U.S. patent application number 14/782987 was filed with the patent office on 2016-10-13 for annular tool.
The applicant listed for this patent is Bernhard FEISTRITZER. Invention is credited to Bernhard FEISTRITZER.
Application Number | 20160298451 14/782987 |
Document ID | / |
Family ID | 50729301 |
Filed Date | 2016-10-13 |
United States Patent
Application |
20160298451 |
Kind Code |
A1 |
FEISTRITZER; Bernhard |
October 13, 2016 |
ANNULAR TOOL
Abstract
The invention relates to an annular tool (1) having at least one
working region (4) oriented radially outward and having high wear
resistance, and a clamping part (5) closer to the axis, in
particular a roller bit or cutting ring for rock, in particular for
tunnel boring machines, made of a material which is formed from an
iron-based alloy as matrix having incorporated hard material
particles, wherein the hard material particles are formed from
carbide and/or nitride and/or oxide and/or boride, possibly as
carbonitride or oxycarbonitride having a boron component of at
least one of the elements, or in mixed form of the elements from
groups 4 and 5 of the periodic system, and have a density at room
temperature of more than 7400 kg/m.sup.3, preferably of more than
7600 kg/m.sup.3. The invention further relates to methods for the
production thereof.
Inventors: |
FEISTRITZER; Bernhard;
(Pfarrwerfen, AT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FEISTRITZER; Bernhard |
Pfarrwerfen |
|
AT |
|
|
Family ID: |
50729301 |
Appl. No.: |
14/782987 |
Filed: |
April 9, 2014 |
PCT Filed: |
April 9, 2014 |
PCT NO: |
PCT/AT2014/050084 |
371 Date: |
October 7, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/48 20130101; C22C 38/44 20130101; B22D 13/04 20130101; C21D
8/10 20130101; C22C 1/02 20130101; C22C 33/0278 20130101; C22C
38/56 20130101; C22C 38/46 20130101; C22C 33/0228 20130101; C22C
38/00 20130101; E21C 25/18 20130101; C22C 38/02 20130101; C21D 9/40
20130101; B22F 2005/002 20130101 |
International
Class: |
E21C 25/18 20060101
E21C025/18; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; B22D 13/04 20060101
B22D013/04; C22C 38/02 20060101 C22C038/02; C22C 1/02 20060101
C22C001/02; C21D 8/10 20060101 C21D008/10; C21D 9/40 20060101
C21D009/40; C22C 38/56 20060101 C22C038/56; C22C 38/04 20060101
C22C038/04 |
Claims
1. Annular tool (1) having at least one working region (4) oriented
radially outward with high wear resistance and a clamping part (5)
closer to the axis, in particular a roller bit or cutting ring for
rock, in particular for tunnel boring machines, characterized in
that the tool is composed of a material that is formed from an
iron-based alloy as a matrix with incorporated hard material
particles, wherein the hard material particles are formed from
carbide and/or nitride and/or oxide and/or boride, possibly as
carbonitrides or oxycarbonitride with a boron component, at least
of one of the elements, or in mixed form of the elements, from
groups 4 and 5 of the periodic system, and have a density at room
temperature of greater than 7400 kg/m3, preferably of greater than
7600 kg/m3.
2. Tool (1) according to claim 1, characterized in that the hard
material particles are present in the tool to an extent of at least
5 vol. %, in particular of more than 8 vol. %, wherein the hard
material particles are inhomogeneously distributed across the tool
cross-section (2) and have a higher volume fraction in the working
region (4).
3. Tool (1) according to claim 1, characterized in that the working
region (4) has a volume fraction of at least 8.0%, preferably of at
least 14.0%, in particular of approximately 20% to 25%, of the tool
(1), in which working region more than 60 vol. %, preferably more
than 75 vol. %, of the hard material particles are formed with a
size of less than 70 .mu.m.
