Cutting tool

Abstract

The present invention relates to a rotary cutting tool insert comprising a carrier body made of maraging steel and at least one cutting element comprising at least one cutting edge. The rotary cutting tool insert further comprises a brazed joint joining the carrier body and the at least one cutting element, where the brazed joint comprises Ti, and where the brazed joint comprises a Ti-containing bonding layer abutting the cutting element, the thickness of the Ti-containing bonding layer being between 0.03 [mu] m and 5 [mu] m.

Description

Technical Field

[0001] The present invention relates to a rotary cutting tool, which comprises a maraging steel carrier body and a cutting element, wherein the carrier body and the cutting element are joined by brazing. The present invention also relates to a method of manufacturing such a rotary cutting tool. Background Art

[0002] In the art, it is common to manufacture rotary cutting tools for chip formation machining, such as drill bits or end mills, from round blanks of cemented carbide. The shape of the drill bit or end mill (e.g., chip flutes, cutting edges, etc.) is achieved by grinding. Grinding is both time-consuming and expensive. Therefore, the rotary cutting tool can be reconditioned several times by regrinding the worn parts (such as the cutting edges), and is also provided with a new coating.

[0003] There have been several attempts to replace a part of the rotary cutting tool body, such as the shank part, with another less expensive material. It is also known to have a loose, replaceable tip on a drill bit, which is fastened by mechanical means (e.g., with screws).

[0004] Joining a cutting element to a carrier body made of a material different from that of the cutting element by welding or brazing is known in the art.

[0005] Due to problems associated with brazing and the low hardness and / or tensile strength of steel, using steel as a carrier material is generally not considered an option. Since the cutting tool is subjected to large forces during cutting operations, the brazed joint needs to be strong, and the carrier body needs to have an optimal toughness / hardness ratio. In addition, the difference in CTE (Coefficient of Thermal Expansion) between steel and materials such as cemented carbide, polycrystalline diamond (PCD), or cubic boron nitride (cBN) may cause cracks.

[0006] In addition, for a rotary cutting tool that is usually provided with cooling channels, joining the cutting element to the carrier body by brazing may cause the cooling channels to be blocked by brazing material.

[0007] Although cemented carbide seems suitable as a carrier body, it still has its drawbacks. For environmental reasons, the recycling of cemented carbide is preferred, which is a complex process. In addition, cemented carbide is difficult to machine, and a large amount of grinding, etc. is usually required to achieve the final shape of the rotary cutting tool.

[0008] Although maraging steel works well in terms of the strength of the brazed joint, etc., it may still have its drawbacks as a carrier body in certain cutting applications because maraging steel does not reach the same hardness level as cemented carbide. The lower hardness will reduce wear resistance. For example, in drilling, where the workpiece chips will not only impact the cutting element but also the carrier body, the wear resistance of maraging steel may not be good enough.

[0009] One object of the present invention is to provide a rotary cutting tool having a steel carrier body that can withstand the forces during a metal cutting operation.

[0010] Another object of the present invention is to provide a rotary cutting tool having a cutting element joined to the steel carrier body by a high-strength brazed joint.

[0011] Another object of the present invention is to provide a rotary cutting tool in which the carrier body can be shaped with less effort compared to a carbide carrier body.

[0012] Another object of the present invention is to provide a rotary cutting tool in which the carrier body can withstand high wear from the workpiece chips. SUMMARY OF THE INVENTION

[0013] DEFINITIONS

[0014] A rotary cutting tool herein refers to any circular tool used for a chip-forming machining cutting operation. Examples are drill bits, end mills, and reamers.

[0015] A rotary cutting tool (also referred to as a solid circular tool) generally includes a shank portion and a fluted portion integral with the shank portion. The fluted portion refers to the portion of the rotary cutting tool having chip grooves formed in its circumferential surface. One or more internal coolant channels may be provided within the rotary cutting tool.

[0016] A cutting element herein refers to the portion of a cutting tool blade that is engaged during a cutting operation, i.e., the portion that includes at least one cutting edge and contacts the workpiece. Depending on the cutting application, the cutting element may have different shapes. As an example, the cutting element may constitute the entire top portion of the rotary cutting tool, see Figure 1 , or the cutting element may be smaller, such as for example a vein or nip, which is brazed onto the outermost portion of the rotary cutting tool, see Figure 2 .

[0017] A carrier body herein refers to the cutting tool body that does not constitute the cutting element. The carrier body may have a rotary cutting tool of any shape as described above, and preferably, it includes at least a portion of the fluted portion.

[0018] DETAILED DESCRIPTION

[0019] The present invention relates to a rotary cutting tool, the rotary cutting tool comprising a carrier body and at least one cutting element, the at least one cutting element comprising at least one cutting edge, a brazed joint that joins the carrier body and the at least one cutting element, wherein the brazed joint comprises Ti, and wherein the brazed joint comprises a Ti-containing joining layer adjacent to the cutting element, the thickness of the Ti-containing joining layer being between 0.03 μm and 5 μm. The carrier body is made of maraging steel.

[0020] The cutting element can be made of any material known in the field of metal cutting, i.e., one of cemented carbide, cermet, ceramic, polycrystalline diamond (PCD) or sintered cubic boron nitride (PcBN).

