BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the metallurgy field. More specifically, the invention relates to a component produced by powder metallurgy from a hard metal alloy with a binder content of from 0.1 to 20% by weight which contains at least one grain growth-inhibiting additive. The invention, furthermore, pertains to a process for producing the component.
The term hard metal is understood as meaning a composite material which substantially comprises a carbidic component and a binder. The most important carbidic components include the carbides or mixed carbides of the metals W, Ti, Zr, Hf, V, Nb, Ta, Mo and Cr. Typical binder metals are Co, Ni and Fe. Additions of further hard materials, such as for example carbonitrides, are also used.
The properties of hard metals are determined by the ratio of carbide content to binder content, by the chemical composition, the carbide grain size and the carbide grain size distribution. This opens up numerous options for matching the properties of hard metal to the corresponding application area.
For example, increasing the binder content leads to an improvement in the fracture toughness and bending strength, combined, at the same time, with a reduction in the hardness, rigidity and compressive strength. A reduction in the carbide grain size leads to an increase in the hardness, the compressive strength and the bending strength combined with a reduced impact toughness and fracture toughness.
Nowadays, carbidic powders in the grain size range from 0.2 μm to 15 μm are used for the production of hard metal components according to the intended use. To reduce the grain coarsening during the sintering operation when fine-grained carbide powder is used, grain growth inhibitors are added. The most effective grain growth-inhibiting additives are vanadium carbide, chromium carbide, titanium carbide, tantalum carbide and niobium carbide. In many cases, two or more additives are used, such as for example mixtures of VC and Cr3C2 or TaC, NbC and TiC. The grain growth-inhibiting additive can be distributed extremely finely in the main component as early as before or during the carburizing step. However, it is also effective if the grain growth inhibitor is admixed with the hard metal powder or individual constituents of the hard metal powder before, during or after milling.
Hard metal components may be subject to very differing local loads. Therefore, from a very early stage solutions which are based on a material composite comprising two or more hard metal alloys have been discovered and implemented. For example, U.S. Pat. No. 5,543,235 describes a hard metal material composite which is produced by powder metallurgy composite pressing, the individual material regions differing in terms of their composition or microstructure. A rotating composite tool which is composed of two hard metal alloys is also described in U.S. Pat. No. 6,511,265 B1 and international PCT publication WO 01/43899. Production is likewise preferably effected by composite pressing.
A further process technique for the manufacture of a hard metal composite-body is described in U.S. Pat. No. 5,594,931. A surface layer or slip which consists of a powder mixture, a solvent, a binder and a plasticizer is applied to a green compact or core. The composite green compact produced in this way is densified by sintering.
However, a drawback of the material composites described here is that stress concentrations occur in the regions of the composite body where materials with different properties meet one another. Furthermore, account must be taken of the fact that each material component has its own sintering characteristics. This may cause distortion to the component during sintering.
However, if the transition between two material regions is made with a graduated composition, stress peaks can be substantially avoided. A structure of graduated composition is understood to mean that the composition changes gradually and continuously over a certain region. Especially in the case of coated hard metal, graduated formations in the region of the layer, in the region of the layer/base material transition and in the adjacent base material have long been known. This graduation is achieved, for example, by the addition of carbonitrides. During the sintering, the nitrogen in the edge zone of the hard metal body is broken down. The metallic carbide-forming or nitride-forming elements diffuse toward the center of the hard metal body. This results in an increase in the levels of binder in the region of the edge zone and a graduated transition to the matrix composition. For example, disposable cutting tool tips with a binder-rich edge zone adjacent to the hard-material layer have long been in use for the machining of steel. However, the graduation is restricted to a small region close to the surface.
For high loaded components, it is advantageous to establish a structure which is graduated over a wide region. In this way, it is possible to achieve considerable improvements to the service life, specifically if the mechanical demands imposed on the hard metal differ in the edge region and the core region.
