrication properties provided at elevated temperatures by the highly ductile metals. Further, there is a good possibility that a reaction may occur between the sheathing material and the billet, which reaction may result in the formation of undesirable alloys or intermetallics which may render the product unacceptable and incapable of accomplishing its intended function.

It is the aim of the present invention to overcome or substantially minimize the above and other shortcomings or drawbacks by providing such reactive metals with a coating or layer of metal which serves to protect the underlying metal from oxidation and contamination at elevated temperatures and also provides a self-removing lubricant for use during the mechanical deformation of the metal. The metals used for this coating are those which exhibit volatile oxides at elevated temperatures and include tungsten, molybdenum, and alloys thereof as will be set forth in greater detail below.

An object of the present invention is to provide for the mechanical deformation of reactive metals at elevated temperatures in a new and improved manner.

Another object of the present invention is to provide self-removing cladding for reactive metals for use in protecting such metals from oxidation and contamination at elevated temperatures.

Another object of the present invention is to provide a lubricant for facilitating the hot-working of reactive metals whereby the lubricant is derived from a volatile metal oxide cladding which is self-removing so as to provide a hot-worked product that is virtually free from oxidation and contamination.

A further object is to provide reactive metals with a metal coating or layer which serves both as a barrier to oxidation and contamination and as a lubricant for facilitating the deformation of the metal.

Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiments about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

Generally, the present invention comprises a cladded billet consisting of a reactive metal body having thereon a thin layer or coating of a metal which is characterized by possessing a volatile oxide. This coating is placed on the reactive metal body prior to its exposure to the elevated temperatures required for the mechanical deformation thereof. The metals found to possess this volate oxide property include molybdenum, tungsten, and certain al loys thereof such as, for example, Mo-0.5Ti, Mo-0.5Ti- 0.1Zr-0.02C, Mo-l.2Ti-0.25Zr-O.1SC, W-2Mo, and Mo- 0.5Ti-0.08Zr. The above alloys are merely exemplary of the many tungsten and molybdenum alloys which may he used to provide the coating material. However, care should be exercised in selecting an alloy for the coating since the temperature at which the volatile oxide forms may be substantially greater than the desired extrusion temperature of the coated metal. Further, the selected alloy must be capable of providing a sufficient quantity of the volatile oxide to provide satisfactory lubrication of the coated metal during the extrusion of deformation thereof.

The metal coating will protect reactive metals from trace oxide contaminants normally found in the inert at mospheres provided in heating mechanisms such as induction furnaces and the like, and will serve as protection against oxidation and provide lubrication for the reactive metal during high temperature mechanical deformation operations in oxidizing or contaminating atmospheres. The high temperature lubrication is achieved by heating the coated reactive metal body or billet to the desired hotworking temperature in an inert atmosphere, exposing the heated coated billet to an oxidizing atmosphere to cause the metal coating to undergo surface oxidation, and immediately thereafter hot-working the metal. The hotworked metal obtained from mechanical operation is readily amenable to further mechanical Working by conventional low temperature techniques such as rolling, drawing, etc.

The billets of reactive metals may be prepared in any conventional and convenient manner. For example, suitable fabricating techniques which may be used include are casting, powder metallurgy, etc. The reactive metals useable in the present invention include the metals vanadium, niobium, tantalum, W-26Re, Nb-lZr, and similar such refractory metals and alloys. However, this invention could be used with virtually all metals which have oxidation problems at elevated temperature or which suffer galling or seizing during high temperature hot-working operations, such as, for example, stainless steels and mild steels. Perhaps one of the more important features of the present invention is due to the fact that there is no compatibility problem between the coating materials and the underlying reactive metal since the metal used for the coating goes into solid solution and thereby obviates the formation of deleterious intermetallics in the hot-worked metal structure.

The metal coating or layer may be deposited upon the reactive metal body in any suitable manner, such as, for example, conventional thermochemical reduction techniques which are used for depositing oxide-free high purity metal, plasma spraying, electron beam vacuum deposition, ion plating, sputtering, and electrochemical deposition. It may be preferable to use low temperatures, e.g., less than about 700 C., for applying these coatings by thermochemical deposition techniques since undesirable grain growth in the billet may occur at higher temperatures.

In order to provide protection for the metal billet from oxidation and other contamination, as, well as provide a lubricant for the mechanical deformation thereof, the layers should be of sufiicient thickness to remain intact during the entire hot-working operation. However, since it is also desired to obtain a hot-worked product which is substantially free of the layer, the latter may be sufficiently thin so that it boils or volatilizes away essentially at a time corresponding to the end of the hot-working operation. Further, the layers are preferably thin so as to minimize thermal stresses due to the difference in the thermal coefiicient of expansion of the layer metal and the reactive metals. Layers having a thickness in the range of about 0.004 of an inch to 0.010 of an inch have been found to be satisfactory. However, in some instances thicker or thinner layers may be used, depending upon the material being extruded, temperature of extrusion, and the size of the reduction. While it is preferable to select a layer of such thickness that it is essentially completely oxidized after its usefulness as a lubricant ends so as to be self-removing, it is also within the scope of the present invention to employ a layer with a sufficient thickness so that a portion of the layer remains after the completion of the hotworking operation. This portion of the layer remaining on the metal may be in the form of unoxidized metal, oxidized metal, or both, since the temperature of the billet and layer after the mechanical operation may be below that at which the oxide of the layer is volatile. By having some of the protective layer present after the mechanical operation, oxidation or contamination of the metal underlying the layer is obviated or substantially minimized, particularly if the billet is still above a temperature at which such oxidation or contamination may occur and if the billet is exposed to an oxidizing or contaminating atmosphere. The portion of the protective layer remaining on the deformed metal may be readily removed by sand blasting, a suitable etchant, or any other suitable mechanism.

