COATED CUTTING INSERT

Abstract: The present invention provides a coated cutting insert particularly useful for finishing operations in milling of grey cast iron. The invention is characterised by a fine-grained WC-Co cemented carbide body and a coating consisting of multiple layers of TiCxNy deposited at conditions aiming to minimize the decarburisation of the cutting edge without any negative effects on the substrate. The invention also relates to a method of applying a TiCxNy-layer by CVD-technique on a cemented carbide cutting insert without eta-phase formation and without etching of the binder phase.

Description: Coated cutting insert The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for finishing operations in milling of grey cast iron. Grey cast iron is a material which, in general, is reasonably easy to machine with cemented carbide tools and often long tool life can be obtained. However, the machinability of cast iron can vary considerably. The tool life may be influenced significantly by small variations in the chemical composition within the material . These variations may be related to the casting technique used such as the cooling conditions. In the finishing operation the surface finish of the machined work piece is extremely important. It is also as important to avoid frittering of the work piece on the exit side. This puts very high demands on the cutting tool regarding the wear behaviour of the cutting edge, especially close to the corner between the main cutting edge and the parallel land which creates the surface of the work piece. Measures can be taken to improve the cutting performance with respect to a specific property, but often such action may have a negative influence on other cut- ting properties as indicated below: - a very sharp cutting edge, i.e. with an edge radius (ER) of about 15 μ or less, minimises the risk of frittering of the work piece and gives normally a very good surface finish on the component. However, this makes the cutting edge very sensitive to micro-chipping and for coated cutting inserts also sensitive to flaking . - uncoated cutting inserts with high toughness (e.g. a cemented carbide with a high binder phase content) can be used with a very sharp cutting edge but they have a poor wear resistance and a short tool life. - coating of the inserts in order to improve the wear resistance properties are commonly used. But in- serts with very sharp cutting edges are particularly susceptible to decarburisation and eta-phase formation when subjected to a conventional CVD coating process. The eta-phase embrittles the edge line and decreases the flaking resistance and thus also the tool life due to bad surface finish of the work piece. Therefore, conventionally CVD-coated insert generally have an edge radius of about 25 μ as a minimum. - PVD-coating, using known technique, is recognised to make possible the coating of very sharp edges without eta phase formation. However, in milling of grey cast iron severe flaking of this type of coatings often occurs which deteriorates the wear resistance and tool life of the insert. The normally used process parameters in conventional CVD technique e.g. temperature, pressure and gas composition are a compromise in order to obtain the best combination of growth rate, uniformity and cutting properties avoiding e.g. undesirable reactions between gas phase and the cemented carbide body, such as etching of the binder phase or decarburisation of the insert surface especially the edge line. A method to avoid decarburisation using intermediate oxygen-containing layers is described in certain prior art instances e.g. U S pat No 5,135,801. It has surprisingly been found that by combining a straight WC-Co cemented carbide body having a very finegrained WC structure with a coating essentially consisting of one or multiple TiC x Ny-layers an excellent cutting tool for finishing operations in milling of grey cast iron can be obtained. Furthermore, it has been found that by using a carefully controlled CVD-process the inserts can be coated without decarburisation and eta-phase formation and without etching of the binder phase. The edge radius can be down to 20 μm. 