Coated Cutting Tool Insert

Abstract

The present invention discloses a coated cutting tool insert particularly useful for dry and wet machining, preferably milling, in low and medium alloyed steels, stainless steels, with or without raw surface zones. The insert is characterized by a WC-TaC-NbC-Co cemented carbide with a W alloyed Co-binder phase and a coating including an innermost layer of TiCxNyOz with columnar grains and a top layer at least on the rake face of a smooth α-Al2O3.

Description

The present invention relates to a coated cemented carbide cutting tool insert particularly useful for wet and dry machining, preferably milling, of low and medium alloyed steels and stainless steels, with raw surfaces such as cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces.

When machining low and medium alloyed steels and stainless steels with cemented carbide tools the cutting edge is subjected to wear according to different mechanisms, such as chemical wear, abrasive wear, adhesive wear and edge chipping caused by cracks formed along the cutting edge, the so called comb cracks. Under severe cutting conditions bulk and edge line breakages commonly occur.

Different work piece materials and cutting conditions require different properties of the cutting insert. For example, when cutting steel components with raw surface zones or cutting under other difficult conditions the coated cemented carbide insert must be based on a tough carbide substrate and have a coating with excellent adhesion. When machining low alloyed steels and stainless steels the adhesive wear is generally the dominating wear type. Here generally thin (1-3 μm) CVD- or PVD-coatings have to be used.

Measures can be taken to improve or optimize cutting performance with respect to a specific wear type. However, very often such measures will have a negative effect on other wear properties.

The influence of some possible measures is given below:

1.) Comb crack formation can be reduced by lowering the binder phase content. However, low binder content will lower the toughness properties of the cutting inserts which is far from desirable.

2.) Improved abrasive wear can be obtained by increasing the coating thickness. However, thick coatings increase the risk for flaking and will also lower the resistance to adhesive wear.

3.) Machining at high cutting speeds and at other conditions leading to high cutting edge temperatures require a cemented carbide with higher amounts of cubic carbides (solid solution of WC-TiC-TaC-NbC), but such carbides will promote comb crack formation.

4.) Improved toughness can be obtained by increasing the cobalt binder content. However, high cobalt content decreases the resistance to plastic deformation.

So far it has been very difficult to improve all tool properties simultaneously. Commercial cemented carbide grades have therefore been optimized with respect to one or a few of the mentioned wear types and hence to specific cutting application area.

U.S. Pat. No. 6,062,776 discloses a coated cutting tool insert particularly useful for milling in low and medium alloyed steel with or without raw surface zones during wet or dry conditions. The insert is characterized by WC-Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase, a coating including an innermost layer of TiC_xN_yO_z with columnar grains and a layer of κ-Al₂O₃ with a top layer of TiN.

U.S. Pat. No. 6,406,224 discloses a coated cutting tool insert also particularly useful for milling of alloyed steel with or without abrasive surface zones at high cutting speeds. The coated cutting tool insert consists of a cemented carbide body with a composition of 7.1-7.9 wt % Co, 0.2-1.8 wt % cubic carbides of the metals Ta, Nb and Ti and balance WC. The insert is coated with an innermost layer of TiC_xN_yO_z with columnar grains and a layer of κ-Al₂O₃ with a top layer of TiN.

EP-A-736615 discloses a coated cutting insert particularly useful for dry milling of grey cast iron. The insert is characterized by having a straight WC-Co cemented carbide substrate and a coating consisting of a layer of TiC_xN_yO_z with columnar grains and a top layer of fine grained textured α-Al₂O₃.

Swedish patent application 0500435-3 discloses a coated cutting tool insert suitable for machining of metals by turning, milling, drilling or by similar chip forming machining methods. The tool insert is particularly useful for interrupted toughness demanding cutting operations.

U.S. Pat. No. 6,200,271 disclose a coated turning insert particularly useful for turning in stainless steel. The insert is characterised by WC-Co-based cemented carbide substrate having a highly W-alloyed Co-binder phase and a coating including an innermost layer of TiC_xN_yO_z with columnar grains and a top layer of TiN and an inner layer of fine grained κ-Al₂O₃.

