US2894837A - Method for producing cemented carbide articles - Google Patents

Description

7 July 14, 1959 E. 1. CNSTOTT' ET AL 2,894,837 METHOD FOR PRODUCING CEMENTED CARBIDE ARTICLES 7 Filed Jan; 16, 1946 INVENTORS- WITNESSES Edward I. 022.5 to 15$ jg? M g o ye D. Cnemer United METHOD FOR PRODUCING CEMENTED. CARBIDE ARTICLES Application January 16, 1946, Serial No. 641,619

Claims. (Cl. 75-201) The present invention relates to molded materials and a method for making the same. More particularly, the invention relates to a method which has particular utility in the preparation of molded articles of intricate shape, the materials of said articles consisting of mixtures of one or more hard metal carbides or oxides and matrix metals or binder metals therefor. 1

Shaped articles of hard metal carbides or oxides such as tungsten carbide, tantalum carbide, zirconium oxide or the like, in a matrix of a relatively low melting temperature binder metal such as cobalt or nickel, are normally' known as cemented carbides, and are generally prepared by the process which includes pressing the mixed powder ingredients in a steel mold and thereafter sintering the pressed or cold compacted material, or by the hot pressing method which includes the steps of simultaneously applying pressure to the mixture while it is heated to or near to the sintering temperature. In either of these two methods, it has previously been found desirable to facilitate the cold or hot compacting operations with such lubricants as camphor or parafiin, or similar substances, the same being applied by immersing the powder in a solution of a solvent containing dissolved paraffin or camphor and also containing in some cases a wetting agent to attain better dispersion of the lubricating agent. Such an impregnation is described for example, in US. Patent 2,363,575 to De Lamatter et al; In such cases it has been found essential that the powder be rendered substantially dry before compacting in ord erto avoid an explosion during the heating processes or at least to avoid porosity in the molded article due to occlusion of moisture during the molding operation.

, While these methods have been found useful in the preparation of regular geometric shapes such as have a substantially rectangular or circular cross section, they have not been effective in the preparation of thin walled articles or shells which, though symmetric about a particular axis, present forming problems which require hydrostatic pressure. These defects arise from the fact that the amount of shrinkage during the sintering operation is dependent upon the cold compacting pressure employed in the preliminary step. Thus for example, where the cold compacting pressure is in the neighborhood of ten tons per square inch, about 17.5% shrinkage is encountered during the sintering operation, whereas if a compacting pressure in the neighborhood of one ton per square inch is applied in cold compacting, the shrinkage during sintering amounts to about 21%. Thus, while only slight differences in density are found to exist in the finished article, in the preparation of a shell-like member, for example in the shape of a hemisphere, differential shrinkage will result in distortion, shrinkage cracking and/ or non-uniformity of wall thickness. While hot. pressing alone has been found effective in. eliminating some of these defects, there are many disadvantages in the employment of the process. For example, a good deal of reaction takes place between the molded article and the graphite die in which the operation is generally 2,894,837 Patented July 14, 1959 4 carried on, leading to a rough surface on the article which requires an excessive and expensive amount of grinding with diamond wheels to be removed.

Furthermore, in such a process critical values of the amount of pressure and the rate of pressure application must be observed to avoid defects such as non-uniformity of density, wall thickness and binder distribution in the finished product.

It is therefore seen to be an object of the present invention to provide a method for preparing shaped articles of the hard carbides.

It is a further object of the present invention to provide simple steps in the preliminary forming operation in the preparation of such articles so that deformation shrinkage in a cemented hard metal compound body during sintering is avoided.

It is a further object of the present invention to provide a simplified cold forming practice to be used in the process of manufacturing cemented carbide or oxide articles which eliminates the necessity for excessive cold forming pressures and results in a homogeneous structure in the finished article. l

Other objects and advantages of the present invention will be apparent from the description of the general principles hereof, and specific illustrative examples in which the method is employed.

These objects are accomplished by preparing a sludge of the metal powder and a solvent in which is dissolved a binder material, compacting the sludge under low compressive pressures to the desired shape in a manner which serves to eliminate from the mold a substantial part of the moisture, baking the compact to reduce it to substantial dryness and subsequently sintering the article at a predetermined temperature. It has been found that by following the procedure hereinabove outlined, thin-walled sections of the refractory or hard carbides or oxides such as tungsten carbide, tantalum carbide or zirconium oxide, can be readily prepared. It has been determined that even on a quantity production basis such. articles have a uniform density throughout, are free from lamination, distortion or porosity, and also have desirable surface characteristics. Furthermore, when the method is applied to the fabrication of slugs of these materials which have a regular geometrical transverse cross section, that is, a substantially circular, square, rectangular or regular polygon transverse cross section, defects such as porosity or laminationresulting from non-uniformity present in the cold compacted slug may be avoided without the use of excessively high pressures. Thus, for example, where the ratio of the length of the slug to the smallest cross sectional dimension is greater than about 3, the present method avoids the difiieulties which resulted from the employment of methods known prior to the present invention wherein cold compacting generally resulted in non-uniformity or lamination of the article, a result attributable to the failure to obtain uniform slippage of the particles relative to one another, throughout the metal powder mass.

