US3752655A

US3752655A - Sintered hard metal product

Info

Publication number: US3752655A
Application number: US00152448A
Authority: US
Inventors: L Ramqvist
Original assignee: Rederi Nordstjernan AB
Current assignee: Rederi Nordstjernan AB
Priority date: 1969-02-07
Filing date: 1971-06-11
Publication date: 1973-08-14
Anticipated expiration: 1990-08-14
Also published as: DE2005707B2; DE2005707A1; NL7001753A; SU455520A3; FR2034038A5; AT303405B; FI48575B; FI48575C; GB1264093A; DE2005707C3; JPS5020933B1; SE329799B

Abstract

AN IMPROVED SINTERED HARD METAL COMPOSITION IS PROVIDED COMPRISED ESSENTIALLY OF REFRACTORY METAL COMPOUND GRAINS OR PARTICLES COATED WITH A REFRACTORY CARBIDE LAYER TO IMPART WETTABILITY TO REFRACTORY METAL COMPOUND GRAINS WHICH ARE DIFFICULT TO WET WITH SUCH BINDER METALS AS THE IRON GROUP METALS. THE WETTABLE REFRACTORY CARBIDE COATING IS FORMED BY DEPOSITING A LAYER OF THE CORRESPONDING REFRACTORY METAL UPON THE GRAINS FROM A HALIDE OF THE METAL, WHICH LAYER IS THEREAFTER CARBURIZED; OR THE CARBIDE COATING MAY BE PRODUCED SIMULTANEOUSLY ON THE SURFACE OF THE GRAINS FROM AN ATMOSPHERE COMPRISING HALIDE VAPOR IN THE PRESENCE OF A CARBURIZING AGENT, IN WHICH HYDROGEN MAY OR MAY NOT BE PRESENT. THE COATED REFRACTORY METAL COMPOUND GRAINS MAY OPTIONALLY BE MIXED WITH WETTABLE REFRACTORY METAL CARBIDE GRAINS IS PRODUCING SINTERED HARD METAL COMPOSITIONS.

Description

United, States Patent US. Cl. 29-1825 31 Claims ABSTRACT OF THE DISCLOSURE 'An improved sintered hard metal composition is provided comprised essentially of refractory metal compound grains or particles coated with a refractory carbide layer to impart wettability to refractory metal compound grains which are difiicult to wet with such binder metals as the iron group metals. The wettable refractory carbide coating is formed by depositing a layer of the corresponding refractory metal upon the grains from a halide of the metal, which layer is thereafter carburized; or the carbide coating may be produced simultaneously on the surface of the grains from an atmosphere comprising halide vapor in the presence of a carburizing agent, in which hydrogen may or may not be present. The coated refractory metal compound grains may optionally be mixed with wettable refractory metal carbide grains in producing sintered hard metal compositions.

This application is a continuation of application Ser. No. 7,970, now abandoned, filed Feb. 2, 1970.

This invention relates to improved sintered hard metal products, such as cemented refractory metal carbides, and also to a method for producing such sintered products from those refractory metal compound grains not readily wetted by iron group type binder metals.

Recent developments in new alloy materials, such as new and improved high temperature alloys for use as structural elements in heat engine components of jet aircraft and in other structures calling for high strength and corrosion resistance at elevated temperatures, have placed heavy demands for new and improved hard metal products, e.g. cemented carbides, capable of machining such alloys.

' The trend has been towards producing harder sintered hard metals having improved wear resistance; however, many of the hard metal compounds which show any promise are not readily wetted by such binder metals as the iron group metals iron, nickel and cobalt. Thus, due to poor sintering, the elasticity or ductility of the binder metal is not fully utilized in' the final product. The term sinteredhard metal employed herein relates to those p'r'oductsmade by sintering togethera compressed powder mixture of refractory metal compounds, such as refractory metal carbides, borides and nitrides, and a ductible binder matrix metal, such as iron, nickel, cobalt, and the like.

