Patent Application
20160298451
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.
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.
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.
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.
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.
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.
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.
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.
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.
The object of the invention is to create a generic annular tool that enables an increased service life during harsh, bedrock-breaking operation.
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.
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° C. commonly found in practice.
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.
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.
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/m3. A desired high concentration of the hard material particles is achieved when the particles have a density greater than 7600 kg/m3 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.
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 μm.
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×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.
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.
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.
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.
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.
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° C. to 1630° C. This base melt is used to introduce most of the alloy elements into the melt for the subsequent finished alloy.
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. %:
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×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 3.5
Iron (Fe) and impurity elements as the remainder.
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 Mx(C+N+B)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.
If the temperature of the base melt is kept between 1550° C. and 1630° 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.
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.
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.
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.
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/s2 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° C. above the liquidus temperature.
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° 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.
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° 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.
The invention is described below with the aid of an executed example.
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° 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° 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° 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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AT2014/050084 | 4/9/2014 | WO | 00 |
