Titanium based carbonitride alloy with binder phase enrichment

Abstract: According to the present invention there is now provided a sintered body of titanium based carbonitride alloy containing hard constituents based on, in addition to titanium, one or more of the metals Zr, Hf, V, Nb, Ta, Cr, Mo or W in 5 - 30 % binder phase based on cobalt and/or nickel. The body according to the invention has a binder phase enriched surface zone with higher binder phase content than in the inner portion in combination with an enrichment of simple hard constituents i.e. the share of grains with core-rim structure is lower in the surface zone than in the inner of the body.

Description: The present invention relates to a sintered body of a carbonitride alloy with titanium as main component which has improved properties particularly when used in cutting tool inserts in intermittent cutting operations under particularly toughness demanding conditions. This has been done by different distribution of hard constituents and binder phase between the surface layer and inner (bulk) and different form of the hard constituents in the surface zone and bulk regarding complex structure particularly different core-rim-situations. Titanium based carbonitrides (so-called cermets) are today well established in the metal cutting industry and are primarily used as tools for finishing. They consist of hard constituents of carbonitride embedded in a binder phase of cobalt and/or nickel. The hard constituents generally have a complex structure with a core surrounded by a rim with different composition. For tungsten-carbide-cobalt-based hard metals, the so-called gradient sintered grades, particularly when coated with one or more CVD layers, have now gained strong foothold in metal cutting inserts. Gradient sintering means that sintering is performed in such a way that a some 10 µm wide surface zone of the material gets another composition than its inner in the form of a higher binder phase content. Examples of patents within this area are US 4,277,283, US 4,610,931, US 4,497,874, US 4,649,048 US 4,548,786, US 4,830,930 just to mention a few. US 4,911,989 describes a coated hard metal where the hardness increase monotonously in a 50-100 µm wide surface zone. Different forms of gradient sintering for titanium based carbonitride alloy exist since a number of years. For example, grades exist with a few µm thick coating with strong binder phase enrichment and below that a binder phase depletion which extends 200-400 µm into the material under gradual increase up to the bulk level. This gradient type gives increased wear resistance which takes place with a certain loss of the toughness behaviour. As expected, a hardness maximum is obtained just below the binder phase enriched zone where the enrichment of hard constituents is the greatest. One way of improving the toughness behaviour is through a relatively moderate binder phase enrichment to a depth of about 20-50 µm from the surface followed by an enrichment of hard constituents which then gives a hardness maximum. The binder phase enrichment gives a better toughness behaviour but increases at the same time the risk for plastic deformation. The hard constituent enrichment increases the wear resistance (when the wear has reached this area) but increases the risk of crack propagation, i.e., deteriorates the toughness behaviour at the same time as the resistance to plastic deformation increases. An example of a variant of the above is EP-A-368 336, which discloses a hard surface layer with a hardness maximum situated between 5 and 50 µm from the surface and an outer surface zone with a hardness of between 20 and 90 % of the maximum hardness. This is accomplished by starting the sintering process in an non-oxidizing atmosphere up to 1100°C followed by a nitriding atmosphere which is finished by a denitriding atmosphere. The denitriding period comprises at least the cooling but can also comprise the whole or part of the sintering holding time. Thus, normally gradient sintered hard alloys get a depletion of binder phase, i.e., an enrichment of hard constituents just below the binder phase enrichment. This leads to increased wear resistance in this area with increased resistance to plastic deformation, but unfortunately also leads to a worsened toughness behaviour. According to the present invention an enrichment of binder phase in the surface is accomplished but without an accompanying depletion of binder phase just below the enrichment in combination with a special structure in the surface zone. In such way the above mentioned negative behaviour is avoided. The resistance to plastic deformation is kept on an acceptably high level with the aid of an advanced core-rim-structure known through the Swedish patent SE 459 862. Figure 1 shows the microstructure in about 5000 X magnification of the surface zone in an alloy according to the invention and Fig 2 shows a microprobe recording of the distribution of Co, W, Ti and Mo in the surface of an alloy according to the invention. In both figures the letter A indicates the outer surface. The present invention comprises a sintered body of a carbonitride alloy with titanium as main component, having the features defined in claim 1. Remaining hard constituent formers are Zr, Hf, V, Nb, Ta, Cr, Mo and/or W. Further, 5-30 % by weight binder phase is included containing cobalt and/or nickel but also other hard constituent forming elements can be found in the binder phase. The alloy is further characterized in that it is built up of complex hard constituent grains with a core-rim structure of the type described in Swedish patent application 8902306-3. It has been given toughness increasing properties through an enrichment of binder phase in a <25 µm, preferably 5-10 µm, wide surface zone without the above mentioned depletion of binder phase and corresponding enrichment of hard constituents in a zone just below the surface zone in combination with a certain microstructure. The binder phase content in the surface zone shall be at least 1.2, preferably 1.5-3, times greater than the binder phase content in the inner of the alloy. Certain hard constituent elements can also show a slight enrichment in the binder phase enrichment. In the surface zone grains with core-rim-structure are essentially missing, i.e., in the surface zone mainly 'simple' grains are present. The mean grain size in the surface zone is in addition finer, about 0.5 µm, whereas the inner portion of the material has a more normal mean grain size for the alloy of about 1-2 µm. This is illustrated by Figs. 1 and 2. In a preferred embodiment the alloy comprises, in weight-%, <20 % WC, 40-60 % TiC+TiN, <10 % of each of TaC, VC and Mo₂C and 10-20 % Co+Ni-binder phase. When the alloy contains molybdenum, the binder phase enrichment is accompanied by a slight enrichment of said element. In addition, the content of W, Mo, Ta and/or V increases slightly, <15 % relatively, in a 150-200 µm wide surface zone whereas the titanium content decreases in the corresponding degree. The above mentioned increase in wear resistance in a hard constituent enriched layer is not obtained with the present invention. Since such an effect, however, does not appear until after a considerable wear and the area of use for titanium based carbonitride alloys is finishing with maintained sharp edge such an increase in wear resistance is of less interest in order to obtain well functioning finishing tools. If a further increased wear resistance is of interest of a body according to the present invention, it is best obtained by coating with one or more layers according to in itself known technique. The alloy according to the present invention is very suitable as a substrate for coating with TiN and/or TiCN, e.g., by PVD-technique. The good toughness behaviour through an outer binder phase enriched layer of a body according to the invention has been further increased in that the hard constituents in the outer zone have another structure than those in the inner portion of the body where, as above has been pointed out, there is a pronounced core-rim-structure. In the surface layer, the cores have not been dissolved and no rim formation has taken place which results in the hard constituent grains in the surface layer to a considerable extent having a homogeneous structure, i.e., not so much core-rim. The absence of the brittle rim phase gives further increased toughness. The invention also relates to a powder metallurgical method, the features of which are defined in claim 6, for manufacturing a sintered titanium based carbonitride alloy. According to the method, powders forming binder phase and powders forming the hard constituents are mixed to a mixture with desired composition. From the mixture bodies are pressed which are sintered. The powder mixture is strongly substoichiometric regarding the interstitial balance and is sintered under such conditions that possible substoichiometric phases completely are transformed to stoichiometric. The sintering is started in a vacuum at 1100-1200°C. This is followed by a deoxidizing treatment in vacuum at 1200°C for 30 min which afterwards is replaced by a deoxidizing H₂-atmosphere for 15-30 minutes at about 1200°C. The temperature is increased to sintering temperature, 1400-1600°C, in a nitrogen atmosphere. During temperature increase and/or sintering time, a gradual decrease of the nitrogen content to zero may take place. Up to about 10⁴ Nm⁻² Ar can with advantage be introduced during the sintering period. The cooling to room temperature takes place in vacuum or in inert gas.