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 N y C - grey TiC x N y D - yellow TiC x N y Fig 2 is an X-ray diffraction pattern from a coating according to the invention showing the (422) -peaks from TiN and TiC x N y resp after blasting. Fig 3 is an X-ray diffraction pattern from a coating according to the invention showing the (422) -peaks from TiN and TiC N y resp after blasting and heat treatment. According to the present invention a cutting tool insert is provided consisting essentially of a cemented carbide body and a coating whereby the cemented carbide has the composition 5-15 wt-% Co, preferably 6-11 wt-% Co, <0.5 wt-%, preferably <0.2 wt-%, cubic carbides of the metals Ti , Ta and/or Nb and balance WC . The mean grain size of the WC is <2 μ , preferably 0.5-1.5 μm, most preferably 0.7-1.3 μm. The cobalt binder phase may be alloyed with small amounts of e.g. Cr <0.5 wt-% as a grain growth inhibitor. The coating comprises - a firs (innermost) , CVD-layer of TiC x N y with x+y=l, y>0.7, preferably y>0.9, with a thickness of 0.1-2 μm, preferably 0.5-1.5 μm. - a second CVD-layer of TiC x N y with x+y=l, with x>0.3 and y>0.3, with a thickness of 0.5-3 μm, preferably 1-2.5 μm. - a third, yellow CVD-layer of TiC x N y with x+y=l and x<0.05, with thickness of 0.5-2.5 μ , preferably 0.5-1.5 μm. The coating may contain oxygen as an impurity only. The total thickness of the coating should be between 2 and 6 μm, preferably 3 and 5 μ . The second layer is preferably thicker than each of the other two. The coating may comprise only one or two of the above mentioned CVD-layers. It is also within the scope of the present invention to make the coating a multiple of the above mentioned CVD-layers in any combination. The total coating thickness should be between 2 and 6 μm, preferably 3 and 5 μm. Preferably the coating contains only carbides, nitrides and/oi carbonitrides of Ti . According to method of the invention a WC-Co-based cemented carbide body having a very fine grained WC- structure is coated with using CVD-technique - a first (innermost), layer of TiC x N y with x+y=l, y>0.7, preferably y>0.9, with a thickness of 0.1-2 μ , preferably 0.5-1.5 μm using a carefully controlled CVD- process which enables the inserts to be coated without decarburisation and eta-phase formation except in occasional small spots and without etching of the Co-binder phase. The pressure should during the whole process be controlled to be between 750 mbar up to atmospheric, preferably above 900 mbar and the temperature should be kept between 900 and 970°C. The gases are introduced into the reactor vessel in the order H2 , 2 and TiCl4 followed by HCl. The amount of TiCij should be kept between 1.5 and 5%, preferably between 2.0 and 3.5% and the amount of HCl should be cont olled to be between 1 and 6% preferably, 1.5-4%. The exact conditions, however, depend to a certain extent on the des gn of the equip- ment used. - a second layer of TiC x N y with x+y=l, with x>0.3 and y>0.3, with a thickness of 0.5-3 μm, preferably 1-2.5 μm. The pressure and temperature are kept the same as for the innermost coating layer, but CH4 is added to the mixture to an amount of 8-20%, preferably 10-18%. The process may be performed with HCl concentrations between 0 and 3% and the amount of N2 shall be kept at 1-40%, preferably 5-20%. - a third, yellow layer of TiC x N y with x+y=l and x<0.05, with thickness of 0.5-2.5 μm, preferably 0.5-1.5 μm deposited at process conditions mainly as for the first innermost layer. A smooth coating surface layer with a surface roughness R max <0.4 μm over a length of 10 μm is obtained using a wet or dry blasting treatment of the coated cutting insert surface with fine grained (400-150 mesh) alumina powder as has been disclosed in Swedish patent application 9402534-4. It has been found that this blasting treatment con- siderably changes the X-ray spectrum of the coating e.g. when using Cu Kα-_ 2 as X-ray radiation source. This can be measured as a difference in peak height of the (422)- reflection of the TiN and/or TiCN of a blasted and a blasted and heat treated coated insert. The heat treat- ent may be performed at about 800°C in hydrogen atmosphere for 1 hour. Dividing the peak height values before and after the heat treatment a ratio, from here on referred to as the " (422 ) -ratio" , is obtained, see Figs 2 and 3. ( 422 peak height ) blasted and heat treated ; 422 ) - ratio = ( 422 peak height ) blasted The (422) -ratio of the TiCN-layer with x>0.3 and y>0.3 has been found to be >1.5, preferably >1.75. Furthermore, it has been found that the blasting treatment