COATED TURNING INSERT AND METHOD OF MAKING IT

Abstract: The present invention discloses a coated turning insert particularly useful for turning in stainless steel. The insert is characterised by a WC-Co-based cemented carbide substrate having a highly W-alloyed Co-binder phase and a coating including an inner layer of TiCxNyOz with columnar grains followed by a layer of fine grained kappa -Al2O3 and a top layer of TiN. The layers are deposited by using CVD-methods.

Description: COATED TURNING INSERT AND METHOD OF MAKING IT The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for wet turning of toughness demanding stainless steels com¬ ponents like square bars, flanges and tubes, with raw surfaces such as cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces. When turning stainless steels with cemented carbide tools the cutting edge is worn according to different wear mechanisms, such as adhesive wear, chemical wear, abrasive wear and by edge chipping caused by cracks formed along the cutting edge, the so called comb cracks . Different cutting conditions require different pro¬ perties of the cutting insert. For example, when cutting in steels with raw surface zones a coated cemented car¬ bide insert must consist of a tough carbide and have very good coating adhesion. When turning in stainless steels the adhesive wear is generally the dominating wear type. Measures can be taken to improve the cutting per¬ formance with respect to a specific wear type. However, very often such action will have a negative effect on other wear properties . So far it has been very difficult to improve all tool properties simultaneously. Commercial cemented car¬ bide grades have therefore been optimised with respect to one or few of the wear types and hence to specific application areas. Swedish patent application 9503056-5 discloses a coated cutting insert particularly useful for turning in hot and cold forged low alloyed steel components . The inserts is characterised by a cemented carbide substrate consisting of Co-WC and cubic carbides having a 15-35 μm thick surface zone depleted from cubic carbides, a coat¬ ing including a layer of TiC x NyO z with columnar grains, a layer of smooth, fine grained K-AI2O3, 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 consis¬ ting of Co-WC and cubic carbides, a coating including a layer of TiC x NyO z with columnar grains, a layer of smooth, fine grained K-AI2O3 and preferably an outer layer of TiN. It has now been found that combinations of the sub¬ strates and coatings described in the above patent app- lications give rise to excellent cutting performance in stainless steels turning. A cemented carbide substrate with a cubic carbide depleted surface zone combined with a coating in accordance with patent application, 9503056-5, has been found to be especially suitable for high speed turning in easy stainless steel, such as turning of machineability improved 304L, In more diffi¬ cult work piece materials such as 316-Ti and in opera¬ tions with a high degree of thermal cycling such as turning of square bars a straight WC-Co substrate of the type described in patent application 9504304-8 has been found the most suitable. A turning tool insert according to the invention useful for turning of steel consists of a cemented car¬ bide 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 NyO z -layer, a K-AI2O3-layer, a TiN-layer and optionally followed by smoothening the cutting edges by brushing the edges with e.g. a SiC based brush. 