The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for machining of cast iron parts by turning.

Cast iron materials may be divided into two main categories, grey cast iron and nodular cast iron. Cast iron materials typically often have an outer layer of cast skin, which may contain various inclusions of sand, rust and other impurities, but also a surface zone which is decarburized and contains a larger amount of ferrite than the rest of the material .

The wear when machining grey cast iron materials with AI2O3-coated cutting tools is dominated by che i- cal, abrasive and so called adhesive wear. In order to protect the cutting tool against chemical wear it is de¬ sirable to use as thick AI2O3-layers as possible. This is contradicted by the properties regarding adhesive wear that this type of layer generally possesses. Adhe- sive wear occurs when fragments or individual grains of the layer are pulled away from the cutting edge by the work piece chip formed. Especially the surface zone with high amounts of ferrite puts severe demands on the adhe¬ sive properties of the coating, and in combination with the inclusions in the cast skin on the work piece this causes notch wear at the depth of cut at the main cut¬ ting edge.

Another critical factor in the machining of grey cast iron is the sensitivity for excessive amounts of Co binder phase in the interface between the cemented car¬ bide cutting insert and the coating. Excessive amounts of Co binder phase deteriorate the adhesion between coating and cemented carbide and lead to flaking of the coating during machining.

Swedish patent application 9502640-7 discloses a coated cutting insert tool consisting of a cemented car¬ bide body of a composition 5-11 wt-% Co, <10%. prefer¬ ably 1.5-7.5 wt-%, cubic carbides of the metals Ti, Ta and/or Nb and balance WC, especially suited for machin¬ ing of low alloyed steel components by turning.

It has surprisingly been found that by combining the following features: a cemented carbide body with a highly W-alloyed binder phase, a low content of cubic carbides and a well defined surface composition result¬ ing from a specific sintering process, a columnar TiC _x NyO _z -layer, a textured 01-AI2O3-layer, a TiN-layer, fulfilling the demands of easy identification of used edges, and a post-treatment of the coated cutting edge by brushing, an excellent cutting tool for machining of cast iron materials, especially grey cast iron, can be obtained.

Fig 1 is a micrograph in 2000X magnification of a coated insert according to the present invention in which

A - cemented carbide body B - TiC _x NyO _z -layer with equiaxed grains C - TiC _x NyO _z -layer with columnar grains D - TiC _x NyO _z -layer with equiaxed or needle like grains

E - textured α-Al2θ3-layer with columnar like grains F - TiN-layer

According to the present invention a cutting tool insert is provided with a cemented carbide body of a composition 5-10 wt-% Co, preferably 5-8 wt-% Co, <2 wt- %, preferably <0.5 wt-%, most preferably 0 wt-% cubic carbides of the metals Ti, Ta and/or Nb and balance WC. The grain size of the WC is in the range of 1-2.5 μm. The cobalt binder phase is highly alloyed with W. The content of W in the binder phase can be expressed as the

CW-ratio= M _s /(wt-% Co ^• 0.0161), where _s is the measured saturation magnetization of the cemented carbide body in kA/m and wt-% Co is the weight percentage of Co in the ce- mented carbide. The CW-value is a function of the W con¬ tent in the Co binder phase. A low CW-value corresponds to a high W-content in the binder phase.

It has now been found according to the present in¬ vention that improved cutting performance is achieved if the cemented carbide body has a CW-ratio of 0.75-0.93, preferably 0.80-0.90. The cemented carbide body may con¬ tain small amounts, <1 volume-%, of eta phase (MgC) , without any detrimental effect.

The surface composition of the cemented carbide in- sert is well defined and the amount of Co on the surface is within -4 wt% to +4 wt% of the nominal content.

Alternatively the cemented carbide according to the invention consists of WC and Co and has an about 100 - 350 μm, preferably 150-300 μm, wide binder phase depleted surface zone in which the binder phase content increases monotonously and in a non-step-wise manner without maximum up to the nominal content in the inner of the cemented carbide body. The average binder phase content in a 50 μm surface zone is 25-75%, preferably 40-60 %, of the nominal binder phase content.