affects the presence of the so called Kα 2 - reflection of the (422) peak for especially the TiC x N γ - layer with x<0.05. Before the blasting treatment and also after the heat treatment of the blasted insert this reflection appears as a shoulder or a small peak towards higher 2θ-values, whereas in the blasted condition this Kα 2 -reflection cannot be detected due to the broadening of the main Ko^-peak, see Figs 2 and 3. Instead of blasting other mechanical treatment giving the same result may be used. Example 1 A. Cemented carbide milling inserts of a special finishing type N260.8-1204-F with the composition 6.0 wt-% Co and balance WC with a mean WC-grain size of about 1.3 μm, were edge-rounding treated to an ER-size of about 20 μm and coated with a TiN-TiC x N y -TiN- layer (about x=0.45 and y * =0.55) with the individual coating thicknesses 1.4, 2.0, 1.2 μm, respectively. The CVD-process conditions were as follow: Gas mixture : Step 1 Step 2 Step 3 TiCl 2.5% 3.3% 2.5% N 2 45% 8% 45% HCl 3.3% - 3.3% CH 4 - 14% - H bal bal bal Pressure : atmospheric in all steps Temperature : 930°C 930°C 930°C Duration : 1.5 h 2 h 1.5 h The inserts were smoothed by wet blasting after the coating procedure. No eta phase was found in the edge line. One insert was investigated by X-ray diffraction and then heat treated in hydrogen atmosphere at 800 °C for 1 hour and then X-ray investigated again. The (422)- ratio of the TiCN-layer was measured to be 2.0 and no Kθc -reflection could be detected for the TiN-peak in the blasted condition. B. Cemented carbide milling inserts of the same type as in A with the composition 10 wt-% Co, 0.4 wt-% Cr and balance WC with a mean WC-grain size of about 0.9 μ and with an ER-size of about 20 μm were coated under the procedure given in A. The thicknesses of the individual coating layers TiN-TiC x N y -TiN were 1.4, 2.0, 1.2 μ re- spectively. After coating, the inserts were smoothed by wet blasting. No eta phase was found in the edge line. One insert was investigated by X-ray diffraction and then heat treated in hydrogen atmosphere at 800°C for 1 hour and then X-ray investigated again. The (422) -ratio of the TiCN-layer was measured to be 1.8 and no Kα 2 -re- flection could be detected for the TiN- (422 ) -peak in the blasted condition. C. Cemented carbide milling inserts with the composition and coating as in A, but without the blasting treatment of the coating. In the X-ray spectrum of these inserts the Ktt 2 -reflection for the TiN- ( 422 ) -peak could clearly be observed. D. Milling inserts from the same batch as in A were coated with an equiaxed 2.0 μm TiC-coating and a 1.0 μm AI2O3- layer according to prior art CVD-technique. The eta phase zone in the edge line was about 5 μm thick. E. Milling inserts from the same batch as in A were coated with a 3.5 μm PVD-TiCN coating according to prior art technique. No eta phase was found in the edge line. F. Milling inserts from the same batch as in A were coated with an 4 μ PVD-TiN coating according to prior art technique. No eta phase was found in the edge line. G. Cemented carbide milling inserts of the same type as in A with the composition 7.3 wt-% Co, 7.7 wt-% TiC and balance WC with a mean grain size for WC of about 1.5 μm, were ER-treated to an ER-size of about 15 μ and used uncoated. Example 2 Inserts from A-G in example 1 were tested as below: Operation: Face milling with SANDVIK NF (N260.8) Cutter diameter: 200 mm Workpiece : Specially designed component with pre-ma- chined surface. Material: SS0125 (grey cast iron HB * =205) Cutting speed: 140 and 290 m/min, respectively Feed rate: 0.18 mm/tooth Depth of cut: 0.5 mm Insert-type: N260.8-1204-F The operation was run both with and without coolant (wet and dry) . Tool-life criterion was surface finish of the work piece and the tool-life given below is a mean value of two or three tested edges/variant. RESULTS: Cutting speed: 140 m/min Tool- life, number of components Grade variant wet dry A. (ace. to invention) 38 35 B. (ace. to invention) 35 30 C. (ace. to invention) 29 23 D. (prior art, CVD) 15 14 E. (prior art, PVD-TiCN) 8 30 F. (prior art, PVD-TiN) 5 27 G. (prior art, uncoated) 29 23 Insert D, E and F were susceptible to flaking close to the edge line especially on the parallel land. This behaviour was most pronounced for inserts E and F in wet-milling. Insert G got a more rapid flank wear than inserts A and B. RESULTS : Cutting