Claims: Sintered titanium based carbonitride alloy body containing hard constituents consisting of both those having a core-rim structure and those having a simple structure and being based on, in addition to titanium, one or more of the metals Zr, Hf, V, Nb, Ta, Cr, Mo or W in 5 - 30 % binder phase based on cobalt and/or nickel, wherein said body has a binder phase enriched surface zone with higher binder phase content than in the inner portion of the body characterized in that the binder phase content just below the binder phase enriched surface zone is the same level as the remainder of the body and that said surface zone has an enrichment of said simple hard constituents without a core-rim structure. Sintered body according to claim 1 characterized in that the binder phase content in the surface zone is at least 1.2, preferably 1.5-3 times higher that the binder phase content in the inner portion of the body. Sintered body according to any of the preceding claims characterized in that the grain size of the hard constituents in the surface zone is about 0.5 µm and in the rest of the body is about 1-2 µm. Sintered body according to any of the preceding claims characterized in that said alloy comprises <20 % WC, 40-60 % TiC+TiN, <10 % of each of TaC, VC and Mo₂C and 10-20 % Co+Ni-binder phase. Sintered body according to any of the preceding claims characterized in that in a 150-200 µm wide surface zone the content of W, Mo, Ta and/or V increase slightly, <15 % relatively, whereas the titanium content decreases in the corresponding degree. Method of manufacturing a sintered titanium based carbonitride alloy comprising: wet milling of powders forming binder phase and powder forming hard constituents to a 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 vacuum at 1100-1200 °C b) in vacuum at about 1200 °C for about 30 minutes c) in deoxidizing H₂-atmosphere for 15-30 minutes at about 1200 °C d) in N₂-atmosphere during heating to sintering temperature 1400 - 1600 °C e) cooling to room temperature in vacuum or inert gas wherein a strongly substoichiometric powder mixture regarding the interstitial balance is used and the sintering is performed to completely transform the substoichiometric phases to stoichiometric.

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