The coating comprises'

- a first (innermost) layer of TiC _x NyO _z with x+y+z=l, preferably y>x and z<0.1, most preferably y>0.8 and z=0, with a thickness of 0.1-2 μm, and with equiaxed grains with size <0.5 μ . In an alternative embodiment the TiC _x NyO _z layer preferably has the composition z<0.5 and y<0.1, most preferably 0.1<z<0.5 and y=0

- a layer of TiC _x NyO _z x+y+z=l, preferably with z=0, x>0.3 and y>0.3, most preferably x>0.5, with a thickness of 4-12 μm, preferably 5-10 μm, most preferably 6-9 μm

with columnar grains and with a diameter of <5 μm, preferably <2 μm

- a layer of TiC _x NyO _z , x+y+z=l with z<0.5, prefer¬ ably x>y, most preferably x>0.5 and 0.1<z<0.4, with a thickness of 0.1-2 μm and with equiaxed or needle-like grains with size <0.5 μm, this layer being the same as or different from the innermost layer

- a layer of textured, fine-grained (with average grain size 0.5-2 μm) 01-AI2O3 with a thickness of 3-8 μm, preferably 3-6 μ and

- an outer layer of TiC _x N _y O _z . This TiC _x N _y O _z -layer comprises one or more layers with the composition x+y+z=l, z<0.05 preferably y>x. Alternatively, this outer layer can be a multilayer of TiN/TiC/TiN in one or several sequences and a total thickness of 0.5-3 μm, preferably 1-2 μm. This layer exhibits a grain size <1 μm.

In order to obtain a smooth cutting edge line suit¬ able for machining, the edge of the coated insert is subjected to a brushing treatment giving a surface roughness Rmax-≤-^ •^ M- ^{m over a} length of 10 μ according to the method described in Swedish patent application 9402543-4. This treatment removes the top layer of TiC _x NyO _z along the cutting edge line. It is also within the scope of this invention that the surface might be smoothed by a wet blasting treatment.

Furthermore, as disclosed in US 5,654,035 or Swedish patent application 9304283-6 or 9400089-0, the α-Al2θ ₃ - layer has a preferred crystal growth orientation in either the (104)-, (012)- or (110) -direction, preferably in the (012) -direction, as determined by X-ray Diffrac¬ tion (XRD) measurements. A Texture Coefficient, TC, can be defined as:

, , , , , , - 1 TC (hkl ) = >

where

I (hkl) = measured intensity of the (hkl) reflection Io(hkl) = standard intensity of the ASTM standard powder pattern diffraction data n = number of reflections used in the calculation, (hkl) reflections used are: (012), (104), (110), (113) , (024) , (116)

TC for the set of (012) , (104) or (110) crystal pla¬ nes ought to be larger than 1.3, preferably larger than 1.5.

According to method of the invention a WC-Co-based cemented carbide body having a highly W-alloyed binder phase with a CW-ratio according to above is subjected to a sintering process and removing the surface cobalt by etching as disclosed in US 5,380,408. Alternatively for cemented carbide consisting of WC and Co cooling at least to below 1200 °C may be performed in a hydrogen atmosphere of pressure 0.4-0.9 bar as disclosed in Swedish patent application 9602750-3. The insert is coated with

- a first (innermost) layer of TiC _x NyO _z with x+y+z=l, preferably y>x and z<0.1, most preferably y>0.8 and z=0, with a thickness of 0.1-2 μm, and with equiaxed grains with size <0.5 μ . In an alternative embodiment the TiC _x NyO _z layer preferably has the composition z<0.5 and y<0.1, most preferably 0.1<z<0.5 and y=0. - a layer of TiC _x NyO _z x+y+z=l, preferably with z=0, x>0.3 and y>0.3, most preferably x>0.5, with a thickness of 4-12 μm, preferably 5-10 μm, with columnar grains and with a diameter of <5 μm, preferably <2 μ , deposited preferably by MTCVD-technique (using acetonitrile as the carbon and nitrogen source for forming the layer in the

temperature range of 700-900 °C) . The exact conditions, however, depend to a certain extent on the design of the equipment used.