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 car- bide substrate in kA/m and wt% Co is the weight per¬ centage 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 turning tool insert is provided particularly useful for difficult stainless steel turning is provided with a cemented carbide substrate with a composition of 6-15 wt% Co, preferably 9-12 wt% Co, most preferably 10-11 wt% Co, 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 the preferred composition of 10-11 wt-% Co, the preferred grain size is 1.5-2 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 η-phase (M5C) , 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=l, preferably z<0.5, with equiaxed grains with size <0.5 μ and a total thickness <1.5 μm and prefe¬ rably >0.1 μm. - a layer of TiC x N v O z with x+y+z=l, preferably with z=0 and x>0.3 and y>0.3, with a thickness of 1-15 μm, preferably 2-8 μm, with columnar grains and with an ave¬ rage diameter of <5 μm, preferably 0.1-2 μm. Most pre¬ ferred thickness of the TiC x NyO z layer is 2-5 μ , par¬ ticularly in extremely edgeline-toughnesε demanding work-piece materials such as Ti-stabilised stainless steel. - a layer of a smooth, fine-grained (grain size about 0.5-2 μ ) AI2O3 consisting essentially of the K- phase. However, the layer may contain small amounts, 1-3 vol-%, of the θ- or the α-phases as determined by XRD- measurement. The Al2θ3-layer has a thickness of 0.5-6 μ m, preferably 0.5-3 μ , and most preferably 0.5-2 μm. Preferably, this Al2θ3-layer is followed by a further layer (<1 μm, preferably 0.1-0.5 μ thick) of TiN, but the AI2O3 layer can be the outermost layer. This outer- most layer, AI2O3 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. According to the 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, a content of cubic carbides of 0.2-1.8 wt%, preferably 0.4-1.8 wt%, most preferably 0.5-1.7 wt% of the metals Ta, Nb and Ti, with 6-15 wt% Co, preferably 9-12 wt% Co, most preferably 10-11 wt% Co at which Co-content the WC grain size 1.5-2 μm, most preferably about 1.7 μm. The body is coated with: - a first (innermost) layer of TiC x NyO z with x+y+z=l, preferably z<0.5, with a thickness of <1.5 μ , and with equiaxed grains with size <0.5 μm using known CVD-methods . - a layer of TiC x NyO z x+y+z=l, preferably with z=0 and x >0.3 and y>0.3, with a thickness of 1-13 μm, pre- ferably 2-8 μm, with columnar grains and with an average diameter of <5 μm, preferably <2 μm, using preferably 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, de- pend to a certain extent on the design of the equipment used. - a smooth l2θ3-layer essentially consisting of K- AI2O3 is deposited under conditions disclosed in e.g. EP-A-523 021. The AI2O3 layer has a thickness of 0.5-6 μ , preferably 0.5-3 μm, and most preferably 0.5-2 μm. Preferably, a further layer (<1 μ , preferably 0.1-0.5 μ m thick) of TiN is deposited, but the AI2O3 layer can be the outermost layer. This outermost layer, AI2O3 or TiN, has a surface roughness Rχnax <( - ) - 4 111 over a length of 10 μm. The smooth coating surface can be obtained by a gentle wet-blasting the coating surface with fine grained (400-150 mesh) alumina powder or by brushing (preferably used when TiN top coating 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. Example 1 A. A cemented carbide turning tool insert in style CNMG120408-MM with the composition 10.5 wt-% Co, 1.16 wt-% Ta, 0.28 wt-% Nb and balance WC, with a binder phase highly alloyed with W corresponding to a CW-ratio of 0.87, was coated with an innermost 0.5 μm equiaxed TiCN-layer with a high nitrogen content, corresponding to an estimated C/N ratio of 0.05, followed by a 4.3 μm thick layer of columnar TiCN deposited using MT-CVD technique. In subsequent steps during the same coating process a 1.1 μm layer of AI2O3 consisting of pure K- phase according to procedure disclosed in EP-A-523 021. A thin, 0.5 μ , TiN layer was deposited, during the same cycle, on top of the AI2O3-layer. The coated insert was brushed by a SiC containing nylon straw brush after coating, removing the outer TiN layer on the edge. B. A cemented carbide turning tool insert in style CNMG120408-MM with the composition of 7.5 wt-% Co, 1.8 wt-% TiC, 3.0 wt-% TaC, 0.4 wt-% NbC, balance WC and a CW-ratio of 0.88. The cemented carbide had a surface zone, about 25 μm thick, depleted from cubic carbides. The insert was coated with an innermost 0.5 μm equiaxed TiCN-layer with a high nitrogen content, corresponding to an estimated C/N ratio of 0.05, followed by a 7.2 μm thick layer of columnar TiCN deposited using MT-CVD technique. In subsequent steps during the same coating process a 1.2 μ layer of AI2O3 consisting of pure K- phase according