US2731711A - Sintered tungsten carbide composition - Google Patents

Description

Jan. 24, 1956 ca. W. LUCAS SINTERED TUNGSTEN CARBIDE COMPOSITION Filed May 13, 1954 Imventor: George W Lucas,

H IS Attorney.

2,731,711 SINTERED TUNGSTEN CARBIDE COMPOSITION George W. Lucas, St. Clair Shores, Mich., assignor to General Electric Company, a corporation of New York Application May 13, 1954, Serial No. 429,445 5 Claims. (Cl. 29--182.8)

This invention relates to hard metal compositions and more particularly to improvements in such. compositions commercially known as cemented tungsten carbides which are composed essentially of tungsten carbide grains held in a matrix of an auxiliary metal binder such as cobalt, nickel or iron, or combinations of these metals.

Hard metal compositions and, more particularly, cemented carbides currently used in fabricating machine tools are, generally speaking, and depending on their ultimate uses, divided into two main categories, one providing maximum wear resistance and the other maximum shock resistance. In the first category are placed compositions presently employed to obtain maximum wear resistance. These compositions are typified by fine grain size of the tungsten carbide crystal component, together with the use of a low percentage of the auxiliary metal binder. Satisfactory maximum wear resistant compositions have been made in which the predominant tungsten carbide crystals measure, generally, within the range of 0.5 to 3.0 microns with possibly a scattering of crystals measuring, generally, in the order of about 6' microns and ordinarily the binder comprises from about 3% to about 6% by weight of the total composition.

Applications of the second category requiring compositions having maximum toughness and resulting shock resistance are not satisfied by the compositions utilized in the first category which prove inadequate due to breaking, chipping or spalling. At the present time, the commercial optimum in the series of compositions utilized in the second category is typified by a composition consisting of 75% tungsten carbide and 25% cobalt in which the tungsten carbide crystals measure predominantly from about 3 to about 10 microns with a sporadic scattering of crystals up to about 20 microns in size. However, the improvements in toughness with the shock resistance thus gained are accompanied byundesirable sacrificesv in wear resistance. Accordingly, it has generally been considered impractical and disadvantageous to exceed either 25% of the auxiliary metal as a maximum, or tungsten carbide crystal size greater than 20 microns.

The industry has constantly strived for performance improvement in hard metal carbides aimed at increasing the Wear resistance of the metal without unduly sacrificing toughness, which is an equally essential prerequisite to satisfactory use. For example, a tool or tool member having maximum Wear resistance is of little or no practical utility if it breaks or cracks apartl On the other hand, a material which is exceedingly tough and henceshock-resistant is of little practical utility if it is lacking in Wear resistance.

For the first time, the present invention provides cemented tungsten carbide compositions which are extremely tough and at the same time possess and retain a degree of Wear resistance far exceeding the 'known compositions of this type. It' has now unexpectedly been found, contrary to present concepts in this art, that the above very substantial advantages may be realized with tungsten carbide compositions in which over 50% (by States Patfi 0 weight or volume) of the tungsten carbide constituent is in the form of crystals measuring from 20 to microns or more. Cemented carbide products produced from these compositions, having the aforementioned grain size, exhibit vast improvement in toughness and resulting shock resistance, while at the same time maintaining an excellent degree of Wear resistance.

The magnitude of dilference in the size of tungsten carbide crystal-s employed in the present invention as compared to conventional compositions is best illustrated by reference to the accompanying drawings wherein:

Figs. 1 and 2 are photomicrographs showing magnifications at 1500 and 200 diameters respectively, of a tungsten carbide composition of the invention, and Figs. 3 and 4 are photomicrographs showing magnifications at 1500 and 200 diameters respectively, of a tungsten carbide composition having conventional grain size tungsten carbide crystals. In these figures the darker angular regions are the tungsten carbide crystals and the lighter interstitial area is the binder metal matrix.

In the present invention, macro-crystalline tungsten carbides of from more than 20 microns up to 250 microns are employed. The high degree of toughness obtained in the cemented carbide compositions of this invention may be attributed to very substantially decreased total surface area of the tungsten carbide, and accordingly the increased depth of cobalt or other binder between adjacent carbide crystals. Moreover, the high degree of Wear resistance may be attributed to the extreme coarseness of the carbide crystals, which is retained in the final product, since the relatively huge size of the crystals inhibits wear and erosion of cobalt or other binder in the carbide interstices. Additionally, the large crystals are more secure in the binder metal and resist pulling out under conditions of use. This is notably substantiated by the comparative data hereinafter disclosed for both microand macrocrystalline tungsten carbide compositions.

Accordingly, it becomes apparent that this invention is predicated on a radical departure high as 10 microns in size would be considered extremely coarse grained. Similarly, sporadic crystals as large as 25 microns which may occasionally be, encountered as a result of inadequate metallurgical control during manufacture are, by present quality standards, considered undesirable.