- a layer of TiC _x NyO _z , x+y+z=l with z<0.5, prefer- ably x>y, most preferably x>0.5 and 0.1<z<0.4, with a thickness of 0.1-2 μm and with equiaxed or needle like grains with size <0.5 μm, using known CVD-methods this layer being the same as or different from the innermost layer. - an intermediate layer of a smooth textured OC-AI2O3 according to US 5,654,035 or Swedish patent applications 9304283-6 or 9400089-0 with a thickness of 3-8 μm, pre¬ ferably 3-6 μm.

-an outer layer of TiC _x NyO _z , comprising one or seve- ral individual layers each with composition x+y+z=l and z<0.05 preferably y>x. Alternatively, this outer layer can be a multilayer of TiN/TiC/TiN in one or several se¬ quences . The total coating thickness of these outer la¬ yers is 0.5-3.0 μm, preferably 0.5-2.0 μ . The grain size in this outer layers is <1.0 μm.

The edge line of the inserts is smoothed e g by brushing the edges with brushes based on e g SiC as been disclosed in the Swedish patent application 9402543-4. When a TiC _x NyO _z -layer with z>0 is desired, CC>2 and/or CO are/is added to the reaction gas mixture.

Example 1

A. Cemented carbide cutting tool inserts of style CNMG 120412-KM with the composition 6.0 wt-% Co and ba- lance WC were sintered in a conventional way at 1410°C and cooled down to 1200°C in 0.6 bar H2 giving inserts with a binder phase highly alloyed with W, corresponding to a CW-ratio of 0.85 and a Co-content on the surface corresponding to 7 wt-% as measured with Energy Disper- sive Spectroscopy. After conventional ER-treating the

inserts were coated with a 0.5 μm equiaxed TiC _x NyO _z - layer, x=0.1, y=0.9, z=0 and an average grain size of about 0.2 μm, followed by a 8.0 μm thick TiC _x N _y -layer x=0.55, y=0.45, with columnar grains with an average grain size of 2.5 μ , by using MTCVD-technique (process temperature 850 °C and CH3CN as the carbon/nitrogen source) . In subsequent process steps during the same coating cycle, a 1 μ thick layer of TiC _x NyO _z (about x=0.6, y=0.2 and z=0.2) with equiaxed grains and an ave- rage grain size of 0.2 μ was deposited followed by a 5.0 μm thick layer of (012) -textured CX-AI2O3 , with ave¬ rage grain size of about 1.2 μm, deposited according to conditions given in US 5,654,035. On top of the α-Al2θ3~ layer, TiN/TiC/TiN/TiC/TiN was deposited in a multilayer structure with a total coating thickness of 1.5 μm and average grain size <0.3 μm in each individual layer. Fi¬ nally, the inserts were subjected to a brushing treat¬ ment in which the cutting edge lines were smoothed with a 320 mesh brush containing SiC as grinding material, the outer TiN/TiC-multilayer was removed by the brushing treatment along the cutting edge line.

B. Cemented carbide cutting tool inserts of style CNMG 120412-KM with the composition 6.0 wt-% Co and ba¬ lance WC were coated under the procedure as described in A) . The inserts had a CW-ratio of 0.88 and they were subjected to a conventional sintering without H2 during the cooling step. Prior to the coating process the in¬ serts were subjected to surface cleaning process involv¬ ing an electrolytic etching process according to US 5,380,408. The Co content on the insert surface was about 5 wt-% as measured with Energy Dispersive Spectro¬ scopy. The cemented carbide had an about 250 my wide binder phase depleted surface zone in which the binder phase content increased monotonously and in a non-step- wise manner without maximum up to the nominal content in

the inner of the cemented carbide. The average binder phase content in a 25 mym surface zone was 3 wt-%. The inserts were subjected to the same final brushing treat¬ ment as in A. ) . C. Cemented carbide cutting tool inserts of style CNMG120412-KM from the same batch as in A.) were coated with a 4 μm equiaxed Tic-layer with grain size <2.0 μm followed by a 6 μm thick layer of AI2O3 according to prior art technique. XRD-analysis showed that the AI2O3- layer consisted of a mixture of α and K-AI2O3 , in the ratio of about 60/40. The OC-AI2O3 showed no preferred growth orientation as measured by XRD. The grain size of the K-AI2O3 was 2.0 μ while the OC-AI2O3 exhibited grains up to 5.5 μm. D. Cemented carbide cutting tool inserts from the same batch as in C. The inserts had been subjected to a wet blasting treatment after coating.

