US20040093985A1

US20040093985A1 - Hard metal body with hardness gradient, such as punching tools

Info

Publication number: US20040093985A1
Application number: US10/363,607
Authority: US
Inventors: Eric Carton; Eric van Eijkeren
Original assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Current assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Priority date: 2000-09-06
Filing date: 2001-09-06
Publication date: 2004-05-20
Also published as: EP1319090A1; CA2421429A1; AU2001294389A1; WO2002020863A1; NL1016112C2

Abstract

The invention relates to a method for the production of a body made of hard metal, consisting of a hard compound A and a binder B, wherein pulverulent A and B, or an optionally precompacted article that contains A and B, are introduced into a container and the material containing A and B is compacted in order to increase the relative density (RD) to a value that is higher than 70% of the theoretical maximum density (TMD). The invention further relates to a body of hard metal comprising a hard compound A and a binder B, the mass ratio of A:B gradually changing over a cross-section of the body in order to impart to said body different mechanical properties in one zone Za or to one end (T) and hardness in one zone Zb or to another end (H). The invention also relates to the use of dynamic compaction techniques for the production of such bodies.

Description

The invention relates to a body made of gradual hard metal, such as punching tools, and a method for the production thereof. The use of static and dynamic compaction techniques known per se for the production of new bodies made of hard metal, such as tools having a hard first end and a tough second end, for example cutting and shaping tools such as punching tools, plays an essential role according to the invention. An explanation of the terms used in this description and claims and of the techniques employed according to the present invention is first given below, before discussing the invention in detail.

Bodies made of hard metal

Bodies made of hard metal are understood to be products which contain a hard compound such as a metal carbide and metallic binder and which have been subjected to sintering or hot isostatic pressing. Their relatively high carbide content makes the hard metal stiff, hard and resistant to wear. The binder imparts the necessary toughness and strength to the whole.

In the present description and claims the hard compound is indicated by A and the metallic binder by B.

The hard compounds A are, for example, carbides, borides, nitrides and diamond.

The hard compounds A have the following characteristics: high hardness, low toughness, high compressive strength, low tensile strength and a high melting point. In addition they are not ferromagnetic or very slightly ferromagnetic. Tungsten carbide is the most widely used hard metallic compound in hard metal products.

The tough compounds B are, for example, metals such as Co, Cr, Ni, Fe (stainless steel) and alloys thereof.

A great deal of information on hard metal products and the methods for the production thereof, including sintering and hot isostatic pressing, is to be found in the general technical literature and in the patent literature. For a general review of “cemented carbides” reference can be made to the ASM Metals Handbook, 9 ^thEdition, Vol. 16, “Machining”, pp 71-83, published in 1989. The contents of this publication must be regarded as incorporated here.

Reference can also be made to the book entitled Cemented Tungsten Carbides, Production, Properties and Testing, 1998, by Gopal S. Upadhyaya, published by Noyes Publications, U.S.A., for the production and properties of and test methods for hard metal products based on tungsten carbide. In this context reference is made in particular to the information on sintering and hot isostatic pressing (HIP) including sinter HIP. The relevant text on pp 110-130 of this publication by Upadhyaya must be regarded as incorporated here.

Static and dynamic compaction techniques

Static and dynamic compaction techniques are known per se for the compaction of pulverulent materials.

Static compaction techniques which can be used in the context of the present invention are therefore generally known and appear to require no further explanation. For a review of static compaction techniques which (can) play a role in the context of the present invention reference can be made to J. S. Reeds, Principles of Ceramic Processing, 2 ^nded, J. Wiley & Sons, New York (1995).

For dynamic compaction techniques which (can) play a role in the context of the present invention reference can be made to the publication entitled “The Dynamic Compaction of Powdered Materials” by S. Clyens and W. Johnson in Materials Science and Engineering, 30 (1977), 121-139. In this publication four main areas in which powder compaction processes are used are indicated: the powder metallurgy, fuels, ceramics and pharmaceutical industries. The following is stated with regard to these industries: “Powder metallurgy fabrication techniques have been developed for three principal reasons. Firstly, because for some components and materials established methods of forming are not suitable, e.g. refractory metals have long been fabricated by powder metallurgy because of the difficulties encountered in melting and casting. Secondly, in some cases powder metallurgy processes are more economical; many iron-base alloys are fabricated from powders into finished components because the savings in materials and machining make it worthwhile. Thirdly, the greater control of both grain size and component distribution afforded by the processes often results in a more homogeneous metallurgical structure.