Since the usual binder metals, such as for example cobalt, have a high diffusivity at the sintering temperature, it is possible to achieve concentration compensation in the transition zone between two hard metal alloys which have a differing cobalt content by means of diffusion processes. In this way, it is possible to establish a continuous transition. A process for that purpose is described, for example, in U.S. Pat. No. 5,762,843 and European patent EP 0 871 556. A composite body which comprises at least two regions which differ in terms of their binder content is produced by composite pressing. During the sintering, the temperature is to be set in such a way that the binder metal diffuses out of the composite region with the higher binder content into the composite region with a lower binder content. A drawback of this process is that the sintering temperature has to be set very accurately in order not to produce complete concentration balancing and thereby lose the different materials properties. A further drawback is that composite pressing is associated with higher production costs than is the case when a monolithic green compact is being produced. European patent applications EP 0 247 985 and EP 0 498 781 likewise describe hard metal bodies with a binder phase gradient and a process for producing them. In this case, first of all a sintered body with a uniformly distributed η phase is produced by means of standard process steps using a reduced-carbon starting powder mixture. A subsequent treatment in a carburizing atmosphere leads to partial dissolution of the η phase in the region of the edge zone. The level of η phase increases gradually and the binder content decreases gradually in the direction of the center of the hard metal body. However, a drawback is that the η phase has an embrittling action. Moreover, the additional carburizing step is time-consuming and energy-consuming.
European patent application EP 0 111 600 describes a highly loaded tool for rock drilling. The device comprises an inner region and an outer region, with a continuous transition of the mechanical properties between these regions. The proposed process technology is a complex powder feed making it possible to continuously adjust the powder concentration during the filling operation. A powder feed of this nature requires complex apparatus and is difficult to control in terms of process technology.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a hard metal component with a graduated structure and a manufacturing process which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, an article of manufacture made from a hard metal alloy, comprising:
at least one carbide, mixed carbide or carbonitride of the metals selected from the group consisting of W, Ti, Ta, Mo, Zr, Hf, V, Nb, Cr, and V;
at least one grain growth-inhibiting additive selected from the group consisting of V, Cr, Ti, Ta, and Nb or a compound thereof, at least one of said grain growth-inhibiting additives, at least locally, having a graduated concentration profile; and
at least one metallic binder selected from the group consisting of Co, Ni and Fe with a binder content of 0.1-20% by weight.
In accordance with an added feature of the invention, the hard metal alloy has a graduated grain size profile, at least locally. Also preferably, the hard metal alloy has a graduated hardness profile.
In accordance with an additional feature of the invention, the concentration of the grain growth-inhibiting additive decreases gradually from the edge zone of the component toward the center of the component. Conversely, the carbide grain size increases gradually from the edge zone of the component toward the center of the component.
In accordance with an alternative feature of the invention, the concentration of the grain growth-inhibiting additive increases gradually from the edge zone of the component toward the center of the component. Conversely, the carbide grain size decreases gradually from the edge zone of the component toward the center of the component.
In accordance with a preferred embodiment, the grain growth-inhibiting additive consists of Cr and/or V, or a compound thereof. A maximum content of the grain growth-inhibiting additive, based on the hard metal alloy, is 2% by weight, and its content decreases gradually to a value x, where 0<x<1.0% by weight.
With the above and other objects in view there is also provided, in accordance with the invention, a method of producing the component outlined above. The method comprises the following steps:
producing a green compact from a hard metal alloy, containing at least one carbide, mixed carbide or carbonitride of the metals selected from the group consisting of W, Ti, Ta, Mo, Zr, Hf, V, Nb, Cr, and V, at least one metallic binder selected from the group consisting of Co, Ni, and Fe, an optional addition of wax or a plasticizer, with a standard powder metallurgy compacting process or a standard shaping process;
producing a dispersion or solution containing at least one grain growth-inhibiting additive selected from the group of metals consisting of V, Cr, Ti, Ta, and Nb or a compound thereof, in finely distributed or dissolved form;
applying the dispersion or solution to a surface of the green compact, such as, for example, by dipping, spraying, and/or brushing;
targeted action to establish the concentration gradient; and
subjecting the article to heat consolidation.