[0021] Ceramics herein refers to a material comprising transition metal carbide, nitride or carbonitride particles (such as WC, Si 3 N 4 , SiAlON, Al 2 O 3 , SiC whiskers, etc.) embedded in an oxide ceramic matrix (such as alumina), wherein the amount of the transition metal carbide, nitride or carbonitride particles is 5 to 45% by volume. They are usually sintered in a hot isostatic pressing process.

[0022] The cemented carbide used as the cutting element can be made of any cemented carbide known in the art. The cemented carbide comprises a hard phase embedded in a metal binder phase matrix.

[0023] Cemented carbide herein means that at least 50% by weight of the hard phase is WC.

[0024] Suitably, the amount of the metal binder phase is between 3% and 20% by weight of the cemented carbide, preferably between 4% and 15% by weight. Preferably, the main component of the metal binder phase is selected from one or more of Co, Ni and Fe, and more preferably, the main component of the metal binder phase is Co.

[0025] Main component herein means that no other elements are added to form the binder phase except the above-mentioned elements. However, if other components are added, such as Cr for example, it will inevitably dissolve in the binder during sintering.

[0026] In one embodiment of the present invention, the cemented carbide may further comprise other components common in cemented carbide elements, which are selected from Cr, Ta, Ti, Nb and V, and exist in the form of elements or in the form of carbides, nitrides or carbonitrides.

[0027] Cermets, as used herein, refer to materials that contain a hard component in a metallic binder phase, where the hard component comprises one or more carbides or carbonitrides of Ta, Ti, Nb, Cr, Hf, V, Mo, and Zr, such as TiN, TiC, and / or TiCN.

[0028] PCD (polycrystalline diamond), as used herein, refers to a material that contains diamond crystals sintered together, where the amount of the diamond crystals is between 50% and 100% by volume. The diamond crystals typically have a grain size between 0.5 μm and 30 μm. PCD may also include one or more components selected from Al, Cr, Co, Ni, V, Fe, and Si.

[0029] PcBN, as used herein, refers to a material that contains cBN grains embedded in a metallic and / or ceramic binder, where the amount of the cBN grains is between 30% and 99% by volume. The ceramic binder may contain one or more components that are carbides, nitrides, carbonitrides, borides, or oxides of Co, Ni, and elements from Groups 4 - 6 of the Periodic Table.

[0030] Polycrystalline diamond (PCD) and sintered cubic boron nitride (PcBN) can be provided as such, i.e., so-called "standalone", or can be provided with a cemented carbide support, i.e., so-called "carbide supported". Polycrystalline diamond (PCD) and sintered cubic boron nitride (PcBN) are typically manufactured by providing a suitable powder mixture that is subjected to a high-temperature - high-pressure (HP / HT) sintering step to form a sintered compact (usually at 1400 °C, 5 GPa).

[0031] When polycrystalline diamond (PCD) and sintered cubic boron nitride (PcBN) are provided with a cemented carbide support, this is prepared prior to the sintering of the polycrystalline diamond (PCD) and sintered cubic boron nitride (PcBN). One way to do this is to use a cup with a cemented carbide disk at the bottom. Then, the cup is filled with a selected PCD or cBN powder mixture, and then the cup is sealed. Then, the sealed cup is subjected to a high-temperature - high-pressure (HPHT) sintering step. The diamond or cBN material binds to the cemented carbide during the sintering step. Then, the disk can be machined into a suitable workpiece using, for example, a laser or WEDM (wire electrical discharge machining).

[0032] The cemented carbide used as a support for polycrystalline diamond (PCD) and sintered cubic boron (PcBN) can be made of any cemented carbide common in the art, see the definition above.

[0033] Maraging steels suitably contain 8 wt% to 25 wt% Ni and alloying elements in a total amount of 7 wt% to 27 wt% selected from one or more of Co, Mo, Ti, Al, and Cr, preferably 7 wt% to 23 wt%. Maraging steels generally contain less carbon than conventional steels, suitably 0.03 wt% or less. The balance is Fe.

[0034] In one embodiment of the present invention, the maraging steel contains 11 wt% to 25 wt% Ni, preferably 15 wt% to 25 wt% Ni. The alloying elements are suitably in an amount of 7 wt% to 15 wt% Co, preferably 8.5 wt% to 12.5 wt% Co, 3 wt% to 10 wt% Mo, preferably 3 wt% to 6 wt% Mo, 0.1 wt% to 1.6 wt% Ti, preferably 0.5 wt% to 1.2 wt% Ti, 0 wt% to 0.15 wt% Cr, 0 wt% to 0.2 wt% Al, and less than 0.03 wt% C. The balance is Fe.

[0035] In one embodiment of the present invention, the composition of the maraging steel is 17 wt% to 19 wt% Ni, 8.5 wt% to 12.5 wt% Co, 4 wt% to 6 wt% Mo, 0.5 wt% to 1.2 wt% Ti, 0 wt% to 0.15 wt% Cr, 0 wt% to 0.2 wt% Al, and less than 0.03 wt% C. The balance is Fe.

