COATED TURNING INSERT

Abstract: The present invention discloses a coated turning insert particularly useful for turning of forged components in low alloyed steel. The insert is characterised by a WC-Co cemented carbide body having a highly W-alloyed Co-binder phase and a coating including an innermost layer of TiCxNyOz with columnar grains and a top layer of fine grained kappa -Al2O3.

Description: Coated turning insert The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for difficult cutting conditions such as turning in hot and cold forged low alloyed steel components like gear rings and axles used in the automotive industry and turning in stainless steel components like bars, tubes and flanges. Stainless and low alloyed steels are materials which, in general, are difficult to machine with coated or uncoated cemented carbide tools . Smearing of work piece material onto the cutting edge and flaking of the coating often occur. The cutting conditions are particu¬ larly difficult during the turning of forged low alloyed components under wet conditions (using coolant) . The hot forged skin (0.05-0.2 mm) is generally decarburized and thus softer than the bulk material due to a mainly ferritic structure. The cold forged skin (less than 0.05 mm) is cold-worked and, thus, harder due to a deforma- tion hardening effect. Furthermore, the ferrite/pearlite bulk structure of such a material is often "ferrite- striated" , i. e. the ferrite and pearlite are forming parallel stripes. This mixture of hard and soft materi¬ als makes the cutting conditions very difficult. Further, when turning stainless and low alloyed steels by coated cemented carbide tools the cutting edge is worn by chemical wear, abrasive wear and by a so called adhesive wear. The adhesive wear is often the tool life limiting wear. Adhesive wear occurs when fragments or individual grains of the layers and later also parts of the cemented carbide are successively pulled away from the cutting edge as work piece chips are formed. Further, when wet turning is employed the wear may also be accelerated by an additional wear mechanism. Coolant and work piece material may penetrate into the cooling cracks of the coatings. This pene¬ tration often leads to a chemical reaction between work piece material and coolant with the cemented carbide. The Co-binder phase may oxidise in a zone near the crack and along the interface between the coating and the cemented carbide. After some time coating fragments are lost piece by piece. Swedish patent application 9501286-0 discloses a coated cutting insert particularly useful for dry mil- ling of grey cast iron. The insert is characterised by a straight WC-Co cemented carbide body and a coating including a layer of TiC x NyO z with columnar grains and a top layer of fine grained (-AI2O3. Swedish patent application 9502640-7 discloses a coated turning insert particularly useful for intermit¬ tent turning in low alloyed steel. The insert is charac¬ terised by a WC-Co cemented carbide body having a highly W-alloyed Co-binder phase and a coating including a layer of TiC x NyO z with columnar grains and a top layer of a finegrained, textured α-Al2C>3. It has surprisingly been found that by replacing the textured OC-AI2O3-layer of the above mentioned patent applications with a K-AI2O3-layer a cutting tool with excellent properties for turning stainless and forged components in low alloyed steel can be obtained. Fig 1 is a micrograph in 5000X 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 - K-AI2O3-layer with columnar like grains E - TiN-layer (optional) According to the present invention a turning tool insert is provided with a cemented carbide body of a composition 5-11, preferably 5-8, most preferably 6.5-8, wt-% Co, 2-10, preferably 4-7.5, most preferably 5-7, wt-% cubic carbides of the metals from groups IVb, Vb or VIb of the periodic table of elements preferably Ti, Ta and/or Nb and balance WC. The grain size of the WC is about 2 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 M 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-ratio is a function of the W content in the Co binder phase. A low CW-ratio corre- sponds to a high W-content in the binder phase. It has now been found according to the invention that improved cutting performance is achieved if the ce¬ mented carbide body has a CW-ratio of 0.76-0.92, prefe¬ rably 0.80-0.90. The cemented carbide body may contain small amounts, <1 volume-%, of η-phase (MgC) , without any detrimental effect. In a preferred embodiment an about 15-35 μm thick surface zone depleted of cubic carbides and often enriched (generally more than 25 % enrichment) in binder phase can be present according to prior art such as disclosed in US 4,610,931. In this case the cemented carbide may contain