Patent Grant
7135221
This application is based on and claims priority under 35 U.S.C. §119 to Swedish Application No. 0302842-0, filed Oct. 27, 2003, the entire contents of which are incorporated herein by reference.
The present disclosure relates to a coated cemented carbide cutting tool insert particularly useful for toughness demanding machining, such as medium and rough turning of steels and also for turning of stainless steels. The disclosure preferably relates to coated inserts in which the substrate has been provided with a tough surface region in such a way that wear resistance and edge strength are obtained in the same grade.
In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
Today, coated cemented carbide inserts with binder phase enriched surface zone are commonly used for turning of steel and stainless steel materials. The binder phase enriched surface zone widens the application towards tougher cutting operations.
It has been known for some time how to produce binder phase enriched surface zones on cemented carbides containing WC, binder phase and cubic carbide phase, e.g., Tobioka et al. (U.S. Pat. No. 4,277,283), Nemeth et al. (U.S. Pat. No. 4,610,931) and Yohe (U.S. Pat. No. 4,548,786).
EP-A-1 026 271 relates to a coated cemented carbide with binder phase highly alloyed with W. The insert has a binder phase enriched surface zone of a thickness <20 μm and, along a line in the direction from the edge to the centre of the insert, the binder phase content increases essentially monotonously until it reaches the bulk composition. The insert is coated with 3–12 μm columnar Ti(C,N)-layer followed by a 2–12 μm thick Al2O3-layer.
EP-A-1 348 779 relates to a coated cemented carbide insert with a binder phase enriched surface zone with a thickness of >20 μm and a Co-content of 4–7 wt-%. The insert is coated with a 3–15 μm thick Ti(C,N) layer followed by 3–15 μm α-Al2O3 and an uppermost 1–10 μm thick layer of a carbide, carbonitride or carboxynitride.
Swedish Patent Application 0201417-3 discloses a method to produce α-Al2O3 coatings with high wear resistance and toughness. The α-Al2O3 coating is formed on a bonding layer of (Ti,Al)(C,O,N) with increasing aluminum content towards the outer surface. The α-Al2O3 coating has a thickness ranging from 1 to 20 μm and is composed of columnar grains. The length/width ratio of the alumina grains is from 2 to 12. The coating is characterized by a strong (012) growth texture, measured using XRD, and by the almost total absence of (104), (110), (113) and (116) diffraction reflections.
The coatings for cutting tools designed for toughness demanding applications are usually composed of layers of Ti(C,N) and kappa alumina. It has been thought that the kappa phase shows better toughness properties than the alpha phase. Consequently, alpha alumina has so far only been applied for applications where wear resistance is the main concern, i.e., for grades with relatively low Co-contents.
It has surprisingly been found that a relatively thick coating including alpha alumina (α-Al2O3) with strong texture in combination with a substrate of relatively high cobalt content shows enhanced edge strength and toughness in medium and rough turning of steels and turning of stainless steels.
An exemplary embodiment of a cutting tool insert comprises a cemented carbide substrate and a coating. The cemented carbide substrate comprises WC, 7 to 12 wt-% Co and 5 to 11 wt-% of cubic carbides of metals from groups IVb, Vb and VIb of the periodic table, a Co-binder that is highly alloyed with W and has an S-value of 0.79 to 0.89 and a tungsten carbide phase having a mean intercept length of 0.7 to 1.4 μm. The coating comprises at least one 2 to 12 μm alumina layer including columnar α-Al2O3 grains and a texture coefficient TC(hkl) of (hkl) reflections (012), (104), (110), (113), (024), (116), wherein TC(012)>2.2 and TC(024)>0.6×TC(012), and wherein texture coefficient TC(hkl) is defined as
where I(hkl)=measured intensity of (hkl) reflection, I0(hkl)=standard intensity according to JCPDS card no 46-1212, and n=number of reflections used in calculation.
Another exemplary embodiment of a cutting tool insert comprises a cemented carbide substrate and a coating. The cemented carbide substrate consists essentially of WC, 7 to 12 wt-% Co and 5 to 11 wt-% of cubic carbides of metals from groups IVb, Vb and VIb of the periodic table, a Co-binder that is highly alloyed with W and has an S-value of 0.79 to 0.89, a tungsten carbide phase having a mean intercept length of 0.7 to 1.4 μm. The coating consists essentially of at least one 2 to 12 μm alumina layer including columnar α-Al2O3 grains and a texture coefficient TC(hkl) of (hkl) reflections (012), (104), (110), (113), (024), (116), wherein TC(012)>2.2 and TC(024)>0.6×TC(012) and wherein texture coefficient TC(hkl) is defined as
where I(hkl)=measured intensity of (hkl) reflection, I0(hkl)=standard intensity according to JCPDS card no 46-1212, and n=number of reflections used in calculation.