4. Tool (1) according to claim 1, characterized in that the hard
material particles are essentially formed as niobium-vanadium mixed
carbides, possibly with a nitrogen component, and that they have a
ratio of atom % of Nb to atom % of V of greater than 5, preferably
greater than 10. Nb [atom %]/V [atom %]>5, preferably>10
5. Tool (1) according to claim 1, characterized in that the matrix
alloy has a chemical composition by wt.% within the limits of
Carbon (C) 0.28 to 2.3 Silicon (Si) 0.01 to 2.0 Manganese (Mn) 0.05
to 25.0 Chromium (Cr) up to 6.0 Nickel (Ni) up to 2.5 Molybdenum
(Mo) up to 2.2 Tungsten (W) up to 1.5 (1.5.times.Mo+W) up to 3.5
Vanadium (V) up to 0.8 Niobium (Nb) up to 0.4 Cobalt up to 3.0
Aluminum (Al) up to 3.0, possibly Titanium (Ti) up to 0.2 Zirconium
(Zr) up to 0.2 Hafnium (Hf) up to 0.1 Tantalum (Ta) up to 0.25 Iron
(Fe) and impurity elements as the remainder.
6. Tool (1) according to claim 5, characterized in that the matrix
alloy is composed of tool steel with a hardness of greater than 44
HRC, preferably of 50 HRC and higher.
7. Tool (1) according to claim 5, characterized in that the matrix
alloy is composed of austenitic manganese steel with a manganese
concentration of 6 to 25 wt. % Mn, preferably of 8 to 15 wt. %
Mn.
8. Method for producing annular tools (1) having at least one
working region (4) oriented radially outward and a clamping part
(5) closer to the axis, in particular roller bits or cutting rings
for rock, in particular for tunnel boring machines, formed from an
iron-based alloy as a matrix in which hard material particles, such
as carbides and/or nitrides and/or carbonitrides and/or borides,
possibly in mixed form of the elements from groups 4 and/or 5 of
the periodic system, are incorporated, possibly for the production
of a tool according to at least one of the preceding claims,
wherein a base alloy is melted and heated to a temperature of
1350.degree. C. to 1630.degree. C. in a first step and an addition
or a formation of hard material particles with a higher density to
or in the melt of the base alloy occurs in a second step, whereupon
in a third step, the matrix melt with the hard material particles
is subjected to a rotational motion about the longitudinal axis in
a mold for the annular tool and is allowed to solidify.
9. Method according to claim 8, wherein in a first step, a base
alloy is melted with a chemical composition by wt. % of Carbon (C)
up to 2.5 Silicon (Si) 0.01 to 3.0 Manganese (Mn) 0.05 to 28.0
Chromium (Cr) up to 9.0 Nickel (Ni) up to 4.3 Molybdenum (Mo) up to
3.5 Tungsten (W) up to 2.2 (1.5.times.Mo+W) up to 5.1 Vanadium (V)
up to 6.0 Niobium (Nb) up to 35.0 Aluminum (Al) up to 3.5, possibly
Titanium (Ti) up to 2.0 Zirconium (Zr) up to 3.0 Hafnium (Hf) up to
1.0 Tantalum (Ta) up to 5.0 Cobalt (Co) up to 3.0 Iron (Fe) and
impurity elements as the remainder.
10. Method according to claim 8, wherein in the second step, the
hard material particles, such as carbides and/or nitrides and/or
oxycarbonitrides and/or borides, possibly as carbonitrides and/or
oxycarbonitrides with boron components, at least of one of the
elements, or in mixed form of the elements, from groups 4 and 5 of
the periodic system, are introduced into the liquid base alloy by
means of a solid or liquid metallic premelt or by means of a
similar mixture of metal and hard material particles with a
diameter of the hard particles of less than 70 .mu.m and
homogeneously distributed in the base alloy, whereupon in the third
step, a solidification of the mixture of hard material particles
and a matrix alloy, formed from the base alloy and the metal
component of the premelt, occurs during rotational motion in the
mold.
11. Method according to claim 8, wherein the base alloy with a
carbon content of under 0.6 wt. % C is melted and heated to a
temperature of 1550.degree. C. to 1630.degree. C., whereupon in a
second step, an addition of the alloy elements carbon and/or
nitrogen and/or boron, possibly as a pre-alloy, takes place and
wherein these elements form, with the dispersed elements from group
4 and/or group 5 of the periodic system, primary carbides and/or
nitrides and/or borides and/or compounds or mixtures thereof in the
melt, wherein the hard material particles being formed have a total
proportion of carbon, nitrogen and boron of atomic ratios from 0.4
to 0.55 and a higher density than the melt, and that 0.3 to 2.3 wt.