[0036] In another embodiment of the present invention, the composition of the maraging steel is 8 wt% to 11 wt% Ni, preferably 9 wt% to 10 wt% Ni, 2.5 wt% to 4 wt% Cr, preferably 3 wt% to 3.5 wt% Cr, 3.5 wt% to 5 wt% Mo, preferably 4 wt% to 4.5 wt% Mo, 0.4 wt% to 1.1 wt% Ti, preferably 0.7 wt% to 0.9 wt% Ti, less than 0.4 wt% Si, less than 0.4 wt% Mn, and the balance is Fe.

[0037] As with all alloys, maraging steels may also contain unavoidable impurities. Impurities herein refer to any of the following elements that may be present in maraging steels in such small amounts that they have no effect on the properties of the steel. The total amount of impurities is less than 0.50 wt%, preferably less than 0.15 wt%. Examples of such elements are Mn, P, Si, B, and S.

[0038] In one embodiment of the present invention, the amount of Mn is less than 0.05 wt%, the amount of P is less than 0.003 wt%, the amount of Si is less than 0.004 wt%, and S is less than 0.002 wt%.

[0039] The average hardness of the maraging steel part will depend on whether any aging / nitriding steps have been carried out, see below.

[0040] In one embodiment of the present invention, the carrier body made of maraging steel is not provided with a hardness gradient, and the average hardness is between 300 HV1 and 1200 HV1, preferably between 500 HV1 and 1100 HV1. The standard deviation of the hardness values is suitably between 0 HV1 and 150 HV1, preferably between 0 HV1 and 100 HV1.

[0041] In one embodiment of the present invention, the carrier body made of maraging steel is provided with a hardness gradient, that is, the carrier body has an increased hardness in the surface area compared to that in the core. This means herein that the hardness has its highest value at the surface and then gradually decreases towards the core. Then, the carrier body made of maraging steel has an average core hardness between 300 HV1 and 700 HV1, preferably between 500 HV1 and 700 HV1. The standard deviation of the core hardness values is suitably between 0 HV1 and 20 HV1, preferably between 0 HV1 and 15 HV1. Then, the average surface hardness of the surface made of maraging steel is between 300 HV1 and 1200 HV1, preferably between 500 HV1 and 1100 HV1. The standard deviation of the hardness values is suitably between 0 HV1 and 150 HV1, preferably between 0 HV1 and 100 HV1. The surface hardness is at least 30% higher than the core hardness, preferably at least 40% higher than the core hardness.

[0042] "Core" herein refers to the inner part of the maraging steel carrier body where the hardness no longer changes when measured in cross-section.

[0043] The depth of the hardness gradient measured from the surface, i.e., the nitriding depth, is determined by making a hardness-depth curve on the cross-section of the maraging steel carrier provided with a hardness gradient, and measuring HV0.3 or HV0.5 according to the standard DIN EN ISO6507-1, which starts from near the surface and towards the core until the hardness no longer changes. The nitriding depth is given by the vertical distance from the surface of the nitrided carrier body to the point of the limiting hardness, where the limiting hardness is defined as the average core hardness + 50 HV0.3 or 50 HV0.5, see Figure 4 。

[0044] The average nitriding hardness depth of the maraging steel carrier body is between 0.001 mm and 0.8 mm, preferably between 0.01 mm and 0.3 mm. The standard deviation of the hardness values is suitably between 0 mm and 0.03 mm, preferably between 0 mm and 0.02 mm.

[0045] By increasing the hardness of the surface of the maraging steel carrier, increased wear resistance will be achieved. This can be a great advantage when the cutting tool insert according to the invention is used in a cutting application in which the chips from the workpiece material impact the maraging steel carrier.

[0046] The brazing technique is so-called active brazing. This means that the joint is not only formed by melting the filler material and forming a metallic bond, but it also involves a chemical reaction with one or both of the materials to be joined. The joining element in the filler material is typically Ti, however elements such as Hf, V, Zr and Cr are also considered to be active elements. According to the invention, Ti is the active element.

[0047] The brazed joint herein refers to the area or mass between the cemented carbide and the maraging steel part, which is filled with the filler material and formed during the brazing process, see below.

[0048] The thickness of the brazed joint is suitably between 5 μm and 200 μm, preferably between 15 μm and 100 μm.

[0049] The brazed joint is not a homogeneous phase. On the contrary, after brazing, the elements in the filler material form different alloy phases.

[0050] The brazed joint after brazing includes a Ti-containing joining layer adjacent to the cutting element. Ti is very reactive and will react with one or more of the elements present in the cutting element during brazing. Most commonly, covalent bonds are formed with one or more of carbon, nitrogen, oxygen and boron, and a strong Ti-containing joining layer is formed at the interface between the brazed joint and the cutting element.

[0051] The composition of the Ti-containing joining layer will vary depending on what material the cutting element is made of, but typically consists of TiC, TiN, TiO x and TiB x one or a mixture thereof. Since the formed joining layer has a ceramic nature, the joint may become brittle if the layer growth is not controlled.

[0052] For example, if the material closest to the brazed joint is PCD (polycrystalline diamond) or cemented carbide, the entire cutting element is made of the cemented carbide, or if it is a carbide-supported PCD or PcBN cutting element, the Ti-containing joining layer is a TiC layer. The Ti in the brazed joint will react with the carbon in WC or diamond and form TiC.