A further exemplary embodiment of a cutting tool insert comprises a cemented carbide substrate and a coating. The cemented carbide substrate comprises WC, 7 to 12 wt-% Co and 5 to 11 wt-% of cubic carbides of metals from groups IVb, Vb and VIb of the periodic table, a Co-binder that is highly alloyed with W and has an S-value of 0.79 to 0.89, a tungsten carbide phase having a mean intercept length of 0.7 to 1.4 μm and wherein the cemented carbide substrate includes a binder phase enriched and an essentially cubic carbide free surface zone of a thickness of 10 to 40 μm. The coating comprises at least one 2 to 12 μm alumina layer including columnar α-Al2O3 grains having a length to width ratio from 2 to 10 and a texture coefficient TC(hkl) of (hkl) reflections (012), (104), (110), (113), (024), (116), and wherein TC(012)>2.2 and TC(024)>0.6×TC(012), and wherein texture coefficient TC(hkl) is defined as
where I(hkl)=measured intensity of (hkl) reflection, I0(hkl)=standard intensity according to JCPDS card no 46-1212, and n=number of reflections used in calculation. The coating further comprises a first layer adjacent the cemented carbide substrate including CVD Ti(C,N), CVD TiN, CVD TiC, MTCVD Ti(C,N), MTCVD Zr(C,N), MTCVD Ti(B,C,N), CVD HfN or combinations thereof, the first layer having a thickness of from 1 to 10 μm.
The following detailed description of preferred embodiments can be read in connection with the accompanying drawings in which like numerals designate like elements and in which:
A coated cutting tool insert is provided consisting of a cemented carbide body with a composition of 7 to 12 wt-%, preferably 8 to 11 wt-%, most preferably 8.5 to 9.5 wt-% Co, 5 to 11 wt-%, preferably 6.5 to 9.5 wt-%, cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table, preferably Ti, Nb and Ta, and balance WC. The ratio between the weight concentrations of Ta and Nb, e.g., Ta:Nb, is within 1.0 to 3.0, preferably 1.5 to 2.5. The ratio between the weight concentrations of Ti and Nb, e.g., Ti:Nb, is within 0.5 to 1.5, preferably 0.8 to 1.2.
The cobalt binder phase is highly alloyed with tungsten. The concentration of W in the binder phase may be expressed as the S-value=σ/16.1, where σ is the measured magnetic moment of the binder phase in μTm3kg−1. The S-value depends on the content of tungsten in the binder phase and increases with a decreasing tungsten content. Thus, for pure cobalt, or a binder that is saturated with carbon, S=1 and for a binder phase that contains W in an amount that corresponds to the borderline to formation of η-phase, S=0.78.
It has now been found that improved cutting performance is achieved if the cemented carbide body has an S-value within the range 0.79 to 0.89, preferably 0.81 to 0.85.
Furthermore, the mean intercept length of the tungsten carbide phase measured on a ground and polished representative cross section is in the range 0.7 to 1.4 μm, preferably 0.9 to 1.3 μm, most preferably 1.1 to 1.3 μm. The intercept length is measured by means of image analysis on images with a magnification of 10000× and calculated as the average mean value of approximately 1000 intercept lengths.
In one embodiment, the cemented carbide is made with all three phases homogeneously distributed in the material.
In a preferred embodiment, the cemented carbide is provided with a 10 to 40 μm thick, preferably 20 to 40 μm thick, most preferably 20 to 30 μm thick, essentially cubic carbide phase free and binder phase enriched surface zone with an average binder phase content in the range 1.2 to 2.5 times the nominal binder phase content.
