Titanium based carbonitride alloy with wear resistant surface layer

Abstract: A sintered body of titanium based carbonitride alloy according to the invention comprises carbonitride hard constituents in 5-25 % binder phase where the hard constituents contain, in addition to Ti, one or more of the metals Zr, Hf, V, Nb, Ta, Cr, Mo and/or W and the binder phase is based on cobalt and/or nickel. The sintered body has at least one outer surface with a <50 µm thick surface layer, A, of a titaniumrich cubic carbonitride. Below this layer there is a <100 µm thick binder phase enrichment, B, in which the binder phase content has a maximum of >1.2 of that in the inner of the body, D. Under the binder phase enrichment there is a <250 µm thick binder phase depleted zone, C. The binder phase content in this zone has a lowest level <0.9 of the binder phase content in the inner of the body, D. Sintered bodies according to the invention are manufactured by heat treatment in an atmosphere of N₂ and/or NH₃ possibly in combination with at least one of CH₄, CO and CO₂ at 1100 - 1350°C for 1-25 hours at atmospheric pressure or higher.

Description: The present invention relates to a sintered body of titanium based carbonitride having improved properties particularly when used as insert material in cutting tools for machining of metals such as turning, milling and drilling especially under heavy wear conditions. Titanium based carbonitride alloys, so-called cermets, are today well established as insert material in the metal cutting industry and are especially used for finishing. They contain mainly carbonitride hard constituents embedded in a binder phase. The hard constituent grains generally have a complex structure with a core surrounded by a rim of other composition. Their grain size is usually <2 µm. In addition to Ti, other metals of the groups IVa, Va and VIa, i.e., Zr, Hf, V, Nb, Ta, Cr, Mo and/or W, are normally found in the carbonitride hard constituents but may also be present as carbide and/or nitride hard constituents. The binder phase generally contains cobalt as well as nickel. The amount of binder phase is generally 3 - 25 % by weight. US Patent 4,447,263 discloses inserts of a titanium based carbonitride alloy provided with a wear resistant surface layer of carbonitride or oxycarbonitride only or in combination. The layer is obtained by a heat treatment at 1100 - 1350°C in an atmosphere of N₂, CO and/or CO₂ at subpressure. Inserts according to the mentioned patent, thus, consist of a brittle layer on a brittle substrate why the tool life becomes inadequate at toughness demanding operations. It has now surprisingly been found that, if the heat treatment is performed at atmospheric pressure, preferably overpressure, an enrichment of binder phase under the above mentioned surface layer is obtained, which gives improved toughness. The invention is defined in claims 1 and 4. Preferred embodiments are defined in claims 2-3. Fig 1 shows the microstructure in 1000X (light optical image) and the element distribution in a cross section of the surface zone in a body according to the invention. Fig 2 shows the structure in a cross section of the surface layer and binder phase enrichment in 2500X (electron optical image) in a body according to the invention. Fig 3 is a light optical structure image in 1200X of a slightly etched cross section of a cutting edge of an insert according to the invention. Fig 4 is an electron optical image in 1000X of a cutting edge according to the invention in cross section and Fig 5 is a corresponding image of the cobalt-distribution. In the figures A(=A1+A2) - surface layer, B - binder phase enrichment, C - hard constituent enrichment, D - unaffected inner part, E - grain with core-rim, F - matrix in the surface layer and G - Ti-N-rich grains in the binder phase enrichment. According to the invention there is now provided a sintered body of titanium based carbonitride alloy in which preferably N/(C+N)>0.1 where C=carbon content and N=nitrogen content. At least one outer surface of the body has a <50 µm, preferably 1-35 µm, thick surface layer, A, containing Ti-N-rich cubic phase formed as a result of the heat treatment. Below this layer there is a <100 µm, preferably 10-50 µm, thick binder phase enriched surface zone, B, also present in the cutting edge, fig 4 and 5. The binder phase content in this zone has a maximum >1.2, preferably 1.5-4, times larger than the binder phase content in the inner of the body, D. Below the binder phase enrichment there is a <250 µm, preferably 50-150 µm, binder phase depleted zone, C. The binder phase content in this zone has a lowest level <0.9, preferably <0.75, of the binder phase content in the inner of the body, D. The surface layer exhibits a complex microstructure, see fig 2, comprising nitrided original grains (after sintering) of which many consist of core-rim, E, and a Ti-N-rich cubic phase, F, forming a more or less interconnecting matrix. In the outer part of the surface layer, A1 in fig 2, binder phase is present with a maximum of <1.2, preferably <0.9, most preferably <0.6, of the binder phase content in the inner part of the body, D. In the inner part of the surface layer, A2 in fig 2, the binder phase content has a minimum of <0.5, preferably <0.3 of the binder phase content in the inner of the body, D. The Ti-N-rich cubic phase also contains other elements found in the inner of the body, e.g., tantalum and vanadium, if present. Tungsten and/or molybdenum, if present, are found mainly in the rims in the core-rim grains. The total content of tungsten and/or molybdenum in the surface layer compared to the inner of the body is, however, distinctly lower, <0.75, preferably <0.5. The cubic phase can also contain oxygen and carbon. The oxygen content of said phase can be higher than in the inner of the body. The carbon content of the cubic phase, on the contrary, is usually lower than in the inner. Carbon and/or oxygen and nitrogen can be present evenly distributed in the whole surface layer, A, or as gradient. Also in the cutting edge itself the binder phase enrichment is present, fig 4 and 5, which effectively contributes to the increased toughness. In relatively sharp cutting edges, usually used in milling, the hard surface layer becomes thinner in the edge line and, in addition, the TiN-rich matrix is present in the form of small grains in a triangular area, G in fig 3, in the outer part of the binder phase enrichment. Sintered bodies according to the invention may further be provided with thin wear resistant coatings, preferably of oxide, known in the art. Bodies of titanium based carbonitride alloy according to the invention are manufactured by heat treatment of sintered bodies, mechanically treated to finished dimension, in an atmosphere of N₂ and/or NH₃ possibly in combination with at least one of CH₄, CO and CO₂ at 1100 - 1350°C for 1-25 hours at atmospheric pressure or higher, preferably >1.1 bar. By varying the contents of the ingoing gases during different periods of the heat treatment the distribution of carbon, nitrogen and/or oxygen can be influenced. After the heat treatment the bodies may be ground on at least one of its surfaces and, possibly, coated by CVD or PVD-technique according to in itself known technique. Sintered bodies according to the present invention are useful as inserts in tools for machining, such as turning and milling and as wear parts such as seal rings etc. Example Turning inserts of type TNMG 160408-QF were manufactured with the following composition in weight-%: TiN 20, TiC 29, TaC 6.3, Mo₂C 9.3, WC 15.9, VC 3.9, Co+Ni 16.2. After sintering in 10 mbar protective atmosphere (Ar) at 1430°C for 90 minutes a mechanical surface treatment was performed to final dimension. Thereafter the inserts were heat treated according to the invention in N₂ atmosphere at 1300°C for 15 hours at 1200 mbar whereby a surface structure according to fig 1 was obtained. The inserts were tested in a turning operation with the following cutting data: Work piece: SS 2541 Cutting depth: 2 mm Feed: 0.2 mm Speed: 3000 m/min The flank wear, VB, was measured continuously every 5:th minute. Three tests were performed. As tool life criterion VB>0.3 mm was chosen for a reference with the same composition but without heat treatment according to the invention. For the inserts according to the invention the following data for VB were obtained after about 20 minutes engagement time where VB for the reference was >0.3 mm. Test VB, mm 1 0.17 2 0.20 3 0.18 As a further reference inserts according to the earlier mentioned US-patent were used. In all of the test these inserts fractured after 5-12 min engagement time. The reason to fracture was brittle fracture without any previous appreciable wear, i.e., lack of toughness.