% of carbon remains in the liquid metal, whereupon in a third step,
the melt is subjected to a rotational motion about the longitudinal
axis in a mold for the annular tool and is allowed to cool, and a
working and a heat treatment of the tool take place in additional
steps.
12. Method according to claim 11, wherein the elements from groups
4 and 5 of the periodic system are selected in terms of their
respective concentration in the base alloy such that the density of
the primarily precipitated hard material particles is greater than
that of the melt at a temperature 50.degree. C. above the liquidus
temperature.
Description
[0001] The invention relates to an annular tool having at least one
working region oriented radially outward with high wear resistance
and a clamping part closer to the axis, in particular a roller bit
or cutting ring for rock, in particular for tunnel boring
machines.
[0002] Furthermore, the invention relates to a method for producing
annular tools having at least one working region oriented radially
outward and a clamping part closer to the axis, in particular
roller bits or cutting rings for rock, in particular for tunnel
boring machines, formed from an iron-based alloy as a matrix in
which hard material particles, such as carbides and/or nitrides
and/or carbonitrides and/or borides, possibly in mixed form of the
elements from groups 4 and/or 5 of the periodic system, are
incorporated.
PRIOR ART
[0003] Boring devices for rock formations or bedrock and the like
are, for larger diameters, typically equipped with annular tools
which comprise a working region oriented radially outward and which
roll off of the rock base under pressure and thereby cause a
removal or breaking-away of the rock base.
[0004] Tunnel boring machines, for example, have a large
disk-shaped tool holder in which a plurality of what are referred
to as roller bits or cutting rings are installed in a rotatable
position. When driven forward, the tool holder is rotated and
pressed against the rock with a high force, wherein the roller bits
arranged at different radii of the tool holder have a breaking
effect in the respective regions of the rock and wherein the
removed rock, or what is referred to as the chippings, is
transported away behind the tool holder.
[0005] In accordance with the mechanical requirements, the annular
tool with a tapered working region oriented radially outward is to
have in this region high wear resistance as well as high hardness
and high toughness of the material.
[0006] In most cases, the tool blank is shrink-fitted onto an axle,
wherein tensile stresses are inevitably produced in the clamping
region, which forces are, in the heavy operation breaking the hard
rock, respectively overlapped the compression stresses on the
material necessary for the operation and do not produce any
essentially stationary stresses on the tool material.
[0007] Roller bits are thus to comprise a working region with the
highest possible wear resistance and a clamping region with
sufficiently high hardness and high toughness and are to have
overall superior breakage protection of the material under
alternating mechanical stress, since a failure of a tool causes
costly repair work with downtime of the boring machine.
[0008] The cutting rings are normally composed of a tool steel. The
shaping generally occurs via a forging process, wherein the desired
material properties are achieved by a subsequent heat treatment. It
is known to the ordinarily skilled artisan that, for tool steels, a
highest possible wear resistance can only be achieved with a high
hardness of the structure. Here, it must be assumed that the
toughness of the structure decreases as the hardness increases. To
achieve the best properties for tool steels with respect to the
harsh use as a cutting ring, a compromise must be made between
superior wear resistance and high toughness.
[0009] Various attempts were made to extend the service life of the
cutting rings by combining extremely wear-resistant materials with
hard but tough materials. DE 10 2005 039 036 B3, for example,
describes a roller bit made of steel that comprises welded-on
segments in the working region, wherein these segments contain hard
metal particles of tungsten carbide. From JP 2000001733 A, a
similar cutting ring is known which has a hard metal ring attached
to a base body of nodular cast iron at the outer circumference.
Furthermore, from the documents JP 2007138437 A, GB 1188305, GB
1379151, DE10300624A1 and DE 101 61 825 A1, cutting rings for
tunnel boring machines are known which have segments or cylindrical
and other specially formed parts made of hard metal arranged at the
outer circumference which are connected to the base body by
soldering, compression or molding. CA 2 512 737 A1 also describes a
cutting ring in which segments of hard metal are axially clamped
between two disks. All of these known attempted solutions involve
either very costly and difficult production or result in the
premature failure of the cutting ring during use, for example, due
to high thermal stresses during use or due to the softening of the
solder. In JP 59144568 A, a production method for cutting rings is
described in which a melt that contains tungsten carbide-based hard
metal particles is cast into a rotating mold, whereupon the hard
metal particles are concentrated in the outer region of the cast
body. This method has the disadvantage that the hard metal
particles added to the melt are partially dissolved by the melt and
that undesired, brittle structural components can form in the
structure of the tool during the solidification. The minimum size
of the added hard metal particles is also limited by the
dissolution process.