[0053] Another example is that if the cutting element is made of solid (also known as "free-standing") PcBN, the bonding layer will be TiN because Ti will react with the nitrogen in cBN, but may also contain a smaller amount of TiBx, such as TiB 2 。

[0054] When the cutting element is made of a ceramic (such as Al 2 O 3 / WC sintered ceramic composite), the bonding layer will be a TiC / TiOx layer.

[0055] Depending on the type of equipment used, there are several ways to detect the presence of the bonding layer.

[0056] If a scanning electron microscope (SEM) with a high enough resolution is used, the bonding layer is clearly visible near the cutting element. To verify the composition of the layer, SEM-EDS (energy-dispersive spectroscopy) with WDS (wavelength-dispersive spectroscopy) and / or SEM-EPMA (electron probe microanalysis) can be used to identify the individual elements in the bonding layer.

[0057] In one embodiment of the present invention, the thickness of the bonding layer is between 0.03 μm and 5 μm, preferably between 0.05 μm and 1 μm, more preferably between 0.05 μm and 0.5 μm, and most preferably between 0.05 μm and 0.25 μm.

[0058] If the SEM images used do not have enough resolution to detect the bonding layer, SEM-EDS or SEM-EPMA with WDS, for example, can be used to observe the accumulation of Ti and / or C at the interface between the filler material and the cutting element. The accumulation of Ti is referred to herein as the Ti accumulation layer and is an indication of the formation of the bonding layer, even if not visually detected in the SEM image. The Ti accumulation layer is much thicker than the actual bonding layer, which may mean that not all Ti will form TiC / TiN / TiO x / TiB x 。Moreover, the thickness of the Ti accumulation layer is partly affected by the analysis method.

[0059] Preferably, in addition to Ti, the brazed joint further contains one or more elements selected from Ag, Cu, Sn, In, Zr, Hf, and C, more preferably Ag, Cu, and In.

[0060] The brazed joint may also contain smaller amounts of other elements that are considered to be inevitable impurities. Inevitable impurities herein refer to small amounts of elements (other than those listed above) that may be present in the brazing material prior to the brazing step and elements from the materials to be joined, such as Co, W, etc. from cemented carbide and Fe, Ni, etc. from maraging steel. When subjected to elevated temperatures during the brazing step, small amounts of elements from the parts to be joined inevitably dissolve in the brazing material, whereby the brazing material melts and allows diffusion from the joined parts. As long as the brazing process parameters (such as temperature and time) are within the scope of the present invention, the total amount of inevitable impurities will be so small that it will not affect the performance of the brazed joint.

[0061] The composition of the brazed joint after brazing is difficult to determine because the elements are not evenly distributed. If available, the simplest way is to look at the filler material that has been used, as the paste or foil is a homogeneous blend. Moreover, the brazed joint may include small amounts of elements from the materials to be joined, such as Co, W from cemented carbide and Fe, Ni, etc. from maraging steel.

[0062] The amounts of Ti and possibly other elements in the brazed joint can also be measured using energy dispersive X-ray spectroscopy (EDS). However, due to the non-uniform distribution of the precipitated elements in the brazed joint, many measurement points are required and the standard deviation will be large. Preferably, the brazed joint contains on average 30 wt% to 80 wt%, preferably 40 wt% to 75 wt% of Ag, 15 wt% to 50 wt%, preferably 15 wt% to 40 wt% of Cu, 0.3 wt% to 15 wt%, preferably 0.5 wt% to 5 wt% of Ti, 0 wt% to 10 wt%, preferably 0 wt% to 2 wt% of Sn and 0 wt% to 30 wt%, preferably 5 wt% to 25 wt% of In.

[0063] At the interface between the brazed joint and the maraging steel part, Ti also accumulates in the brazed joint, where Ti forms a metal bond with iron in the steel. The thickness of the Ti accumulation layer at the surface of the maraging steel is preferably between 1 μm and 10 μm, preferably between 2 μm and 5 μm, and can be measured by, for example, EDS.

[0064] In one embodiment of the present invention, the cutting element forms the top of the rotary cutting tool, also known as the head. Herein, this means that the cutting element includes the at least one cutting edge and a part of the chip fluting portion. If the rotary cutting tool is provided with one or more cooling channels, the cooling channels pass through both the carrier body and the cutting element. An example of such a rotary cutting tool can be seen in Figure 1 in.

[0065] In one embodiment of the present invention, the cutting element is a tip or texture. An example of such a rotary cutting tool can be seen in Figure 2 in.

[0066] The present invention also relates to a method of manufacturing a cutting tool according to the above, the method comprising the following steps: - Providing a carrier body made of maraging steel, - Providing at least one cutting element including at least one cutting edge; - Placing a filler material containing Ti in an amount of 0.3 wt% to 15 wt% of the filler material between the carrier body and the cutting element and in contact with the carrier body and the cutting element; - Subjecting the carrier body, the cutting element, and the filler material therebetween to a brazing step in a furnace at a temperature between 600 °C and 830 °C for a period of 1 to 60 minutes, and wherein the brazing is carried out in a vacuum.

