Note: Descriptions are shown in the official language in which they were submitted.
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HARD METAL BODY WITH HRADNESS GRADIENT, SUCH AS PUNCHING TOOLS
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"' Edition, Vol.
16,
"Machining", pp 71 - 83, published in 1989. The contents of this publication
must be
regarded as incorporated here.
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2
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, USA, 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°a ed, 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
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3
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
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4
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 glcm3) 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 duringwse 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
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edge as a result of the gradual change in the composition. With regard to this
problem
reference is made, for example, to US Patents 4 820 492 and 5 543 235.
For instance, it is described in the preamble of US Patent 4 820 482 that
diffusion of
the binder phase takes place during sintering of bodies made of hard metal,
which leads to
5 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, weax-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 US Patent 5 543 235, in which "multiple grade" products of
hard metal and a method for the production thereof axe described, is of more
recent date
(1996). It is also reported in the preamble of this patent (column 1, lines 56
- 64) that
"cemented caxbide 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 US 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
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6
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 HIP 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,
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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
S 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 subj ected 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.
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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
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9
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,
Cr3C2, MozC, nitrides such as TiN, HfN and BN and borides such as TiB2 and
ZrBz, 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
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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
5 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.
10 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
Figures
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).
In Figure 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.
In Figures 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.
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 WCIGo 70/30 with, respectively,
8 and
30 % (m/m) Co. In' order to make gradual hard metal these starting materials
are mixed
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11
with one another in various ratios, as shown in the table below; see also the
"stacking" in
Fig. 5.
WC/Co 70/30 Co fraction Hardness of TMD of
Grade 8 fractionfraction in end mixture [g/cm3]
mixture product [GPa]
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 Figure 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.
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.
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Dynamic compaction
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 Figure 5, in which the reference numerals have the
following
meaning:
1. = detonator;
2. = powder explosive;
3. = PVC tube; .
4. = metal tube;
5. = hard metal powder;
6. = metal stopper;
7. = substrate.
