The present invention relates to a cutting tool comprising a superhard part comprising a polycrystalline diamond (PCD) or a cubic boron nitride (cBN) sintered compact bonded to a cemented carbide substrate and a maraging steel part wherein the parts are joined by brazing. The present invention also relates to the making of such cutting tool.

Background

Using polycrystalline diamond (PCD) or a cubic boron nitride (cBN) in cutting tools are known in the art. In many cases, the PCD or cBN material is bonded to a cemented carbide substrate. This has several reasons, e.g. to simplify production and also since the Co in the cemented carbide infiltrates into to PCD or cBN material during manufacturing and thus function as a catalyst or binder in the PCD or cBN material.

Joining steel with cemented carbide by brazing or welding has been known for a long time in the art of making tools. There are several challenges when joining steel with cemented carbide, e.g. differences in CTE (coefficient of thermal expansion), strength of the braze joint, undesired hardness profiles in the steel etc.

There are several solutions that can improve each of these problems individually, but the solutions often result in problems in other areas and not all problems can be solved.

The principle of brazing is that you use a braze material that joins the two pieces when heated. There are several ways to heat the braze joint, where one of the most common ways is induction heating using an induction coil. One of the benefits with using a coil is that only the local area around the braze joint is heated and leaving the rest of the tool unaffected. This local heating can however lead to unwanted hardness profiles in the steel part which can cause problems when the steel part will be provided with threading etc. for fastening rotary tools and other cutting tools etc. It can also lead to residual stresses caused by uncontrolled cooling, inhomogenous heat distribution (skin effect) and unwanted lattice transformation.

Another disadvantage with the heating using a coil, is that each tool must be handled individually, and a more automatic industrial process would be preferred.

Heating the whole steel and cemented carbide part would make the hardness profile more even but then the increased temperature would affect the whole steel part and thus lead to less hardness overall.

Another problem that can occur when the steel part is provided with threading in order to fasten a cutting tool, is wear. Since the same tool e.g. a shank, is preferably used for a long time, many changes of cutting tool will take place and wear of the threading can affect the fastening of the cutting tool in a negative way, e.g. the tool might be stuck and cannot be removed.

Another problem is that high brazing temperatures can affect the PCD or cBN material in a negative way. High temperatures can cause stresses which can lead to cracks. For PCD, graphitization of the diamond particles can also occur at higher temperatures.

One object of the present invention is to provide a cutting tool which have both a strong braze joint and a steel part with an even hardness profile and a high hardness and consequently an improved wear resistance.

Another object of the present invention is to provide a process of joining steel and cemented carbide which is easy to use and lead to a predictable joint with high strength and a steel part that has a predictable hardness.

Another object of the present invention is to provide a process of joining steel and a superhard part comprising a polycrystalline diamond (PCD) or a cubic boron nitride (cBN) sintered compact bonded to a cemented carbide substrate which does not have a negative effect on the PCD or cBN material.

Detailed description of the invention

The invention relates to a cutting tool comprising:

- a superhard part comprising a polycrystalline diamond (PCD) or a cubic boron nitride (cBN) sintered compact bonded to a cemented carbide substrate;

- a maraging steel part with a hardness of between 350 and 580 HV1 with a standard deviation between 0 and 20 HV1. The cutting tool further comprises a braze joint joining said cemented carbide substrate and said maraging steel part where said braze joint comprises Ti and wherein said braze joint comprises a TiC layer with a thickness of between 0.03 and 5 pm adjoining to the cemented carbide.

By PCD (polycrystalline diamond) sintered compact is herein meant a material comprising diamond crystals sintered together where the amount of diamond crystals is between 50 to 100 vol%. The diamond crystals typically have a grain size of between 0.5 and 30 pm. The PCD sintered compact can also comprise one or more constituents selected from Al, Cr, Co, Ni, V, Fe and Si.

By cBN sintered compact is herein meant a material comprising cBN grains embedded in a metallic and/or ceramic binder where the amount of cBN grains is between 30 to 99 vol%. The ceramic binder can contain one or more constituents being carbides, nitrides, carbonitrides, borides or oxides of elements selected from Co, Ni and groups 4-6 in the periodic table of elements.

Polycrystalline diamond (PCD) and cubic boron nitride (cBN) sintered compacts are usually manufactured by providing a suitable powder mixture which is subjected to a high temperature-high pressure (HPHT) sintering step to form a sintered compact. When the compact is to be bonded to a cemented carbide substrate, one way of doing this is to use a cup with a cemented carbide disc in the bottom. The cup is then filled with the PCD or cBN powder mixture of choice and the cup is then sealed. The sealed cup is then subjected to a high temperature-high pressure (HPHT) sintering step. The PCD or cBN material is bonded to the cemented carbide during the sintering step. The disc can then be cut into suitable pieces using e.g. laser or WEDM (wire electrical discharge machining).

The cemented carbide used as a substrate for the PCD or cBN sintered compact can be made of any cemented carbide common in the art. The cemented carbide comprises a hard phase embedded in a metallic binder phase matrix.

By cemented carbide is herein meant that at least 50 wt% of the hard phase is WC.