speed: 290 m/min Tool-life, number of components Grade variant wet dry A. (ace. to invention) 23 15 B. (ace. to invention) 20 10 C. (ace. to invention) 15 14 D. (prior art, CVD) 15 E. (prior art, PVD-TiCN) 13 13 F. (prior art, PVD-TiN) 10 7 G. (prior art, uncoated) 13 7 At this higher cutting speed inserts D, E and F got flaking on the edge- line but they also got a more rapid normal flank wear than insert A. Insert G was also less wear resistant than insert A, especially in dry milling. At this high cutting speed insert B was susceptible to plastic deformation during the dry milling operation. It is evident that inserts according to the invention perform well in both wet and dry machining in a wide cutting speed range. Example 3 Inserts from A-G in example 1 were tested as below: Operation: Face milling with SANDVIK NF (N260.8) Cutter diameter: 200 mm Workpiece : Specially designed component with several holes with cast skin at the surface inside the holes but pre-machined upper surface. Material: SS0125 (grey cast iron HB=205) Cutting speed*. 130 m/min Feed rate: 0.20 mm/tooth Depth of cut: 0.5 mm Insert-type: N260.8-1204-F The operation was run with coolant. Tool-life criterion was chipping of the edge line due to inclusions in the cast skin. The tool-life given below is a mean value of three tested edges/variant. RESULTS: Tool-life, Grade variant number of components A. (ace. to invention) 29 B. (ace. to invention) 38 C. (ace. to invention) 22 D. (prior art, CVD) 11 E. (prior art, PVD-TiCN) 19 F. (prior art, PVD-TiN) 21 G. (prior art, uncoated) 20 Insert B was superior due to its outstanding toughness behaviour. From these milling tests it is evident that inserts according to the invention show a broader and higher performance level compared to inserts according to prior art .

Claims: Claims 1. A cutting tool insert for finishing milling of grey cast iron comprising a cemented carbide body and a coating applied by CVD technique c h a r a c t e r i s e d in said cemented carbide body being free of eta-phase particularly in the edgeline and consisting of WC with a mean grain size of <2 μm, 5-12 wt-% Co and < 0.5 wt-% cubic carbides of metals from groups IVb, Vb or VIb of the periodic table of the ele- ments and said coating comprising a 0.5-3 μ layer of TiC x N y with x+y=l and x>0.3 and y>0.3. 2. Cutting tool insert according to claim 1 c h a r a c t e r i s e d in that in the X-ray diffraction pattern from said coating the peak height of the (422) -reflection of the TiC x N y after and before heat treatment at about 800°C in hydrogen for 1 hour has a ' (422) -ratio' of >1.5, where the (422) -ratio is defined as (422 peak height ) after heat treatment (422)-ratio = (422 peak height ) before heat treatment 3. Cutting tool insert according to claim 1 c h a r a c t e r i s e d in that said coating comprises in addition to said TiC x N y - layer - an inner layer of TiC x N y with x+y=l, preferably x<0.3 and y>0.7, most preferably y>0.9, with a thickness of 0.1-2 μm, preferably 0.5-1.5 μm - an outer layer of TiC x N y with x+y=l, preferably x<0.1, with thickness of 0.5-2.5 μm, preferably 0.5-1.5 μm. 4. Cutting tool insert according to claims 1 or 2 c h a r a c t e r i s e d in that said coating is blasted . 5. Cutting insert according to the preceding claim c h a r a c t e r i s e d in that the o^-reflection of the TiN (422) peak cannot be detected in the X-ray spectrum. 6. Cutting insert according to any of the preceding claims c h a r a c t e r i s e d in that the mean grain size of the WC in the cemented carbide insert is 0.5-1.5 μm, preferably 0.7-1.3 μm. 7. Method of depositing a TiC x N y - layer on a cemented carbide insert by CVD-technique without eta-phase formation and without etching of the binder phase c h a r a c t e r i s e d in a pressure during the whole process between 750 mbar and atmospheric, preferably above 900 mbar and a temperature between 900 and 970°C, introducing the gases in the order Hp , Np and optionally CH4 and TiCl4 followed by HCl, keeping the amount of TiCl4 between 1.5 and 5%, preferably between 2.0 and 3.5% and the amount of HCl between 1.0 and 6 %, preferably 1.5 and 4%.

Inventors: AAKESSON LEIF, KUTSCHER AASA, SOEDERBERG STAFFAN, AAKESSON, LEIF, KUTSCHER, AASA, SOEDERBERG, STAFFAN
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 1997
Main classification: C23C16
IPC codes: B23B27, B23C5, C23C16, C23C30