to procedure disclosed in EP-A-523 021. A thin, 0.5 μm, TiN layer was deposited, during the same cycle, on top of the AI2O3-layer. The coated insert was brushed by a SiC containing nylon straw brush after coating, removing the outer TiN layer on the edge. C. A competitive cemented carbide turning tool insert in style CNMG120408 from an external leading cemented carbide producer was selected for comparison in a turning test. The carbide had a composition of 9.0 wt- % Co, 0.2 wt-% TiC, 1.7 wt-% TaC, 0.2 wt-% NbC, balance WC and a CW-ratio of 0.90. The insert had a coating consisting of 1.0 μm TiC, 0.8 μm TiN, 1.0 μm TiC and, outermost, 0.8 μ TiN. Examination in light optical microscope revealed no edge treatment subsequent to coating. D. A competitive cemented carbide turning tool in¬ sert in style CNMG120408 from an external leading ce¬ mented carbide producer was selected for comparison in a turning test. The cemented carbide had a composition of 5.9 wt-% Co, 3.1 wt-% TiC, 5.6 wt-% TaC, 0.1 wt-% NbC, balance WC and a CW-ratio of 0.95. The cemented carbide had a surface zone, about 30 μm thick, which was en¬ riched in Co content. The insert had a coating consist¬ ing of 5.3 μm TiC, 3.6 μm TiCN, outermost, 2.0 μ TiN. Examination in light optical microscope revealed no edge treatment subsequent to coating. E. A competitive cemented carbide turning tool in¬ sert in style CNMG120408 from an external leading ce¬ mented carbide producer was selected for comparison in a turning test. The carbide had a composition of 8.9 wt-% Co, balance WC and a CW-ratio of 0.84. The insert had a coating consisting of 1.9 μm TiC, 1.2 μm TiN, 1.5 μ AI2O3 laminated with 3 0.1 μm tick layers of TiN and, outermost, 0.8 μm TiN. Examination in light optical microscope revealed no edge treatment subsequent to coating. F. A competitive cemented carbide turning tool in¬ sert in style CNMG120408 from an external leading ce¬ mented carbide producer was selected for comparison in a turning test. The carbide had a composition of 5.4 wt-% Co, 2.7 wt-% TiC, 3.5 wt-% TaC, 2.3 wt-% NbC, balance WC and a CW-ratio of 0.94. The cemented carbide had a sur¬ face zone, about 40 μm thick, which was enriched in Co content. The insert had a coating consisting of 5.3 μm TiC, 3.6 μm TiCN, outermost, 2.0 μm TiN. Examination in light optical microscope revealed no edge treatment sub¬ sequent to coating. Inserts from A, B, C, D, E and F were compared in facing of a bar, diameter 180, with two, opposite, flat sides (thickness 120 mm) in 4LR60 material. Feed 0.25 mm/rev, speed 180 m/min and depth of cut 2.0 mm. The wear mechanism in this test is chipping of the edge. The inserts with gradient substrates (B, E and F) looked good after three cuts but broke suddenly after about four. Insert Number of cuts A (ace. to invent.) 15 B (outside invention) 5 C (external grade) 9 D (external grade) 9 E (external grade) 4 F (external grade) 4 Example 2 Inserts A, and B from above were selected for a turning test, longitudinal and facing in machineability improved AISI304 stainless steel. Cutting speed was 250 m/min, feed 0.3 mm/rev and depth of cut 2 mm. Cutting time 1 minute/cycle. The wear mechanism was plastic deformation. Insert Number of cycles B (outside invention) 7 A (ace. to invent.) 4 Example 3 G. Inserts in geometry TNMG160408-MM with composi¬ tion and coating according to A above. H. Inserts in geometry TNMG160408-MM with composi¬ tion and coating according to B above. I. Inserts in geometry TNMG160408 with composition and coating according to C above. The inserts G, H and I were tested in longitudinal, dry, turning of a shaft in duplex stainless steel. Feed 0.3 mm/rev, speed 140 m/min and depth of cut 2 mm. Total cutting time per component was 12 minutes. Insert G and I got plastic deformation whereas in¬ sert H got some notch wear. Two edges of insert G were worn out to produce one component whereas one edge of insert H completed one component and four edges were required to finalise one component using insert I. Insert Number of edges/component H (outside invention) 1 G (ace. to invent.) 