OBJECT OF THE INVENTION
[0010] The object of the invention is to create a generic annular
tool that enables an increased service life during harsh,
bedrock-breaking operation.
[0011] It is also the object of the invention to specify a method
of the type named at the outset for producing annular tools which,
according to the respective demands, have an optimal material
structure.
[0012] The aforementioned object of creating a generic annular tool
that enables an increased service life during harsh,
bedrock-breaking operation is achieved in that the tool is composed
of a material which is formed from an iron-based matrix alloy with
hard material particles incorporated therein. The hard material
particles can thereby be formed from carbide, nitride, oxide or
boride or as compounds thereof, such as carbonitride, carboboride
or oxycarbonitride with a boron component. Depending on the case of
application, it can be advantageous that mixtures of these
different types of hard materials are contained in the tool. The
metal component in the hard material particles comes primarily from
groups 4 and 5 of the periodic system (Ti, Zr, Hf, V, Nb, Ta),
wherein here, too, only individual elements from these groups, or
mixtures thereof, can be contained in the hard materials. Unlike
the hard materials often used in iron metallurgy, whose metallic
components stem from group 6 of the periodic system (for example,
tungsten carbide), hard materials of metals from groups 4 and 5
have the advantage that they exhibit only a slight solubility in an
iron base melt at the melting and casting temperatures of
iron-based alloys of up to 1650.degree. C. commonly found in
practice.
[0013] It is known that hard materials which are formed or
precipitated during the solidification of an iron base melt and
during the subsequent cooling of the resulting workpiece preferably
form eutectic crystalline structures or are precipitated at grain
boundaries. The hard materials formed in such a manner can
significantly reduce the toughness of the structure. The advantage
of the low solubility of the above hard materials in an iron base
melt is then that, on the one hand, large quantities of these hard
materials can be contained as solid particles in the melt, whereas
on the other hand, only small amounts of additional hard material
particles are formed or precipitated in the structure during the
solidification of the melt and during the subsequent cooling of the
workpiece. These small amounts of brittle hard materials have only
a slight negative influence on the toughness of the structure.
These can even increase the toughness, however, if the precipitated
particles are fine enough to reduce a grain growth of the matrix
during a heat treatment.
[0014] In order to achieve a high wear resistance and long service
life of the roller bits, a minimum amount of hard material
particles is to be present in the structure and the hard material
particles are also to be distributed in the cutting ring in such an
inhomogeneous manner that a high proportion thereof is located in
the working region of the roller bit, which region is oriented
radially outward. For what is considered a sufficient volume
fraction of the wear-resistant working region of approximately 8
percent by volume (vol. %), a hard material content of at least 5
vol. %, based respectively on the entire workpiece, has proven to
be suitable. At least 8 vol. % of hard material particles is
necessary if harsh working conditions are intended for the cutting
ring. The possible service life of the roller bit can be increased
with a larger volume fraction of the working region. For example,
the proportion of the working region can be increased up to
approximately 25 vol. % and higher in order to enable long service
life under simultaneously difficult conditions of use.
[0015] The desired distribution of the hard material particles in
the cutting ring is achieved when the density thereof is higher
than the density of the melt, and if the particles thus move
outward in the centrifugal casting process. Tests have shown that
good results are already achieved when the density of the hard
material particles at room temperature is greater than 7400
kg/m.sup.3. A desired high concentration of the hard material
particles is achieved when the particles have a density greater
than 7600 kg/m.sup.3 at room temperature. Hard materials with this
density are, for example, carbides, nitrides and carbonitrides of
niobium, which have proven effective in tests. It has also been
shown that a small addition of vanadium to these niobium hard
materials can advantageously influence the growth and the
properties of the particles, but that the density of the particles
decreases with the addition of vanadium. A ratio of Nb atom %/V
atom %>5 for niobium-vanadium mixed carbides, which can also
possibly be carbonitrides, should be maintained in any case. Higher
concentrations of these particles in the working region are
achieved with a ratio of Nb atom %/V atom %>10.
