Coated grooving or parting insert

The present application is a reissue of U.S. Pat. No. 6 , 261 , 673 B 1 , which claims the benefit of priority to Swedish Application No. 9802488 - 8 filed Jul. 9 , 1998 .

The present invention relates to a coated cemented carbide body. The present invention further relates to a coated cutting tool insert useful for grooving or severing steel components such as steel or stainless steel tubes and bars.

When steel or stainless steel tubes or bars are severed using an appropriate coated cutting tool or cutting tool insert, the cutting edge is frictionally heated to a high temperature with the consequence that the material being cut becomes welded onto the cutting edge of the cutting tool insert. As a result, the coating can flake and small cemented carbide chips can be torn out of the cutting tool or cutting tool insert substrate body.

On the other hand, for grooving of the same materials, a high coating wear resistance in combination with a stiff substrate material is required. Finding a cutting tool material that effectively fulfils both these requirements is a challenge.

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

Swedish patent application 9602413-8 discloses a coated cutting insert particularly suited for wet turning or grooving stainless steel components. The inserts are characterised by a cemented carbide body consisting of WC—Co and cubic carbides coated by one layer of TiC x N y O z with columnar grains, one layer of smooth, fine grained κ—Al 2 O 3 , and preferably an outer layer of TiN.

Swedish patent application 9504304-8 discloses a coated cutting insert particularly useful for wet and dry milling of low and medium alloyed steels. The insert is characterised by a cemented carbide substrate consisting of Co—WC and cubic carbides, a coating including a layer of TiC x N y O z with columnar grains, a layer of smooth, fine grained κ—Al 2 O 3 and preferably an outer layer of TiN.

According to the present invention, a combination of the cemented carbide substrates, coatings and insert styles has been developed which gives rise to excellent cutting performance in grooving as well as severing of steel or stainless steel.

According to the principles of the present invention, a cutting tool insert is provided which comprises:

a cemented carbide body comprising

6-15 weight % Co, 0.2-1.8 weight % cubic carbides of Ti, Ta and/or Nb, a highly W-alloyed binder phase with a CW-ratio of 0.78-0.93, and the balance WC; and

a coating comprising

a first innermost layer of TiC x N y O z wherein x+y+z=1, the first layer having a thickness of 0.1-1.5 μm and equiaxed grains with size<0.5 μm, a second layer of TiC x N y O z wherein x+y+z=1, the second layer having a thickness of 0.4-3.9 μm, preferably 1.5-3.0 μm, with coluanar grains with an average diameter of 0.1-5.0 μm, a third layer of a smooth fine-grained κ—Al 2 O 3 layer with a thickness of 0.5-5.5 μm, and the total thickness of the first innermost TiC x N y O z and the second TiC x N y O z layer is 0.5-4.0 μm, and the total thickness of all layers is 2.0-6.0 μm, preferably 3.0-5.0 μm.

Further according to the present invention, a method of making a cutting tool insert comprising a WC—Co-based cemented carbide body with a highly W-alloyed binder phase and a CW-ratio of 0.78-0.93, the method comprising coating the body by:

forming a first innermost layer of TiC x N y O z with a CVD-based technique, wherein x+y+z=1, the first layer having a thickness of 0.1-1.5 μm and equiaxed grains with a size <0.5 μm,

forming a second layer of TiC x N y O z by a MTCVD-technique, wherein x+y+z=1, the second layer having a thickness of 0.4-3.9 μm and columnar grains with an average diameter of 0.1-5.0 μm, forming a third layer of a smooth κ—Al 2 O 3 having a thickness of 0.5-5.5 μm, and forming the layers such that the total thickness of the first and second layers is 0.5-4.0 μm, and the total thickness of all layers is 2.0-6.0 μm.

FIG. 1 shows in 1800× magnification of a cross section of an insert according to the present invention.

A cutting tool insert according to the invention useful for severing or grooving of steel and stainless steel includes a cemented carbide substrate with a highly W-alloyed binder phase and with a well balanced chemical composition and grain size of the WC, a columnar TiC x N y O z -layer which layer thickness should be kept as low as possible, a κ—Al 2 O 3 -layer, a TiNayer and optionally followed by smoothening the cutting edges. For example, the edges may be smoothed by brushing, e.g.—by a SiC-based brush. Surprisingly, beneficial properties have been achieved with a relatively thin coating.

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 M s is the measured saturation magnetisation of the cemented carbide substrate in kA/m hAm 2 /kg and wt % Co is the weight percentage of Co in the cemented carbide. The CW-value is a function of the W content in the Co binder phase. A low CW-value corresponds to a high W-content in the binder phase. According to the present invention improved cutting performance is achieved if the cemented carbide substrate has a CW-ratio of 0.78-0.93.