The inventors have after carrying out a lot of experiments and extensive testing at tool users on the market surprisingly found that by combining a number specific features an improved cutting tool insert, preferably for milling, can be obtained. The combined features are: a specific cemented carbide composition, a certain WC grain size, alloyed binder phase, an inner coating consisting of a number of defined layers and a smooth top rake face layer of α-Al₂O₃.

The insert has improved cutting performance in low and medium alloyed steel, with or without raw surface zones preferably under stable conditions in both wet and dry machining. It is also surprising that the invented cutting tool insert also works well in stainless steel. The cutting tool according to the invention shows improved cutting properties compared to prior art inserts with respect to many of the wear types earlier mentioned. In particular comb crack toughness behaviour has been improved.

FIGS. 1 and 2 illustrates in 6000× a SEM image of an alumina layer in top view before blasting, FIG. 1, and after blasting, FIG. 2.

FIG. 3 shows the wear pattern of an exemplary insert according to the present invention and FIG. 4 that of an insert according to prior art, when subjected to a face milling performance test.

FIG. 5 shows the wear pattern of an exemplary insert according to the present invention and FIG. 6 that of an insert according to prior art, when subjected to a face milling—roughing performance test.

The cutting tool insert according to the invention consists of a cemented carbide body with a W alloyed Co-binder phase, a well balanced chemical composition and a well selected grain size of the WC, a coating consisting of a columnar TiC_xN_yO_z-inner layer followed by a smooth α-Al₂O₃-top layer, a TiN-layer is preferably the top layer at the clearance faces of the insert.

According to the present invention a coated cutting tool insert is provided consisting of a cemented carbide body with a composition of 8.5-9.7 wt % Co, preferably 8.6-9.8 wt % Co, most preferably 8.7-9.3 wt % Co, and 1.00-1.45 wt % TaC, preferably 1.18-1.28 wt % TaC and 0.10-0.50 wt % NbC, preferably 0.25-0.35 wt % NbC and balance WC. The cemented carbide body may also contain smaller amounts of other elements, but then at a level corresponding to a technical impurity. The coercivity is in the range 11.7-13.3 kA/m, preferably within 12.1-12.9 kA/m.

The cobalt binder phase is alloyed with a certain amount of W giving the invented cemented carbide cutting insert its desired properties. W in the binder phase influences the magnetic properties of cobalt and can hence be related to a value CW-ratio, defined as

CW-ratio=magnetic-% Co/wt-% Co

where magnetic-% Co is the weight percentage of magnetic Co and wt-% Co is the weight percentage of Co in the cemented carbide.

The CW-ratio varies between 1 and about 0.75 dependent on the degree of alloying. A lower CW-ratio corresponds to higher W contents and CW-ratio=1 corresponds practically to an absence of W in the binder phase.

It has been found that improved cutting performance is achieved if the cemented carbide has a CW-ratio of 0.85-<1.00, preferably 0.86-0.95, most preferably 0.88-0.93.

The cemented carbide may also contain small amounts, <1 volume %, of η-phase (M₆C), without any detrimental effects. From the specified CW-ratios (<1) it also follows that no free graphite is allowed in the cemented carbide body according to the present invention.

The cemented carbide insert is at least partly coated with a 4.1-6.9 μm thick coating including at least three layers of TiC_xN_yO_z. The three layers form an inner coating with an α-Al₂O₃-layer as the outer layer at least on the rake face. The TiC_xN_yO_z-layers having a total thickness of 1.9-3.6 μm comprise:

A first TiC_xN_yO_z layer adjacent to the cemented carbide having a composition of x+y=1, x>=0, preferably x<0.2 and z=0.

A second TiC_xN_yO_z layer having a composition of x>0.4, y>0.4 and 0=<z<0.1, preferably z=0.

A third TiC_xN_yO_z layer adjacent to the α-Al₂O₃-layer having a composition of x+y+z>=1 and z>0, preferably z>0.2, x+y+z=1 and y<0.2.

The outer α-Al₂O₃-layer has a thickness of 1.8-3.6 μm with flattened grains on the surfaces that have been subjected to the blasting treatment.

In a preferred embodiment the insert has a thin 0.1-1 μm coloured top layer at the flank faces preferably of TiN, TiCN, TiC, ZrN or HfN most preferably deposited by CVD technique.