More specifically, in the present method the employment of a sludge during the cold compacting operation results in an approach to hydrostatic conditions, so that the pressure exerted everywhere in the metal powder mass is substantially uniform and relative movement of the discreet particles is likewise substantially uniform.

In the formation of the sludge, it is preferred to employ a metal powder of the hard carbide or oxide upon which a thin coating of the bonding or matrix metal is applied by any well-known method, as for example, ball milling or the like. The paste to which the metal powder is added may comprise simply methyl alcohol, toluene, naphtha, acetone or the like, with or without a wetting 3 agent or lubricating material such as a stearate, camphor, parafiin' or the like admixed therewith. I

A preferred paste contains a hydrogenous materialsuch as parafiin, a cyclic material as camphor or its substitute, the aromatic material naphthalene, or a mixture of the latter two, and toluene or a similar solvent. The mixture should preferablybe one that has a boiling point above 70 C. and the composition of the paste should preferably besuch that the paste is solid at room or cold compacting temperature but liquid in the neighborhood of 50 C. to70 C., though this is not critical in the practice of the invention. The mixture of the metal powder and thebinder-lubricant paste is performed at about 60 C. so that the immersion of each of the discreet particles in the liquid is readily accomplished. It is further desirable in the preparation of the paste to employ only such materials as can be volatilized at relatively low temperatures or which burn to carbon during the sintering operation, in order to avoid the possibility of inclusion in the metal body. As a specific example of the hereinabove described method, the fabrication of a relatively thin-walled metal hemisphere of cobalt bonded tungsten carbide will be described, reference being made to the attached drawing (made a part of this specification) to clarify the procedure. However, it should be noted at this point that it is not the intention to limit the scope of the invention by the specific description here given.

Referring to the drawing, there is shown in elevational cross section, a graphite and metal die employed in the cold forming and baking steps of the novel process. The die comprises an outer sleeve 5 having a step-bored passage therethrough. In the larger diameter section the sleeve 5 accommodates a porous mold 6 of graphite or other fine pored material. The mold is provided with a suitable recess, of a size and shape to produce a compact which shrinks during sintering to a predetermined size, as hereinafter described. Concentric plungers 7 and 8 are disposed in the smaller diameter bore of sleeve 5 and may be moved vertically, separately or in any relative position'with respect to each other by the use of spacer sleeves and plugs of predetermined length (not shown).

Tungsten carbide and cobalt powders are ball-milled together in a cemented carbide mill using cemented carbide tumblers until a thin, substantially uniform layer of cobalt is found to exist on each carbide particle. The resultant metal powder mixture is then passed through a 200 mesh sieve to eliminate from the subsequent steps agglomerates that may have been formed during the milling which might lead to inhomogeneity of structure in the finished article. A paste is prepared of the metal powder and paraffin, camphor, naphthalene and toluene in the following weight percentages; 90.2% tungsten carbide and cobalt binder, 1.5% paraffin, 3.0% camphor, 3.5% naphthalene and 1.8% toluene. At room temperature the condition of this entire mixture is such that it is substantially solid. However, it is heated to between 50 C. to 70 C., and thereupon becomes liquid. The heated liquid mass is poured into the mold 6, which is normally fairly porous or at least suificiently porous to permit a fluid material to be forced therethrough or impregnated therein. During this pouring operation the mold is maintained at a temperature of about 60 C. in order that the fluid condition of the mixture of metal powder and paste is maintained. The mold 6 is then incorporated in the die sleeve 5 in the position shown, and the forming plungers 7 and 8 are forced into the mass 9 under a pressure of about 400 pounds per square inch (the specific pressure being determined by the transverse area of the forming plungers and not the surface of the forming ends thereof). This application of pressure results in forcing some of the fluid constituents of the paste into the porous structure of the die. Further increments of pressure are then imposed upon the still plastic mass until a total pressure of about 1000 pounds per square inch is exerted thereon. The end result of this forming operation is to squeeze most of the fluid out of the mixture and into or through the walls of the mold, so that a compact substantially uniform in all respects is provided. The forming plunger is then withdrawn, and the mold is removed from the sleeve and cooled from 60 C, temperature at which it was maintained to room temperature. However, if it is desired to accomplish volatilization of the remaining constituents of the paste after removal from the sleeve, the mold 6 may be heated to. about 70 C., at which temperature the paste vaporizes rapidly without detriment to the compact. This heating step may be carried on until the compact is reduced to substantial dryness or substantially all of the paste constituents are removed therefrom. It has also been found useful, where extremely rapid evaporation is sought, to carry out the vaporizing heating operation in a vacuum, although this expedient is not necessary to the success of the process. i

The mold is then heated to about 800 C. to facilitate removal therefrom of the compact, which at this tem perature. acquires a pre-sintered or chalky state. The

pre-sintered article may then be incorporated in a graphite boat, completely surrounded by a layer of crystalline alumina and sintered in a hydrogen atmosphere in a manner well known in the art. It is apparent that if surface considerations are not too important, the presintering step, with consequent heating loss upon cooling, is not necessary. In other words; the graphite moldmay beernployedas'the sintering boat and the sintering operation carried on in air or a hydrogen atmosphere, depending upon the qualities desired.

