Wear resistant hard metal free of tungsten carbide

Abstract

A wear-resistant hard metal free of tungsten carbide. The hard metal comprises: molybdenum carbide; a carbide of another transition metal; boron, boron nitride, boron carbide, or a mixture thereof, in an amount of 0.1 to 1% by weight boron, based on the weight of the molybdenum carbide, other transition metal carbide and boron-containing material, and a binder metal selected from the group consisting of metals of the iron group of the Periodic Table of Elements, and alloys thereof.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a wear resistant hard metal free of tungsten carbide and containing mixed carbides of molybdenum carbide and a carbide of a further transition metal as well as a metal or an alloy of the metals from the iron group of the Periodic Table of Elements as a binder metal.

Hard metals comprising of hard transition metal carbide and a ductile binder metal have been known and commercially available for several years. They are used mainly as tools for material working, particularly for cutting work, for stone processing or for noncutting shaping, and also in the area of wear protection. A very high percentage of these commercially available hard metals has tungsten carbide as the major component, sometimes with additions of titanium carbide, niobium carbide or tantallum carbide and cobalt as the binder metal. Tungsten carbide is preferred due to its mechanical properties, namely high hardness, very high modulus of elasticity, and high compressive strength, and also due to its excellent wettability and its high-temperature dependent solubility in the liquid cobalt binder.

Over the years, a technology has been established which permits the reproducible manufacture of hard metals having certain specific properties by changing the state of the starting materials, the manufacturing parameters and by the use of additives. However, it has been and is still difficult to introduce new materials into this established technology.

Recent studies concerning the exhaustion of supplies of certain raw materials have indicated that for some elements there are only very limited minable reserves and that for other elements the majority of the supplies are concentrated in only a few countries. The use of these raw materials may lead to dependencies which may become a risk, particularly for highly industrialized nations. One of these raw materials is tungsten, which has a relatively high specific weight and is not very plentiful.

It has been proposed to produce a hard metal utilizing titanium carbide as the hard substance and nickel as the binder metal, and to improve the wetting behavior between this hard substance and binder by adding molybdenum carbide. Particularly fine grained hard metals were obtained from (Ti,Mo) (C,N) mixed phases. However, the additions of molybdenum carbide were limited. As can be seen from the phase diagram for the molybdenum-carbon system, there exist four binary molybdenum carbides. Of these, only Mo.sub.2 C is stable over the entire temperature range. Mo.sub.2 C, however, is not very hard, is brittle and, moreover, converts to an ordered orthorhombic modification at temperatures around 1700.degree. K. Therefore, binary molybdenum carbides do not offer promising possibilities for industrial utilization as hardness carriers in hard metals.

The situation is different, however, with the ternary molybdenum-tungsten carbide which is rich in molybdenum and with which a (W,Mo)C mixed crystal can be obtained and processed. Molybdenum contents up to 60 Mol % seem to be possible with this system. It has been shown that in the presence of the binder metals iron, cobalt, or nickel, the nucleation of the (Mo,W)C mixed crystals is substantially improved and it has been proposed to produce hard metals by simultaneously melting or sintering for extended periods MoC+WC+Co mixtures.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a wear-resistant hard metal free of tungsten carbide which has a stabilized structure and which is equivalent in characteristics to the family of WC-Co hard metals and which is able to replace the family of WC-Co hard metals.

It is another object of the invention to provide a wear-resistant hard metal free of tungsten carbide which has a lower density than tungsten carbide hard metals.

It is a further object of the invention to provide a wear-resistant hard metal based on components expected to be available in the future.

To achieve these objects, and in accordance with its purpose, the present invention provides a wear-resistant hard metal free of tungsten carbide, comprising:

(a) molybdenum carbide;

(b) a carbide of another transition metal;

(c) boron, boron carbide, boron nitride or a mixture thereof, present in an amount of 0.01 to 1% by weight boron, based on the weight of (a), (b) and (c); and

(d) a binder metal selected from the group consisting of metals of the iron group of the Periodic Table of Elements, and alloys thereof.

It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the hardness at ambient and higher temperatures of boron-stabilized molybdenum-based mixed carbides.