For many years the ceramic industry has employed compaction techniques to form dry- or slightly moist ceramic powders into a wide range of products. The dry-pressing technique has two main advantages; unlike the wet-forming methods, dry pressing may be fully automated with high rates of production and one operator may take charge of several presses. Also, as the powder is pressed dry, the expense of filter pressing and drying is avoided, so that dimensional tolerances to better than 1% in the fired product may be achieved.

The pharmaceutical industry employs powder compaction processes to form medicinal powders into tablets. Tabletting has been used mainly as a convenient and simple form of dosage and there are now few countries in which tablets are not manufactured.

In the fuel industries many processes lead to the production of powdered fuels which cannot be handled conveniently or re-used. Such powders have often been disposed of as waste representing an economic loss to industry and, in some instances, a major cause of pollution. Compaction processes have been used to form these powders into briquettes which may be handled easily and used again.”

It is pointed out that the above information does not relate to relatively substantial compaction. The compaction technique that is used in the ceramics industry relates, for example, to the pre-pressing of ceramic powders before they are sintered, so that the “dimensional tolerance is better than 1%.” What is concerned here is, in fact, a compaction that is used as possible (non-mandatory) precompaction in the method according to the invention, as is described in more detail further below.

Following the introductory remarks on the static compaction methods that were conventional in 1977, the publication by Clyens et al. gives consideration to the dynamic compaction methods. The following is stated with regard to the difference compared with the compaction methods customary up to that time: “These methods differ from the more conventional consolidation techniques in respect of the compacting pressure and the speed or rate of compaction used. There is now some evidence to suggest that increasing the rate of compaction results in a more uniform density distribution, improved green strength, and in the case of die compaction lower compact ejection forces.” The following is stated in the sentence that runs from page 121 to page 122: “In the powder metallurgy and ceramic industries efforts have concentrated mainly upon developing methods capable of producing high-density components of complex shape or large, semi-finished articles, whereas, in the pharmaceutical and fuel industry, interest has concentrated upon increasing production rates.”

The compaction referred to in the abovementioned literature of course results in a reduction in the porosity. This means that the pore volume is reduced and thus that the density increases. In the present context—including that of the invention discussed further below—the density is not cited as an absolute value (in g/cm ³) but as relative density (RD) with respect to the density of the solid without pores, or the theoretical maximum density (TMD) of the substance. It is clear that with the known compaction discussed above relatively little compaction takes place in the field of hard metal production, the RD of the materials concerned never being raised to a value that is above approximately 65% TMD, which will be immediately apparent to those skilled in the art.

For the dynamic compaction techniques and the mechanisms on which these are based, reference is made to the cited publication by Clyens et al., the contents of which must be regarded as incorporated here.

The problem on which the present invention is based

As indicated in the first paragraph of this description, the invention relates to bodies made of gradual hard metal and the production thereof. The problem on which the present invention is based is explained with reference to punching tools, but comparable problems are experienced with a wide variety of tools which are produced from hard metal and which (have to) possess different mechanical properties in different locations.

Punching tools must be hard (wear-resistant) at the punching edge and be impact-resistant (tough) at the recoil edge. To date it has not been possible to combine these properties within one material and punching tools are therefore in general made up of two materials. The recoil edge is generally made of fast steel (a type of wear-resistant steel), whilst the punching edge consists of hard metal (tungsten carbide with cobalt, WC/Co). These materials are mechanically joined to one another. As a result play develops in the joint during use of the tool, which leads to a reduction in the product quality. This limits the punching time of the punching tool (such as punching stamps), as a result of which this tool has to be replaced prematurely, which has the effect of increasing costs and as a result of which the production process has to be interrupted many times. It would therefore be desirable to have punching tools that are constructed entirely of hard metal, but in which there is nevertheless an impact-resistant (tough) edge and a wear-resistant (hard) punching edge as a result of the gradual change in the composition. With regard to this problem reference is made, for example, to U.S. Pat. Nos. 4,820,492 and 5,543,235.