carbonitride or even nitride. The coating preferably comprises - a first (innermost) layer of TiC x NyO z with x+y+z=l, preferably z<0.5, with a thickness of 0.1-2 μ and with equiaxed grains with size <0.5 μm - 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 3-15 μm, prefe¬ rably 5-8 μm, with columnar grains and with an average diameter of <5 μm, preferably <2 μm. In an alternative embodiment the outer part of this layer may contain oxygen, z<0.5 - a layer of a smooth, fine-grained (grain size 0.5- 2 μm) 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-measure- ment. The AI2O3-layer can have a thickness of 1-9 μm, preferably 1-3 μm or alternatively 4-8 μm and a surface roughness R max <0.4 μm over a length of 10 μm. Preferab- ly, this AI2O3-layer is the outermost layer but it may also be followed by further layers such as a thin (about 0.1-1 μm) decorative layer of e g TiN. According to the 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 and preferably with a binder phase enriched surface zone 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 0.1-2 μm, and with equiaxed grains with size <0.5 μ using known CVD-methods. - a layer of TiC x NyO z x+y+z=l, preferably with z=0 or alternatively z<0.5 and x>0.3 and y>0.3, with a thickness of 3-15 μ , preferably 5-8 μm, with columnar grains and with an average diameter of <5 μm, preferably <2 μ , using preferably MTCVD-technique (using acetonit¬ rile 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. - an outer layer of a smooth AI2O3-layer essentially consisting of K-AI2O3 is deposited under conditions disclosed in EP-A-523 021. The AI2O3-layer has a thick¬ ness of 1-9 μm, preferably 1-3 μm or alternatively 4-8 μm, and a surface roughness Rmax-0 •4 ™ 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 the edges with brushes based on e g SiC as disclosed in Swedish patent application 9402543-4. Example 1 A. Cemented carbide turning tool inserts of style CNMG 120408-PM with the composition 7.5 wt-% Co, 1.8 wt- % TiC, 0.5 wt-% TiN, 3.0 wt-% TaC, 0.4 wt-% NbC and balance WC, with a binder phase highly alloyed with W corresponding to a CW-ratio of 0.88 were coated with a 0.5 μm equiaxed TiCN-layer (with a high nitrogen content corresponding to an estimated C/N-ratio of 0.05) fol- lowed by a 7 μm thick TiCN-layer with columnar grains by using MTCVD-technique (temperature 885-850 °C and CH3CN as the carbon/nitrogen source) . In subsequent steps du¬ ring the same coating cycle, a 1.5 μm thick layer of AI2O3 was deposited using a temperature 970 °C and a concentration of H2S dopant of 0.4 % as disclosed in EP- A-523 021. A thin (0.5 μm) decorative layer of TiN was deposited on top according to known CVD-technique. XRD- measurement showed that the AI2O3-layer consisted of 100 % K-phase. The cemented carbide body had a surface zone about 25 μm thick, depleted from cubic carbides and with an about 30 % enrichment in binder phase. The coated in¬ serts were brushed by a nylon straw brush containing SiC grains . Examination of the brushed inserts in a light microscope showed that the thin TiN-layer had been brushed away only along the cutting edge leaving there a smooth, R a =0.3 μm, AI2O3-layer surface. Coating thick¬ ness measurements on cross sectioned brushed samples showed no reduction of the coating along the edge line except for the outer TiN-layer that was removed. B.) A strong competitive cemented carbide grade in style CNMG 120408 from an external leading carbide producer was selected for comparison in a turning test. The carbide had a composition of 9.8 wt-% Co, 0.2 wt-% TiC, 2.0 wt-% TaC, balance WC and a CW-ratio of 0.86. The insert had a coating consisting of a 5 μm TiCN-layer followed by a 1.5 μm thick Al2θ3~layer and a 0.5 μ TiN- layer. Light microscope examination showed that the insert had not been smoothed along the edgeline after the coating step. An insert from A was compared against an insert from B in a turning test in a hot forged ring gear (diameter 206 mm, in TSCM815H material) . Each turning cycle per¬ formed on each component consisted of one facing cut, one longitudinal cut and one chamfering cut. The feed was 0.35 mm/rev and cutting speed around 230 m/min. First, 150 components were machined with both insert A and B and obtained flank wear was measured and com¬ pared. Since the wear was much less developed on insert A it was allowed to cut further components, altogether 354 components. Obtained flank wear is shown in the table below: Number of measured flank components wear, mm insert A 150 0.07 (according to the invention) " 354 o.08 insert B 150 0.10 (external grade) Microscope examination of the tested inserts showed tiny flaking on insert B while no visible flaking had occurred on insert A, not even after 354 machined compo¬ nents. It is obvious