COATED CUTTING TOOL

Abstract

The present invention relates to a coated cutting tool including a Cr-containing cemented carbide substrate having WC, a binder phase and a gamma phase. The cemented carbide includes a gradient surface zone with a thickness of between 2 to 100 μm, which is binder phase enriched and depleted of gamma phase. The cemented carbide includes M.sub.7C.sub.3 carbides in an amount of between 0.5 to 7 area % measured in the bulk, where M is elements being Cr, W and at least one binder metal. The coated cutting inserts shows an improved edge line toughness.

Claims

1. A coated cutting tool comprising a Cr-containing cemented carbide substrate comprising WC, a binder phase and a gamma phase, wherein the cemented carbide includes a gradient surface zone with a thickness of between 2 to 100 μm, which is binder phase enriched and depleted of gamma phase, wherein the cemented carbide comprises M.sub.7C.sub.3 carbides in an amount of between 0.5 to 7 area % measured in a bulk of the substrate, and wherein M is elements comprising Cr, W and at least one binder metal.

2. The coated cutting tool according to claim 1, wherein the cemented carbide substrate comprises M.sub.7C.sub.3 carbides in an amount of 1.5 to 6 area %.

3. The coated cutting tool according to claim 1, wherein the M.sub.7C.sub.3 carbides is present in both the bulk and the gradient surface zone.

4. The coated cutting tool according claim 1, wherein the thickness of the surface zone is between 3 to 70 μm.

5. The coated cutting tool according to claim 1, wherein the binder phase content in the gradient surface zone is at least 1.3 times the binder phase content in the bulk.

6. The coated cutting tool according to claim 1, wherein the gamma phase comprises (W,M)C and/or (W,M)(C,N) wherein M is one or more of Ti, Ta, Nb, Hf, Zr, V and Cr.

7. The coated cutting tool according to claim 1, wherein the amount of gamma phase is between 3 to 25 area %.

8. The coated cutting tool according to claim 1, wherein the binder phase content is between 2 to 20 wt %.

9. The coated cutting tool according to claim 1, wherein the cemented carbide substrate is provided with a CVD or PVD coating.

10. The coated cutting tool according to claim 1, wherein the coated cutting tool is a turning insert.

11. A method of making a coated cutting tool comprising a cemented carbide substrate according to claim 1 comprising the following steps: providing powders forming hard constituents comprising WC and powders forming gamma phase; providing powders forming the binder phase; providing powders comprising Cr; providing a milling liquid; milling, drying, pressing and sintering the powders into a cemented carbide substrate; and providing the cemented carbide substrate with a coating.

12. The method according to claim 11, wherein at least one of the powders forming gamma phase is a nitride or a carbonitride.

13. The method according to any of claim 11, wherein the coating is a CVD coating.

14. The method according to any of claim 11, wherein the coated cutting tool is subjected to a shot peening step.

15. The method according to claim 14, wherein the shot peening step is performed at a temperature of/or above 100° C.

Description

DETAILED DESCRIPTION OF DRAWINGS

[0063] In FIG. 1 a SEM image of a cemented carbide microstructure according to the present invention is shown.

[0064] In FIG. 2, a more detailed part of FIG. 1 is shown where 1 is WC, 2 is binder phase, 3 is M.sub.7C.sub.3 carbide and 4 is gamma phase.

EXAMPLE 1 (INVENTION)

[0065] A coated cutting insert was prepared by first make a cemented carbide substrate from the powders given in Table 1 in weight % and with balance WC having an average particle size (FSSS) of 3±0.5 μm.

TABLE-US-00001 TABLE 1 Co WC TaC (Ta.sub.0.8Nb.sub.0.2)C Ti(C.sub.0.5N.sub.0.5) (Ti.sub.0.5W.sub.0.5)C Cr.sub.3C.sub.2 W 7.82 85.63 0.52 0.71 0.28 1.09 2.71 1.23

[0066] The powders were milled together with a milling liquid (water/ethanol) and an organic binder (PEG) 2 wt % calculated from the total dry powder weight. The formed slurry was then dried, and the dried powder was then subjected to a pressing operation to form a green body.

[0067] The green body was then sintered at a temperature of 1450° C. for 1 h/min in vacuum.

[0068] The sintered body was studied in a light optical microscope (LOM) and the gradient zone was determined to 35 μm.

[0069] When looking at a SEM image of Invention 1 (see FIGS. 1 and 2), M.sub.7C.sub.3 carbide was observed both in the surface zone as well as in the bulk. The amount of M.sub.7C.sub.3 carbides was determined by image analysis using the software Fiji on a high contrast SEM image of the bulk of the cemented carbide. The M.sub.7C.sub.3 carbides were detected as greyscale values from 0 to 50 of the 256 available grey scale values. No visible pores or graphite may be present in the region of interest for accurate measurements to be possible. Noise reduction function “Despeckle” was used before quantification. The amount of M.sub.7C.sub.3 carbide was measured as 2.5 area %.

[0070] The area fraction of gamma phase was also measured, 8 area %.

[0071] The substrate was then coated with a TiCN (9.6 μm)/Al.sub.2O.sub.3 (6 μm) CVD coating.

EXAMPLE 2 (COMPARATIVE)

[0072] A coated cutting insert was prepared by first prepare a cemented carbide substrate from the raw material powders 7.2 wt % Co, 2.87 wt % TaC, 0.46 wt % NbC, 1.87 wt % TiC and 0.40 wt % TiN and balance WC with an average particle size (FSSS) of 5.5-6.3 μm. Although the particle size of this sample is higher than the particle size in Example 1, the grain size of the actual WC grains will be comparable after milling.

[0073] The powders were milled together with a milling liquid (water/ethanol) and an organic binder (PEG) 2 wt % calculated from the total dry powder weight. The formed slurry was then pan dried and the dried powder was then subjected to a pressing operation to form a green body.

[0074] The green body was then sintered at a temperature of 1450° C. for 1 h/min in vacuum.

[0075] The sintered body was studied in a light optical microscope (LOM) and the first gradient zone was determined to 26 μm.

[0076] The area fraction of gamma phase was 11.5 area %.

[0077] No M.sub.7C.sub.3 carbide was detected.

[0078] The substrate was then coated with the same coating process as Invention 1 resulting in a TiCN (8.2 μm)/Al.sub.2O.sub.3 (5.5 μm) coating.

EXAMPLE 3 (WORKING EXAMPLE)

[0079] The inserts according to the Invention (Invention 1) and prior art (Comparative 1) were tested in a longitudinal turning operation in steel (SS1672) under the following working conditions:

[0080] Vc: 300 m/min.

[0081] f: 0.2 mm/rev

[0082] ap: 2.64 mm

[0083] Cutting fluid: Yes

[0084] Number of cuts: 7 (1 cycle)

Three edges for each variant were tested.
The tool life criterion was Fracture damage edge line ≥50%.
The results are shown in Table 2 where the number of cycles is an average of three edges.

TABLE-US-00002 TABLE 2 No of cycles Invention 1 17.8 Comparative 1 11.6

[0085] It is clearly seen in Table 2 that the insert according to the invention has a considerably improved edge line toughness compared to the comparative insert.