FIG. 2 is a photomicrograph showing the structure of a (Ta.sub.0.25 Mo.sub.0.75)C.sub.1-x (B)-10Ni hard metal in accordance with one embodiment of the present invention.

FIG. 3 shows the hot hardness of boron stabilized, Mo based hard metals of the present invention compared to conventional hard metals.

FIG. 4 shows the hardness of (Ti,Mo)C.sub.1-x mixed crystals with and without boron stabilization.

FIG. 5 shows the micro structure of a (Ti,Mo)C.sub.1-x -20Ni hard metal (BN stabilized) with a Ti/Mo ratio of 1/1 (atom %) in accordance with a further embodiment of the present invention.

FIG. 6 shows the micro structure of a (Ti,Mo)C.sub.1-x -20Ni hard metal (BN stabilized) with a Ti/Mo ratio of 1/2 (atom %) in accordance with a still further embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a hard metal free of tungsten carbide which is based on molybdenum carbide, and a carbide of at least one other transition metal. The preferred transition metals for this purpose are Ti, Hf, Zr, V, Nb, Ta, and Cr.

The hard metal will also contain a binder metal from the iron group of the Periodic Table of Elements as published in the Handbook of Physics and Chemistry, 59th Edition, CRC Press, 1978, especially Fe, Ni, Co, or an alloy thereof.

In order to stabilize the structure of the hard metal and to increase its stability both at room temperature and under heating, the hard metal will contain a low boron content. The boron can be added in the form of elemental boron, or boron nitride (BN), boron carbide, (B.sub.4 C), or a mixture thereof.

The boron content of the hard metal will be 0.01 to 1% by weight, based on the weight of the hard substance. As used herein, the term "hard substance" refers to the hard metal without the binder metal. Thus, the term "hard substance" as used with respect to the present invention refers to the combination of the transition metal carbides (Mo and others) and boron-containing substance.

In a particular embodiment of the present invention, mixed carbides of the formula (M,Mo)C.sub.1-x are present, where M is at least one of the transition metals Ti, Zr, Hf, V, Nb, Ta, Cr and O is less than or equal to x which is less than or equal to 0.5. The ratio of MC to MoC.sub.1-x in this embodiment, is in the range between 5 mol percent MC:95 mol percent MoC.sub.1-x and 95 mol percent MC:5 mol percent MoC.sub.1-x. The binder metal is Fe, Co, Ni or an alloy of these metals in an amount of between 5 and 40 weight percent based on the hard metal.

MC-MoC.sub.1-x systems without the addition of boron or a suitable boron compound present disadvantages. At low temperatures, the mixed phases on the molybdenum carbide rich side tend to decompose, particularly when processed with binder metals. This undesirable decomposition is prevented in the hard metals having a composition according to the present invention, by the very low boron content.

The present invention also provides methods for producing the wear-resistant hard metals. In one such method for producing a wear-resistant hard metal free of tungsten carbide, the starting components for the mixed carbides of the hard metal comprise metal or carbide powders of the transition metals Ti, Zr, Hf, V, Nb, Ta, and Cr, and molybdenum powder, carbon powder and boron powder. These materials are mixed in predetermined quantities with a powdered binder metal which is iron, cobalt, nickel or an alloy of more than one of these binder metals, and the mixture is sintered at temperatures between 1373.degree. and 1873.degree. K., either under a protective gas atmosphere or in a vacuum. Instead of molybdenum powder, powdered Mo.sub.2 C or a powder mixture of Mo and Mo.sub.2 C, can also be used as a starting component. Further, instead of boron powder, powdered boron nitride or powdered boron carbide or a mixture of these boron-containing components can be used.

In a variation of the method for producing the hard metal free of tungsten carbide according to the present invention, molybdenum and carbon powder or molybdenum carbide or mixtures thereof and the carbides of other transition metals are initially mixed with the desired proportions of boron, BN or B.sub.4 C to form a powdered mixture of the starting components, and this mixture is sintered at temperatures above 1773.degree. K. The sintered product is then ground and the resulting powder is mixed with powdered binder metal and sintered to yield the final hard metal.