from the obtained flank wear that in¬ sert A according to the invention is superior and pos¬ sesses longer tool life. Example 2 D. ) A strong competitive cemented carbide grade in style CNMG 120408 from another external leading carbide producer was selected for comparison in a turning test. The chemical composition of the cemented carbide was: 7.6 wt-% Co, 2.4 wt-% TiC, 0.5 wt-% TiN, 2.4 wt-% TaC, 0.3 wt-% NbC and balance WC. The cemented carbide had a surface zone, about 20 μm thick, depleted from cubic carbides. The composition of the cemented carbide was similar to that of the invention but had a higher CW-ra- tio of 0.93 and a different coating which consisted of a 5 μm TiCN-layer followed by a 3.5 μm TiC-layer, a 1.5 μm Al2θ3-layer and a 0.5 μm TiN-layer. Light microscope examination showed that the insert had not been smoothed along the edgeline after the coating step. Inserts from A and D were compared in a facing turn¬ ing test in a hot forged ring gear (outer diameter of 180 mm and inner diameter of 98 m in a SCr420H mate¬ rial) with feed = 0.25-0.35 mm/rev and cutting speed = 220 m/min. The inserts were run to a predetermined flank wear value of 0.08 mm and the number of produced compo¬ nent was the evaluation criteria. Number of measured flank components wear, mm insert A edge 1 203 0.08 (ace. to invent. ) n e dge 2 226 0.08 insert D 182 0.08 (external grade) Example 3 C.) Cemented carbide turning tool inserts of style WNMG 080408-PM with the same composition and CW-ratio of 0.88 as insert A were coated according to A. XRD-mea- surement showed that the AI2O3-layer consisted of 100 % K-phase. The inserts were brushed according to A. E.) An insert in style WNMG 080408 from the same ce¬ mented carbide producer as in D and with the same CW- ratio, carbide composition and coating as in D was selected for comparison in a turning test. Light micro¬ scope examination showed that the insert had not been smoothed along the edgeline after the coating step. Inserts from C and E were compared in a facing tur¬ ning test of an forged axle (length of 487 mm and diame- ter of 27-65 mm, material 50CV4) with feed = 0.28-0.30 mm/rev and cutting speed = 160 m/min. Three axles were run per each cutting edge and the wear of the cutting edges was examined in a light microscope insert C flank wear less than 0.07 mm (ace. to invent.) no flaking insert E flank wear less than 0.07 mm (external grade) flaking and chipping along the edge Example 4 F.) Cemented carbide turning tool inserts of style CNMG 120408-PM from the same batch as in A were coated according to Swedish patent application 9502640-7 with 0.5 equiaxed TiCN followed by a 7 μm thick layer TiCN with columnar grains, 1 μm equiaxed TiCN and a 4 μm thick 012-textured CC-AI2O3. The inserts were wet-blasted using a water/AI2O3-slurry in order to smooth the coa¬ ting surfaces. G.) Cemented carbide turning tool inserts of style CNMG 120408-PM with the composition 6.5 wt-% Co and 8.8 wt-% cubic carbides (3.3 wt-% TiC, 3.4 wt-% TaC and 2.1 wt-% NbC) and balance WC were coated under the procedure given in A) . The cemented carbide body had a CW-ratio = 1.0 and a surface zone about 23 μm thick depleted in cu¬ bic phase and enriched in binder phase. XRD-measurement showed that the AI2O3-layer consisted only of the K- phase. Inserts from A, F, G and B were compared in a tur¬ ning test in a hot and cold forged ring gear in material SCr420H. The ring had an outer diameter of 190 mm and an in¬ ner diameter of 98 mm. Each turning cycle performed on each component consisted of three facing cuts and one longitudinal cut. Feed = 0.25-0.40 mm/rev and cutting speed around 200 m/min. 170 components were machined and the wear of the cutting edges was examined. insert A no visible flaking of the (ace. to invent. ) coating, flank wear less than 0.07 mm insert F some removal of the coating (CW-ratio = 0 . 88 ] along the cutting edge, flank wear less than 0.08 mm insert G substantial flaking along the (CW-ratio = 1.0) cutting edge and flank wear more than 0.10 mm insert B some removal of coating along (external) the cutting edge, flank wear less than 0.08 mm Although insert F produced according to the Swedish patent application 9502640-7 generally performs superior when turning low alloyed steels it can not always com¬ pete with insert A produced according to the present in- vention when turning some hot and cold forged low al¬ loyed steel components. Example 5 H.) Inserts from the same batch as in A in Example 1 were coated according to the procedure given in Example 1 with the exception that the process time for the AI2O3 coating step was prolonged to 7.5 hours giving a 5.5 μm thick layer of AI2O3. A thin (0.5 μm) decorative layer of TiN was deposited on top using prior art technique. I.) Inserts from the