[0067] The filler material according to the present invention (also known as brazing metal) contains Ti in an amount of 0.3 wt% to 15 wt%, preferably 1 wt% to 5 wt% of the filler material in total. The filler material of the present invention suitably has a solidus temperature between 490 °C and 1125 °C, preferably between 600 °C and 700 °C. In addition, the filler material of the present invention has a liquidus temperature between 610 °C and 1180 °C, preferably between 700 °C and 750 °C. In addition to Ti, the filler material further includes one or more elements selected from Ag, Cu, Sn, In, Zr, Hf, and Cr.

[0068] In one embodiment of the present invention, the filler material contains Ag in an amount of 30 wt% to 80 wt%, preferably 40 wt% to 75 wt%, Cu in an amount of 15 wt% to 50 wt%, preferably 15 wt% to 40 wt%, Ti in an amount of 0.3 wt% to 15 wt%, preferably 0.5 wt% to 5 wt%, Sn in an amount of 0 wt% to 10 wt%, preferably 0 wt% to 2 wt%, and In in an amount of 0 wt% to 30 wt%, preferably 5 wt% to 25 wt%.

[0069] Suitably, the filler material is provided in the form of a foil or a paste.

[0070] The filler material is disposed on the joint surface of the cutting element and the maraging steel carrier.

[0071] Before the brazing process, the thickness of the filler material disposed at the joint surface depends on the type of material, i.e., foil or paste. If a paste is used, sufficient material is applied such that the surface to be brazed is covered. Generally, the thickness is between 5 μm and 200 μm, preferably between 15 μm and 100 μm.

[0072] Then, the part (i.e., the cutting element and the maraging steel carrier and the filler material therebetween) is placed in a furnace having an inert or reducing environment (i.e., having a minimal amount of oxygen). Preferably, the brazing temperature in the furnace is between 600 °C and 830 °C, preferably between 650 °C and 820 °C, more preferably between 700 °C and 750 °C.

[0073] The selection of the brazing temperature depends on several factors. For example, if the cutting element is made of PCD, the brazing temperature should be below 750 °C to avoid diamond graphitization, while if the cutting element is made of cemented carbide, the widest temperature range as described above can be used.

[0074] The time for which the part is subjected to the high temperature is between 1 minute and 60 minutes, preferably between 5 minutes and 15 minutes. If the time at the elevated temperature is short, there is not enough time to form the brazed joint and for the Ti to react to achieve the desired strength of the brazed joint. If the time at the elevated temperature is long, the brittle reaction zone containing Ti will grow uncontrollably, which adversely affects the joint properties, such as shear strength.

[0075] The brazing is suitably carried out in a vacuum or in the presence of argon at a low partial pressure. Vacuum herein means that the pressure in the furnace is below 5×10 -4 mbar, preferably below 5×10 -5 mbar. If argon is present, the argon pressure is below 1×10 -2 mbar.

[0076] The brazing furnace used according to the present invention can be any furnace capable of providing such well-controlled conditions as described above with respect to aspects such as evacuation, heating, and cooling rates.

[0077] In one embodiment of the present invention, after the brazing step, the brazed portion is subjected to a high temperature between 300 °C and 600 °C, preferably between 350 °C and 500 °C, and most preferably between 400 °C and 440 °C for a period between 5 minutes and 12 hours, preferably between 30 minutes and 8 hours, and more preferably between 3 hours and 6 hours, to subject the portion to an aging step.

[0078] Suitably, the heating rate up to the aging temperature is between 1 °C / minute and 50 °C / minute, preferably between 5 °C / minute and 10 °C / minute. Suitably, the cooling rate from the aging temperature to a temperature at least below the solidus temperature of the filler material (preferably below 300 °C) is between 1 °C / minute and 50 °C / minute, preferably between 5 °C / minute and 10 °C / minute.

[0079] In one embodiment of the present invention, aging occurs exactly after the brazing step in the same furnace where the brazing step takes place.

[0080] In one embodiment of the present invention, the aging is carried out exactly after the brazing step in a furnace different from the vacuum brazing furnace.

[0081] In one embodiment of the present invention, aging is carried out in the same furnace / deposition chamber before or during the deposition of the coating.

[0082] The aging step will increase the total hardness of the maraging steel.

[0083] In one embodiment of the present invention, the aging step is carried out at least partially in a nitriding atmosphere. Due to the temperature during nitriding, the aging effect will also be present, and thus if nitriding is carried out, usually no additional separate aging step is required.

[0084] The nitriding step can be carried out using plasma nitriding or gas nitriding, preferably plasma nitriding. The nitriding atmosphere can be provided by a nitrogen-containing gas, such as N 2 、NH 3 。

[0085] In one embodiment of the present invention, the nitriding step is carried out using plasma nitriding. This means herein that the nitriding occurs in a vacuum vessel provided with a plasma generator, where a nitriding atmosphere can be provided. The temperature is suitably between 300 °C and 600 °C, preferably between 350 °C and 550 °C, and the duration can be between 1 hour and 100 hours. The pressure is preferably low, suitably between 50 Pa and 600 Pa. For plasma nitriding, the gas is preferably N 2 ,which can be mixed with, for example, H 2 。

[0086] In one embodiment of the present invention, the nitriding step is carried out using gas nitriding. The gas nitriding is preferably carried out at a temperature between 450 °C and 600 °C, preferably between 500 °C and 520 °C. The gas nitriding is preferably carried out by dividing into H 2 and N 2 in the reactor of NH 3 . The gas nitriding can be carried out at low pressure, preferably at 0.05 MPa - 0.02 MPa, or near atmospheric pressure.