                TABLE A                                                     

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     Theoretical densities of the hardmetals according to this                 

     invention:                                                                

     Hardmetal            Density                                              

     Composition          g/cm.sup.3                                           

     ______________________________________                                    

     (Ti.sub.0,25 Mo.sub.0,75)C.sub.1-x + 10% Co                               

                          7,6                                                  

     (Zr.sub.0,25 Mo.sub.0,75)C.sub.1-x + 10% Co                               

                          8,3                                                  

     (Hf.sub.0,25 Mo.sub.0,75)C.sub.1-x + 10% Co                               

                          9,8                                                  

     (V.sub.0,25 Mo.sub.0,75)C.sub.1-x + 10% Co                                

                          8,1                                                  

     (Nb.sub.0,25 Mo.sub.0,75)C.sub.1-x + 10% Co                               

                          8,7                                                  

     (Ta.sub.0,25 Mo.sub.0,75)C.sub.1-x + 10% Co                               

                          10,0                                                 

     WC + 6% Co           14,6                                                 

     WC + 9% Co           14,4                                                 

     WC + 12% Co          14,0                                                 

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The useful range of hard substance composition of this invention is 5-95 mol % molybdenum carbide, the remainder being one or more of other transition metal carbides and appropriate boron additions. The binder content of the hardmetal is useful in the range between 5 and 45 weight percent. The preferred composition of the hard substance is between 40 and 80 mol % molybdenum carbide, the remainder being one or more transition metal carbides and boron or its compounds in the range 0.01-1.0 wt %. The preferred binder content is between 5 and 30 wt % of the hardmetal.

The production of hard metals according to the present invention enables the replacement of tungsten by other metals. The hard metals so produced have properties similar to those of tungsten carbide hard metals, but have a substantially reduced density.

The following examples are given by way of illustration to further explain the principles of the invention. These examples are merely illustrative and are not to be understood as limiting the scope and underlying principles of the invention in any way. All percentages referred to herein are by weight unless otherwise indicated.

EXAMPLE 1

Six samples are prepared, each containing a metal carbide MC, where M is Ti, Zr, Hf, V, Nb or Ta--and molybdenum carbide. The carbides are mixed in such ratios that the resultant hard substance compositions contain 15 atom % M, 42 atom % Mo, and 43 atom % C. Boron in an amount of 0.5 weight % is added to each sample.

These starting materials are hot pressed at, for example, at 2573.degree. K., or homogenized in a vacuum (10.sup.-8 to 10.sup.-3 bar) at temperatures between 2073.degree. and 2273.degree. K. This processing results in single-phase, cubic face-centered molybdenum-rich hard substance having a high hardness at ambient and high temperature. The hardness of molybdenum based mixed carbides of different compositions containing boron and Hf, Ti or Ta, so produced, is compared with the hardness of several other hard substances in Table I, below, at 25.degree. C. and in FIG. 1 over a range of temperatures. In FIG. 1, K.sup.-1 =1/.degree.K. and the curve numbers correspond to the hard substances of Table I as indicated.

                TABLE I                                                     

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     Curve No. Hard substance    HV (25.degree. C.)                            

     ______________________________________                                    

     6         TiC               2800                                          

     1         (Hf.sub.0.25 Mo .sub.0.75)C.sub.0.75 + 0.2% B                   

                                 2400                                          

     2         (Ti.sub.0.33 Mo.sub.0.67)C.sub.0.88 + 0.2% B                    

                                 2320                                          

     4         WC                2300                                          

     3         (Ta.sub.0.25 Mo.sub.0.75)C.sub.0.75 + 0.2% B                    

                                 2200                                          

     5         MoC.sub.1-x + 0.3% B                                            

                                 1500                                          

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FIG. 1 shows that at high temperatures, from about 200.degree. to 1400.degree. C., only WC has a hardness comparable with the hard substances of the present invention shown in curves 1, 2 and 3. The remaining hard substances--TiC and MoC.sub.1-x +0.3%B-clearly have lower hardnesses at high temperature, even though TiC is the hardest substance at 25.degree. C.

After the heat treatment, the boron content of the mixed crystal can be determined by chemical analysis as 0.2% by weight. The hard substance phases are then comminuted and together with 10 weight percent of the binder metals Co or Ni wet ground in a ball mill for 48 hours under heptane.