A method of making cutting tool inserts as described herein is also provided. According to a preferred embodiment, a powder mixture is formed containing 7 to 12 wt-%, preferably 8 to 11 wt-%, of binder phase consisting of Co, and 5 to 11 wt-%, preferably 6.5 to 9.5 wt-%, cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table, preferably Ti, Nb and Ta, and balance WC. The ratio between the weight concentrations of Ta and Nb is within 1.0 to 3.0, preferably 1.5 to 2.5. The ratio between the weight concentrations of Ti and Nb is within 0.5 to 1.5, preferably 0.8 to 1.2. Well-controlled amounts of nitrogen are added through the powder, e.g., as nitrides. The optimum amount of nitrogen to be added depends on the composition of the cemented carbide and in particular on the amount of cubic phases and is higher than 1.7%, preferably 1.8 to 5.0%, most preferably 3.0 to 4.0 wt-%, of the weight of the elements from groups IVb and Vb of the periodic table. The exact conditions depend to a certain extent on the design of the sintering equipment being used. It is within the purview of the skilled artisan to determine and to modify the nitrogen addition and the sintering process in accordance with the present specification in order to obtain the desired result.
The raw materials are mixed with a pressing agent and optionally W such that the desired S-value is obtained and the mixture is milled and spray dried to obtain a powder material with the desired properties. Next, the powder material is compacted and sintered. Sintering is performed at a temperature of 1300 to 1500° C., in a controlled atmosphere of about 50 mbar followed by cooling. After conventional post sintering treatments including edge rounding, a hard, wear resistant coating according to the below is applied by CVD- or MT-CVD-technique.
The coating comprises a first layer adjacent the body of CVD Ti(C,N), CVD TiN, CVD TiC, MTCVD Ti(C,N), MTCVD Zr(C,N), MTCVD Ti(B,C,N), CVD HfN or combinations thereof preferably of MTCVD Ti(C,N) having a thickness of from 1 to 10 μm, preferably from 3 to 8 μm, most preferably about 6 μm and α-Al2O3 layer adjacent said first layer having a thickness of from about 2 to 12 μm, preferably from 3 to 10 μm, most preferably about 5 μm. Preferably there is an intermediate layer of TiN between the substrate as well as in the said first layer both with a thickness of <3 μm, preferably about 0.5 μm.
In one embodiment the α-Al2O3 layer is the uppermost layer. In a preferred embodiment, there is a layer of carbide, nitride, carbonitride or carboxynitride of one or more of Ti, Zr and Hf, having a thickness of from about 0.5 to 3 μm, preferably 0.5 to 1.5 μm atop the α-Al2O3 layer.
The total coating thickness is preferably 7 to 15 μm, more preferably 8 to 13 μm.
The α-Al2O3 layer is composed of columnar grains with a strong (012) texture. The columnar grains have a length/width ratio of from 2 to 10, preferably 4 to 8, with a width of 0.5 to 3.0 μm, preferably 0.5 to 2.0, most preferably 0.5 to 1.5.
The texture coefficients (TC) for the α-Al2O3 layer is determined as follows:
where
The texture of the alumina layer is defined as follows:
In an exemplary embodiment, α-Al2O3 is deposited on a Ti(C,N) coating, which is obtained preferably by MTCVD. Several steps can optionally be used to control nucleation as described in Swedish Patent Application 0201417-3. In an exemplary embodiment, a modified bonding layer of α-Al2O3 is deposited on the Ti(C,N) layer characterized by the presence of an Al concentration gradient. The controlled oxidation treatment is performed in this case using a CO2/H2/N2 gas mixture, resulting in a lower O-potential than in SE 0201417-3, enhancing the (012) texture further. The oxidation step is short and may be followed by a short treatment with a AlCl3/H2 mixture, again followed by a short oxidation step. This kind of pulsating (Al-treatments/oxidation) treatments create favorable nucleation sites for α-Al2O3 and a strong (012) texture. The growth of the alumina layer onto the surface modified bonding layer is started by sequencing the reactant gases in the following order: CO, AlCl3, CO2. The temperature is preferably about 1000° C. For contrast, a conventional bonding layer is described in U.S. Pat. No. 5,137,774 (referred to as kappa-bonding), the entire contents of which are incorporated herein by reference.
A cemented carbide substrate in accordance with the present disclosure has a composition 9.0 wt-% Co, 3.6 wt-% TaC, 2.2 wt-% NbC, 2.9 wt-% (Ti,W)C 50/50 (H. C. Starck), 1.1 wt-% TiN and balance WC, with a binder phase alloyed with W corresponding to an S-value of 0.83. Inserts were produced by conventional milling of the raw material powders, pressing of green compacts and subsequent sintering at 1430° C. Investigation of the microstructure after sintering showed that the cemented carbide inserts had a cubic carbide free zone with a thickness of 22 μm. The cobalt concentration in the zone was 1.4 times that in the bulk of the substrate. The mean intercept length of the tungsten carbide phase was 1.2 μm.
