Coated titanium-based carbonitride

Abstract: The present invention discloses a titaniumbased carbonitride grade for finishing and semifinishing turning operations with excellent properties. from a titaniumbased carbonitride grade used for milling operations. Said grade is obtained by using a conventional titaniumbased carbonitride grade used for milling operations and by optimising its carbon content in such a way that it is close to the point where η-phase or other substoichiometric phases are formed and providing it with a thin wear resistant Ti-containing coating deposited preferably by PVD-technique.

Description: The present invention relates to coated titaniumbased carbonitride turning inserts with excellent properties for finishing and semifinishing operations. Cermet grades, i.e. sintered titaniumbased carbonitride alloys with titanium as the main component, are now well established materials used for cutting tool applications like milling, turning and drilling and they are preferably used for finishing and semi-finishing operations owing to their extraordinary wear resistance at high cutting temperatures. The demands on these grades are quite different depending on the operation. Turning operations very often produce high temperatures during cutting together with a highly abrasive wear which leads to flank and crater wear and, in the case of too much wear, to plastic deformation on the cutting edge. Milling operations on the other hand are by nature interrupted cuttings and grades used in such operations principally need good resistance to mechanical and thermal shocks. Many efforts have been made during the last few years in developing cermet grades working very well in their intended applications. There is, however, still a lack of grades which are top performers in both milling and turning operations owing to the apparent impossibility to combine the above mentioned different demands for the grade. One attempt to overcome the said problem has been coating of existing tough grades, often used in milling applications, with for instance TiN and/or TiCN, mainly by a PVD-process. This design gives a definitely more wear resistant product. Other properties of the grade, however, remain unaffected and it does not have the properties needed for turning applications earlier mentioned, for instance, enough resistance to plastic deformation when the temperature is increased. This disadvantage is particularly observed when the coating is worn or flaked. From U.S. patent No. 5,314,657 a titanium based carbonitride alloy with cutting properties well balanced mainly for milling and drilling is known and the said alloy is characterised by the formula: (Ti a ,Ta b ,Nb c ,V d ) x (Mo e ,W f ) y (C g ,N h ) z representing the composition of the hard constituent phase where the indices a-f are the molar index of each element of the carbide, carbonitride or nitride formers, and the indices g-h are the molar index of carbon and nitrogen respectively. The following relations apply: a + b + c + d = 1 e + f = 1 g + h = 1 x + y = 1 and z < 1 The same alloy when sintered is characterised by the following relations: 0.88 < a < 0.96, preferably 0.90 < a < 0.94 0.04 < b < 0.08, preferably 0.05 < b < 0.07 0 ≤ c < 0.04, preferably 0 ≤ c < 0.03 0 ≤ d < 0.04, preferably 0 ≤ d < 0.03 0.60 < f < 0.73, preferably 0.66 < f < 0.72 0.80 < x < 0.90, preferably 0.82 < x < 0.88 0.32 < h < 0.40, preferably 0.34 < h < 0.38 Oxygen is present as impurity. The total amount of binder which is Co+Ni is 12-17%, preferably 14-17% by weight with 0.6 < Co/ (Co+Ni) < 0.7, preferably Co/ (Co+Ni) = 2/3. It has now surprisingly been found that it is possible to obtain an excellent turning grade for finishing and semifinishing operations from the alloy according to the above US 5,314,657 by optimising its carbon content and providing it with a thin coating deposited preferably by PVD-technique. Fig 1 shows the microstructure in 4000X of a coated insert according to the present invention. A titanium based carbonitride alloy according to the invention has a composition according to said US patent but with the carbon content of the starting powder mixture such that the carbon content in the final structure is on the limit of formation η-phase, (M 6 C or M 12 C, e.g. W 3 Co 3 C or other substoichiometric phase). Preferably the structure shall be free of η-phase. However, small amount, <0.02 vol-%, of evenly distributed fine-grained η-phase can be accepted. Cutting inserts according to the invention are made from said alloy and are coated using PVD-methods preferably ion plating with a 2-8 µm coating comprising at least one PVD-layer of carbide, nitride or carbonitride of Ti. Particularly good results have been obtained for a first (innermost) TiCN-layer with a thickness of 5 ± 3 µm and a second <1 µm TiN layer. Due to the method chosen the coating thickness often varies between the different faces of the insert. Other coating techniques may also be used provided that the negative influence of Ni in the binder phase is minimised or eliminated. Titanium based carbonitride inserts according to the invention are manufactured by powder metallurgical methods. Powders forming binder phase and powders forming the hard constituents are mixed to a mixture with desired composition. From the mixture inserts are pressed and sintered in vacuum at a pressure of <10 mbar at 1400-1600°C. The cooling to room temperature takes place in vacuum or in inert gas. After surface treatment the inserts are coated according to above. The carbon content of said powder mixture has to be chosen depending on its oxygen content, milling time and sintering conditions such that the total carbon content of the sintered alloy is as close as possible to the point where η-phase or other substoichiometric phases are formed. Such phases are, of course, well-known in these types of alloys and the carbon content at which η-phase forms can be determined for any particular alloy using conventional techniques known to the skilled artisan. Preferably, said carbon content shall be 0.25 - 0.35, most preferably about 0.3, % by weight above the theoretical stoichiometric composition. This level of carbon content generally results in a structure on the limit of η-phase formation. This carbon content once found for a certain composition and manufacturing conditions is easy to control and measure by for instance the magnetic cobalt content in the sintered alloy. It can also be found by e.g. a light decarburizing treatment upon which η-phase is rapidly formed. In a preferred embodiment a titanium containing powder is used in which the grains are rounded, non-angular with a logarithmic normal distribution standard deviation of <0.23 logarithmic µm most preferably produced directly by carburizing or carbonitriding of the oxides of the metals or of the metals themselves. Inserts according to the invention have shown particularly good results in semifinishing operations of non-alloyed carbon steel, low-alloyed steel and finishing operations of stainless steels and nodular cast iron. They can also be used in light intermittent machining operations. Example 1 Turning inserts TNMG160408MF according to the invention, were made by mixing in weight-% 26.4 (Ti,Ta)(C,N), 11.6 (Ti,Ta)C, 20 Ti(C,N), 1.4 TiN, 1.8 NbC, 17.7 WC, 4.6 Mo 2 C, 11 Co, 5.5 Ni and 0.3 soot pressing and vacuum sintering at 1430°C. The Ti-containing powders used had grains which were rounded, non-angular with a logarithmic normal distribution with a standard deviation of <0.23 logarithmic µm. The composition of the hard constituent of the sintered inserts was according to the formula (Ti a ,Ta b ,Nb c ,V d ) x (MO e ,W f ) y (C g ,N h ) z in which a = 0.906, b = 0.056, c = 0.038, d = 0.0, e = 0.333, f = 0.667, g = 0.616, h = 0.384, x = 0.862 and y = 0.318. The structure was free of η-phase. However, the cobalt magnetic content of the sintered inserts showed a value of 4.3% which indicates that the alloy was close to η-phase formation. Example 2 TNMG160408MF inserts of the alloy from Example 1 were coated with an about 4 µm thick TiCN- + <1 µm TiN-layer using ion plating technique, sample C. At the same time inserts from a commercially available cermet grade according to US 5,314,657 were coated, sample B. The samples B and C were tested together with inserts of said commercially available uncoated prior art alloy, sample A, and of the best competitor coated carbonitride alloy, sample D, in a longitudinal turning wear resistance test with the following cutting data: Work piece material Ovako 825B Speed 250 m/min Feed 0.2 mm/rev Depth of cut 1.0 mm Coolant yes Tool life criterion edge fracture or poor surface on the work piece material. Three edges of each alloy were tested. The result expressed in terms of relative figures, with sample A being valued as 1, is shown in the table below: Alloy Coated Resistance to flank wear Resistance to crater wear Relative tool life Magnetic cobalt content, wt-% A No 1.0±0.1 1.0±0.2 1.0±0.1 6.5 (prior art) B Yes 1.2±0.1 1.9±0.1 1.6±0.2 6.5 (prior art) C Yes 1.5±0.05 2.3±0.1 2.3±0.02 4.3 (according to the invention) D Yes 1.2±0.1 1.6±0.1 1.5±0.2 --- (prior art)