The milled powder is dried in vacuum and then cold pressed under a pressure of about 3 tons/cm.sup.2 and sintered in a vacuum for 3 hours at 1623.degree. K. The resulting hard metal products have a network structure containing cubic (M,Mo) C.sub.1-x where M is Ti, Zr, Hf, V, Nb, or Ta, cubic MoC.sub.1-x, hexagonal MoC.sub.1-x and Co or Ni as binder metal. FIG. 2 shows the micro structure of the hard metal product where M is Ta.

The hardness and hot hardness of these hard metals correspond to the hardness of conventional hard metals based on WC, with about the same amount of 9 volume % binder metal. This can be seen from Table 2, below, and from FIG. 3. The curve numbers in FIG. 3 correspond to those in Table 2. Curves 7 and 9 represent hard metals produced according to the present invention.

                TABLE 2                                                     

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                                 Hardness                                      

     Curve No.                                                                 

             Hard Metal          (HV) at 20.degree. C.                         

     ______________________________________                                    

     8       WC, 2(Nb, Ta)C, 6Co 1680                                          

     10      TiC, 10Mo.sub. 2 C, 13Ni                                          

                                 1560                                          

     7       (Ti.sub.0,25 Mo.sub.0,75)C.sub.1-x (B) + 10Co                     

                                 1600                                          

     9       (Ti.sub.0,25, Mo.sub.0,75)C.sub.1-x (B) + 10Ni                    

                                 1570                                          

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The toughness of the hard metals produced according to the present invention is also comparable with that of conventional WC-based hard metals.

As a result of the considerably reduced density of the products according to the present invention (see table A), a saving in high melting point transition metals can be achieved for a given geometrical size of a product. At the same time tungsten can be replaced by other metals.

EXAMPLE 2

TiC, Mo.sub.2 C, C, and BN are ground and mixed in weight ratios of 36.43%, 61.93%, 1.24%, and 0.4% respectively, for 15 hours in a ball mill under heptane. The resulting mixture is cold pressed at a pressure of 2 tons/cm.sup.2 and homogenized for 4 hours at 2073.degree. K. and in a vacuum of 10.sup.-5 bar to produce a hard substance. Further hard substance samples were produced using different weight ratios of the starting materials, and the hardness of the various samples then was determined. In FIG. 4, the hardness of B-stabilized hard-substances of the formula (Ti,Mo)C.sub.1-x (B,BN), having varying Ti/Mo ratios (curve 12) are compared with the hardness of the equivalent hard substances without boron additions (curve 13). Also shown in FIG. 4 is curve 11 representing the hardness of (Ti,Cr)C.sub.1-x which is not B-stabilized, and points 5, 6, 14 and 15, representing the hardnesses of MoC.sub.1-x +0.3%B, TiC, MoC.sub.0.5, and CrC.sub.0.67, respectively. The hardness of the hard substance used in this example is also shown in FIG. 4.

The hard substance (Ti,Mo)C+(B,BN), produced by employing, by weight, 36.43% TiC, 61.93% Mo.sub.2 C, 1.24%C and 0.4%B, is then comminuted with 20 weight percent nickel and 0.02 weight percent C and milled for 48 hours in a ball mill. The dried powder material is pressed at 4 tons/cm.sup.2 and then sintered for 1 hour at 1723.degree. K. under a vacuum (10.sup.-3 bar). The result is a two-phase hard metal consisting of a carbide phase and a binder phase with a uniform fine grained micro structure as shown in FIG. 5, a density of 6.7 g/cm.sup.3, a hardness of 1330 HV and considerable toughness, the crack resistance being 1950 N/mm.

Among conventional WC-Co hard metals, WC with 9 weight % Co has a density of 14.4 (g/cm.sup.3), a hardness of 1350 HV, a crack resistance of 1110 N/mm, and a bending strength of 2800 N/mm.sup.2, and WC with 12 weight % Co has a density of 14.0 (g/cm.sup.3), a hardness of 1180 HV, a crack resistance of 3330 N/mm, and a bending strength of 3200 N/mm.sup.2.