[0016] It is known to the ordinarily skilled artisan that the wear
resistance of a structure is not only dependent on the hardness of
the matrix and of the incorporated hard material particles, as well
as on the proportions thereof, but also on the size distribution of
the hard material particles. All structural components that are not
the hard material particles referred to above are to be understood
below as meaning the matrix. If the hard material particles are too
small, then they can be stripped from the matrix as whole particles
during grooving wear without notably increasing the wear
resistance. However, if the particles are too large, they can
fracture under the high compressive load while being used to break
bedrock and thus also cannot adequately increase the wear
resistance. In the present case of the roller bits, it has been
shown that superior results can be achieved if at least 60 vol. %,
preferably at least 75 vol. %, of the hard material particles are
formed with a size of less than 70 .mu.m.
[0017] In addition to the properties of the hard material
particles, the properties of the matrix are also of critical
importance in order to achieve high wear resistance in the working
region of the roller bits. In particular, the properties of the
matrix are critical in order to enable a sufficient toughness of
the structure both in the working region and also in the clamping
region. The properties of the matrix are primarily determined by
the chemical composition thereof and by a possible heat treatment.
Carbon is the most important alloy element and influences above all
the hardenability of the steel, wherein approximately 0.28% C is
considered the lower limit for a sufficient hardenability of the
steel for the present purpose of use. With a carbon content of over
1.2% in the matrix, a carbide network can form in the structure,
which network reduces the toughness of the same. Silicon increases
the strength and the wear resistance, as well as the castability of
the melt, but should not exceed 2% in the matrix. Manganese
decreases the critical cooling rate for the formation of the
martensite and, at a sufficient quantity of up to 2%, enables an
air hardening of the cutting rings. By means of higher manganese
contents of up to 25%, the solubility of carbon in the austenite
can be significantly increased and the transformation properties of
the austenite during cooling or mechanical loading can be
influenced. With manganese contents of up to 25%, the carbon
amounts in the matrix can also be up to 2.3%. Similar to manganese,
chromium also increases the hardenability of the steel and forms
secondary and tertiary carbides which are precipitated out of the
austenite and increase the wear resistance, wherein excessively
high chromium contents lead to a chromium carbide network in the
structure. The chromium content should therefore not be higher than
6.0%. Like manganese and chromium, nickel also facilitates the
martensite formation and additionally increases the toughness of
the matrix. For nickel, a content of 2.5% as an upper limit in the
matrix appears to be sufficient for achieving the necessary
properties. For setting a low critical cooling rate, a combination
of Mn, Cr and Ni has proven effective. At up to 2.2%, molybdenum
increases the strength of the matrix and, through the formation of
carbides, increases the wear resistance. In combination with Nb and
V, tungsten forms mixed carbides and mixed nitrides and can thus
increase the density of these hard materials. However, the content
of W in the melt is to be set such that, after the centrifuging out
of the hard materials primarily formed in the matrix, only a
content of max. 1.5% is still contained, since together with Mo a
network of W-Mo mixed carbides can otherwise be produced. For this
reason, 1.5.times.Mo+W is also not to be more than 3.5%. Due to the
high affinity of Nb and V for C or N, only slight amounts of less
than max. 0.8% thereof remain in the matrix.
[0018] Similar to Nb and V, only slight amounts of Ti, Zr, Hf and
Ta also remain in the matrix. To increase the high temperature
strength for cutting rings subjected to particularly high loads,
cobalt can be contained in the matrix up to a content of 3%. For
the purpose of deoxidation, Al is often added to the melt and can
still remain partially dissolved in the matrix after the
solidification. By means of higher contents of Al, the density of
the melt can be reduced and the density difference from the hard
material particles thus increased. An Al amount of up to 3% in the
matrix is possible.
[0019] The alloys of the alloyed tool steels as they are described
in the DIN 10020 standard are particularly well suited as a base
composition for the matrix. Cold work steels, hot work steels and
high-speed steels can be used as a base composition for the matrix.
