The present invention relates to a coated cemented carbide milling insert for wet or dry machining of cast iron such as nodular cast irons. Such inserts are described in EP-A-1 798 310 and EP-A-1 867 741.

During milling of various materials with coated cemented carbide cutting tools, the cutting edges are regarded as being worn according to different wear mechanisms. Wear types such as chemical wear, abrasive wear and adhesive wear, are rarely encountered in a pure state, and complex wear patterns are often the result. The domination of any of the wear mechanisms is determined by the application, and is dependent on properties of the machined material, applied cutting parameters and the properties of the tool material. The machinability of cast irons can vary considerably between the various groups but also within a certain group. Small variation in the chemical composition or the micro-structure, related to the casting technique, can have significant influence on the tool life.

In general, the different cast irons are very demanding when it comes to wear resistance and therefore CVD-coated inserts have been commonly used. However, in some applications these inserts do not have the combination of edge toughness and wear resistance needed.

EP 1205569 discloses a coated milling insert particularly useful for milling of grey cast iron with or without cast skin under wet conditions at low and moderate cutting speeds and milling of nodular cast iron and compacted graphite iron with or without cast skin under wet conditions at moderate cutting speeds. The insert is characterised by a WC-Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase and a coating including an inner layer of TiC_xN_y with columnar grains followed by a layer of K-Al₂O₃ and a top layer of TiN.

EP 1655391 discloses coated milling inserts particularly useful for milling of grey cast iron with or without cast skin under dry conditions at preferably rather high cutting speeds and milling of nodular cast iron and compacted graphite iron with or without cast skin under dry conditions at rather high cutting speeds. The inserts are characterised by a WC-Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase and a coating including an inner layer of TiC_xN_y with columnar grains followed by a wet blasted layer of α-Al₂O₃.

It is an object of the present invention to provide a coated cutting tool with enhanced performance for wet or dry milling of cast irons.

The cutting tool insert according to the present invention includes a cemented carbide substrate with a relatively low amount of cubic carbides, with a relatively low binder phase content, that is medium to highly alloyed with W and a fine to medium WC grain size. This substrate is provided with a wear resistant coating comprising a (Ti_xAl_1-x)N layer.

Fig 1 shows in 40000x a scanning electron microscopy image of a fracture cross section of a cemented carbide insert according to the present invention in which

1. 1. Cemented carbide body and
2. 2. (Ti_xAl_1-x)N layer.

According to the present invention a coated cutting tool insert is provided consisting of a cemented carbide body and a coating. The cemented carbide body has a composition of 5-7, preferably 5.5-6.5, most preferably 5.8-6.2 wt% Co, 0.05-2.0 wt%, preferably 0.08-1.5 wt%, most preferably 0.1-1.2 wt% total amount of the metals Ti, Nb and Ta and balance WC.

In a preferred embodiment, the content of Ti and Nb is on a level corresponding to a technical impurity.

The coercivity (Hc)of the cemented carbide is 14-19 kA/m, preferably 14.8-18.3 kA/m.

The cobalt binder phase is medium to highly alloyed with tungsten. The content of W in the binder phase may be expressed as the S-value=σ/16.1, where o 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 in a cemented carbide 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 n-phase, S=0.78.

The cemented carbide body has an S-value of 0.81-0.96, preferably 0.84-0.95, most preferably 0.85-0.95.

The coating comprises a layer of (Ti_xAl_1-x)N, where x is between 0.25 and 0.50, preferably between 0.30 and 0.40, most preferably between 0.33 and 0.35. The crystal structure of the (Ti,Al)N-layer is of NaCl type. The total thickness of the layer is between 1.0 and 5.0 µm, preferably between 1.5 and 4.0 µm. The thickness is measured on the middle of the flank face.

In a preferred embodiment, the layer is strongly textured in the (200)-direction, with a texture coefficient TC(200) larger than 1.3, preferably between 1.5 and 2.5.

The texture coefficient (TC) is defined as follows: $$\mathrm{TC}\left(\mathrm{hkl}\right)=\frac{I\left(\mathit{hkl}\right)}{I_0\left(\mathit{hkl}\right)}{\left[\frac{1}{n}{\displaystyle \sum _{n=1}^n}\frac{I\left(\mathit{hkl}\right)}{I_0\left(\mathit{hkl}\right)}\right]}^{-1}$$
where
I(hkl) = intensity of the (hkl) reflection
I_O(hkl) = standard intensity according to JCPDS card no 38-1420
n = number of reflections used in the calculation
(hkl) reflections used are: (111), (200), (220).

The layer has a compressive residual stress with a strain of 2.5*10^-3-5.0*10^-3, preferably 3.0*10^-3-4.0*10^-3.

In an alternative embodiment, a layer of TiN between 0.1 and 0.5 µm thick is deposited on the final (Ti_xAl_1-x)N layer.

The present invention also relates to a method of making a cutting 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, comprising a cemented carbide substrate and a coating. According to the method a substrate is provided consisting of 5-7, preferably 5.5-6.5, most preferably 5.8-6.2 wt% Co, 0.05-2.0 wt%, preferably 0.08-1.5 wt%, most preferably 0.1-1.2 wt% total amount of the metals Ti, Nb and Ta and balance WC.

In a preferred embodiment, the content of Ti and Nb is on a level corresponding to a technical impurity.

The manufacturing conditions are chosen to obtain an as-sintered structure with a coercivity, Hc, within 14-19 kA/m, preferably 14.8-18.3 kA/m and with a S-value within 0.81-0.96, preferably 0.84-0.95, most preferably 0.85-0.95.

