The present invention relates to a sintered body of carbonitride alloy with titanium as main component and containing tungsten and cobalt. This alloy is preferably used as an insert material in cutting tools for machining of metals, e.g., turning, milling and drilling. For a given gross composition, it is possible to optimize the relation between toughness and wear resistance of the alloy by choosing the form in which tungsten is added.
Titanium-based carbonitride alloys, so-called cermets, are today well established as insert materials in the metal cutting industry and are especially used for finishing. They consist of carbonitride hard constituents embedded in a metallic binder phase. The hard constituent grains generally have a complex structure with a core surrounded by a rim of other composition.
In addition to titanium, group VIa elements, normally both molybdenum and tungsten and sometimes chromium, are added to facilitate wetting between binder and hard constituents and to strengthen the binder by means of solution hardening. Group IVa and/or Va elements, i.e., Zr, Hf, V, Nb and Ta, are also added, mainly in order to improve the thermomechanical behaviour of the material, e.g., its resistance to plastic deformation and thermal cracking (comb cracks). All these additional elements are usually added as carbides, nitrides and/or carbonitrides. The grain size of the hard constituents is usually &lt;2 .mu.m. The binder phase is normally a solid solution of mainly both cobalt and nickel. The amount of binder phase is generally 3-25 wt%. Furthermore, other elements are sometimes used, e.g., aluminium, which are said to harden the binder phase and/or improve the wetting between hard constituents and binder phase.
As a result of the rather large number of elements generally added to the alloy, it is practically impossible to predict the effect that alterations of the chemical composition may have on the performance of the alloy as cutting tool. However, simple compositions with few alloying elements have hitherto not been available with sufficiently good properties to be able to compete in real cutting tool applications. Also, due to their high nickel content, it has previously not been possible to apply wear resistant coatings (e.g., Ti(C,N)- and Al.sub.2 O.sub.3 - coatings) on titanium based carbonitride alloys using the chemical vapor deposition (CVD) technique common for WC-Co based alloys. The reason for this is the strong catalytic properties of nickel.
However, several previous patents and patent applications deal with the question of in which form the carbide and/or nitride forming elements should be added in order to obtain reasonable wear resistance and toughness of the material. in the Swedish patent SE B 467 257 (which corresponds to U.S. Pat No. 5,308,367) one method is disclosed in which prealloyed raw material powders are used in order to obtain the desired chemical composition of the hard phase cores. By a proper combination of tungsten and carbon rich cores giving high wear resistance, tantalum-rich cores giving high resistance against plastic deformation, and titanium-rich cores giving high toughness it is possible to balance these properties in a desired way. The method relies on the possibility to avoid that the thermodynamically least stable raw materials are dissolved during sintering.
UK patent application GB 2 227 497 (which corresponds to U.S. Pat. No. 5,051,126) A discloses a similar method. The raw materials are prealloyed in such a way that the sintered body contains only two types of hard phase grains. The first type is single phase nitrides or carbonitrides of group IVa metals, i.e. grains which lack the usual core/rim structure. The second type has a core/rim structure where the core contains significantly more group Va metals and tungsten than the surrounding rim. Again, since the desired cores are remnants of the raw material powder it is vital that the raw materials are designed in such a way that they are not dissolved to any large extent during sintering.
The Swedish patent SE B 470 481 (which corresponds to U.S. Pat. No. 3,395,421) also discloses a method to increase the toughness of the material while maintaining a reasonable hardness, using prealloyed raw materials. The basis of the method is to add essentially all tungsten in the form of a quite specific (probably inhomogeneous) (Ti,W)(C,N) powder. The sintered body contains at least four different types of cores, all of which contains significant amounts of tungsten. In more than 5% of these, at least 50 wt % of the metal content is tungsten. For thermodynamic reasons, such a core cannot form during normal liquid phase sintering. Thus, it is vital for the method that the different components of the raw material do not dissolve completely in the sintering process. Apart from titanium and tungsten, the material also contains at least one additional element chosen from the groups IVa, Va and VIa.
U.S. Pat. No. 4 778 521 discloses an alternative method to increase the toughness of the material while maintaining a reasonable hardness. The basis of this method is to add titanium and tungsten exclusively as Ti(C,N) and WC; respectively, and possibly one additional element selected from the groups IVa, Va and VIa. All hard phase grains in the resulting material consist of three components, a titanium-rich tungsten-poor core, a tungsten-poor titanium-rich intermediate rim surrounding the core and an outer rim with intermediate tungsten content surrounding the intermediate rim. This structure, with intermediate rims of fairly homogeneous thickness completely surrounding the cores, is generally obtained using a nickel based binder. Although the method is interesting it has to our knowledge not been commercialized, most probably due to the inferior high temperature properties of nickel as compared to cobalt.