According to the present invention a parting tool insert is provided with a cemented carbide substrate with a composition of 6-15 wt % Co, preferably 9-12 wt % Co, and most preferably 10-11 wt % Co. The composition further includes 0.2-1.8 wt % cubic carbides, preferably 0.4-1.8 wt % cubic carbides, most preferably 0.5-1.7 wt % cubic carbides, of the metals Ta, Nb and Ti and balance WC. The cemented carbide may also contain other carbides from elements from group IVb, Vb or VIb of the periodic table. The content of Ti is preferably on a level corresponding to a technical impurity. The preferred average grain size of the WC depend on the binder phase content. At a preferred composition of 10-11 wt-% Co, the preferred grain size is 1.5-2.0 μm, most preferably about 1.7 μm. The CW-ratio shall be 0.78-0.93, preferably 0.80-0.91, and most preferably 0.82-0.90. The cemented carbide may contain small amounts, <1 volume %, of ρ-phase (M 6 C), without any detrimental effect. From the CW-value it follows that no free graphite is allowed in the cemented carbide substrate according to the present embodiment.

The coating comprises a first innermost layer of TiC x N y O z with x+y+z=1, preferably y>x and z<0.2, most preferably y>0.8 and z=0, with equiaxed grains with a size <0.5 μm, and a total thickness <1.5 μm but >0.1 μm, preferably the thickness is 0.1-0.6 μm;

a second layer of TiC x N y O z with x+y+z=1, preferably with z=0, x>0.3 and y>0.3, most preferably x>0.5, with a thickness of 0.4-3.9 μm, preferably the thickness is 1.5-3.0 μm, the second layer has columnar grains with an average diameter of <5 μm, preferably 0.1-2.0 μm; the total thickness of the first and second layers is 0.5-4.0 μm, preferably 1.5-3.5 μm. Preferably, the first layer is thinner than the second layer; a third layer of a smooth, fine-grained Al 2 O 3 consisting essentially of the κ-phase having a grain size about 0.5-2 μm. However, the layer may contain small amounts, 1-3 vol-%, of the θ or the α-phases as determined by x-ray diffraction (XRD) measurement. The Al 2 O 3 -layer has a thickness of 0.5-5.5 μm, preferably 0.5-3.0 μm; preferably, this Al 2 O 3 -layer is followed by a further layer of TiN having a thickness <1 μm, preferably 0.1-0.5 μm thick. The Al 2 O 3 or the TiN layer can be the outermost layer. This outermost layer, whether Al 2 O 3 or TiN, has a surface roughness R max <0.4 μm over a length of 10 μm. The TiN-layer, if present, is preferably removed along the cutting edge by a suitable removal technique, such as brushing; and the total thickness of the first, second and third layers is 2-6 μm, preferably 3-5 μm.

According to a method of the invention, a WC—Co-based cemented carbide substrate is made with a highly W-alloyed binder phase with a CW-ratio of 0.78-0.93, preferably 0.80-0.91, and most preferably 0.82-0.90. The content of cubic carbides of the metals Ta, Nb and Ti is 0.2-1.8 wt %, preferably 0.4-1.8 wt %, and most preferably 0.5-1.7 wt %. The substrate further includes 6-15 wt % Co, preferably 9-12 wt % Co, and most preferably 10-11 wt % Co. The WC grain size is 1.5-2.0 μm, most preferably about 1.7 μm, when the Co content is in the 10-11 wt. % range. The body is coated by:

a first innermost layer of TiC x N y O z with x+y+z=1, preferably y>x and z<0.2, most preferably y>0.8 and z=0, with equiaxed grains with size <0.5 μm and a total thickness <1.5 μm, preferably >0.1 μm, preferably 0.1-0.6 μm, using known CVD-methods; a second layer of TiC x N y O z with x+y+z=1, preferably with z=0, x>0.3 and y>0.3, most preferably x>0.5, with a thickness of 0.4-3.9 μm, preferably 1.5-3.0 μm, with columnar grains and with an average diameter of about <5 μm, preferably 0.1-2.0 μm, is applied preferably using a MTCVD-technique (e.g.—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; the total thickness of the first and second layers is 0.5-4.0 μm, preferably 1.5-3.5 μm. Preferably, the first layer is thinner than the second layer; a third layer of a smooth, fine-grained Al 2 O 3 consisting essentially of the κ-phase having a grain size about 0.5-2.0 μm. The Al 2 O 3 -layer has a thickness of 0.5-5.5 μm, preferably 0.5-3.0 μm. Preferably, this Al 2 O 3 -layer is followed by a further layer of TiN having a thickness <1 μm, preferably 0.1-0.5 μm thick. The Al 2 O 3 layer can be exposed as the outermost layer. The outermost layer of Al 2 O 3 or TiN has a surface roughness R max <0.4 μm over a length of 10.0 μm. The smooth coating surface can be obtained by gently wet-blasting the coating surface with fine grained (400-150 mesh) alumina powder, or by brushing (preferably used when TiN top layer is present) the edges with brushes based on SiC as disclosed in Swedish patent application 9402543-4. The TiN-layer, if present, is preferably removed along the cutting edge; and the total thickness of the first, second and third layers is 2.0-6.0 μm, preferably 3.0-5.0 μm.

As a way of further illustrating the present invention and the advantages thereof, the following examples are given.