[0087] The exact temperature, duration and selection of the nitriding gas depend on several factors, namely the desired nitriding effect on the maraging steel, the specific type of equipment used, etc.

[0088] In one embodiment of the present invention, the maraging steel part has the following composition: 18 wt% - 19 wt% of Ni, 8 wt% - 10 wt% of Co, 4 wt% - 6 wt% of Mo, 0.5 wt% - 1.2 wt% of Ti, 0 wt% - 0.15 wt% of Cr, 0 wt% - 0.2 wt% of Al, less than 0.03 wt% of C, less than 0.04 wt% of Si, less than 0.05 wt% of Mn, less than 0.003 wt% of P, less than 0.002 wt% of S and less than 0.0005 wt% of B. The balance is Fe. The filler material preferably has the following composition: 40 wt% - 75 wt% of Ag, 20 wt% - 40 wt% of Cu, 0.5 wt% - 5 wt% of Ti, 0 wt% - 2 wt% of Sn, 10 wt% - 25 wt% of In.

[0089] In one embodiment of the present invention, the maraging steel part has the following composition: 9 wt% - 10 wt% of Ni, 3 wt% - 3.5 wt% of Cr, 4 wt% - 4.5 wt% of Mo, 0.7 wt% - 0.9 wt% of Ti, less than 0.4 wt% of Si, less than 0.4 wt% of Mn and the balance of Fe. The filler material preferably has the following composition: 40 wt% - 75 wt% of Ag, 20 wt% - 40 wt% of Cu, 0.5 wt% - 5 wt% of Ti, 0 wt% - 2 wt% of Sn, 10 wt% - 25 wt% of In.

[0090] The most common way to manufacture a rotary cutting tool made of only one material (e.g., cemented carbide) is to start with a cylindrical bar, which is ground into its final shape, i.e., having at least one chip groove and at least one cutting edge. When manufacturing the rotary cutting tool according to the present invention, the brazing step can occur before or after the formation of these chip grooves and the at least one cutting edge. If the rotary cutting tool undergoes an aging step with or without nitriding, the formation of the chip grooves and the at least one cutting edge preferably occurs after the brazing step but before the aging step with or without nitriding.

[0091] In one embodiment of the present invention, the at least one chip groove and the at least one cutting edge can be formed before the brazing step.

[0092] In one embodiment of the present invention, the rotary cutting tool can be coated with a wear-resistant coating to further enhance the cutting performance. PVD or CVD techniques can be used to deposit the coating, however, PVD is the most common technique for rotary cutting tools. Description of the Drawings

[0093] Figure 1 An exemplary drill is shown, where the cutting element A is the drill head, B is the carrier made of maraging steel, and there is a brazed joint C therebetween. The drill has a shank portion 1, a chip-grooved portion 2 with a chip groove 4, and a coolant channel 3.

[0094] Figure 2 A drill is shown, where the cutting element is a texture (which can also be called a tip), where the cutting element A is the texture, B is the carrier body made of maraging steel, and there is a brazed joint C (invisible) therebetween.

[0095] Figure 3 A schematic diagram of a shear test device is shown, where 1 is the steel part, 2 is the cemented carbide part, and F is the applied force.

[0096] Figure 4 An example of a hardness depth curve is shown, which shows the hardness values decreasing from the surface towards the core, where A is the average core hardness, B is the limit hardness, and C is the nitriding depth.

[0097] Figure 5 An SEM image of a brazed joint is shown, where A is the cemented carbide, B is the brazed joint, C is the maraging steel carrier, and D shows the interface where the Ti-containing layer is located. Detailed Description

[0098] Example 1 (of the present invention)

[0099] A steel part in the form of a cylinder made of maraging steel 1.2709 is provided together with a cemented carbide part having a composition of 10 wt% Co, 1 wt% other carbides and the balance WC. The maraging steel has a hardness of about 340 HV1 before brazing.

[0100] The brazing material (Incusil ABA from the WBC Group) is provided in the form of a foil with a thickness of 100 μm. The brazing material has a composition of 59.0 wt% Ag, 27.5 wt% Cu, 12.5 wt% In and 1.25 wt% Ti. The solidus temperature is about 605 °C and the liquidus temperature is about 715 °C.

[0101] The foil is placed between the maraging steel part and the cemented carbide part such that both components are in contact with the foil. Then, the assembled joined components are placed in a Schmetz vacuum furnace (type: EU80 / 1H 30*45*30 6barSystem*2RV*), where the temperature is first raised to 740 °C at a rate of 20 °C / minute. The brazing temperature of 740 °C is maintained for 15 minutes, after which the components are cooled to 300 °C at a rate of 5 °C / minute. After 300 °C, it is allowed to cool freely to room temperature.

[0102] The results of high shear testing demonstrate that excellent wetting can be observed without signs of thermal stress cracking.

[0103] This sample is designated as Invention 1 herein.