In fabricating the tungsten carbide compositions of the present invention, it has been found that tungsten carbide crystals may be employed, which in the final prodnot measure as large as 250 microns or more, and in which the mean tungsten carbide crystal size may be as large as microns. It has also been found, depending upon the end use of the composition that the beneficial results of the invention may be attained with coarse crystals of more moderate size. For example, for usein a header die the tungsten carbide crystal size may be from about 20 microns to about 80 microns. It is stressed that in all instances the predominant tungsten carbide constituent is in the form of crystals in order of magnitude of at least ten times greater in individual crystal dimension than the counter part in current commercial cemented carbide products.

The coarse grain tungsten carbide compositions of the present invention may be manufactured by processes presently employed for the manufacture of conventional carbides, with some modifications. Present processes must be modified to the extent that the normal ball-milling or grinding of the powder constituents is substantially abbreviated so as to preclude the possibility of reducing the crystal size of the macro-crystalline tungsten carbide to a size smaller than the range of the present invention.

In the sintering operation, the sintering temperature required varies with changes in percentages of the binder metal in the composition. In the lower range of binder metal content, for example to 25% by volume, temperatures as high as 1475 C. may be required. In the higher range of binder content, for example 40% to 60% by volume, temperatures as low as 1300 C. may be used to advantage. Although conventional hydrogen sintering is highly satisfactory, somewhat better results have been obtained by employing vacuum sintering as regards the lower, for example less than 25% by volume, binder content.

In preparing the products of the invention, the proportion of tungsten carbide employed may be varied from between 40% to 95% by volume (or approximately 54 to 97.7% by weight), of the sintered product.

The binder metals found most suitable in producing the tungsten carbide compositions include cobalt, nickel and iron, and are employed, on a volume basis, in amounts ranging from 5% to 60% which, on a weight basis, corresponds to from 2.3% to 46%. Of these, cobalt is the preferred binder metal. Alternatively, nickel or iron may be used instead of cobalt, or all of these metals may be used in combination in any proportion and any of these metals may be used alone.

In order that those skilled in the art may better understand how the present invention may be put into effect, the following examples are given in which typical prior art compositions are comparatively considered with a typical composition of the present invention which latter composition is given by way of illustration and not by Way of limitation. All parts and percentages are by weight unless otherwise specified.

Example 1 Another conventional composition was prepared employing 72% tungsten carbide and 28% cobalt in accordance with the procedure outlined in Example 1.

The predominant tungsten carbide crystal size was again in the conventional range, specifically 2 to 8 microns in this particular case.

Example 3 A composition in accordance with the present invention was prepared employing 72% tungsten carbide and 28% cobalt, utilizing in this case macrocrystalline tungsten carbide consisting mainly of grains ranging between 40 microns and 150 microns (-100+325 mesh), the composition being ball-milled with the cobalt powder for 6 hours (as contrasted with the 18 hours in Examples 1 and 2) in the acetone medium, after which the milled slurry was dried in an atmosphere of hydrogen.

The tungsten carbide crystal size of this product measured from about 20 to 200 microns, the predominant crystal size being between 20 and 80 microns.

Dies fabricated of steel and from compositions of Examples 1, 2 and 3, respectively, were field tested in the manufacture of the same production item and the number of production pieces produced using these dies was compared before failure of the individual dies. This particular result, as well as the hardness, Rockwell A and Rockwell C scales, and transverse rupture strength of the products tested, are tabulated as follows: J

Predom- Hardness mam v e i s Produc- Die M Die p Rupture tion ria s1 ion rain size RC Streigith, Pcsfl Mierons Steel 81 60 60, 000 Example 1.. WC+25 00.. 2r-6 85 67 400, 000 80, 000 Example 2.. WC+28 00.. 2-8 83 63 394, 000 105, 700 Example 3-- WC+28 (30.. 20-80 48 105, 000 445, 000

1 Hardness shown both for carbides) and Re (Rockwell 1 Number of pieces produced tion before failure of the die.

The phenomenal increase in the number of pieces produced using the die having the composition of Example 3 is self-evident. However, all of the other data obtained would normally lead those skilled in the art to expect no such actual results in practice from the die material of the invention. For example, it will be noted that as grain size increases, the Rockwell A and C hardness figures decrease and the transverse rupture strength falls off sharply. In present technology, the use of tungsten carbide compositions having these properties for end uses in dies, for example, or for use in any tool requiring a very high degree of toughness as well as a high degree of wear resistance would not be considered feasible. For these reasons, the results obtained with compositions of the present invention are entirely unexpected and completely unobvious.

The optimum tungsten carbide crystal size can be varied Within certain limits. Certain of the factors which determine the grain size in the ultimate sintered tungsten carbide products are determined by the percentages of binder material employed. With compositions containing less than 20% binder, cobalt for example, but more than 10% by weight binder, the mean diameter of the coarse tungsten carbide crystals may preferably be reduced to about 50 microns. In some instances further to enhance wear resistance, the insterstices between the macro-crystals may advantageously be filled with smaller, more conventional grain size tungsten carbide, in which case the macro-crystalline tungsten carbide constituent is preferably at least 50% or more by volume of the total composition. In situations Where fine-grain tungsten carbide is employed to fill the interstices, minor amounts of chromium or chromium carbide, for example, 0.005 to 0.50%, by Weight of the final composition, may also be incorporated to serve as grain growth inhibitors for the finer carbide constituent. Similarly, small amounts of tungsten, for example, 0.10 to 2%, by weight of the final composition, may likewise be used for purposes of grain growth inhibition.