For instance, it is described in the preamble of U.S. Pat. No. 4,820,482 that diffusion of the binder phase takes place during sintering of bodies made of hard metal, which leads to sintered bodies containing a virtually uniform binder phase. The technique with which starting materials with different particle sizes are used or with which the hard metal body is subdivided into zones of different particle sizes is also discussed. These techniques also fail to produce bodies with, on the one hand, impact-resistant locations and, on the other hand, wear-resistant locations in a controlled manner. This patent gives a solution to this problem by the use of “carburizing” the sintered body to such an extent that the so-called eta phase can be completely removed during this operation. With this technique a body is produced that has two zones: a core with a relatively high binder content and a surface zone with a relatively low binder content (and possibly small amounts of free graphite). It is true that a body which has a core that has different properties to the surface can be obtained with this technique, but bodies which have an impact-resistant edge and a hard punching edge, in which there is a gradual transition of the binder content, cannot be produced according to this technique.

The abovementioned U.S. Pat. No. 5,543,235, in which “multiple grade” products of hard metal and a method for the production thereof are described, is of more recent date (1996). It is also reported in the preamble of this patent (column 1, lines 56-64) that “cemented carbide articles are invariably fabricated having a substantially uniform composition and microstructure . . . and substantially uniform properties throughout the volume of the article. Many such substantially homogeneous compositions exist in the prior art.” According to this U.S. patent the starting materials for the hard metal body are introduced into a mould that can be subdivided into various compartments, it being possible to remove the partitions between these compartments. The compartments are filled with different mixtures, after which the partitions are removed and the powder is compressed to give a “single compact” of the desired shape. Sintering is then carried out.

The problem is also discussed in the recent (1998) abovementioned book entitled Cemented Tungsten Carbides by G. Upadhyaya, in particular pp. 365/366, where the following is stated under “Functionally Graded Cemented Carbides”: “Composition gradient cemented carbide tools are expected to offer a number of advantages for specific engineering applications. For example, a tough bulk and a hard surface would be interesting in the case of cutting picks used in the mining industry, which are subjected to shocks.

The first attempt to study a model of such a type of structure was done by Cooper et al. in order to explain the migration of cobalt from the coarse grained to fine grained layers of a hard metal composite. The driving force for migration was the higher capillary forces existing in the fine grained layer during liquid phase sintering.

Coling et al. investigated multilayer graded structures in WC-Co cemented carbides with cobalt content varying from 10-30 wt. % from one side of the structure to the other, prepared by solid state or liquid phase sintering routes. In the case of solid state sintering, the graded structure remained after sintering, as there was no risk of homogenization during such sintering. In the case of liquid phase sintering, the sintering time had to be much shorter because densification occurred much faster with the liquid phase. This required precise control of sintering time, which had to be as short as possible in order to avoid homogenization of the structure. In the former case, to obtain dense material, post-sintering treatment became necessary.”

According to the invention the problem of undesired mass transport, leading to homogenisation, is solved in that a method is provided which makes it possible to produce bodies that consist virtually completely of hard metal with a minor amount of binder, in which the composition of A and B has a gradual transition.

The invention therefore relates to a method for the production of a body made of hard metal, consisting of a hard compound A and a binder B, wherein

chosen amounts of pulverulent A and B, or of an optionally precompacted article that contains A and B in chosen amounts, are introduced into a container and

the material containing A and B is subjected to compaction in one or more steps in order to increase the relative density (RD) to a value that is higher than 70% of the theoretical maximum density (TMD) with the formation of a body made of hard metal, in which

on the one hand at least one zone (Za) containing a relatively large amount of B and, on the other hand, at least one zone (Zb) containing a relatively small amount of B are present and

the amount of B gradually decreases from at least one zone Za to at least one zone Zb,

after which said body is optionally subjected to sintering, hot isostatic pressing (HIP) or sinter HIP.

Where mention is made in the present description and claims of compaction, this does not refer to the consolidation that takes place during a temperature treatment such as sintering, HIP or sinter HIP.