Claims: Coated cutting tool insert for turning applications comprising hard constituents in a binder phase based on cobalt and nickel where the composition of the hard constituent phase is represented by the formula: (Ti a , Ta b , Nb c , V d ) x (Mo e , W f ) y (C g , N h ) z where 0.88<a<0.96, 0.04<b<0.08, 0≤c<0.04, 0≤d<0.04, 0.60<f<0.72, 0.80<x<0.90, 0.31<h<0.40 and with a binderphase content of 12-17 % by weight with 0.6<Co/(Co+Ni)<0.7 characterised in that said insert has a total carbon content on the limit of formation of η-phase and a 2-8 µm coating comprising at least one PVD-layer of carbide, nitride or carbonitride of Ti. Coated insert according to any of the previous claims characterized in that 0.90<a<0.94 0.05<b<0.07 0≤c<0.03 0≤d<0.03 0.63<f<0.70 0.82<x<0.88 and 0.34<h<0.38. Coated insert according to any of the previous claims characterized in that said coating comprises a first (innermost) TiCN-layer and an outermost <1 µm TiN layer. Method of manufacturing a titanium based carbonitride alloy comprising hard constituents in a binder phase based on cobalt and nickel where the composition of the hard constituent phase is represented by the formula: (Ti a , Ta b , Nb c , V d ) x (Mo e , W f ) y (C g , N h ) z by powder metallurgical methods, i.e., milling of a mixture of powders forming binder phase and powders forming the hard constituents, pressing and sintering said mixture wherein: 0.88<a<0.96, 0.04<b<0.08, 0≤c<0.04, 0≤d<0.04, 0.60<f<0.72, 0.80<x<0.90, and 0.31<h<0.40. with a binderphase content of 12-17 % by weight with 0.6<Co/(Co+Ni)<0.7 characterised in adjusting the carbon content of said powder mixture (taking into account its oxygen content) prior to sintering such that the total carbon content of the sintered alloy is as close as possible to the point where η-phase or other substoichiometric phases are formed, preferably said carbon content being 0.25 - 0.35, most preferably about 0.3, % by weight above the theoretical stoichiometric composition and depositing a 2-8 µm coating comprising at least one PVD-layer of carbide, nitride or carbonitride of Ti. Method according to claim 4 characterized in that 0.90<a<0.94 0.05<b<0.07 0≤c<0.03 0≤d<0.03 0.63<f<0.70 0.82<x<0.88 and 0.34<h<0.38. Method according to any of claims 4 or 5 characterized in depositing a coating comprising a first (innermost) TiCN-layer and an outermost <1 µm TiN layer. Method according to any of claims 4 - 6 characterized in using at least one Ti-containing powder the grains of which are rounded, non-angular with a logarithmic normal distribution standard deviation of <0.23 logarithmic µm, preferably produced directly by carburizing orcarbonitriding of the oxides of the metals or of the metals themselves.

Inventors: WEINL GEROLD, WEINL, GEROLD
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1996
Main classification: C22C29
IPC codes: B23B27, C23C14, B23P15, C22C29