Sintered carbonitride alloy with highly alloyed binder phase

Abstract: According to the present invention there is now provided a sintered titanium based carbonitride alloy containing hard constituents based on, in addition to Ti, W and/or Mo, one or more of the metals Zr, Hf, V, Nb, Ta or Cr in 5 - 30 % binder phase based on cobalt and/or nickel. The content of tungsten and/or molybdenum, preferably molybdenum in the binder phase is >1.5 times higher than in the rim and >3.5 times higher than in the core of adjacent hard constituent grains.

Description: The present invention relates to a sintered carbonitride alloy with titanium as main component and containing molybdenum. The alloy is preferably used as an insert for milling and turning. By starting the sintering with an oxidizing treatment it is possible to obtain a high molybdenum-content in the binder phase which gives the alloy improved properties. Classic cemented carbide, i.e., based upon tungsten carbide (WC) and with cobalt (Co) as binder phase has in the last few years met with increased competition from titanium based hard materials, usually called cermets. In the beginning, these titanium based alloys were used only for high speed finishing because of their extraordinary wear resistance at high cutting temperatures. This property depends essentially upon the good chemical stability of these titanium based alloys. The toughness behaviour and resistance to plastic deformation were not satisfactory, however, and therefore the area of application was relatively limited. Development has proceeded and the area of application for sintered titanium based hard materials has been considerably enlarged. The toughness behaviour and the resistance to plastic deformation have been considerably improved. This has been done, however, by partly sacrificing the wear resistance. An important development of titanium based hard alloys is substitution of carbides by nitrides in the hard constituent phase. This decreases the grain size of the hard constituents in the sintered alloy. Both the decrease in grain size and the use of nitrides lead to the possibility of increasing the toughness at unchanged wear resistance. Characteristic for said alloys is that they are usually considerably more finegrained than normal cemented carbide, i.e., WC-Co-based hard alloy. Nitrides are also generally more chemically stable than carbides which results in lower tendencies to stick to work piece material or wear by solution of the tool. Besides Ti, the other metals of the groups VIa, Va and VIa, i.e., Zr, Hf, V, Nb, Ta, Cr, Mo and/or W, are normally used as hard constituent formers as carbides, nitrides and/or carbonitrides. The grain size of the hard constituents is generally <2 µm. As binder phase nowadays both cobalt and nickel are used. The amount of binder phase is generally 3 - 25 % by weight. In addition, also other metals are used, for example aluminium, which sometimes are said to harden the binder phase and sometimes improve the wetting between hard constituents and binder phase, i.e., facilitate the sintering. During sintering the relatively seen less stable hard constituents are dissolved in the binder phase and precipitate then as a rim on the more stable hard constituents. A very common structure in alloys in question is therefore hard constituent grains with a core-rim structure. An early patent in this area is US 3,971,656 which comprises Ti- and N-rich cores and rims rich in Mo, W and C. It is known from US 3,971,656 and DE 3,528,308 to have a relatively high molybdenum content in the binder. It is through Swedish patent application SE 8902306-3 (EP 406201) known that at least two different combinations of duplex core-rim-structures in well balanced proportions give optimal properties regarding wear resistance, toughness behaviour and/or plastic deformation. Further examples of patents in this area are US 4,904,445, US 4,775,521, US 4,957,548 just to mention a few. As a result of the dissolution of the hard constituents in the binder phase during sintering the binder phase will contain a certain part of these in solid solution which affects the properties of the binder phase and thereby those of the whole alloy. The composition of the binder phase is determined by the starting raw materials as well as the way of manufacture, i.a., time and temperature during the sintering. It would be desirable to increase the alloying of group VI elements in order to obtain a more rigid alloy which gives improved resistance against mechanical stresses, i.e., a tougher behaviour. According to the features defined in claim 1, a titanium based carbonitride alloy with improved rigidity is provided. By the way of manufacture defined in claim 2, it has surprisingly turned out to be possible to obtain an alloy with higher content of molybdenum and/or tungsten in the binder phase relative to the hard constituents than previously possible. In an alloy according to the invention, the content of molybdenum and/or tungsten, preferably molybdenum, in the binder phase is >1.5 times greater than the content of said elements in the rim and >3.5 times the content in the core of adjacent hard constituent grains with core-rim-structure. A titanium based carbonitride alloy according to the invention is manufactured with powder metallurgical methods. Powders forming binder phase and powders forming the hard constituents are mixed to a mixture with desired composition, preferably satisfying the relation 0.3<N/(N+C)<0.6 where N is the nitrogen content and C is the carbon content. From the mixture bodies are pressed and sintered. After dewaxing, the sintering is started with an oxidizing treatment in oxygen or air at 100-300°C for 10-30 min whereafter vacuum is pumped and maintained up to 1100-1200°C followed by a deoxidizing treatment in vacuum at 1200°C for 30 min which afterwards is replaced by a deoxidizing H₂-atmosphere during a certain time at about 1200°C whereupon temperature is increased to sintering temperature, 1400-1600°C, in a nitrogen atmosphere. During said temperature increase and/or sintering time a gradual decrease of the nitrogen content to zero can take place. Up to about 10 kPa (100 mbar) Ar can with advantage be introduced during the sintering period. The cooling to room temperature takes place in vacuum or in inert gas. The reason to the relatively seen high content of, e.g., molybdenum in the binder phase using a method according to the invention is not completely clear. It is probably due to the special distribution of nitrogen in the carbide raw material which is obtained through the introductory oxidation-, reduction- and nitriding steps. The oxidation- and reduction steps result in carbon loss leading to an influence on the interstitial balance of the oxycarbonitrides, particularly in carbide surface close areas. During the nitriding steps vacant interstitial positions are filled with nitrogen whereby carbonitrides with in the rim increased content of nitrogen can be expected. The carbonitrides obtained according to above constitute, during the initial stages of the sintering, very effective nitrogen sources whereby an increased nitrogen potential during the period when the core-rim structure is formed, can be expected. The distribution of molybdenum between binder phase and hard constituent is influenced by the nitrogen potential in such a way that high nitrogen potential leads to high content of molybdenum in the binder phase relative to the hard constituent phase. The method gives, thus, high molybdenum-content in the binder phase at the same time as the weighed-in nitrogen content totally is low. Chemical analysis shows that the total nitrogen content increases 10-15% relatively during sintering. Example 1 A powder mixture consisting of (in % by weight) 12.4 % Co, 6.2 % Ni, 34.9 % TiN, 7.0 % TaC, 4.4 % VC, 8,7 % Mo₂C and 26.4 TiC was wetmilled, dried and pressed to inserts of type TNMG 160408-QF which were sintered according to the following steps: a) dewaxing in vacuum b) oxidation in air for 15 minutes at 150°C c) heating in vacuum to 1200°C d) deoxidation in vacuum at 1200°C for 30 minutes e) flowing H₂ at 1 kPa (10 mbar) for 15 minutes at 1200°C f) flowing N₂ during heating to 1200 - 1500°C g) sintering in Ar at 1 kPa (10 mbar) and 1550°C for 90 minutes h) cooling in vacuum X-ray diffraction analysis showed presence of cubic carbonitride and binder phase. The lattice constant of the binder phase was 3.594 Å which shows that the alloying content is increased. For comparison inserts of the same type and the same composition were manufactured according to EP-A-368336. The ratio between the contents of molybdenum in the binder phase and the rim resp. core in hard constituent grains in the alloy according to the invention and according to known technique was determined with EDS-analysis with the following result: Binder phase/rim Binder phase/core According to the invention 1.7 4 According to known technique 1.3 2.9 Example 2 The inserts from example 1 were tested in an intermittent turning operation under the following conditions: Work piece: SS 2244 Cutting speed: 110 m/min Cutting depth: 1.5 mm Feed: 0.11 mm/rev which was increased continuously (doubled every 90:th second) Result: 50% of the inserts according to the invention fractured after 1.41 min which corresponds to a feed of 0.21 mm/rev whereas 50 % of the prior art inserts fractured after 0.65 min which corresponds to a feed of 0.16 mm/rev. Inserts according to the invention, thus, show a significantly better toughness.