The method according to the invention makes it possible to immobilise B with respect to A in a desired and controlled manner. The term immobilisation of B with respect to A is used to indicate that complete mass transport of B does not take place, or there is virtually no mass transport of B, during sintering or hot isostatic pressing (HIP). The invention is based on the insight that this desired immobilisation of B, that is to say incomplete mass transport during sintering or HIP, can be achieved by reducing the pore volume of the pulverulent starting material A and B to a suitable value. This reduction in the pore volume goes further than the precompaction known from the prior art, which precedes sintering or HIP of precursors. As has already been stated above, the precompaction according to the prior art is of the order of magnitude of at most 65% TMD.

In order to obtain the desired immobilisation of B during sintering or HIP it is preferable to achieve a compaction of the starting material in excess of 80% TMD, in particular in excess of 90% TMD.

The greater the compaction the less is the transport of B during sintering or HIP. The transport or immobilisation of binder B can be controlled via the T,t effect. High compaction (even up to 100% TMD) ensures a very appreciable or even complete immobilisation of B with respect to A according to the invention if the compacted product is subjected to sintering or HIP. Thus, too long a sintering time and/or too high a sintering temperature will still (ultimately) lead to a hard metal with a homogeneous composition. On the other hand, too short a sintering time and/or too low a temperature will result in inadequate diffusion of B, as a result of which the (too) steep B gradient will continue to exist in the end product.

The problem with conventional hard metal production is that the T,t combination that is required for removal of the porosity (a primary requirement for strong products) already produces homogenisation of the binder B. The compaction according to the invention makes it possible to sinter gradual hard metal to close all pores before homogenisation takes place to a substantial extent or completely.

No special requirements with regard to the particle sizes of A and B are imposed with the method according to the invention. In this context reference can be made to the values for the particle sizes as are used in conventional hard metal production.

The compaction according to the invention can be carried out in one step or in various steps. It is often effective to subject the powder of A and B, which is introduced into a container in a controlled manner, to a precompaction, as a result of which the subsequent final compaction step or steps are more effective. Therefore, according to a preferred embodiment, the invention relates to a method as described above, wherein the compaction is carried out in two steps:

(a) a first compaction (precompaction) to increase the RD to a value of at most 70% TMD;

(b) a second compaction in which the RD of the precompacted powder or article from step a) is further increased to a value above 70% TMD, preferably above 80% TMD, in particular to above 90% TMD.

The method according to the invention makes it possible for mechanical joins, which are frequently used in tools such as punching tools, to be eliminated because, according to the invention, tools can be produced which are made up entirely of hard metal (with a varying percentage of binder). Because the composition of A and B gradually changes there is still an impact-resistant (tough) edge and a wear-resistant (hard) punching edge. Therefore, the invention also relates to a method as described above wherein different mixtures of A and B are introduced into two or more zones of the container, the mass ratios of A:B in the two or more zones having different values.

For, for example, the production of punching tools, according to the invention a method is employed in which the container has an elongated shape and the container is filled with different mixtures of A and B in such a way that the quantity of binder at the one end of the shape H is lower than that at the other end of the shape T. Possible embodiments are explained in more detail further below in this description with reference to drawings.

Thus, with the method according to the invention, in general use will be made of varying amounts of A and B, the amount of B at the hard end of course being low and the amount of B at the tough end being relatively high. In this context it is preferable that the amount of B at end H of the shape is at least 1% (m/m) and the amount of B at the end T of the shape is at most 50% (m/m), the amounts being based on the mass of the total mixture. The invention makes it possible to allow the amount by mass of B to increase gradually from end H to end T. Where mention is made in this description of “end H” and “end T” this is also intended to refer to zone Zb and zone Za, respectively. By the choice of the “geometry” of the starting materials (sequence) it is possible to define articles having a wide variety of conceivable zones of hard and tough regions, including “within” the articles.

Starting materials A and B that can be used are the known hard compounds on the one hand and the known metallic binders on the other. In this context reference is made to the literature cited in the preamble to this description. Preferably, A is chosen from the group consisting of diamond or carbides such as SiC, WC, TiC, TaC, NbC, ZrC, HfC, Cr ₃C₂, Mo₂C, nitrides such as TiN, HfN and BN and borides such as TiB₂and ZrB₂, in particular tungsten carbide, and B is chosen from the group consisting of the metals Co, Cr, Ni, Fe (for example stainless steel) and alloys thereof, in particular cobalt.