Onto this substrate is deposited a coating comprising a (TLAl_1-x)N layer, where x is between 0.25 and 0.50, preferably between 0.30 and 0.40, most preferably between 0.33 and 0.35. The crystal structure of the (Ti,Al)N-layer is of NaCl type. The total thickness of the layer is between 1.0 and 5.0 µm, preferably between 1.5 and 4.0 µm. The thickness is measured on the middle of the flank face.

In a preferred embodiment, the method used to grow the layer is based on arc evaporation of an alloyed, or composite cathode, under the following conditions: The Ti+Al cathode composition is 25 to 50 at.% Ti, preferably 30 to 40 at.% Ti, most preferably 33 to 35 at.% Ti.

Before coating, the surface is cleaned preferably by applying a soft ion etching. The ion etching is performed in an Ar atmosphere or in a mixture of Ar and H₂.

The evaporation current is between 50 A and 200 A depending on cathode size and cathode material. When using cathodes of 63 mm in diameter the evaporation current is preferably between 60 A and 100 A. The substrate bias is between -20 V and -35 V. The deposition temperature is between 400°C and 700°C, preferably between 500°C and 600°C.

The (Ti,Al)N-layer is grown in an Ar+N₂ atmosphere consisting of 0-50 vol.% Ar, preferably 0-20 vol.%, at a total pressure of 1.0 Pa to 7.0 Pa, preferably 3.0 Pa to 5.5 Pa.

On top of the (Ti,Al)N-layer a TiN-layer of between 0.1 and 0.5 µm thickness may be deposited using Arc evaporation as known.

In a further preferred embodiment, the cutting tool insert as described above is treated after coating with a wet blasting or brushing operation, such that the surface quality of the coated tool is improved.

The present invention also relates to the use of a cutting tool insert according to above in milling of nodular cast iron, in both wet and dry conditions with a cutting speed of 75-300 m/min and feed per tooth of 0.05-0.4 mm.

Grade A: A cemented carbide substrate with the composition 6 wt% Co, 0.2 Ta and balance WC, a binder phase alloyed with W corresponding to an S-value of 0.92 was produced by conventional milling of powders, pressing of green compacts and subsequent sintering at 1430°C. The Hc value for the cemented carbide was 16.5 kA/m, corresponding to a mean intercept length of about 0.65 µm. The substrate was coated in accordance with the invention with a (Ti,Al)N-layer, deposited by using cathodic arc evaporation. The layer was deposited using a Ti+Al cathode composition of 33 at.% Ti and the (Ti,Al)N layer was grown in an Ar+N₂ atmosphere. The thickness of the coating was 2.8 µm, when measured on the middle of the flank face. X-ray diffraction showed that the (Ti,Al)N layer had a TC(20C) of 1.8. Fig 1 shows in 40000x a scanning electron microscopy image of a fracture cross section of the coated cemented carbide.

Grade B: A substrate with composition 6 wt% Co, 0.2 Ta and balance WC, a binder phase alloyed with W corresponding to an S-value of 0.92, and a Hc value of 16.4 kA/m was coated with a 0.3 µm thick layer of TiN layer, a 4.2 µm thick layer of columnar MTCVD TiC_xN_y, and a 3.5 µm thick layer of α-Al₂O₃ deposited at about 1000°C.

Inserts of grade A and B were tested in a square shoulder milling operation in a nodular cast iron. Operation Square shoulder milling Cutter diameter 45 mm Work piece Bridge Material GGG 60 Insert type XOMX180608TR-MD15 Cutting speed 181 m/min Feed 0.25 mm/tooth Depth of cut 14 mm Width of cut 12 mm Coolant No Results Tool life (pieces) Grade A 1000 Grade B 700

The tool life of Grade A was limited by flank wear. The tool life of Grade B was limited by the combination of flank wear, chipping and thermal cracking.

Grade C: A substrate with composition 7.6 wt% Co, 0.9 Ta, 0.3 Nb and balance WC, a binder phase alloyed with W corresponding to an S-value of 0.90, and a Hc value of 14 kA/m was coated with a 0.1 µm thick layer of TiN, a 2.8 µm thick layer of columnar MTCVD TiC_xN_y, a 2.1 µm thick layer of α-Al₂O₃ and a 0.5 µm thick layer of TiN, deposited at about 1000°C.

Grade D: A substrate with composition 8.1 wt% Co, 1.1 Ta, 0.3 Nb and balance WC, a binder phase alloyed with W corresponding to an S-value of 0.89, and a Hc value of 15 kA/m was combined with a coating according to Grade A.

Inserts of Grade A, B, C, and D were tested in a shoulder milling operation in a compacted graphite iron material. Operation Rough shoulder milling Cutter diameter 63 mm Component Pump housing Material CGI Insert type XOMX180608TR-M14 Cutting speed 190 m/min Feed 0.22 mm/tooth Depth of cut 9.5 mm Width of cut 51 mm Coolant No Results Tool life (pieces) Grade A 116 Grade B 70 Grade C 24 Grade D 65

The tool life of Grades A and D was limited by flank wear. The tool life of Grades B and C was limited by the combination of flank wear, chipping and thermal cracking.

Inserts of Grade A and B were tested in a face milling operation performed with a disc mill in nodular cast iron. Operation Face milling Cutter diameter 180 mm Material FGS 400.12 Insert type 335.18-1005T Cutting speed 100 m/min Feed 0.10 mm/tooth Depth of cut 2 mm Width of cut 22 mm Coolant Yes Results Tool life (pieces) Grade A 5480 Grade B 4500

The tool life of Grade A was limited by flank wear. The tool life of Grade B was limited by the combination of flank wear and delamination of the coating.