Claims: Sintered body of titanium based carbonitride alloy comprising mainly carbonitride hard constituents in 5-25 % binder phase where the hard constituents contain, in addition to Ti, one or more of the metals Zr, Hf, V, Nb, Ta, Cr, Mo and/or W and the binder phase is based on cobalt and/or nickel characterized in that said body has at least one outer surface with a <50 µm thick surface layer, A, containing Ti-N-rich cubic phase below which layer there is a <100 µm thick binder phase enrichment, B, in which the binder phase content increases to a maximum of >1.2 of the binder phase content in the inner of the body, D, and below said binder phase enrichment a <250 µm thick binder phase depleted zone, C, in which the binder phase content has a lowest level <0.9 of the binder phase content in the inner part of the body, D. Sintered body according to the previous claim characterized in that N/(N+C)>0.1 where N=nitrogen content and C=carbon content. Sintered body according to claim 1 or 2 characterized in that said body is provided with at least one wear resistant oxide coating deposited by CVD- or PVD-technique. Method of manufacturing a sintered body of titanium based carbonitride alloy comprising mainly carbonitride hard constituents in 5-25 % binder phase where the hard constituents contain, in addition to Ti, one or more of the metals Zr, Hf, V, Nb, Ta, Cr, Mo and/or W and the binder phase is based on cobalt and/or nickel, said body is heat treated in an atmosphere of N₂ and/or NH₃ possibly in combination with at least one of CH₄, CO and CO₂ at 1100 - 1350°C for 1-25 hours characterized in that said heat treatment is made at atmospheric pressure or higher, preferably at a pressure of >1.1 bar.

Inventors: MIKUS MARIAN, WEINL GEROLD, MIKUS, MARIAN, WEINL, GEROLD
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1992
Main classification: C22C29
IPC codes: C04B41, B23B27, C22C29, C23C30, C04B35