EXAMPLE 3

TiC, Mo.sub.2 C, C, and BN are mixed and ground in weight ratios of 22.12%, 76.35%, 1.03% and 0.4% respectively, for 15 hours in a ball mill under heptane. The mixture is then further processed as described in Example 2. The resulting product is a hard metal having a uniformly fine-grained structure as shown in FIG. 6, a hardness of 1430 HV and a crack resistance of 1000 N/mm.sup.2. A WC-Co hard metal with 9 weight % binder, equivalent to 13 volume % binder metal exhibits similar properties, namely a hardness of 1350 HV, and a crack resistance 1100 N/mm. The hard metal produced according to the present invention in this example contains, with its 17 volume percent binder metal, a higher proportion of binder metal and, mainly because of the substantially reduced density of 7.3 g/cm.sup.3 compared to that of 14.0 for WC-Co, requires a significantly less amount of high melting point transition metal, in this case Ti and Mo instead of W.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptions, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Claims

1. Wear-resistant hard metal free of tungsten carbide, comprising the components:

(a) molybdenum carbide;

(b) a carbide of another transition metal;

(c) boron, boron nitride, boron carbide, or a mixture thereof, in an amount of 0.01 to 1% by weight boron, based on the weight of components (a), (b) and (c);

(d) a binder metal selected from the group consisting of metals of the iron group of the Periodic Table of Elements, and alloys thereof; and

(e) wherein said molybdenum carbide and a carbide of another transition metal are present as mixed carbides of the formula (M,Mo)C.sub.1-x, where M is at least one of the transition metals Ti, Zr, Hf, V, Nb, Ta and Cr, and x is less than or equal to 0.5, the amount of MoC.sub.1-x being at least 40 mole % based on the moles of (a), (b) and (c) and wherein the binder metal comprises iron, cobalt, nickel, or an alloy thereof in an amount of between 5 and 40 weight %, based on the hard metal.

2. Wear resistant hard metal according to claim 1, wherein component (c) comprises boron.

3. Wear resistant hard metal according to claim 1, wherein component (c) comprises boron nitride.

4. Wear resistant hard metal according to claim 1, wherein component (c) is present in an amount of 0.01 to 0.5% by weight boron, based on the weight of (a), (b) and (c).

5. Method for producing a wear-resistant hard metal according to claim 1, comprising the steps of:

(i) mixing, in predetermined quantities, the components

(1) at least one powdered carbide of the transition metals Ti, Zr, Hf, V, Nb, Ta and Cr,

(2) Mo powder, Mo.sub.2 C powder, or a mixture thereof,

(3) carbon powder, and

(4) B powder, BN powder or B.sub.4 C powder, or a mixture thereof in an amount 0.01 to 1% by weight boron, based on the weight of components (a), (b) and (c) with a powdered binder metal (d) to form a mixture; and

(ii) sintering the mixture under a protective gas atmosphere or a vacuum at a temperature between 1373.degree. and 1873.degree. K.

6. Method as defined in claim 5, wherein component (2) comprises Mo powder.

7. Method as defined in claim 5, wherein component (2) comprises Mo.sub.2 C powder.

8. Method as defined in claim 5, wherein component (4) comprises B powder.

9. Method as defined in claim 5, wherein component (4) comprises BN or B.sub.4 C powder.

10. Method as defined in claim 5, wherein component (4) comprises BN.

11. Method as defined in claim 5, wherein component (c) is present in an amount of 0.01 to 0.5% by weight boron, based on the weight of components (a), (b) and (c).

12. Method for producing a wear-resistant hard metal according to claim 1, comprising the steps of:

(i) mixing the components

(1) at least one powdered carbide of the transition metals Ti, Zr, Hf, V, Nb, Ta and Cr,

(2) molybdenum carbide powder, and

(3) B powder, BN powder or B.sub.4 C powder, or a mixture thereof in an amount 0.01 to 1% by weight boron, based on the weight of components (a), (b) and (c) to form a powdered mixture;

(ii) sintering the powdered mixture at a temperature above 1773.degree. K.;

(iii) grinding the sintered mixture;

(iv) mixing the ground, sintered mixture with a powdered binder metal; and

(v) sintering the mixture of ground sintered mixture with powdered binder metal.

13. Method as defined in claim 12, wherein component (3) comprises B powder.

14. Method as defined in claim 12, wherein component (3) comprises BN.

15. Method as defined in claim 12, wherein component (c) is present in an amount of 0.01 to 0.5% by weight boron, based on the weight of components (a), (b) and (c).