Cemented carbide cutting inserts from Example 1 were coated with a layer of MTCVD Ti(C,N) according to step 1 (below). The thickness of the MTCVD layer was about 6 μm.
Step 1: MTCVD Coating
Onto this layer of MCTVD Ti(C,N), the following alumina layers, e.g., the layers disclosed at (a), (b) or (c) below, were deposited:
(a) 5 μm α-Al2O3 was deposited according to steps 2–6: (Invention).
Step 2: Bonding Layer
Step 3: Aluminizing Step
Step 4: Oxidizing Step
Step 5: Nucleation Step
Step 6: Deposition
(b) 5 μm κ-Al2O3 was deposited according to the prior art, where the κ-Al2O3 layer was deposited without oxidation resulting in epitaxial growth of κ-Al2O3 on the Ti(C,N) of step 1.
(c) 5 μm α-Al2O3 deposited according to prior art. The nucleation control did not in this case produce 100% pure α-Al2O3 but instead a mixture of κ-Al2O3 and α-Al2O3 was produced. As a result, the κ-Al2O3 phase transformed during the deposition process to α-Al2O3 with a high dislocation density.
The total coating thickness of the experimental coatings was 11 μm in all cases.
The inserts from Example 2a and 2c (alpha oxides) were studied by using XRD. Coating 2c exhibited a random texture but the coating 2a according to this invention showed a clear (012) texture. Table 1 shows the obtained texture coefficients for coating 2a.
Inserts according to coating 2a from Example 2 were tested against inserts according to prior art (coating 2c from Example 2) under the following conditions.
The cutting edges are shown in
This demonstrates that the α-Al2O3 with a texture according to the invention has a much tougher and wear resistant behavior than the α-Al2O3 produced according to prior art.
Inserts from Example 2 with coatings 2a and 2b were compared in metal cutting. Coating 2a is composed of defect free α-Al2O3 according to the invention and coating 2b is composed of κ-Al2O3 according to prior art. The test conditions were the following:
The results are presented in
The results in
The coatings 2a and 2b from Example 2 were tested under the following conditions.
The prior art inserts coated with κ-Al2O3 had severe plastic deformation after 3.6 min cutting while those produced according to the invention exhibited very little plastic deformation. The superiority of α-Al2O3 to prevent plastic deformation is clear.
The following three variants were tested by interrupted turning of stainless steel.
After 7.6 minutes, the flank wear of the three variants was measured:
The results show that the cemented carbide tool with a layer of α-Al2O3 with a texture as disclosed herein exhibits enhanced tool life as compared with competitor products.
The same variants that were tested in Examples 5 and 6 were also tested in continuous turning in ordinary carbon steel. The cutting data were:
The test results show that the cemented carbide disclosed herein, i.e., with a coating layer of α-Al2O3 with a texture as disclosed herein, exhibits longer tool life than prior art material with κ-Al2O3 in continuous cutting. Thus, the examples above show that the cemented carbide according to the invention is superior to prior art materials both regarding wear resistance and toughness.
Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0302842 | Oct 2003 | SE | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4277283 | Tobioka et al. | Jul 1981 | A |
| 4548786 | Yohe | Oct 1985 | A |
| 4610931 | Nemeth et al. | Sep 1986 | A |
| 5137774 | Ruppi | Aug 1992 | A |
| 5487625 | Ljungberg et al. | Jan 1996 | A |
| 5654035 | Ljungberg et al. | Aug 1997 | A |
| 6333100 | Palmqvist et al. | Dec 2001 | B1 |
| 20040028951 | Ruppi | Feb 2004 | A1 |
| Number | Date | Country |
|---|---|---|
| 0 630 744 | Dec 1994 | EP |
| 1 026 271 | Aug 2000 | EP |
| 1 026 271 | Aug 2000 | EP |
| 1 247 789 | Oct 2002 | EP |
| 1 288 335 | Mar 2003 | EP |
| 1 348 779 | Oct 2003 | EP |
| Number | Date | Country | |
|---|---|---|---|
| 20050129987 A1 | Jun 2005 | US |