[0104] Example 2 (of the present invention)

[0105] A steel part in the form of a cylinder made of maraging steel 1.2709 is provided together with a cemented carbide part having a composition of 10 wt% Co, 1 wt% other carbides and the balance WC. The maraging steel has a hardness of about 340 HV1 before brazing.

[0106] The brazing material (TB-651 from Tokyo Braze) is provided in the form of a foil with a thickness of 100 μm. The brazing material has a composition of 65.0 wt% Ag, 28.0 wt% Cu, 2.0 wt% Ti and 5.0 wt% Sn. The solidus temperature is about 700 °C and the liquidus temperature is about 750 °C.

[0107] Place the foil between the maraging steel part and the cemented carbide part such that both components are in contact with the foil. Then, place the assembled joined components into a Schmetz vacuum furnace (type: EU80 / 1H 30*45*30 6barSystem*2RV*) where the temperature is first raised at a rate of 20 °C / minute to 815 °C. Hold the brazing temperature of 815 °C for 15 minutes and then cool the components at a rate of 5 °C / minute to 300 °C. After 300 °C, allow it to cool freely.

[0108] The high shear test results demonstrate that excellent wetting can be observed without signs of thermal stress cracking.

[0109] The sample is designated as Invention 2 in this text.

[0110] Example 3 (Aging treatment)

[0111] After the brazing step, the samples of Invention 1 and Invention 2 are subjected to an aging treatment to maintain the hardness of the maraging steel. Place the components in a furnace where the temperature is first raised at a rate of 5 °C / minute to 490 °C. Hold the temperature of 580 °C for 3 hours and then cool the components at a rate of 5 °C / minute to 300 °C. After 300 °C, allow it to cool freely. The results are shown in Table 1.

[0112] Example 4 (Plasma nitriding)

[0113] At a chamber pressure of 3 mbar, the samples according to Invention 1 and Invention 2 are subjected to a plasma nitriding step in a gas flow of 350:50 ml / min H 2 :N 2 . The temperature in the chamber is 490 °C. The time for which the samples are subjected to the plasma nitriding step is 16 hours. No mask for the brazed joint is used before nitriding. An SEM image of the cross-section of Invention 2 after 16 hours of plasma nitriding is shown in Figure 5 .

[0114] Example 5 (Gas nitriding)

[0115] The sample according to Invention 1 is subjected to a gas nitriding step by NH 3 cracking. The temperature in the chamber is 510 °C. The time for which the sample is subjected to the gas nitriding step is 23 hours or 55 hours. No mask for the brazed joint is used before nitriding.

[0116] Example 6 (Comparison)

[0117] A steel part made of cold-worked steel 1.2714 hardened by carbon is provided together with a cemented carbide part having a composition of 10 wt% Co, 1 wt% of other carbides and the balance WC.

[0118] The brazing metal is provided in the form of a foil with a thickness of 100 μm. The brazing material 1 has a composition of 100.0 wt% Cu. The melting temperature is 1085 °C.

[0119] The foil is placed between the steel part and the hard alloy part, and the assembled joint piece is placed in a furnace, where the temperature is first raised to 650 °C at a rate of 20 °C / min and held for 5 minutes. Then, the temperature is raised from 650 °C to the brazing temperature of 1100 °C at a rate of 10 K / min, with a dwell time of 15 minutes. After the dwell time, natural cooling is started until room temperature.

[0120] Subsequently, the hard alloy - steel joint with a part made of carbon - hardened cold - worked steel 1.2714 is heated to a temperature of 850 °C by a torch for 10 minutes and then quenched in oil to room temperature. After that, stress relaxation is carried out at 200 °C in a vacuum furnace for 2 hours.

[0121] Subsequently, the hard alloy - steel joint with a part made of carbon - hardened cold - worked steel 1.2714 is aged at 500 °C for 2 h.

[0122] The sample is hereinafter referred to as Comparative Example 1.

[0123] Example 7

[0124] The samples are analyzed in terms of shear strength, surface hardness, core hardness, and hardness depth curve.

[0125] The shear strength is analyzed by the shear device setting as Figure 3 shown, where 1 is the steel part in the shape of a steel cylinder (ϕ = 20 mm, h = 5 mm), and 2 is the hard alloy part in the shape of a hard alloy cylinder (ϕ = 10 mm, h = 5 mm). The steel cylinder is positioned in the gap of the shear strength test device and can thus only move in the loading direction. A notch is etched into the surface of the device, which holds the joined parts in the correct position and ensures a uniformly distributed force induction into the brazed joint. The applied force F is continuously increased until the brazed joint fails and the hard alloy cylinder detaches. Then, the ultimate shear strength is calculated by the quotient of the maximum measured force and the initial joint surface (A = 78.5 mm 2 ) without removing the brazing material before determining the shear strength of the brazed joint.

[0126] To determine the nitriding depth according to Example 4, the average nitriding hardness depth is determined at room temperature. This is done by plotting a hardness-depth curve, which is carried out as follows: on the cross-section of the nitrided sample, according to the standard DIN EN ISO 6507-1, starting from the first indentation at a distance of 0.025 mm - 0.1 mm from the edge, and then every 0.03 mm - 0.10 mm until the hardness no longer changes, the HV0.3 is measured. The obtained hardness values are recorded as a function of the distance from the surface. From this hardness curve, the nitriding hardness depth is taken as the distance between the surface and the limiting hardness (where the limiting hardness is the average core hardness (in HV0.3) + 50 HV0.3). For Example 5, the nitriding depth is determined in the same way as in Example 3, except that HV0.5 is used.