It has long been known that several very hard refractory metal compounds, such as titanium carbide, titanium boride, tungsten boride, and the like, are not readily wettable and, therefore, are difficult to bond to iron group binder metals, such that the sintered product generally tends to be quite brittle and of little industrial interest. These refractory compounds, on the other hand,.are at- 3,752,655 Patented Aug. 14, 1973 ice tractive in view of their low initial cost. In attempts to utilize such hard refractory metal compounds, efforts have been made to improve their wettability relative to the binder metal by adding other metals to their binder metal prior to sintering.

Under certain circumstances, the added metal can form a mixed carbide layer on the solid hard materials during sintering. However, this method results in a rather dilfused surface layer on the hard material, wherein the bonding property is not appreciably improved. One system which has been proposed (note US. Pat. Re. 25,815) is the system TiC-Ni (Mo), where molybdenum or molybdenum carbides are added to the nickel binder metal before sintering and where the TiC-powder is substantially free of molybdenum. It is further known according to Swedish Pat. 314,212 that molybdenum metal can be added to a hard metal system containg TiC in such manner that the Mo-metal is deposited on the TiC-cores before sintering to provide improved dispersion of M0 in the system. However, this method results only in a marginal improvement with respect to the first-mentioned method. With regard to the known techniques for adding metal to the binder phase, it is important to note that when the powder of hard material consists of borides and nitrides, no carbides are formed on the surface of the particles as discussed above, but instead mixed borides and nitrides, respectively, are formed, which generally means poor wetting properties and a brittle sintered hard metal body.

It has been found in accordance with the invention that the aforementioned difiiculties can be overcome by directly modifying the surface of the refractory metal compound grains prior to sintering so that they are rendered more wettable and can be mode easily sintered in a matrix of an elastic or ductile binder metal phase consisting substantially of one or more of the iron group metals. The sintered hard metal obtained is generally very hard, has a high water resistance and good modulus of elasticity.

It is thus an object of this invention to provide an irn: proved sintered hard metal composition consisting essentially of refractory metal compound grains not readily wettable in which the surface of the grains has been modilied with a special layer of refractory metal carbide in order to confer improved wettability to the refractory compound grains relative to the binder metal inwhich the grains are dispersed.

Another object is to provide a method of producing an improved sintered hard metal composition from refractory metal compound grains normally difficult to wet with binder metal in which the surface of the grains is first modified by applying a special refractory carbide coating prior to sintering to enhance the wettability of the refractory compound grains relative to the binder metal, such as iron group binder metals.

These and other objects will more clearly appear when taken in conjunction with the following description and the appended claims.

Stating it broadly, the invention provides as a product an improved sintered hard metal composition consisting essentially of refractory carbide-coated hightemperature refractory metal compound grains dispersed through a binder matrix metal selected from the group consisting of Fe, Ni, Co and Fe-base, Ni-base and Co-base alloys, the coated refractory metal compound grains having a core selected from the group consistingvof: 1) monocarbides of the Group IVb and Group Vb elements Ti,

Zr, Hf, V, Nb and Ta, and (2) nitrides and borides of the Group IVb, Group Vb and VIb elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, the refractory carbide coating on each of said cores being selected from the group consisting of carbides of V, Nb, Ta, Cr, Mo and W, the carbides of V, Nb and Ta having the formula selected from the group consisting of Me C and MeC the compound MeC having a cubic structure in which x is less than 1, and more preferably ranging from about 0.75 to about 0.85.

A method which may be employed for producing coated fine granular core material comprising refractory metal compound grains comprises intimately contacting a powder of the core material of desired particle size with a fluid of one or several halides of the metals selected from the group V, Nb, Ta, Cr, Mo and W, preferably an atmosphere of hydrogen which may also contain a hydrocarbon gas. For example, by having a hydrocarbon gas present, the coating deposited directly on the core material may be a carbide of one or more of the elements V, Nb, Ta, Cr, M or W. Alternatively, the method may be carried out in two steps, to wit: substantially pure metal may be deposited on the core material from the halide vapor of the metal at an elevated temperature in preferably an atmosphere of hydrogen; and in the second step, the deposited metal may then be carburized using a hydrocarbon gas.

