Hard-metal

1. A hard-metal comprising at least 13 volume % of a metal carbide selected from the group consisting of TiC, VC, ZrC, NbC, MoC, HfC, TaC, WC or a combination thereof, and a binder phase having a liquidus temperature of 1280 degrees C. or lower, the binder phase comprising one or more of iron-group metals or alloy thereof, 0.1 to 10 weight % Si, 0.1 to 10 weight % Cr, 1 to 20 weight % of dissolved carbon, and 3 to 39 volume % of diamond or cBN grains coated with a protective coating or a mixture thereof.

2. A hard-metal according to claim 1 wherein the protective coating comprises a single layer or multilayers consisting of a metal or metals of the lVa to Vla groups of the periodic table and/or their carbides, carbonitrides or nitrides, the coating having an average thickness of at least 0.2 μm.

3. The hard-metal according to claim 1 wherein the liquidus temperature of the binder phase is less than 1160 degrees C.

4. The hard-metal according to claim 1 wherein the Cr is present in the form of a chromium carbide and/or a solid solution in the binder phase.

5. The hard-metal according to claim 4 wherein the Cr is present in the form of a metal Cr complex carbide (Me,Cr) x C y where Me is Fe, Co and/or Ni, x is 1 to 23 and y is 1 to 6.

6. The hard-metal according to claim 1 wherein the Si is present in the form of a solid solution in the binder phase.

7. The hard-metal according to claim 1 wherein the Si is present in the form of a silicide of Co, Ni and/or Fe.

8. The hard-metal according to claim 1 wherein the binder phase additionally comprises up to 10 weight % of B, Al, S and/or Re.

9. The hard-metal according to claim 1 wherein the diamond and/or cBN grains have average size within the range of 1 to 500 microns.

10. The hard-metal according to claim 1 wherein the binder phase is a low, medium or high-alloyed steel.

11. The hard-metal according to claim 1 having a density equal to or higher than 99.5% of the theoretical density.

12. The hard-metal according to claim 1 which includes a microstructure comprising:

rounded grains of (Cr,Me) x C y of average size within the range of 1 to 30 μm, wherein Me is Fe, Co, and/or Ni, x is 1 to 23 and y is 1 to 6;

a carbide phase of either rounded or facetted grains of average size within the range of 0.2 to 20 μm; and

a metal based phase consisting of a solid solution of C, Cr, Si and components of the carbide phase in Me, where Me is Fe, Co and/or Ni.

13. The hard-metal according to claim 12 wherein the rounded grains of (Cr,Me) x C y have a brown or yellow colour on a metallurgical cross-section after etching in Murakhami reagent at room temperature for 5 minutes or longer.

14. A process for making the hard-metal according to claim 1 including the steps of:

forming a blend of powders comprising at least 13 volume % of a metal carbide, 0.1 to 10 weight % Si, 0.1 to 10 weight % Cr, 1 to 20 weight % dissolved carbon, and one or more iron-group metals or alloy thereof;

admixing diamond or cBN grains coated with a protective coating into the blend of powders to form an admixture;

compacting the admixture to form a green article; and

sintering the green article at less than atmospheric pressure or in an inert atmosphere at an elevated temperature not greater than 1250 degrees C. for 30 seconds to 5 minutes.

15. A process according to claim 14 wherein the sintering time is 30 seconds to 3 minutes.

16. A process according to claim 15 wherein the sintering time is 30 seconds to 2 minutes.

17. A process according to claim 14 wherein the sintering temperature is no greater than 1160 degrees C.