The present invention also relates to a method of making a coated cutting tool insert by powder metallurgical technique, wet milling of powders forming hard constituents and binder phase, compacting the milled mixture to bodies of desired shape and size and sintering, consisting of a cemented carbide body with a composition of 8.5-9.7 wt % Co, preferably 8.6-9.8 wt % Co, most preferably 8.7-9.3 wt % Co, and 1.00-1.45 wt % TaC, preferably 1.18-1.28 wt % TaC and 0.10-0.50 wt % NbC, preferably 0.25-0.35 wt % NbC and balance WC. The cemented carbide body may also contain smaller amounts of other elements, but then on a level corresponding to a technical impurity. The milling and sintering conditions are chosen to obtain an as sintered structure with the coercivity is in the range 11.7-13.3 kA/m, preferably within 12.1-12.9 kA/m. and a CW-ratio of 0.85-<1.00, preferably 0.86-0.95, most preferably 0.88-0.93.

The cemented carbide insert body is at least partly coated with a 4.1-6.9 μm thick coating including at least three layers of TiC_xN_yO_z forming an inner coating with a blasted α-Al₂O₃-layer as the outer layer at least on the rake face. The TiC_xN_yO_z-layers having a total thickness of 1.9-3.6 μm comprise:

A first TiC_xN_yO_z layer adjacent to the cemented carbide having a composition of x+y=1, x>=0, preferably x<0.2 and z=0 using known CVD method using a reaction mixture consisting of TiCl4, H2 and N2.

A second TiC_xN_yO_z layer having a composition of x>0.4, y>0.4 and 0=<z<0.1, preferably z=0 by using the well-known MTCVD-technique, temperature 885-850° C. and CH₃CN as the carbon/nitrogen source.

A third TiC_xN_yO_z layer adjacent to the α-Al₂O₃-layer having a composition of x+y+z>=1 and z>0, preferably z>0.2, x+y+z=1 and y<0.2 using known CVD method using a reaction mixture consisting of TiCl₄, H₂ and N₂,

An α-Al₂O₃-layer with a thickness of 1.8-3.6 μm with flattened grains on the surfaces that have been subjected to the blasting treatment deposited by using known CVD-technique.

• • a third TiC_xN_yO_z layer adjacent to the α-Al₂O₃-layer having a composition of x+y+z>=1 and z>0, preferably z>0.2, x+y+z=1 and y<0.2 using known CVD method using a reaction mixture consisting of TiCl₄, H₂ and N₂,
  • the α-Al₂O₃-layer with a thickness of 1.8-3.6 μm deposited by using known CVD-technique and
  • subjecting the insert to a blasting treatment at least on the rake face.

In one embodiment an additional 0.1-1 μm coloured layer is deposited on top of the α-Al₂O₃-layer preferably of TiN, TiCN, TiC, ZrN or HfN preferably using CVD technique prior to the blasting treatment.

In another embodiment after the blasting treatment an additional 0.1-1 μm coloured top layer is deposited at least at the flank faces preferably of TiN, TiCN, TiC, ZrN or HfN using CVD- or PVD-technique, preferably using CVD technique.

The present invention also relates to the use of an insert according to above for wet and dry machining, preferably milling, of low and medium alloyed steels and stainless steels, with raw surfaces such as cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces at cutting speeds and feed rates according to following table:

Milling with 90° entering angle:

Cutting speed: 25-400 m/min, preferably 150-300 m/min and feed rate: 0.04-0.4 mm/tooth

Face milling (45-75° entering angle):

Cutting speed: 25-400 m/min, preferably 150-300 m/min and feed rate: 0.05-0.7 mm/tooth

High feed and round insert milling concepts:

Cutting speed: 25-500 m/min and feed rate: 0.30-3.0 mm/tooth, preferably 0.3-1.8 mm/tooth.