As stated above, it is known from the literature that a carbide of the type MeC, in which Me is a metal from Group IVb and Vb in the periodic system, i.e., TiC, ZrC, HfC, VC, NbC and TaC, is not readily wetted by liquid metals from the iron group, i.e. Ni, Fe and Co. The wetting of the carbides MeC and the bonding to the iron metals can, however, be considerably improved if the carbon content is reduced in the homogeneity range of the crystal lattice. Carbides of the type Me C, wherein Me is a metal from the Group Vb, are also more easily wetted than the indicated carbides of the type MeC. The carbides of the metals in Group VIb, e.g. Cr C M0 0 and WC, are readily wetted by the metals of the iron group. Generally speaking, refractory metal carbides are usually more readily wetted than, for instance, refractory metal nitrides.

It has been found that hardness as a function of the carbon content of carbides of metals in the Groups Nb and Vb behave rather strangely. The intrinsic hardness is greatly reduced with decreasing carbon content for carbides in Group IVb, i.e. for TiC, ZrC and HfC and for VC; whereas, the intrinsic hardness increases with decreasing carbon content for NbC and TaC. The variations of the hardness with the carbon content are quite significant for all the carbides. In the following table, the hardnesses are shown measured as Vickers hardness using a 50 g. weight:

I-I (Vickers hard- Refractory carbide: ness), kg/mm. TaC TaC 3000 NbC 1400 NbC 3200 maa 3000 C 2000 TiC 3000 TiC 1800 According to the present invention, a core or grain of refractory metal compound not readily wettable by binder metal is coated with a thin layer of wettable carbide firmly 'bonded to the core. The thus coated core is now easily bonded to and wetted by metals of the iron group. It is important that this layer not react adversely over a large temperature range with the material in the core or with the actual binder phase, which usually consists of cobalt metal.

In carrying out the present method, a metal halide in the flu d or g eous state is bro ght in contact with the 4 i hard material powder cores under conditions which provide the formation of the desirable metal carbide layer on the cores. Where the cores are comprised of refractory metal carbides, the contact with the metal halide can be made directly so as to result in an exchange of reaction; for instance, TiC+WCl TiCl +WC. Where the cores are borides or nitrides, an atmosphere comprising hydrocarbon and preferably hydrogen is used together with the metal halide vapor. The thickness of the layer can be varied from some 0.01 micron up to several microns according to the temperature employed, the relation of the halide to reducing agent present, for instance hydrocarbon (e.g. methane), the grain size of the core and the reaction time.

According to the present method, it is possible to coat grains of titanium carbide (TiC) having a diameter of about 1 micron with a 0.01-0.1 micron thick layer of pure WC. All the suitable layer types, for instance WC, M0 0, Cr C Ta C Nb C and V C wherein y=1 or 2 and x21, can with advantage be obtained by using the corresponding metal halide together with either hydrogen and/or a suitable hydrocarbon. Where y=1, the x should be less than 1 and range preferably from about 0.75 to 0.85. It is also possible to carry out the deposition of the layer on the core in two steps, substantially pure metal being deposited in the first step and the metal then being thereafter carburized in a second step. Carburization is preferably carried out with gaseous hydrocarbons (e.g. methane) and the temperature may be in the neighborhood of about 1000 C., although other carburizing agents can be employed. This step-wise treatment is especially suitable when a substoichiometric carbide layer is desired, that is, a carbide layer based on the formula MeC where x is less than 1 and preferably ranges from about 0.75 to about 0.85. The particular carbide layer desired can be determined by experiment by the controlling amount of reactants employed.

The invention will now be described in its more detailed aspects as follows. Very hard, normally non-wettable core grains of TiC are coated with a thin layer of carbide from Group Vb, e.g. NbC or TaC where x 1.0 but optimally ranges from about 0.75 to 0.85. The coated grains are hard throughout with H 3000 kg./mm. yet; the grains can easily be bonded by either Co, Ni or Fe. The hard phase in the sintered product is present as angular grains. The layer material for imparting improved wettability may also be V 0, Nb C or Ta C.