To avoid eutectic carbides, it is, in the case of high-speed
steels, sometimes necessary to reduce the carbon content compared
to the standard composition. With these matrix alloys, the hardness
of at least 44 HRC that is necessary for a trouble-free use of the
cutting rings can be achieved by a suitable heat treatment, which
is generally composed of a hardening process and an annealing
process. It has been shown that particularly good wear resistance
is achieved when the matrix of the cutting rings has a hardness of
50 HRC and higher. This hardness is required where boring takes
place in hard, particularly abrading rock formations. The heat
treatment of the cutting rings must always be adapted to the
specific case of use of the application in order to achieve a
balanced relationship between the hardness and toughness of the
structure.
[0020] If the matrix composition is selected such that it
corresponds to an austenitic manganese steel, then the advantage of
a particularly tough and impact-resistant base structure can be
utilized together with a surface which hardens under pressure and
is thus wear resistant. `Houdremont, Handbuch der Sonderstahlkunde,
Springer Verlag, 1956` and other literature sources describe
austenitic manganese steels of this type, which are also named
Hadfield steels after their inventor and, according to their
structure, are austenitic manganese tool steels. These steels have
a manganese content of approximately 8 wt. % to 15 wt. %, in
exceptional cases 6 to 25 wt. %, and a carbon content of
approximately 0.8 to 2.3 wt. %. The ratio of wt. % of Mn to wt. %
of C is roughly 10:1. Austenitic manganese steels are, after a
corresponding heat treatment, characterized in that their structure
is composed of a metastable, extremely tough austenite. By applying
pressure to the surface, the metastable austenite can transform
into a hard and wear-resistant martensite, whereby a part with a
hard surface and a tough core is obtained. The transformation
behavior can be influenced depending on the amount of Mn and C in
the steel and the proportion thereof to one another.
[0021] To form the hard martensitic surface, the load during use
may be sufficient on its own. If the compressive load during use is
not enough to induce the required transformation of the structure
in the region of the surface, then the surface region that is to be
hardened can already be hardened prior to use, for example, by
hammering or a different mechanical treatment. The composition of
the matrix alloy can also be set such that the surface or the
entire tool body can be transformed at least partially into
martensite by a cooling below room temperature, preferably by means
of liquid nitrogen.
[0022] Roller bits described above or similar annular tools which
contain at least one working region oriented radially outward and a
clamping part closer to the axis and are composed of an iron-based
alloy as a matrix, in which hard material particles, such as
carbides and/or nitrides and/or carbonitrides and/or borides,
possibly in mixed form of the elements from groups 4 and/or 5 of
the periodic system, are incorporated, can be produced in that, in
a first step, a base alloy is melted, for example in an induction
furnace, and heated to a temperature of 1350.degree. C. to
1630.degree. C. This base melt is used to introduce most of the
alloy elements into the melt for the subsequent finished alloy.
[0023] The base melt can, depending on the desired matrix
composition and depending on the selection of the design of the
second step subsequent thereto, have the following composition by
wt. %:
[0024] Carbon (C) up to 2.5
[0025] Silicon (Si) 0.01 to 3.0
[0026] Manganese (Mn) 0.05 to 28.0
[0027] Chromium (Cr) up to 9.0
[0028] Nickel (Ni) up to 4.3
[0029] Molybdenum (Mo) up to 3.5
[0030] Tungsten (W) up to 2.2
[0031] (1.5.times.Mo+W) up to 5.1
[0032] Vanadium (V) up to 6.0
[0033] Niobium (Nb) up to 35.0
[0034] Aluminum (Al) up to 3.5,
[0035] possibly
[0036] Titanium (Ti) up to 2.0
[0037] Zirconium (Zr) up to 3.0
[0038] Hafnium (Hf) up to 1.0
[0039] Tantalum (Ta) up to 5.0
[0040] Cobalt (Co) up to 3.5
[0041] Iron (Fe) and impurity elements as the remainder.