To practice the present invention, a metal billet coated with a layer of molybdenum, tungsten, or a certain alloy thereof may be heated to the hot-working or extrusion temperature of the metal billet in a suitable furnace, e.g., an argon atmosphere induction furnace. Inasmuch as the extrusion temperatures for the coated metals and alloys may vary over a wide range of temperatures, care should be exercised to assure the selection of a range of temperatures within which the particular metal or alloy possesses sufiicient plasticity to allow the shaping process to be conducted within the capacity of the available extrusion press or other metal deforming mechanism. Further, the deformation operation should be conducted within a range of temperatures where the effects of the deformation are dissipated rapidly enough to prevent the resistance of the material from being deleteriously affected by work-hardening such as occurs at lower temperatures. Such a temperature range will depend for example, not only upon the particular metal or alloy, but also upon the reduction ratio employed and the capacity of the reduction mechanism.

In a typical extrusion operation, after the coated metal billet is heated to a desired extrusion temperature, it is removed from the furnace and inserted immediately into a billet-receiving chamber of an extrusion press. This extrusion press may be of any suitable commercially available type which is capable of mechanically deforming the particular metal or alloy used into the desired configuration. Upon contact with room air during this transfer, the heated metal layer on the billet undergoes rapid surface oxidation, such as to cause white clouds of oxide to evolve from the layer surface when the higher extrusion temperatures are employed, as will be discussed below. The coated billet is preferably extruded into desired configurations immediately after it is placed in the billet chamber to assure that sufiicient oxide remains to complete the extrusion. However, if the coated billet is placed in pressurized atmosphere or in a sealed volume, the volatilization of the oxide may be sufliciently retarded so as to allow a greater time delay between the placement of the billet in the extrusion press and the extrusion of the billet.

The mechanism by which the volatile oxides provide lubrication of the coated metal billet during deformation thereof is not clearly understood, particularly since oxides in both a solid and molten or liquid state have been found to provide a satisfactory bearing surface between the billet and the billet deforming mechanism. It may be that the bearing surface is provided by a thin layer of liquefied oxides in all instances, even when the initial oxide is in solid state. This premise is believed plausible due to the fact that the temperatures and pressures exerted upon the solid oxide layer during deformation may be sufiicient to at least partially liquefy the oxide and thereby provide the necessary lubrication. A temperature of about 900 C. is adequate to produce sufiicient oxide to provide the lubricant when molybdenum metal is used to form the layer. The oxide produced at this temperature is somewhat solid, but evaporates or evolves away from the surface upon which it was deposited. The oxides formed from molybdenum and tungsten at higher temperatures, e.g., above about 1500 C., are believed to be in a molten or liquid state, but these oxides, like the solid oxide, provide highly desirable lubrication characteristics.

Another important feature of the present invention is due to the fact that no oxidation of the billet naturally occurs during or after the deformation process, even though an oxide layer is in contact with the surface of the billet during this entire operation. This unexpected result is believed to be due to an oxygen scavenging effect provided by the oxide layer during the volatilization thereof.

In order to more clearly illustrate the quantitative and procedural aspects of the present invention, the following examples are provided. These examples are illustrative to niobium, Nb-lZr, and W-26Re, and mild steel, but it is to be understood that other materials and alloys may be similarly extruded at the same or different temperatures and with thicker or thinner coatings of molybdenum and tungsten and alloys thereof.

Example I A niobium billet of a desired size for producing the desired product configuration is coated with a thin layer of tungsten. This tungsten coating is thermochemically deposited upon the billet to provide a layer having a uniform thickness of about 0.005 of an inch. The temperature used for the formation of this layer is preferably in the range of 580 C. to 700 C. When ready to extrude the coated niobium billet, it is heated in an inert atmosphere to a temperature at which the niobium is sufficiently plastic to facilitate extrusion thereof. Satisfactory results have been achieved with niobium at about 1700 C. When the billet is heated to the desired temperature, it is removed from the heating mechanism and exposed to an oxidizing atmosphere. The oxide formed during this exposure causes white clouds of tungsten oxide (W to evolve from the billet. The billet with the oxide layer thereon is placed into the billet-receiving chamber of the extrusion press as rapidly as possible to assure that sufficient oxide will be present on the billet during the entire extrusion operation. The billet is then extruded at a reduction ratio of approximately 5 to 1. Visual inspection of the product formed by the extrusion indicated that the surface contained no significant defects and possessed a good surface finish. Nondestructive tests conducted on the extruded product revealed no significant defects in the structure of the finished product. The oxide content of the extruded product was not significantly different from that originally in the starting material.