In another embodiment, the very hard grain of TiC is coated with a layer of tungsten carbide. While the coated grain is also quite hard, it is less hard than TiC. The coated grain can easily be bonded to Co, Ni or Fe binder metal. The hard phase in the product appears as slightly rounded grains after sintering. The grains of TiC as core material may be replaced with other hard metal compounds such as TiB WB', TiN. Where the layer consists of TaC it does not react adversely with the TiC core, and the diffusion is also moderate. The layer of TaC which is firmly bonded to the surface of TiC, is easily wetted by nickel binder metal and a fine grain alloy is obtained after sintering at 1450 C. The sintered body obtained exhibits hardnesses (H 3 kgs.) of up to about 1600 kgs./mm. when the binder metal consists of less than about 20% by weight. The bending strength (trans verse rupture) is above 250 kg/mm. and compares very favorably with conventional products, very elastic but less hard metal of WC-Co, showing a bending strength of about 250 kg./mm. and a hardness (H 3 kgs.) of about 1300 kg./mm.

It has been noted that a deposited layer of WC does not react adversely with a core grain of TiC and that the diffusion is relatively inconsequential. Therefore, a deposited layer of WC remains quite thin and wellas about 1200 C. Such temperatures are known to be obtained at the tip of cuttin tools in machining operations. This has been confirmed by microprobe analyses. The layer of WC is firmly bonded to the surface of the titanium carbide grain and is detectable as a well defined layer by the microprobe. The deposited layer of WC is easily wetted and forms a firm bond with the elastic or ductile cobalt phase when sintered at about 1400 C. The resulting hard metal alloy has a high hardness, high bending strength (transverse rupture) and exhibits high resistance to wear. In a hard metal body comprising 60 weight percent of TiC, weight percent of WC and 30 weight percent of Co, a hardness (H S kg.) of 1600 kg./mm. and a bending strength of more than 300 kg./ mm. are obtained. The titanium carbide is present as somewhat rounded fine grains. By comparison, the system of 87 weight percent of WC and 13 weight percent of Co exhibits a hardness (H 3 kg.) of about 1200 kg./mm.? and a bending strength of about 250 kg./mm. The average grain size of the carbide in the two systems is about 3 microns. Uncoated TiC generally grows to a coarse grain, e.g. l5 microns, compared to sintered coated TiC.

As illustrative of additional embodiments of the invention, the following examples are given.

EXAMPLE 1 A kg. of TiC powder, averaging about 3 microns in size, was reacted with gaesous WCl at 950 C. for about 1 mour. An analysis showed that the coated TiC powder contained a little more than 9% W. The coated powder was treated in pure hydrogen at 1350 C. for about /2 hour, after which the powder was analyzed again. Only T iC and WC phases could be detected by X-ray measurements using the Guinier-H'zigg method. The coated TiC powder was then mixed for 50 hours in a ball mill together with 30% Co binder metal and 2% wax. After pressing a compact at about 6 to 10 t.s.i., the presintering of the compact was carried out at about 900 C. for about one hour in hydrogen and the final sinterin at about 1420 C. for about 1 /2 hours under vacuum. The sintered bodies were tested as regards transverse rupture strength by applying a load to a bar 6 mm. high, 4 mm. wide and mm. long to failure and the porosity tested according to ASTM B 276-54, the hardness according to Vickers using 3 kgs. load, and a microstructure obtained at 150 times magnification. All tests were carried out-in comparison with a standard body of TiC-30% Co, which standard body was also treated analogously with the schedule recited above. The geometry of the WC layer was studied indetail by a microprobe and the layer was shown to be very well defined and narrow. The results obtained are given as follows:

TABLE 1 Uncoated TiC Coated TiC Composition TiC-30% Co Tig(10% WC)-30% Transverse rupture 129 kgJmm. 284=|=19 kg./mm.

strength. Porosity Large pores B4 A2. Hardness Dtfiicult to measure... 1,598 kgJmm. Microstructure at 1,500 Coarse TiC-grains Fine granular TiC times magnification. generally over 15 generally less than 4 microns.

microns.

sintering and appears quite difiuse as compared to Example 1 where the WC layer is formed directly'on the grain before sintering. Only a marginal improvement in properties was obtained in comparison with the TiC-30% Co system. The results obtained are as follows:

Table 2 5 Transverse rupture strength 160:21 kg./mm.