[0042] If the metallic components of the hard material particles
that are subsequently to be formed (elements from groups 4 and 5)
are already contained in the base melt, and if the content of C, N
and B is at the same time kept as low as possible, then carbon
and/or nitrogen and/or boron are introduced into the base melt in a
second step, whereupon these elements combine with the elements
from group 4 and/or 5 of the periodic system, which are already
present in the base melt, to form hard material particles that have
a higher density than the melt. The hard materials formed have the
structure M.sub.x(C+N+B).sub.y, wherein the total proportion of
carbon, nitrogen and boron in the hard materials formed is between
the atomic ratios 0.4 and 0.55, or the ratio x:y is between 1.5 and
0.8. The amount of alloyed carbon is to be chosen such that a
carbon content of 0.3 to 2.3 wt. % C remains in the residual melt.
There is thus sufficient carbon available for forming martensite in
the matrix during the subsequent heat treatment. The amount of the
other alloy elements, except for those from the fourth and fifth
groups, is based on the desired properties of the matrix
surrounding the hard material particles, wherein the formation of a
eutectic carbide network is to be avoided in order to achieve a
highest possible toughness. Here, particular attention must be paid
to the heat treatment properties of the matrix.
[0043] If the temperature of the base melt is kept between
1550.degree. C. and 1630.degree. C., there results a rapid
formation of the hard materials simultaneously with low wear on the
melting vessel. The alloying of carbon, nitrogen and boron can
occur by means of solid materials, such as for example coke,
ferrochrome with a high carbon content, silicon carbide,
ferronitrogen and ferroboron, or by the addition of melts or gases
containing carbon and/or nitrogen and/or boron. This component or
these components can also contain other alloy elements. Depending
on the carbon, nitrogen and boron content of the added
component(s), very large amounts thereof can be necessary in order
to achieve the desired carbon, nitrogen and boron content in the
final melt. The amount of the added carbon, nitrogen and boron
carriers can thus also be significantly larger than the amount of
the base melt, which means that the alloy element components in the
base melt can take on very large contents, for example, up to 35
wt. % of niobium.
[0044] The melting of an alloy rich in the elements from the fourth
and/or fifth main group with small contents of carbon, nitrogen and
boron has the advantage that the ferroalloys, via which the
elements from the fourth and fifth groups are generally alloyed,
liquefy quickly. If the carbon, nitrogen and boron contents in the
melt are too high, a hard material layer can form on the surface of
the ferroalloy pieces used, which layer markedly impedes the
liquefaction. It has been shown in tests that the proportion of
carbon in the base melt is to be less than 0.6 wt. % in the above
case.
[0045] It is also possible to set the composition of the base melt
in the first step such that the melt does not contain the elements
for forming the hard material particles and that, in the second
step, the hard material particles are added by means of a solid or
liquid metallic premelt, or by means of a similar mixture of metal
and hard material particles, and are distributed homogeneously in
the base melt. These hard material particles can be carbides and/or
nitrides and/or oxycarbonitrides and/or borides, possibly as
carbonitrides and/or oxycarbonitrides with boron components, at
least of one of the elements, or in mixed form of the elements, of
groups 4 and 5 of the periodic system. The homogeneous distribution
of the hard material particles in the base melt can be facilitated
by mechanical processes, for example by stirring, or by blowing in
gases in the lower region of the melting vessel.
[0046] Depending on the melt composition and the composition of the
formed or introduced hard material particles, it can be
advantageous, for example in order to prevent an oxidation of
components in the melt, to perform process steps 1 and/or 2
completely or merely partially under an inert gas atmosphere or
under reduced ambient pressure.
[0047] Following the homogeneous distribution of the hard metal
particles in the second step, the matrix melt with the hard
material particles contained therein is, in a third step, cast into
a rotating mold and allowed to solidify. Produced by the rotational
motion about the longitudinal axis of the mold and the centrifugal
force acting on the melt and the hard material particles as a
result, the hard material particles migrate outwards into the
eventual working region of the roller bit, where a crystalline
structure highly rich in hard materials forms. At the same time, a
crystalline structure forms in the interior region, which structure
has only small contents of the primarily precipitated or introduced
hard materials. The resulting amount of hard materials in the outer
region is mainly determined by the process parameters of rotational
speed of the mold, the density difference between the hard material
particles and the melt, the size distribution of the hard material
particles, and the cooling rate of the melt in the rotating mold.