Suitably, the amount of metallic binder phase is between 3 and 20 wt%, preferably between 4 and 15 wt% of the cemented carbide. Preferably, the main component of the metallic binder phase is selected from one or more of Co, Ni and Fe, more preferably the main component of the metallic binder phase is Co.

By main component is herein meant that no other elements are added to form the binder phase, however, if other components are added, like e.g. Cr, it will inevitably be dissolved in the binder during sintering.

In one embodiment of the present invention, the cemented carbide can also comprise other components common in cemented carbides elements selected from Cr, Ta, Ti, Nb and V present as elements or as carbides, nitrides or carbonitrides.

Maraging steel is a type of steel which is hardened by precipitation of intermetallic compounds. Maraging steels suitably contains from 13 to 25 wt% Ni and one or more alloying elements selected from Co, Mo, Ti, Al and Cr in a total amount of between 10 to 27 wt%, preferably between 11 to 23 wt% of alloying elements. Maraging steels typically contain less carbon than conventional steel, suitably 0.03 wt% or less. The balance being Fe.

The maraging steel according to the present invention preferably contains from 11 to 25 wt% Ni, preferably 17 to 25 wt% Ni. The alloying elements are suitably Co in an amount of from 7 to 15 wt%, preferably 8.5 to 12.5 wt% Co, Mo in an amount of from 3 to 10 wt%, preferably 3 to 6 wt% Mo, Ti in an amount of from 0.1 to 1.6 wt% preferably from 0.5 to 1.2 wt% Ti, from 0 to 0.15 wt% Cr, Al in an amount of from 0 to 0.2 wt%, less than 0.1 wt% of any of Mn, P and S and less than 0.03 wt% C. The balance is Fe.

In one embodiment of the present invention, the maraging steel have a composition of from 17 to 19 wt% Ni, from 8.5 to 12.5 wt% Co, from 4 to 6 wt% Mo, from 0.5 to 1.2 wt% Ti, from 0 to 0.15 wt% Cr, from 0 to 0.2 wt% Al, less than 0.1 wt% of any of Mn, P and S and less than 0.03 wt% C. The balance is Fe.

The average hardness of the maraging steel part is suitably between 350 and 580 HV1, preferably between 400 and 580 HV1 and more preferably 410 to 580 HV1. The hardness is measured by a Vickers hardness tester, by applying a load of 1 kgf (kilogram force) and a loading time of 15s. A pattern of at least 15 indents evenly distributed over the cross section (not surface) of the maraging steel parts should be applied. By evenly distributed over the whole cross section is herein meant that the indents should be placed so that they cover the whole cross section with a small variation in the distance between them.

The average value is an average of these measurement points. The standard deviation of the hardness values is suitably between 0 to 20 HV1, preferably between 0 and 14 HV1.

The brazing technique is the so-called active brazing. By that is meant that the joint is not just formed by melting the braze material and forming a metallic bond, it also involves a chemical reaction with one or both of the materials that are to be joined. The reactive element in the braze material is usually Ti, however elements such as Hf, V, Zr and Cr are also considered to be active elements. According to this invention, Ti is the active element.

By braze joint is herein meant the area or mass between the cemented carbide and the maraging steel part that is filled by the braze material and formed during the brazing process, see below.

The thickness of the braze joint is suitably between 5 and 200 pm, preferably between 15 and 100 pm.

The braze joint is not a homogenous phase. Instead, after brazing, the elements in the braze material form different phases.

The braze joint contains Ti. During brazing Ti will react with the carbon in tungsten carbide in the cemented carbide part and form a TiC layer at the interface between the braze joint and the cemented carbide part.

There are several ways to detect the presence of a TiC layer depending on which type of equipment that is used.

If a Scanning Electron Microscope (SEM) with a high enough resolution is used, the TiC layer is clearly visible adjacent the cemented carbide part. See e.g. Figures 2 and 3. To verify the composition of the layer, SEM-EDS (energy dispersive spectroscopy) and/or SEM- EPMA (electron probe microscopy analysis) with WDS (wave length dispersive spectroscopy) can be used to identify the individual elements in the TiC layer.

If the SEM used does not have enough resolution to show the TiC layer, the accumulation of Ti and/or C at the interface between the braze material and the cemented carbide substrate can be seen using e.g. SEM-EDS or SEM-EPMA with WDS. The accumulation of Ti is herein after called the Ti-accumulation layer (see e.g. Figure 4) and is one indicator that a TiC layer is formed, even if not visually detected in the SEM image. The Ti-accumulation layer is considerably thicker than the actual TiC -layer which could mean that not all Ti will form TiC. The thickness of the Ti-accumulation layer is also partly affected by the analyze method.

In one embodiment of the present invention, the thickness of the TiC layer is between 0.03 and 5 pm, more preferably between 0.05 and 0.5 pm and most preferred between 0.05 and 0.25 pm.

Preferably, the braze joint further comprises one or more elements selected from Ag, Cu, Sn, In, Zr, Hf, Cr. More preferably from Ag, Cu and In.