Porosity A3. Hardness 1315 kg./mm. Microstructure 1500 times magnification TiC, 6-7 microns.

EXAMPLE 3 A kg. of WB powder, averaging 3 microns in size, was reacted with gaseous TaCl and CH at 950 C. for about 1 hour. Analysis showed that the WB powder contained about 6% Ta. The powder was treated at about 1350 C. in hydrogen for /2 hour after which WB and TaC phases were detected using X-ray and microprobe analyses. The coated WB powder was mixed for 50 hours in a ball mill together with 10% Ni and 2% wax. A compact was produced as in Example 1 and the compact then presintered at 900 C. in hydrogen for about 1 hour and finally sintered at about 1450 C. for about 1 /2 hours in vacuum. The sintered body was tested in comparison to a standard body containing WB-10% Ni as-recited in Ex ample 1. The results obtained are as follows:

The TaC coating on the WB core corresponded to the formula MeX with x less than 1 and ranging from about 0.75 to 0.85.

While the invention has been described with regard to' producing sintered hard metal compositions from normally difiicult-to-wet refractory metal compound grains or particles, it will be appreciated that the advantages of the invention can also be utilized in producing hard metal compositions in which part of the coated grains may be replaced with refractory metal carbide grains or particles. which are wettable by iron group metals and alloys thereof. Examples of wettable carbides which can be mixed with the coated hard metal grains in producing sintered hard metal compositions are WC, MOgC, Cr C mixed crystals of at least two carbides thereof, as well as mixed crystals (e.g. solid solution) of the system WC-TiC, among others. The amount of wettable refractory metal carbide that can be mixed with and replace some of the coated hard metal grains may range optionally from about 0 to 10 times the amount ofcoated grains present.

Broadly speaking, the total amount of hard metal particles in the sintered hard metal composition may range from about 30% to 96% by weight, with substantially the balance an iron group binder metal ranging from about 70% to 4% by weight. Advantageously, the sintered composition may range in its more preferred aspects from about 5 0% to 95% by weight of hard metal particles, with substantially the balance binder metal ranging from about 50% t0 5% by weight. The binder metal is selected from the group consisting of iron, nickel and cobalt, and ironbase, nickel-base and cobalt-base alloys. An example of an iron-base alloy is a steel comprising 5% Cr, 5% Mo, 0.5%

C and the balance'essentially iron. A nickel-base alloy is' TABLE 4 Composition by weight of coated Composition by weight N 0. hard metal particles of binder metal 1 30% (TiC coated with WC) 70% cobalt. 2 40% (ZrC coated with V 10) 60% nickel. 3 50% (NbC coated with 'IaCn-s) 50% iron alloy. 4 60% (WE coated with M020) 40% (80% Ni-20% Mo). 5 80% (TlBg coated with @1302) 20% cobalt alloy. 6 90% (CrN coated with Nbcu's) 10% cobalt.

X 5% Cr, 5% M0, 0.5% C and Fe balance. 9 20% Cr, 5% W, 1% C and Co balance.

As illustrative of cemented hard metal compositions in which wettable refractory metal carbide particles are employed mixed with the coated hard metal particles, the following examples are given in Table 5 below:

TABLE 5 Ratio by weight: of Composition by weight of coated uucoated Composition and uncoated hard metal to coated by weight of No. particles particles binder metal 7--. 40% (30% T10 coated with W0, 10% 0.33 60% nickel CIzCz grains). alloy. 8 50% (25% WE coated with TaCo-a, 1.0 50% cobalt.