[0127] The core hardness given in Table 1 is in HV1 and is measured on the cross-section of the maraging steel part by a Vickers hardness tester, with a load of 1 kgf (kilogram-force) and a loading time of 15 seconds applied.

[0128] According to the standard, a pattern of 5 indentations placed 1.5 mm apart is carried out, and the values given in Table 1 are the average of the 5 indentations.

[0129] The surface hardness measurement is carried out on the nitrided surface, and at least 5 indentations 1.5 mm apart are made, and the values given in Table 1 are the average of the 5 indentations. This measurement is carried out using a Vickers hardness tester, with a load of 1 kgf (kilogram-force) and a loading time of 15 seconds applied.

[0130] Table 1

[0131] Sample Aging treatment / Nitriding TiC layer (nm) Cumulative Ti layer (µm) Average nitriding hardness depth (mm) Shear strength (MPa) Core hardness (HV1) Surface hardness (HV1) Invention 1 Aging treatment 131 1-1.5 - - - - Invention 1 Plasma nitriding - -- 0.07 150.2 522.8 878 Invention 1 Gas nitriding (23 hours) - - 0.16 127.7 487 739 Invention 1 Gas nitrogen (55 hours) 142 - 0.17 108.2 505.3 821 Invention 2 Aging treatment 131 1-1.5 - - - - Invention 2 Plasma nitriding - 1-1.5 0.08 121.3 599.2 898 Comparison 1 Aging treatment - - - Joint failure 468 -

[0132] As can be seen from Table 1, the nitriding will produce a surface with a hardness significantly higher than that of the core, which will lead to an increase in wear resistance.

Claims

1. A rotary cutting tool, comprising: - a carrier body, and; - at least one cutting element including at least one cutting edge; - a brazed joint that joins the carrier body and the at least one cutting element, wherein the brazed joint includes Ti, and wherein the brazed joint includes a Ti-containing bonding layer adjacent to the cutting element, and the thickness of the Ti-containing bonding layer is between 0.03 μm and 5 μm, wherein the carrier body is made of maraging steel.

2. The rotary cutting tool according to claim 1, wherein, the cutting element is made of one of cemented carbide, ceramic, polycrystalline diamond (PCD) or cubic boron nitride (PcBN).

3. The rotary cutting tool according to any one of the preceding claims, wherein, The composition of the Ti-containing bonding layer is TiC, TiN, TiO x and TiB x or a mixture of them.

4. The rotary cutting tool according to any one of the preceding claims, wherein, the Ti-containing bonding layer has a thickness between 0.05 μm and 0.5 μm.

5. The rotary cutting tool according to any one of the preceding claims, wherein, the maraging steel contains 8 wt% to 25 wt% of Ni, one or more alloying elements selected from Co, Mo, Ti, Al and Cr in a total amount between 7 wt% and 27 wt%, less than 0.03 wt% of C and the balance of Fe.

6. The rotary cutting tool according to any one of the preceding claims, wherein, the brazed joint contains Cu, Ag and In.

7. The rotary cutting tool according to any one of the preceding claims, wherein, the brazed joint contains 30 wt% to 80 wt% of Ag, 15 wt% to 50 wt% of Cu, 0.3 wt% to 15 wt% of Ti, 0 wt% to 10 wt% of Sn and 0 wt% to 30 wt% of In.

8. The rotary cutting tool according to any one of the preceding claims, wherein, the average core hardness of the carrier body made of maraging steel is 300 to 700 HV1, and the average surface hardness is 300 to 1200 HV1.

9. The rotary cutting tool according to any one of the preceding claims, wherein, the carrier body made of maraging steel has a hardness distribution such that the surface hardness is at least 30% higher than the core hardness.

10. The rotary cutting tool according to any one of the preceding claims, wherein, the cutting element is a head, and the head includes at least a part of a chip flute.

11. A method of manufacturing a rotary cutting tool according to any one of claims 1-10, comprising the steps of: - providing a carrier body made of maraging steel, - providing at least one cutting element including at least one cutting edge; - placing the filler material containing 0.3 wt% to 15 wt% of Ti of the filler material between the carrier body and the cutting element and in contact with the carrier body and the cutting element; - Subject the carrier body, the cutting element, and the filler material therebetween to a brazing step in a furnace at a temperature between 600 °C and 830 °C for a period between 1 minute and 60 minutes, and wherein the brazing is carried out in a vacuum.

12. The method according to claim 11, wherein, after the brazing step, subject the carrier body and the cutting element to an aging step at a temperature between 300 °C and 600 °C for a duration between 5 minutes and 12 hours.

13. The method according to claim 11, wherein, after the brazing step, subject the carrier body and the cutting element to a nitriding step in a nitriding atmosphere at a temperature between 300 °C and 600 °C.

14. The method according to claim 13, wherein, the nitriding step is plasma nitriding for 1 hour to 100 hours at a temperature between 300 °C and 600 °C and a pressure between 50 Pa and 600 Pa in a nitriding atmosphere.

Images