Cemented multi-carbide compositions containing tungsten carbide crystals of large grain size and a secondary carbide of conventional grain size, e. g., titanium carbide, zirconium carbide, tantalum carbide, or columbium carbide which multi-carbide compositions are eminently suitable for machining or cutting of steel and ferrous alloys are the subject of and are claimed in the copending application of George W. Lucas and Carl S. Jiiedrnan Serial No. 429,446 filed concurrently herewith and assigned to the assignee of the present invention.

Tungsten carbide'grains are generally of the same inherent hardness and wear resistance regardless of their size. However, during normal use in conventional grade tungsten carbide compositions, the fine grains may be erodedand pulled out of the matrix, bodily, far in advance of the time that the wear potential has been utilized. On the other hand, coarse macro-crystals since they are more deeplyv and securely embedded in the matrix, that is the bindermaterial, resist pull-out and erosion and hence .a higher degree of wear resistance potential is utilized. Although this theory has been mentioned as Rs (Rockwell A" scale used essentially 0 scale used essentially for steels). in the same conventional heading operapreviously, it must be considered in connection with relative surface area per unit volume of crystals of varying sizes, in order to be appreciated. For example, one cubic millimeter of carbide grains measuring one micron has a combined surface area of 600 sq. mm.; at microns the surface area is reduced to 600 sq. mm., at 25 microns to 240 sq. mm., at 50 microns to 120 sq. mm., at 100 microns to 60 sq. mm., and at 250 microns to 24 sq. mm. Obviously, the exposed surface area of carbide crystals of size 250 microns, and those of intermediate size, are considerably decreased as compared to crystals measuring only one micron.

Generally speaking, and without being limited as to the theory postulated, the foregoing probably explains why, as the crystal size increases, the use of considerably less of the soft binder metal is made possible without sacrifice of essential toughness characteristics while concurrently maintaining enhanced wear resistance. Conversely, when normal amounts of binder material are employed, a final product of greatly enhanced toughness results. However, a sacrifice in wear resistance also accompanies this latter condition, although not to the usually expected degree, since the large crystals of carbide deeply embedded in the matrix metal aiford an excellent barrier to wear and erosion of the soft binder.

While the invention has been described and particularly illustrated with reference to the production of a header die, it will be obvious to those skilled in the art that the invention is of wide application in the cemented carbide field. Obvious uses will readily suggest themselves and include other wear parts, percussion type mining tools, cutting tools and the like. In general, the products of the invention are useful wherever the combined properties of wear resistance and toughness are desired.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A sintered hard metal composition consisting essentially of from about 40 to 95% by volume of tungsten carbide in which over 50% of the tungsten carbide constituent is in the form of crystals larger than 20 microns,

and 5 to 60% by volume of a binder selected from the group consisting of cobalt, nickel, iron and mixtures thereof.

2. A sintered hard metal composition consisting essentially of from about 40% to 95% by volume of the sintered product of tungsten carbide crystals ranging in size from about 20 to 250 microns with a mean crystal size of 150 microns, and 5% to 60% by volume of the sintered product, of a binder selected fromthe group consisting of cobalt, nickel, iron and mixtures thereof.

3. A sintered hard metal composition consisting essentially of from about 40% to 95%, by volume, of the sintered product of tungsten carbide and in which over of the tungsten carbide constituent is in the form of crystals measuring from about 20 to 80 microns and from 5% to by volume of the sintered product of cobalt.

4. A sintered hard metal composition consisting essentially of 60% by volume of the sintered product, of tungsten carbide and in which over 50% of the tungsten carbide constituent is in the form of crystals measuring from about 20 to microns, and approximately 40%, by volume of the sintered product, of cobalt.

5. A sintered hard metal composition consisting essentially of from about 40% to by volume of the sintered product of tungsten carbide crystals ranging in size from about 20 to 250 microns with a mean crystal size of from 50 to microns, and 5% to 60% by volume of the sintered product, of a binder selected from the group consisting of cobalt, nickel, iron and mixtures thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,254,975 Pickus Sept. 2, 1941 FOREIGN PATENTS 705,844 Great Britain Mar. 17, 1954

Claims (1)

1. A SINTERED HARD METAL COMPOSITION CONSISTING ESSENTIALLY OF FROM ABOUT 40 TO 95% BY VOLUME OF TUNGSTEN CARBIDE IN WHICH OVER 50% OF THE TUNGSTEN CARBIDE CONSTITUENT IS IN THE FORM OF CRYSTALS LARGER THAN 20 MICRONS, AND 5 TO 60% BY VOLUME OF A BINDER SELECTED FROM THE GROUP CONSISTING OF COBALT, NICKEL, IRON AND MIXTURES THEREOF.

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