25% W grains). 0"..- 60% (25% TiN coated with NbCQ- 1.4 40% iron.

35% M020 grains). 10--.- 75% (25% CrN coated with MozC, 2.0 25% cobalt.

50% WC-TiC mixed crystals). 11-..- 90% (60% TiC coated with VCo-a, 0.67 10% cobalt.

40% WC grains). 12..-- 90% (10% TiO coated with W0, 10% 3.5 Do.

TaC goated with W0, 70% W0 grains 13.-.- 94% (8% TiC coated with TaCo-s, 10.75 6% cobalt.

86% W0 grains).

1 Cr, 5% M0, 0.1% O and Ni balance.

The foregoing examples of Table 5 illustrate how part of the coated grains can be replaced by wettable refractory metal carbides, such as grains of Cr C WC, M0 0, WC-TiC (mixed crystals), etc. As stated hereinbefore, part of the coated grains can be replaced optionally by wettable refractory carbide grains in amounts ranging from about 0 to times the amount of coated grains present, the total hard metal particles present falling within the range of about 30% to 96% by weight or, more advantageously, from about 50% to 95% by weight.

As has been stated hereinbefore, the wettable refractory metal carbide coating is produced from a halide of the refractory metal, the refractory metal being deposited from the halide in the fluid state heated to the appropriate temperature. The fluid halide may be either in the liquid or gaseous state (vapor), the gaseous state being particularly preferred. Following the production of the metal coating, the coating is thereafter carburized using solid or gaseous carburizing agents; or the carbide coating may be produced in situ using a mixture of the halide vapor and a hydrocarbon gas, with or without hydrogen present.

Sintered hard metal compositions produced in accordance with the invention, depending upon the amount of binder metal present, may be employed in the production of cutting ools, tools in wh ch h gh hardness and ear resistance are prime requisites, such as earth drilling tools, rests for centerless grinders, liners for brick mold, facings for hammers in hammermills, balls and seats for check valves, sandblast nozzles, ring and plug gages, gage blocks, wear pads for machinery, and the like.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be with the purview and scope of the invention and the appended claims.

What is claimed is:

1. An improved sintered hard metal composition comprised essentially of hard metal particles of refractory carbide-coated high temperature refractory metal compound grains dispersed through a binder matrix metal selected from the group consisting of Fe, Ni, Co and Febase, Ni-base and Co-base alloys, said coated refractory metal compound grains having a core selected from the group consisting of: (l) carbides of Ti, Zr, Hf, V, Nb and Ta, and (2) nitrides and borides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, the refractory carbide coating on said core being selected from the group consisting of carbides of V, Nb, Ta, Cr, Mo and W, the carbides of V, Nb and Ta of said refractory carbide coating having the formula selected from the group consisting of Me C and MeC the compound MeC having a cubic structure in which x is less than 1.

2. The sintered hard metal composition of claim 1 wherein x of the formula MeC ranges from about 0.75 to 0.85.

3. The sintered hard metal composition of claim 1, wherein the amount of hard metal particles by weight ranges from about 30% to 96% and the binder metal from about 70% to 4% by weight, and wherein the hard metal particles may include optionally refractory metal carbide grains wettable by said binder metal to replace part of the coated grains, the total hard metal particles present falling within the range of about 30% to 96% by weight.

4. The sintered hard metal composition of claim 3, wherein the amount of the wettable refractory metal carbide ranges up to about 10 times the amount of coated grains present.

5. The sintered hard metal composition of claim 3, wherein the wettable refractory metal carbide is selected from the group consisting of WC, Mo C, Cr C mixed crystals of at least two of said carbides and mixed crystals of the system WC-TiC.