EXAMPLE 1

(Invention)

Cemented carbide milling inserts in the following styles R390-11T308M-PM, R390-170408M-PM, R245-12T3M-PM, R300-1648M-PH and R300-1240M-PH having a composition of 9.1 wt-% Co, 1.25 wt-% TaC, 0.28 wt-% NbC and balance WC and with a coercivity of 12.5 kA/m, corresponding to a WC grain size of about 1.7 μm, and a CW-ratio of 0.91 as measured in the FÖRSTER KOERZIMAT CS 1.096 from Foerster Instruments Inc. The inserts were coated as follows:

• • a first layer of 0.5 μm TiC_xN_yO_z with a composition of about x=0.05, y=0.95 and z=0 using known CVD method using a reaction mixture consisting of TiCl₄, H₂ and N₂,
  • a second layer of 1.7 μm columnar TiC_xN_yO_z with a composition of about x=0.55, y=0.45 and z=0 by using the well-known MTCVD-technique, temperature 885-850° C. and CH₃CN as the carbon/nitrogen source and
  • a third, bonding layer of 0.5 μm TiC_xN_yO_z. The grains of this third layer were needle shaped and the composition was about x=0.5, y=0 and z=0.5
  • a fourth layer consisting of 2.5 μm α-Al₂O₃ and finally a top layer of about 0.3 μm TiN was deposited by using known CVD-technique. XRD-measurements confirmed that the Al₂O₃-layer consisted to 100% of the a-phase.

After the coating cycle the top side (rake face) of the inserts was subjected to intense wet blasting with a slurry consisting of Al₂O₃ grits and water. The blasting treatment removed the top TiN-layer on the rake face exposing a smooth α-Al₂O₃ with most grains flattened. FIGS. 1 and 2 illustrates, in a 6000× SEM image in top view, an alumina layer before blasting, FIG. 1, and after blasting with most grains flattened according to the invention, FIG. 2.

EXAMPLE 2

(Prior Art)

Cemented carbide milling inserts in the following styles R390-11T308M-PM, R390-170408M-PM, R245-12T3M-PM, R300-1648M-PH and R300-1240M-PH with a composition of 9.1 wt-% Co, 1.25 wt-% TaC, 0.28 wt-% NbC and balance WC and with a coercivity of 12.3 kA/m, corresponding to a WC grain size of about 1.7 μm, and a CW ratio of 0.92 as measured in the FORSTER KOERZIMAT CS 1.096 from Foerster Instruments Inc. The inserts were coated as follows:

• • a first layer of 0.5 μm equiaxed TiC_xN_y-layer (with a high nitrogen content corresponding to an estimated x=0.95 and y=0.05) followed by a
  • 4 μm thick TiCN-layer, with columnar grains by using MTCVD-technique at a temperature 885-850° C. and with CH₃CN as the carbon/nitrogen source. In subsequent steps during the same coating cycle, a 1.0 μm thick layer of Al₂O₃ was deposited using a temperature 970° C. and a concentration of H₂S dopant of 0.4% as disclosed in EP-A-523 021. A thin, 0.3 μm, layer of TiN was deposited on top according to known CVD-technique. XRD-measurement showed that the Al₂O₃-layer consisted of 100% κ-phase.

EXAMPLE 3

Inserts of the different styles from Examples 1 and 2 were compared in cutting tests.

Operation 1:     Face milling, Coromill 245-80 mm
Work-piece:      Test piece
Material:        SS2541 (P20), 300HB, medium alloyed
                 steel
Cutting speed:   280 m/min
Feed rate/tooth: 0.24 mm/tooth.
Depth of cut:    2 mm
Insert-style:    R245-12T3M-PM
Note:
1 insert, wet, very toughness demanding due to heavy chip load at exit of the milling cutter.

Tool-life criterion was chipping of the edge line and breakages. Improved edge line toughness resulted in less comb cracks, higher security and better surface finish.

Results:         Wear after 8 minutes in cut:
Invention A:     see FIG. 3
Prior art B:     see FIG. 4
Operation 2:     Profile milling, Coromill 200-2 inch
Work-piece:      Trim steel
Material:        SS2260 (A2), 250HB, High alloyed steel
Cutting speed:   446 m/min
Feed rate/tooth: 0.9 mm/tooth
Depth of cut:    0.9 mm
Insert-style:    RCKT 130400-PH
Note:
Five inserts in the cutter, dry.

Tool-life criterion was flank wear and edge-line chipping. A combination of better wear resistance and better edge line toughness gave a considerable increase in tool life.