same batch as in H were coated with a 7 μm equiaxed layer of TiCN followed by a 5 μm thick layer of Al2θ3-layer and a 0.5 μm top coating of TiN using prior art techniques. XRD-analysis showed that the AI2O3-layer consisted of a mixture of α- and K- AI2O3, approximately in the ratio 30/70. Inserts from H and A were brushed after coating in order to remove the TiN-layer and smooth the cutting edge. Inserts from H, A and H were tested in an intermit¬ tent longitudinal turning operation. The work piece material was a low alloyed low carbon steel (SCr420H) in the shape of a 22 mm thick ring with an outer diameter of 190 mm and an inner diameter of 30 mm. Each longitu¬ dinal passage over the ring thickness consisted of 22 in-cuts of one mm each. The number of passages over the ring thickness until flaking occurred was recorded for each insert. Insert number of passages before edge flaking A.) ace. to the invention 240 1.5 μm AI2O3 H. ) ace. to the invention 180 5.5 μm AI2O3 I.) ace. to prior art 40 5 μ AI2O3 Inserts from H and A were also compared in a cutting test in a ball-bearing steel (SKF25B, v=250 m/min, f=0.3 mm/r, depth of cut= 2 mm) . In this test crater wear was predominant. The inserts were run for 15 min and the formed crater wear was measured as crater area in m 2 . Insert crater area, mm 2 A.) ace. to the invention 0.9 1.5 μm AI2O3 H.) ace. to the invention 0.5 5.5 μm AI2O3 From the test results above it is clear that insert I have inferior flaking resistance compared to insert H and A. Insert H show both good result with respect to crater wear resistance and flaking resistance. Insert A show the best flaking resistance and can be used in cutting operations demanding extremely high flaking resistance. Example 6 H. A cemented carbide turning tool insert in style TNMG160408-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 μm 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. I. A competitive cemented carbide turning tool in¬ sert in style TNMG160408 from an external leading ce- mented 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 con¬ sisting of 1.0 μm TiC, 0.8 μm TiN, 1.0 μm Tie and, outermost, 0.8 μm TiN. Examination in light optical microscope revealed no edge treatment subsequent to coating. The inserts 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 I got plastic deformation whereas insert H got some notch wear. One edge of insert H according to the invention com¬ pleted one component whereas four edges were required to finalise one component using insert I.

Claims: Claims 1. A cutting tool insert for turning of steel com¬ prising 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, 5-11 wt-% Co and 2-10 wt-% cubic carbides of Ti, Ta and/or Nb and a highly W-alloyed binder phase with a CW-ratio of 0.76-0.92 and in that said coating comprises - a first (innermost) layer of TiC x NyO z with a thickness of 0.1-2 μm, and with equiaxed grains with size <0.5 μm - a layer of TiC x NyO z with a thickness of 3-15 μm with columnar grains with a diameter of <5 μm - an outer layer of a smooth, fine-grained (0.5-2 μ m) K-AI2O3-layer with a thickness of 1-9 μm. 2. Cutting insert according to claim 1 c h a r a c t e r i s e d in that the K-AI2O3-layer has a thickness of 1-3 μm. 3. Cutting insert according to claim 1 c h a r a c t e r i s e d in that the K-AI2O3-layer has a thickness of 4-8 μm. 4. 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 cemented carbide body has a surface zone 15-35 μm thick depleted from cubic carbides . 5. 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 cemented carbide has the composition 6.5-8.0 wt-% Co and a CW- ratio of 0.80-0.90. 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 outermost layer is a thin 0.1-1 μm TiN-layer. 7. Cutting insert according to claim 6 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. 8. Method of making a turning insert comprising a cemented carbide body and a coating c h a r a c t e r i s e d in that WC-Co-based cemented carbide body with a highly W-alloyed binder phase with a CW-ratio of 0.76-0.92 is coated with - a first (innermost) layer of TiC x NyO z with a thickness of 0.1-2 μm, with equiaxed grains with size <0.5 μm using known CVD-methods - a layer of TiC x NyO z with a thickness of 3-15 μm with columnar grains with a diameter of <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 1- 9 μ . 9. Method according to the previous claim c h a r a c t e r i s e d in that said cemented carbide body has a binder phase enriched surface zone. 10. Use of an insert according to claims 1-7 for turning in hot and cold forged low alloyed steel

Inventors: AAKESSON LEIF, LJUNGBERG BJOERN, AAKESSON, LEIF, LJUNGBERG, BJOERN
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 1996
Main classification: C23C30
IPC codes: C23C16, C23C30