6. An improved sintered hard metal composition comprised essentially of hard metal particles of refractory carbide-coated refractory compound grains dispersed through a matrix metal selected from the group consisting of Fe, Ni, Co and Fe-base, Ni-base, and Co-base alloys, said coated refractory compound grains having a core selected from the group consisting of carbides of Ti, Zr, Hf, V, Nb and Ta, the refractory carbide coating on said core being selected from the group consisting of carbides of V, Nb, Ta, Cr, Mo and W, the carbides of V, Nb and Ta or said refractory carbide coating having the formula selected from the group consisting of Me C and MeC the compound MeC having a cubic structure in which .1: is less than 1 and ranges from about 0.75 to about 0.85.

7. The sintered hard metal composition of claim 6, wherein the amount of hard metal particles by weight ranges from about 30% to 96% and the binder metal from about 70% to 4% by weight, and wherein the hard metal particles may include optionally refractory metal carbide grains wettable by said binder metal to replace part of the coated grains, the total hard metal particles present falling within the range of about 30% to 96% by weight.

8. The sintered hard metal composition of claim 7, wherein the amount of wettable refractory metal carbide ranges-up to about 10 times the amount of coated grains present. l

9. The sintered hard metal composition of claim 8, wherein the wettable refractory metal carbide is selected from the group consisting of WC, Mo C, Cr C mixed crystals of at least two of said carbides and mixed crystals of the system WC-TiC.

10. An improved. sintered hard metal composition comprised essentially of hard metal particles of refractory carbide-coated high temperature refractory metal compound grains dispersed through a binder matrix metal selected from the group consisting of Fe, Ni, Co and Febase, Ni-base and Co-base alloys, said coated refractory metal compound grains having a core selected from the group consisting of nitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, M and W, the refractory carbide coating on said core being selected from the group consisting of carbides of V, Nb, Ta, Cr, Mo and W, the carbides of V, Nb, and Ta of said refractory carbide coating having the formula selected from the group consisting of Me C and MeC the compound MeC having a cubic structure in which x is less than 1 and ranges from about 0.75 to about 0.85.

11. The sintered hard metal composition of claim 10, wherein the amount of hard metal particles by weight ranges from about 30% to 96% and the binder metal from about 70% to 4% by weight, and wherein the hard metal particles may include optionally refractory metal carbide grains wettable by said binder metal to replace part of the coated grains, the total hard metal particles present falling within the range of about 30% to 96% by weight.

12. The sintered hard metal composition of claim 11, wherein the amount of wettable refractory metal carbide ranges up to about times the amount of coated grains present.

13. The sintered hard metal composition of claim 11, wherein the wettable refractory metal carbide is selected from the group consisting of WC, Mo C, Cr C mixed crystals of at least two of said carbides and mixed crystals of the system WC-TiC.

14. An improved sintered hard metal composition comprised essentially of hard metal particles of refractory carbide-coated high temperature refractory metal compound grains dispersed through a binder matrix metal selected from the group consisting of Fe, Ni, Co and Febase, Ni-base and Co-base alloys, said coated refractory metal compound grains having a core selected from the group consisting of borides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, the refractory carbide coating on said core being selected from the group consisting of carbides of V, Nb, Ta, Cr, Mo and W, the carbides of V, Nb and Ta of said refractory carbide coating having the formula selected from the group consisting of Me C and MeC the compound MeC having a cubic structure in which x is less than 1 and ranges from about 0.75 to about 0.85.

15. The sintered hard metal composition of claim 14, wherein the amount of hard metal particles by weight ranges from 30% to 96% and the binder metal from about 70% to 4% by weight, and wherein the hard metal particles may include optionally refractory metal carbide grains wettable by said binder metal to replace part of the coated grains, the total hard metal particles present falling within the range of about 30% to 96% by weight.

16. The sintered hard metal composition of claim 15, wherein the amount of wettable refractory metal carbide ranges up to about 10 times the amount of coated grains present.

17. The sintered hard metal composition of claim 15, wherein the wettable refractory metal carbide is selected from the group consisting of WC, Mo C, Cr C mixed crystals of at least two of said carbides and mixed crystals of the system WC-TiC.