In order to achieve a high concentration of hard material particles
in the outer region and thus high wear resistance, the rotational
speed of the mold and therefore the centrifugal acceleration acting
on the melt and on the hard material particles should be as high as
possible. Centrifugal accelerations of 700 m/s.sup.2 and higher,
measured at the outer diameter of the cast piece, have proven
effective. A large density difference between the hard material
particles and the melt can mainly be achieved with high proportions
of niobium, tantalum and hafnium in the hard materials. For cost
reasons, hard materials particularly rich in niobium, specifically
niobium-vanadium mixed carbides, have proven advantageous for
achieving a high hard material content. The hard material particles
precipitated or added in the second step are in any case to have a
density that is greater than that of the matrix melt at a
temperature 50.degree. C. above the liquidus temperature.
[0048] The migration of the hard material particles outward
requires a different length of time depending on the dimensions of
the cast piece, and in order to achieve a maximum possible
concentration of hard materials in the outer structure, the
duration between the time at which the melt is cast into the mold
and the solidification of the melt should be as long as possible.
Here, preheating the mold to several 100.degree. C. can provide
small advantages. The solidification rate can be reduced to a
particularly significant extent if the mold is composed, wholly or
in parts that face the cast piece, of a material that exhibits only
very poor thermal conductivity. Here, quartz sand and molding
materials with an aluminum-silicate-ceramic base should be
mentioned. A ceramic- or carbon-based heat-insulating coating on
the inner side of the mold provides advantages in this case.
[0049] After the blank has been cast, it can, in order to keep
stresses in the ring low, be removed from the mold at a temperature
of up to 1000.degree. C., the temperature can be equalized across
the entire ring in a furnace, and the blank can then be slowly
cooled such that the matrix structure is present in a soft state at
room temperature. Here, the cooling rate is based on the alloy
composition of the matrix. If necessitated by the subsequent
conditions of use of the roller bit, for example boring into
particularly hard rock, then after the blank is emptied from the
mold, it can be brought to the proper forging temperature in a
furnace and can then be plastically deformed in one or more steps
in a drop forging process. By means of this process, the toughness
of the structure can be significantly increased. The forging
process is then followed by the controlled cooling to room
temperature. The blank can then be mechanically preworked, for
example by turning, whereupon a heat treatment of the ring follows.
The heat treatment can, in the case of a matrix composition similar
to a tool steel, be composed of a hardening process and at least
one annealing process. For a matrix composition similar to an
austenitic manganese steel, a rapid cooling generally occurs after
an annealing in order to achieve a metastable austenitic structure.
After the heat treatment, the mechanical final working of the
cutting ring occurs, for example by turning and/or grinding.
[0050] The invention is described below with the aid of an executed
example.
[0051] A premelt with 0.28% C, 1.3% Si, 0.9% Mn, 1.34% Cr, 2.2% Ni,
0.1% Mo, 0.8% V and 10.0% Nb was melted in an induction furnace,
brought to a temperature of 1590.degree. C., held at this
temperature for 5 minutes and then, at a constant temperature,
brought to a carbon content of 2.35% using petroleum coke. After
the carburization, the final melt was reduced to a temperature of
1570.degree. C., kept there for 3 minutes and subsequently cast in
a centrifugal casting process. A steel mold was used as a
centrifugal casting mold, into which a core of bound silicon
dioxide was inserted. This core had previously been coated on the
inner surface with a 1-mm thick zirconium oxide-based layer. The
cast piece was removed from the mold at approximately 800.degree.
C. and, after an equalization phase of 60 minutes in the furnace,
cooled to room temperature in the furnace, after which it was
preworked and brought to a hardness of 53 HRC in the clamping
region by means of a hardening and double annealing.
[0052] FIG. 1 shows, by way of example, a cut-open annular roller
bit 1 with the cross section 2. The part 3, which is enriched with
hard material particles, comprises the working region 4 positioned
at the outer diameter of the ring 1. The clamping region 5 is
positioned at the inner diameter of the ring 1 and contains only a
low proportion of hard materials.
[0053] FIG. 2 shows, by way of example, the structure in the
working region 4, wherein the hard material particles are shown in
a light depiction and the matrix is shown in a dark depiction. The
hard material content is approximately 20%.
[0054] FIG. 3 shows, by way of comparison, the structure in the
clamping region 5 with only a low proportion of hard materials.
* * * * *