Results:         Tool-life, parts made
Invention A:     14
Prior art B:     11
Operation 3:     Milling with helical interpolation,
                 Coromill 390-63 mm
Work-piece:      Engine plate
Material:        Low alloy steel
Cutting speed:   290 m/min
Feed rate/tooth: 0.21 mm/tooth
Depth of cut:    3-4.5 mm
Insert-style:    R390-170408M-PM
Note:
Dry, five inserts, toughness demanding operation.

Tool-life criterion was chipping. Due to better edge line security, the wear behaviour was more predictable.

Results:         Tool-life, min:
Invention A:     93
Prior art B:     28
Operation 4:     Face milling-roughing, Coromill 245-
                 100 mm
Work-piece:      Die support
Material:        High alloyed steel, 200HB
Cutting speed:   176 m/min
Feed rate/tooth: 0.33 mm/tooth
Depth of cut:    2-3 mm
Insert-style:    R245-12T3M-PH
Note:
Dry, seven inserts

Tool-life criterion was chipping of the edge line. The improved results lead to better reliability during unmanned machining.

Results:         Wear after 90 minutes in cut:
Invention A:     see FIG. 5
Prior art B:     see FIG. 6
Operation 5:     Face milling, R245-100 mm
Work-piece:      Damper 90 degree
Material:        SS2225-23 Cast low alloy steel 270HB,
                 uneven hardness
Cutting speed:   180 m/min
Feed rate/tooth: 0.2 mm/tooth
Depth of cut:    0-5 mm
Insert-style:    R245-12T3M-PH
Note:
Dry, seven inserts, toughness sand inclusions

Tool-life criterion was chipping. The better edge line toughness increase the tool life.

Results:     Tool-life, min:
Invention A: 39
Prior art B: 31

From the result from the cutting tests it is clear that the inserts from Example 1 outperform the prior art inserts according to Example 2.

Claims

1. A cutting tool milling insert for machining of low and medium alloyed steels, stainless steels, with or without raw surfaces during wet or dry conditions comprising a cemented carbide body and a coating,

wherein said cemented carbide body has a composition of 8.5-9.7 wt % Co, 1.00-1.45 wt % TaC, 0.10-0.50 wt % NbC, and balance WC, a coercivity in a range 11.7-13.3 kA/m, and a CW-ratio of 0.85-<1.00, and

wherein said insert is at least partly coated with a 4.1-6.9 μm thick coating including at least three layers of TiCxNyOz of which one layer is adjacent to the cemented carbide and a blasted α-Al2O3-layer is an outer layer at least on a rake face, the TiCxNyOz layers having a total thickness of 1.9-3.6 μm, the coating comprising:

a first TiCxNyOz layer adjacent to the cemented carbide having a composition of x+y=1, x>=0,

a second TiCxNyOz layer having a composition of x>0.4, y>0.4 and 0=<z<0.1,

a third TiCxNyOz layer adjacent to the α-Al2O3-layer having a composition of x+y+z >=1 and z>0, and

the α-Al2O3-layer having a thickness of 1.8-3.6 μm with flattened grains on surfaces that have been subjected to a blasting treatment.

2. A Cutting insert according to claim 1, wherein the cemented carbide has the composition 8.6-9.5 wt-% Co and 1.00-1.45 TaC and 0.10-0.50 NbC.

3. A cutting tool insert according to claim 1, wherein the coating includes a 0.1-1 μm coloured top layer at the flank faces.

4. A cutting tool insert according to claim 3 wherein the coloured layer consists of TiN, TiCN, TiC, ZrN and/or HfN deposited by CVD- or PVD-technique, preferably CVD-technique.

5. Method of making a cutting insert comprising a cemented carbide body and a coating by powder metallurgical technique, the method comprising:

wet milling of powders forming hard constituents and binder phase;

compacting the milled mixture to bodies of desired shape and sizes and

wherein said cemented carbide body has a composition of 8.5-9.7 wt % Co, 1.00-1.45 wt % TaC, 0.10-0.50 wt % NbC, and balance WC, a coercivity in a range 11.7-13.3 kA/m, and a CW-ratio of 0.85-<1.00, and