2 I (external grade) 4 Example 4 Inserts A and E from above were selected for a turn¬ ing test, mainly facing, in a cover rotorcase made in cast AISI316 stainless steel. The cutting was interrupt- ed due to component design. Cutting speed was 180 m/min, feed 0.2 mm/rev and depth of cut 0-2 mm (irregular shape of casting) . Cut¬ ting time 10.5 minutes/component. The wear mechanism was a combination of edge chip- ping and plastic deformation. Insert Number of components A (ace. to invent.) 2 E (external grade) 1 Example 5 Inserts according to A, B, C and D were selected for a turning test. Internal turning of AISI304 stainless steel valve substrate. Cutting speed was 130 m/min and feed 0.4 mm/rev. The stability was poor due to the bor¬ ing bar. The wear was chipping of the edge for inserts D and B whereas inserts A and C got plastic deformation. Insert Number of components A (ace. to invent.) 9 D (external grade) 7 C (external grade) 5 B (outside invention) 2 Example 6 Inserts A and C from above were selected for a turn¬ ing test, roughing of a square bar in AISI316Ti stain¬ less steel. The cutting was interrupted due to component design. Cutting speed was 142 m/min, feed 0.2 mm/rev, depth of cut 4 mm. and cutting time 0.13 minutes/component. The wear was chipping of the edge. Insert Number of components A (ace. to invent.) 25 C (external grade) 15

Claims: Claims 1. A cutting tool insert particularly for turning of steel comprising a cemented carbide body and a coating c h a r a c t e r i s e d in that said cemented carbide body consists of WC, 6-15, preferably 9-12, wt-% Co and 0.2-1.8 wt-% cubic carbides of Ti, Ta and/or Nb and a highly W-alloyed binder phase with a CW-ratio of 0.78- 0.93, preferably 0.80-0.91 and in that said coating com¬ prises - a first (innermost) layer of TiC x NyO z with a thickness of <1.5 μm, and with equiaxed grains with size <0.5 μm - a layer of TiC x NyO z with a thickness of 2-5 μ with columnar grains with an average diameter of <5 μm - an outer layer of a smooth, fine-grained (0.5-2 μm) κ-Al2θ3~layer with a thickness of 0.5-6 μm. 2. 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 outermost layer is a thin 0.1-1 μm TiN-layer. 3. Cutting insert according to claim 2 c h a r a c t e r i s e d in that the outermost TiN- layer has been removed along the cutting edge. 4. Method of making an insert for turning comprising a cemented carbide body and a coating c h a r a c t e r i z e d in that a WC-Co-based cemented carbide body with a highly W-alloyed binder phase with a CW-ratio of 0.78-0.93 is coated with - a first (innermost) layer of TiC x NyO z with x+y+z=l, preferably z<0.5, with a thickness of 0.1-1.5 μ m, with equiaxed grains with size <0.5 μ using known CVD-methods - a layer of TiC x NyO z with x+y+z=l, preferably with z=0 and x>0.3 and y>0.3, with a thickness of 2-8 μm with columnar grains with a diameter of about <5 μm deposited by MTCVD-technique, using acetonitrile as the carbon and nitrogen source for forming the layer in a preferred temperature range of 850-900 °C . - a layer of a smooth K-AI2O3 with a thickness of 0.5-6 μm and - preferably a layer of TiN with a thickness of <1 μm. 5. Method according to the previous claim characteri z ed in that said cemented carbide body has a cobalt content of 9-12 wt% and 0.4-1.8 wt% cubic carbides of Ta and Nb. 6. Method according to claim 4 or 5 c h a r a c t e r i s e d in that said cemented carbide body has a cobalt content of 10-11 wt% . 7. Method according to claim 4 , 5 or 6 c h a r a c t e r i z e d in a CW-ratio of 0.82-0.90. 8. Method according to any of the claims 4, 5, 6 and 7 c h a r a c t e r i z e in that the outermost TiN- layer, if present, is removed along the cutting edge.

Inventors: GUSTAFSON PER, LINDSKOG PER, LJUNGBERG BJOERN, OESTLUND AAKE, GUSTAFSON, PER, LINDSKOG, PER, LJUNGBERG, BJOERN, OESTLUND, AAKE
Applicants: GUSTAFSON PER, LINDSKOG PER, LJUNGBERG BJOERN, OESTLUND AAKE, SANDVIK AB, GUSTAFSON, PER, LINDSKOG, PER, LJUNGBERG, BJOERN, OESTLUND, AAKE, SANDVIK AB, (PUBL)
Application year: 1996
Main classification: C23C16
IPC codes: B23B27, B23P15, C22C29, C23C16, C23C30