Claims: Sintered titanium based carbonitride alloy containing hard constituents based on, in addition to Ti, W and/or Mo, one or more of the metals Zr, Hf, V, Nb, Ta or Cr in 5 - 30 % binder phase based on cobalt and/or nickel characterized in that the content of molybdenum and/or tungsten in the binder phase is >1.5 times higher than in the rim and >3.5 times higher than in the core of adjacent hard constituent grains with core-rim structure. Method of manufacturing a sintered carbonitride alloy comprising: wetmilling of powders forming binder phase and powder forming hard constituents to a powder mixture with desired composition, compaction of said mixture to compacts and sintering of said compacts characterized in that the sintering is performed under the following sequential conditions: a) in oxygen or air at 100-300°C for 10-30 minutes, b) in vacuum to 1100-1200°C, c) in vacuum at about 1200°C for about 30 minutes, d) in deoxidizing H₂-atmosphere for 15-30 minutes at about 1200°C, e) in N₂-atmosphere during heating to sintering temperature 1400 - 1600°C, f) cooling to room temperature in vacuum or inert gas. Method according to the preceding claim characterized in that in said powder mixture 0.3<N/(N+C)<0.6 , where N is the nitrogen content and C is the carbon content. Method according to any of the preceding claims 2 and 3 characterized in that at the heating and/or sintering holding time the nitrogen content is gradually reduced to zero and that preferably up to about 10 kPa (100 mbar) Ar is added.

Inventors: GUSTAFSON PER, OSKARSSON ROLF, WEINL GEROLD, GUSTAFSON, PER, OSKARSSON, ROLF, WEINL, GEROLD
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1992
Main classification: C22C29
IPC codes: C04B35, B22F3, C22C1, C22C29, B23B27