wherein said body is at least partly coated with a 4.1-6.9 μm thick coating including at least three layers of TiCxNyOz forming an inner coating with an α-Al2O3-layer as the outer layer at least on a rake face, the TiCxNyOz-layers have a total thickness of 1.9-3.6 μm, the coating comprising:

a first TiCxNyOz layer adjacent to the cemented carbide having a composition of x+y=1, x>=0, using a CVD method using a reaction mixture consisting of TiCl4, H2 and N2,

a second TiCxNyOz layer having a composition of x>0.4, y>0.4 and 0=<z<0.1, by using a MTCVD-technique, temperature 885-850° C. and CH3CN as the carbon/nitrogen source,

a third TiCxNyOz layer adjacent to the α-Al2O3-layer having a composition of x+y+z>=1 and z>0, using a CVD method using a reaction mixture consisting of TiCl4, H2 and N2,

wherein the α-Al2O3-layer with a thickness of 1.8-3.6 μm is deposited by using a CVD-technique, and

wherein the method includes subjecting the insert to a blasting treatment at least on the rake face.

6. Method according to claim 5, wherein the method includes depositing an additional 0.1-1 μm coloured layer on top of the α-Al2O3-layer prior to the blasting treatment.

7. Method according to claim 5, wherein the method includes depositing after the blasting treatment an additional 0.1-1 μm coloured top layer at the flank faces.

8. Method of wet and dry milling of low and medium alloyed steels and stainless steels, with raw surfaces or pre-machined surfaces under stable conditions at cutting speeds and feed rates according to the following:

milling with 90° entering angle at a cutting speed of 25-400 m/min, or

face milling at 45-75° entering angle at cutting speed of 25-400 m/min, or

high feed and round insert milling applications at cutting speed of 25-500 m/min and feed rate: 0.30-3.0 mm/tooth,

wherein an insert for any of the applications of milling, face milling and high feed and round insert milling is the insert according to claim 1.

9. A cutting tool insert according to claim 1, wherein the composition includes 8.6-9.8 wt % Co.

10. A cutting tool insert according to claim 1, wherein the composition includes 1.18-1.28 wt % TaC.

11. A cutting tool insert according to claim 1, wherein the composition includes 0.25-0.35 wt % NbC.

12. A cutting tool insert according to claim 1, wherein the coercivity is in the range 12.1-12.9 kA/m.

13. A cutting tool insert according to claim 1, wherein the CW-ratio is 0.86-0.95.

14. A cutting tool insert according to claim 1, wherein the composition of the first TiCxNyOz layer adjacent to the cemented carbide has x<0.2 and z=0.

15. A cutting tool insert according to claim 1, wherein the composition of the second TiCxNyOz layer has z=0.

16. A cutting tool insert according to claim 1, wherein the composition of the third TiCxNyOz layer adjacent to the α-Al2O3-layer has z>0.2, x+y+z=1 and y<0.2.

17. Method according to claim 5, wherein the composition includes 8.6-9.8 wt % Co.

18. Method according to claim 5, wherein the composition includes 1.18-1.28 wt % TaC.

19. Method according to claim 5, wherein the composition includes 0.25-0.35 wt % NbC.

20. Method according to claim 5, wherein the coercivity is within 12.1-12.9 kA/m.

21. Method according to claim 5, wherein the CW-ratio 0.86-0.95.

22. Method according to claim 5, wherein the composition of the first TiCxNyOz layer adjacent to the cemented carbide has x<0.2 and z=0.

23. Method according to claim 5, wherein the composition of the second TiCxNyOz layer has z=0.

24. Method according to claim 5, wherein the composition of the third TiCxNyOz layer adjacent to the α-A2O3-layer has z>0.2, x+y+z=1 and y<0.2.

25. Method according to claim 6, wherein the coloured layer includes TiN, TiCN, TiC, ZrN or HfN.

26. Method according to claim 7, wherein the coloured top layer includes TiN, TiCN, TiC, ZrN or HfN.

27. Method according to claim 8, wherein the cutting speed for milling is 150-300 m/min.

28 Method according to claim 8, wherein the cutting speed for face milling is 150-300 m/min.

29. Method according to claim 8, wherein the cutting speed for high feed and round insert milling applications is 150-400 m/min and the feed rate for high feed and round insert milling applications is 0.3-1.8 mm/tooth.