18. A method of producing an improved sintered hard metal composition comprising hard metal particles of refractory metal compound grains normally diflicult to wet with a binder matrix metal selected from the group consisting of the iron group metals Fe, Ni, Co and Febase, Ni-base and Co-base alloys which comprises, selecting a batch of refractory metal compound core grains from the group consisting (l) carbides of Ti, Zr, Hf, V, Nb and Ta, and (2) nitrides and borides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, applying to said core grains a firmly bonded coating of at least one metal carbide selected from the group consisting of V, Nb, Ta, Cr, Mo and W, the carbides of V, Nb and Ta of said metal carbide coating having the formula selected from the group consisting of Me C and MeC the compound MeC having a cubic structure in which x is less than 1, forming a powder mixture of said coated grains and said binder matrix metal, compressing said mixture into a compact, sintering said compact at an elevated temperature at which said binder melts, and then cooling the sintered compact, whereby a strongly bonded hard metal composition is obtained.

19. The method of claim 18, wherein x of the formula MeC ranges from about 0.75 to about 0.85.

20. The method of claim 18, wherein the amount of hard metal particles by weight ranges from about 30% to 96% and the binder metal from about 70% to 4% by weight, and wherein the coated hard metal particles may include optionally in mixture therewith refractory metal carbide grains wettable by said binder metal to replace part of the coated grains, the total hard metal particles present falling within the range of about 30% to 96% by weight.

21. The method of claim 20, wherein the amount of wettable refractory metal carbide ranges up to about 10 times the amount of coated grains present.

22. The method of claim 20, the wettable refractory metal selected from the group consisting of WC, Mo C, Cr C mixed crystals of at least two of said carbides and mixed crystals of the system WC-TiC.

23. The method of claim 18, wherein said metal carbide coating is produced on the core grains by contacting the core grains with a halide of at least one of said metals V, Nb, Ta, Cr, Mo and W at an elevated temperature with the halide in the fluid state.

24. The method of claim 23, wherein the halide is a vapor and wherein the treatment is carried out in an atmosphere of hydrogen.

25. The method of claim 23, wherein the core grains are coated by heating them to an elevated temperature in an atmosphere comprising a vapor of said halide and a hydrocarbon gas.

26. The method of claim 23, wherein said metal carbide coating is formed by applying a coating of a metal from the group consisting of V, Nb, Ta, Cr, Mo and W by reduction of a halide gas of staid metal in an atmosphere containing hydrogen, followed by a second step of carburizing said metal coating.

27. Refractory carbide-coated refractory metal compound grains suitable for use in the production of sintered hard metal compositions, the coated grains having a core selected from the group consisting of: (1) carbides of Ti, Zr, Hf, V, Nb and Ta, and (2) nitrides and borides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, the refractory carbide coating on said core being selected from the group consisting of carbides of V, Nb, Ta, Cr, Mo and W, the carbides of V, Nb and Ta of said refractory carbide coating having the formula selected from the group consisting of Me C and MeC the compound MeC having a cubic structure in which x is less than 1.

28. The coated grains of claim 27, wherein x is the formula MeC ranges from about 0.75 to 0.85.

29. The coated grains of claim 27, wherein the core of the grains is selected from the group consisting of carbides of Ti, Zr, Hf, V, Nb and Ta.

30. The coated grains of claim 27, wherein the core of the grains is selected from the group consisting of nitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W.

l1 12 31. The coated grains of claim 27, wherein the core 5,489,541 1/1970 Steinberg- 51--295 of the grains is selected from the group consisting of 3,663,191 5/1972 Kroder 51295 borides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W. V

CA'RL D. QUA-RFORTH, Primary Examiner References Cited 5 B. HUNT, Assistant Examiner UNITED STATES PATENTS Y 3,520,667 7/1970 Taylor 51-308 1 3,264,102 8/1966 Scrugg 75 212 29182-7, 182.8; 75-05 BC,- 201, 203, 204, 205, 212;

2,1 19,487 5/1938 Padowiz 75-212 10643, 47 R; 117-100 B, 106 C