The compaction according to the invention is preferably carried out at a temperature at which no mass transport of the one component into the other component takes place, that is to say diffusion of both B and A is avoided. This means that no special measures have to be taken with regard to the temperature. Compaction is preferably carried out at ambient temperature. This contributes to simple implementation of the method according to the invention.

It will be clear that the reduction in the pore volume of pulverulent A and B constitutes the core of the present invention. This reduction in pore volume can be achieved in accordance with methods known per se. With the method according to the invention use is generally made of static or dynamic compaction techniques, preferably (iso)dynamic compaction techniques, such as pneumomechanical uniaxial compaction, ballistic compaction, explosive compaction, including shock compaction, and magnetic compaction, for the compaction. For the compaction techniques reference is made to the information and literature given in the preamble to this description.

The invention also relates to hard metal bodies which are obtainable according to the abovementioned methods according to the invention, as well as bodies of hard metal comprising a hard compound A and a binder B, the mass ratio of A:B changing over a cross-section of the body in order to impart to said body different mechanical properties such as, on the one hand, toughness in at least one zone Za or at at least one end (T) and, on the other hand, hardness in at least one zone Zb or at at least one other end (H), the change in the ratio of A:B being gradual. The invention also relates to the use of dynamic compaction techniques such as pneumomechanical uniaxial compaction, ballistic compaction, explosive compaction including shock compaction and magnetic compaction for the production of one-piece bodies made of hard metal having at least one hard zone (Zb) or hard end (H) and at least one tough zone (Za) or tough end (T). It is not known from the prior art to use compaction techniques as specified above for the substantial reduction in pore volume according to the invention (that is to say to in excess of 70% TMD) of pulverulent mixtures or materials which serve as starting material for bodies made of hard metal.

An important field of application of bodies according to the invention is punching tools.

Examples of gradual embodiments according to the invention are shown in FIGS. [0053] 1-4.
In the figures the differences in concentration of the components A and B are shown by grey tints. In this context white indicates a relatively low content of hard compound (a relatively large amount of binder) and black indicates a relatively large amount of hard compound (relatively little binder). [0054]
In FIG. 1 it can be seen that as a result of the very steep concentration gradient in the compression surface after precompaction (midway between T and H) (some) transport of B still takes place during HIP, which levels off the concentration gradient of B and a gradual hard metal is spontaneously formed. [0055]
In FIGS. [0056] 2-4, in each of which a different sequence of A and B is used as the starting point, it can be seen that a shallow gradient yields too little driving force for diffusion of B, as a result of which the gradual composition is retained during HIP.
From the above examples it can thus clearly be seen that gradual patterns of diverse types can be defined.[0057]

EXAMPLES

According to these examples hard metal is produced from a major fraction of tungsten carbide and a minor fraction of cobalt. The starting materials used are the Grade 8 material known to those skilled in the art and WC/Co 70/30 with, respectively, 8 and 30% (m/m) Co. In order to make gradual hard metal these starting materials are mixed with one another in various ratios, as shown in the table below; see also the “stacking” in FIG. 5. [0058]

TMD

Grade WC/Co 70/30 Co fraction Hardness of end of mixture

8 fraction fraction in mixture product [GPa] [g/cm³]