Example II A billet of niobium alloy, namely, Nb-lZr, with a central core portion removed so that the billet may be extruded into the configuration of a tube is provided with a molybdenum coating in the same manner as set forth in Example I. The coating thickness in this case was also 0.005 of an inch. The formation of the coating oxide and the extrusion steps were similar to those carried out in Example I. However, in this example, the extrusion temperature of this alloy was approximately 1550 C. The extruded tube possessed properties similar to those noted in Example 1.

Example III A tungsten-rhenium alloy (W-26Re) was prepared and coated with a 0.005-of-an-inch layer of tungsten in the same manner as set forth in Example 1. Although the extrusion of this alloy required a higher temperature (about 2200 C.) than those used for the niobium or niobium alloys, the extrusion operation was conducted in a manner similar to the procedure set forth in Example I and provided an acceptable product exhibiting virtually no gain in oxygen content.

It was necessary to coat the W-26-Re billet with tungsten because it was found that the tungsten in the alloy did not volatilize rapidly enough to provide sufficient lubrication for the extrusion. Apparently, the presence of the rhenium depressed the vapor pressure of the tungsten sufficiently to prevent any substantial evolution or formation of a tungsten oxide.

Example IV A billet of mild or low-carbon steel (about 0 .2 percent carbon) was coated with a 0.005-of-an-inch layer of molybdenum in the same manner as set forth in Example I. The coated billet was heated to a temperature of 1000 C., the coating was oxidized, and the coated billet was thereafter extruded through a conventional reduction die at a reduction ratio of approximately 5 to 1. No seizing of the billet was encountered during the extrusion operation and the billet possessed virtually the same oxygen content as it had before the deformation.

While the present invention has been described as being useable primarily in the deformation of metal by extrusion, it is to be understood that metal shaping procedures such as drawing, hot-rolling, etc., may also benefit from the teachings of the present invention. Accordingly, the term extrudable metal as used herein is intended to be applicable to deformable metal whether or not the metal is deformed by an extrusion procedure.

It will be seen that the present invention sets forth a novel mechanism for facilitating the shaping of niobium, tantalum, tungsten, rhenium, molybdenum, zirconium, and other refractory metals and alloys. Further, stainless steel and perhaps any metal or alloy having extrusion temperatures greater than about 900 C., galling and seizing problems, or contamination and oxidation problems may enjoy usage of the present invention.

As various changes may be made in the method and the arrangement of the method steps herein Without departing from the spirit and scope of the invention and without sacrificing any of its advantages, it is to be understood that all matter herein is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of mechanically deforming a body of metal which normally undergoes oxidation and other contamination at elevated temperatures and has a melting point greater than about 900 C., comprising the steps of coating the metal body with a metal for protecting the metal body underlying the coating from oxidation and contamination upon exposure to elevated temperatures, subjecting the coated metal body to a temperature greater than about 900 C. in an inert atmosphere for rendering the coated metal body sufficiently plastic to effect mechanical deformation thereof, exposing the coated metal body to an oxidizing atmosphere for oxidizing at least exposed surface portions of the coating to form a volatile compound characterized by its capability of providing a lubricating bearing surface on the underlying metal, said metal for providing the coating being selected from the group consisting of the metals molybdenum, tungsten, and alloys thereof which provide sufficient quantities of the volatile compound to provide the lubricating bearing surface, and thereafter mechanically deforming the coated metal body.

2. The method claimed in claim 1, wherein said coating is about 0.004 to 0.010 of an inch in thickness prior to the oxidizing step, and wherein the coating metal and compound are essentially nonexistent after the mechanical deforming step.

3. A mechanically deformable metal structure comprising a metal body subject to oxidation and other contamination oxidizable at elevated temperatures and having a melting point greater than about 900 C., and a layer of metal different from and covering said body for protecting said metal body from oxidation and contamination when exposed to said elevated temperatures and for providing a lubricant during mechanical deformation of the metal body when at least surface portions of said layer are oxidized, said layer of metal being selected from a class consisting of molybdenum, tungsten, and alloys thereof and being further characterized by the formation of a volatile oxide when exposed to temperatures greater than about 900 C. in an oxidizing atmosphere.

4. The deformable metal structure claimed in claim 3, wherein said layer of metal has an essentially uniform thickness of about 0.004 to about 0.010 of an inch when in an unoxidized state, and wherein said metal body is essentially devoid of said layer when the mechanical deformation of the body is completed.

References Cited UNITED STATES PATENTS 8/1959 Milnes 29S28 9/ 1959 Hanink et al 29S28 7/ 1961 Finlay 29S28 X 9/1967 Durfee et a1 29S28 11/1967 McDonald et al. 72-38 U.S. Cl. X.R.