E. Cemented carbide cutting tool inserts of style CNMG120412-KM from the same batch as in A. ) were coated with a 2 μ equiaxed layer of TiC _x NyO _z , the average grain size of which was 0.2 μm, followed by a 8 μm thick columnar TiCN deposited according to prior art technique with a grain size of about 3.0 μm and a 6 μm thick layer of (012) -textured CC-AI2O3 deposited according to the same process conditions as in A. ) . The inserts were sub¬ jected to the same brushing treatment as in A. ) .

F. Cemented carbide cutting tool inserts of style CNMG120412-KM with the composition 6 wt-% Co, 4 wt-% cu¬ bic carbides and balance WC were subjected to the same coating process as in A. ) The CW-ratio of the inserts was 0.88 and they were subjected to a sintering process using H2 during the cooling step, the Co content on the surface was 9% as measured by Energy Dispersive Spectro¬ scopy. The inserts were subjected to the same final brushing treatment as in A. ) .

G. Cemented carbide cutting inserts of style CNMG120412-KM with the composition 6.0 weight % Co and balance WC were coated under the procedure described in insert E. The cemented carbide had a CW-ratio of 0.98 and the inserts were subjected to a conventional sinte¬ ring without H2 during the cooling step. The inserts were ER-treated and cleaned in conventional processes. The Co content on the surface prior to coating was 30 weight % as measured with Energy Dispersive Spectros- copy. The inserts were subjected to the same final brushing treatment as in insert A.

The inserts were tested in a facing operation. The work piece material was nodular cast iron, SS717. The workpiece shape causes intermittent cutting conditions during each revolution. Cutting speed was 250 m/min, feed 0.10 mm/rev and cutting depth 2.0 mm. The operation was performed using coolant.

This type of operation typically causes severe flak¬ ing of the coating. The coating is torn off the insert in fragments. The wear can be measured as the part of the edge line on which the coating has flaked off com¬ pared to the total length of the edge line used in the cutting operation.

Insert type % of edge line with flaking

A <5

B <5

C 100

D 70

E 25

F 20

G 50

Example 2.

Inserts of type A, B, D and E in Example 1 above were tested in an intermittent cutting operation in grey cast iron, SS0125. The cutting conditions put high de- mands on the flaking resistance of the coating as well as the chemical and abrasive wear resistance of the coating. The shaping of the work piece is such that for each revolution two entrances in the work piece will be made giving intermittent cutting conditions. The cutting speed was 300 m/min, the cutting feed 0.25 mm/rev and the cutting depth 2.0 mm. The machining was made without using any coolant.

Insert type number of passes before the edge was worn out A 60

B 55

D 48

E 48

Example 3.

The same cutting conditions as in example 2 and in¬ serts of type A, B, C, D, and F from the same batches as in example 1. In this test coolant was used during ma¬ chining. Insert type number of passes state of the edge A 60 minor chipping of edge line, not worn out B 60 undamaged edge line not worn out C 48 fracture in cutting edge, insert worn out D 24 fracture in cutting edge, insert worn out F 24 fracture in cutting edge, insert worn out

Example 4.

Cemented carbide inserts of type A, B and G in exam¬ ple 1 above were tested in a turning test that causes deformation of the cutting edge leading to flaking of the coating and enhanced wear of the insert. The test was performed in a nodular cast iron SS0737 and for a certain combination of feed and cutting depth in a lon¬ gitudinal turning operation. The highest possible cut¬ ting speed before deformation of the cemented carbide occurs was sought.

Insert type Highest possible cutting speed, m/min A 475

B 450 G 400