It will be seen that what has been described herein is a method for the preparation of a massive cemented carbide body which avoids the necessity of excessive compacting pressures and results in a body of uniform density characteristics and homogeneous structure. This method, in employing a fluid mass in the operations before final sintering, provides hydrostatic conditions without the use of starches or other materials which tend to contaminate the finished product.

Therefore, while emphasis has been placed upon a specific example relating to tungsten carbide and a preferred paste mixture, it will be understood that the in vention should not be limited thereto because the methods disclosed herein are equally applicable when simple carriers such as those hereinabove mentioned are-*em ployed. Furthermore, the method and steps thereof are applicable to other refractory metals or their compounds, for example, tantalum carbide, zirconium oxideor various mixtures of the metals and metal compounds. Of the various modifications and changes that will suggest themselves to those skilled in the art, many of such modifications and changes may be made without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. In a process for producing hollow shell-like articles of metal-bonded refractory metal compounds, the steps of incorporating at least one finely comminuted refractory metal compound together with at least one finely comminuted' metallic bonding agent into a slurry with avolatile organic liquid containing in solution a major proportion of an organic lubricant Whose melting point lies in the range from about 50 C. to about 70 C., said incorporation taking placeat a temperature at which said organic system is liquid and below the boiling point of said volatile organic liquid; placing said slurry in a mold, at least one surface of which is porous to said fluid organic system; compacting said slurry by the application of pres. sure-in increments, said placing and compacting steps being carried out at a temperature similar to thatof said incorporating step; removing at least a portion of said mold from contact with the formed compact, and heating said compact to remove remaining organic matter and to sinter said compact.

2. In a process for producing hollow shell-like articles of metal-bonded refractory metal carbides, the steps of incorporating at least one finely comminuted refractory metal carbide together with at least one finely comminuted metallic bonding agent into a slurry with a volatile organic liquid containing in solution a major proportion of an organic lubricant whose melting point lies in the range from about 50 C. to about 70 C., said incorporation taking place at a temperature at which said organic system is liquid and below the boiling point of said volatile organic liquid; placing said slurry in a mold, at least one surface of whch is porous to said fluid organic system; compacting said slurry by the application of pressure in increments, said placing and compacting steps being carried out at a temperature similar to that of said incorporating step; removing at least a portion of said mold from contact with the formed compact, and heating said compact to remove remaining organic matter and to sinter said compact.

3. In a process for producing hollow shell-like articles of cobalt-bonded tungsten carbide, the steps of incorporating finely comminuted tungsten carbide powder together with finely comminuted cobalt powder into a slurry with a volatile organic liquid containing in solution a major proportion of an organic lubricant Whose melting point lies in the range from about 50 C. to about 70 C., said incorporation taking place at a temperature at which said organic system is liquid and below the boiling point of said volatile organic liquid; placing said slurry in a mold, at least one surface of which is porous to said fluid organic system; compacting said slurry by the application of pressure in increments, said placing and compacting steps being carried out at a temperature similar to that of said incorporating step; removing at least a portion of said mold from contact with the formed compact, and heating said compact to remove remaining organic matter and to sinter said compact.

4. In a process for producing hollow shell-like articles of metal-bonded refractory metal carbides, the steps of incorporating at least one finely comminuted metal carbide together with at least one finely comminuted metallic bonding agent into a slurry with a fluid organic system consisting of about 15 percent paraifin, about 30 percent camphor, about 35 percent naphthalene and the remainder toluene, said incorporation taking place at a temperature in the range from about C. to about C.; placing said slurry in a mold, at least one surface of which is porous to said fluid organic system; compacting said slurry by the application of pressure in increments, said placing and compacting steps being carired out at a temperature similar to that of said incorporating step; removing at least a portion of said mold from contact with the formed compact, and heating said compact to remove the remaining organic matter and to sinter said compact.

5. In a process for producing hollow shell-like articles of cobalt-bonded tungsten carbide, the steps of incorporating 90.2 percent finely comminuted tungsten carbide together with finely comminuted cobalt bonding agent into a slurry with a liquid organic system consisting of 1.5 percent paraifin, 3.0 percent camphor, 3.5 percent naphthalene and 1.8 percent toluene, said incorporation taking place at a temperature in the range from about 50 C. to about 70 C.; placing said slurry in a mold, at least one surface of which is porous to said fluid organic system; compacting said slurry by the application of pressure in increments, said placing and compacting steps being carried out at a temperature similar to that of said incorporating step; removing at least a portion of said mold from contact with the formed compact, and heating said compact to remove the remaining organic matter and to sinter said compact.

References Cited in the file of this patent UNITED STATES PATENTS

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