In accordance with an alternative process, the manufacture comprises the following steps:
producing a green compact from a hard metal alloy, containing at least one carbide, mixed carbide or carbonitride of the metals selected from the group consisting of W, Ti, Ta, Mo, Zr, Hf, V, Nb, Cr, and V, at least one metallic binder selected from the group consisting of Co, Ni, and Fe, an optional addition of wax or a plasticizer;
producing a solution containing at least one grain growth-inhibiting additive selected from the group of metals consisting of V, Cr, Ti, Ta, and Nb or a compound thereof;
applying the solution to a surface of the green compact, for example, by dipping, spraying, and/or brushing;
targeted action to establish the concentration gradient or complete infiltration;
gradually removing the grain growth inhibitor from regions close to the surface by heat treating and/or with a solvent; and
subjecting the article to heat consolidation.
In other words, the above objects are achieved by a component made from a hard metal alloy and by a process for producing it, in which the hard metal alloy contains at least one carbide, mixed carbide or carbonitride of the metals from the group consisting of W, Ti, Ta, Mo, Zr, Hf, V, Nb, Cr and V, at least one grain growth-inhibiting additive from the group consisting of V, Cr, Ti, Ta and Nb or a compound of these metals, and at least one metallic binder from the group consisting of Co, Ni and Fe, at least one of the grain growth-inhibiting additives, at least locally, having a graduated concentration profile.
The graduated concentration profile of the grain growth-inhibiting additive leads to a graduated profile of the carbide grain size. Consequently, the mechanical properties also have a graduated profile. This is advantageous, for example, where a high wear resistance and bending fracture strength is required at the surface and, at the same time, a high toughness is required in the core, such as for example in forming tools or tools for diamond production. If the concentration profile of the gain growth-inhibiting additive is now set in such a way that the concentrations are higher in the regions of the edge zone and decrease in the direction of the center of the component, the edge zone is in fine-grain form, with a graduated transition to the more coarse-grained center. As a result, it is possible to produce components with an excellent wear resistance and bending fracture toughness in the region of the edge zone, in combination with a high toughness in the center. These components have an improved tool service life. A high fracture toughness in the region of the edge zone may also be advantageous in the event of a high cyclical or impact shock loading. This is achieved by a reduced grain growth-inhibiting additive content in the region of the edge zone. The compressive and bending strength properties in the core of the component are improved by a graduated profile of the grain size and a more fine-grained center. This embodiment is also favorable for coated components. The action of the invention is also achieved if the hard metal alloy contains further, non-carbidic hard material phases, provided that the mechanical properties are not significantly adversely affected as a result.
Advantageous grain growth-inhibiting additives worthy of mention are vanadium and chromium compounds, the maximum concentration being 2% by weight. Higher contents lead to embrittlement effects. A particularly advantageous process is the application of a dispersion or solution to the surface of a green compact. The dispersion contains the grain growth-inhibiting additive in extremely finely distributed form. The green compact may be in the as-pressed state. If the green compact contains additions of wax and/or plasticizer, it may also, according to an advantageous configuration of the present invention, be in the dewaxed or partly dewaxed state. The application of the dispersion or solution can be carried out, for example, by dipping, spraying or brushing. The dispersion or solution then penetrates into the interior of the green compact along open pores. The duration of action and the grain growth-inhibiting additive content in the dispersion or solution substantially determine the introduction quantity and the penetration depth. Therefore, depending on the profile of requirements, it is possible to set a graduation which extends only on the micrometer scale. However, it is also possible to make the graduation such that it extends all the way to the center of the component. Furthermore, the process can also be carried out in such a way that first of all the green compact is completely impregnated with the dispersion. This is then removed again from the regions close to the surface by means of suitable solvents or by thermal processes. Furthermore, the dispersion may be applied to the entire surface or alternately only to local parts of the surface. In particular the local application makes it possible to produce components or tools which only have a high hardness where resistance to wear is required. The remaining regions have a coarser microstructure with a high fracture toughness. Furthermore, it has proven advantageous if the carbidic component of the green compact has a mean grain size of less than 2 μm.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a hard metal component with a graduated structure, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.