100 0 8 1250 14.74

75 25 13.5 1120 14.18

50 50 19 1010 13.67

25 75 24.5 900 13.19

0 100 30 810 12.74
The compositions (powder compacts) indicated in the above table are precompacted to approximately 50% TMD by means of cold uniaxial pressing. This pre-pressing is carried out with the aid of a tube made of stainless steel with an internal diameter of 20 mm and a wall thickness of 1.5 mm, which is closed off at one end by a stainless steel stopper (see also FIG. 5, which is discussed in more detail below). The bottom half of the tube is then in each case filled with a hard metal powder in the WC/Co mass ratio as indicated in the above table. The top half of the tube is filled with hard metal powder containing a Co fraction as indicated in the above table. After each introduction of a small amount of powder the powder is subjected to uniaxial initial pressing under a pressure of approximately 100 MPa. After this filling and precompaction process the tube is closed off at the top with a stainless steel stopper. [0059]
An alternative embodiment of the precompaction of the powder is to make use of a cold isostatic press (CIP). In this case the powder (or the various powder mixtures) are poured into a cylindrical rubber container, after which the container is closed off by a stopper, that is likewise made of rubber. The container containing the powder is placed in the CIP filled with fluid, after which the CIP is hermetically sealed. The fluid is brought up to pressure (3000 bar) using a pump, the powder in the rubber container being isostatically precompacted. An initial density of the powder of 64% TMD is achieved by this method. After the cylindrical powder compact has been removed from the rubber container a metal foil (copper foil with a thickness of 0.1 mm) is wrapped around it, by which means the diameter of the compact can be matched to the internal diameter of a metal tube that is just somewhat larger. The tube is then closed off at both ends again using metal stoppers. [0060]
Dynamic compacti n [0061]
For the dynamic (explosive) compaction of the powder the tube is glued in place centred in a PVC cylinder having a length of 175 mm, an internal diameter of 76 mm and a wall thickness of 4 mm. The intermediate remaining space is filled with an explosive powder based on ammonium nitrate. The detonation speed of the explosive powder is 3.6 km/s. On detonation of the explosive (mainly) the diameter of the metal tube is reduced and by this means the hard metal powder is compacted to a relative density of approximately 90% TMD. A diagrammatic representation of such a set-up for explosive compaction is shown in FIG. 5, in which the reference numerals have the following meaning: [0062]
[0063] 1.=detonator;
[0064] 2.=powder explosive;
[0065] 3.=PVC tube;
[0066] 4.=metal tube;
[0067] 5.=hard metal powder;
[0068] 6.=metal stopper;
[0069] 7.=substrate.

Claims

1. Method for the production of a body made of hard metal, consisting of a hard compound a and a binder b, wherein

2. Method according to claim 1, wherein the material containing A and B is compacted to a RD value in excess of 80% TMD, preferably in excess of 90% TMD.

3. Method according to claim 1 or 2, wherein the compaction is carried out in two steps:

4. Method according to one or more of the preceding claims, wherein different mixtures of A and B are introduced into two or more zones of the container, the mass ratios of A:B in the two or more zones having different values.

5. Method according to one or more of the preceding claims, wherein the container has an elongated shape and the container is filled with different mixtures of A and B in such a way that the quantity of binder at the one end of the shape (H) is lower than that at the other end of the shape (T).

6. Method according to one or more of the preceding claims, wherein the amount of B in zone Zb is at least 1% (m/m) and the amount of B in zone Za is at most 50% (m/m), the amounts being based on the mass of the total mixture.

7. Method according to one of the preceding claims, wherein the amount by mass of B, optionally gradually, increases from zone Zb to zone Za.

8. Method according to one or more of the preceding claims, wherein A is chosen from the group consisting of diamond or carbides such as SiC, WC, TiC, TaC, NbC, ZrC, HfC, Cr₃C₂and Mo₂C, nitrides such as TiN, HfN and BN and borides such as TiB₂and ZrB₂, preferably tungsten carbide, and wherein B is chosen from the group consisting of the metals Co, Cr, Ni, Fe (for example stainless steel) and alloys thereof, in particular cobalt.

9. Method according to one or more of the preceding claims, wherein the compaction is carried out at a temperature at which no or virtually no mass transport of B or A takes place, preferably ambient temperature.

10. Method according to one or more of the preceding claims, wherein use is made of static or (iso)dynamic compaction techniques, preferably dynamic compaction techniques such as

pneumomechanical uniaxial compaction;

ballistic compaction;

explosive compaction, including shock compaction, and

magnetic compaction,

for the compaction.

11. Body obtainable according to the method of one or more of the preceding claims.

12. Body of hard metal comprising a hard compound A and a binder B, the mass ratio of A:B changing over a cross-section of the body in order to impart to said body different mechanical properties such as, on the one hand, toughness in at least one zone Za or at at least one end (T) and, on the other hand, hardness in at least one zone Zb or at at least one other end (H), the change in the ratio of A:B being gradual.

13. Body made of hard metal according to claim 11 or 12, comprising a punching tool.

14. Use of dynamic compaction techniques as specified in claim 10 for the production of one-piece bodies made of hard metal having at least one hard zone (Zb) or hard end (H) and at least one tough zone (Za) or tough end (T).