The composition of the braze joint after brazing is difficult to determine since the elements are not evenly distributed. If available, the easiest way is to look at the braze material that has been used since the paste or foil are a homogenous blend. Also, the braze joint might comprise small amounts of elements from the materials to be joined, e.g. Co, W from the cemented carbide and Fe, Ni etc. from the maraging steel.

The amount of Ti and the other elements in the braze joint could also be measured using Energy-dispersive X-ray spectroscopy analysis (EDS). However, due to the uneven distribution of the elements in the braze joint, many measuring points need to be used and the standard variation will be large. Preferably, the braze joint comprises, in average, Ag in an amount of from 30 to 80 wt%, preferably from 40 to 75 wt%, Cu in an amount of from 15 to 65 wt%, preferably from 20 to 40 wt%, Ti in an amount of from 0.3 to 15 wt%, preferably from 0.5 to 5 wt%, Sn in an amount of from 0 to 10 wt%, preferably from 0 to 2 wt% and In in an amount of from 0 to 30 wt%, preferably from 10 to 25 wt% where the balance is unavoidable impurities.

By unavoidable impurities are herein meant small amounts of elements possibly present in the braze material, other than those listed above, prior to the brazing step as well as elements from the material to be joined, e.g. Co, W etc. from the cemented carbide and Fe, Ni etc. from the maraging steel. Small amounts of the elements from the parts to be joined are unavoidably dissolved in the braze material when subjected to the increased temperature during the brazing step whereby the braze material melts and allow diffusion from the joining parts. As long as the brazing process parameters such as temperature and time are within the ranges according to the present invention, the total amount of the unavoidable impurities will be so small that it does not affect the performance of the braze joint.

The braze joint suitably has a shear strength of at least 130 MPa, preferably at least 140 MPa more preferably between 140 and 300 MPa. The shear strength is measured by shear testing.

At the interface between the braze joint and the maraging steel part Ti is also accumulated in the braze joint where it forms a metallic bond with the iron in the steel. The thickness of the accumulation layer of Ti at the maraging steel surface is preferably between 1 and 10 pm, preferably between 2 to 5 pm and can be measured by e.g. EDS. In one embodiment of the present invention the maraging steel is 1.2709 and the cemented carbide has a composition of 4 to 15 wt % Co, 0.1 to 1 wt% Cr and the rest WC. The braze joint has an average composition of 58 to 62 wt% Ag, 22 to 26 wt% Cu, 13 to 15 wt% In and 1.5 to 2.5 wt% Ti.

The cutting tool can be a tool or part of a tool where polycrystalline diamond (PCD) or a cubic boron nitride (cBN) sintered compact bonded to a cemented carbide substrate is joined with a steel part by brazing, eg. a cutter as is shown in figure 11.

The present invention also relates to a method of making a cutting tool according to the above comprising the steps of:

-providing a superhard part comprising a polycrystalline diamond (PCD) or a cubic boron nitride (cBN) sintered compact bonded to a cemented carbide substrate; and a maraging steel part;

-placing a braze material comprising Ti in an amount from 0.3 to 15 wt% of the braze material between and in contact with the cemented carbide substrate and the maraging steel part.

-subjecting the cemented carbide supported superhard part and the maraging steel part with the braze material in between to a brazing step in a furnace at a temperature between 600 and 780°C, preferably from 650 to 750°C, more preferably from 700 to 740°C, for a time period of between 1 and 60 minutes and wherein the brazing takes place in vacuum;

-subjecting at least the maraging steel part to an ageing step at a temperature of between BOO and 600°C for between 5 minutes and 12 hours.

The superhard part comprising a polycrystalline diamond (PCD) or a cubic boron nitride (cBN) sintered compact and the maraging steel part have a composition as described above. The hardness of the maraging steel, prior to brazing, can differ from that described above depending on maraging steel grade and if the steel has been aged or not. In one embodiment the maraging steel is supplied solution annealed with a hardness of around 340 HV1.

The shape and size of the superhard part and the maraging steel part are depending on the type of tool that is to be made.

The braze material (also called filler metal or solder) according to the present invention contains Ti in a total amount of from 0.3 to 15 wt%, preferably 1 to 5 wt% of the braze material. The braze material of the present invention suitably has a solidus temperature of between 488 and 1123°C, preferably between 600 and 700°C. Further, the braze material of the present invention has a liquidus temperature of between 612 and 1180°C, preferably between 700 and 750°C. The braze material further comprises, in addition to Ti, one or more elements selected from Ag, Cu, Sn, In, Zr, Hf and Cr.

In one embodiment of the present invention, the braze material comprises Ag in an amount of from 30 to 80 wt%, preferably from 40 to 75 wt%, Cu in an amount of 15 to 65 wt%, preferably from 20 to 40 wt%, Ti in an amount of 0.3 to 15 wt%, preferably from 0.5 to 5 wt%, Sn in an amount of 0 to 10 wt%, preferably from 0 to 2 wt% and In in an amount of from 0 to 30 wt%, preferably from 10 to 25 wt%.

Suitably, the braze material is provided as a foil or paste.

The braze material is provided onto the joining surfaces of the cemented carbide substrate and the steel part.

The thickness of the braze material prior to the brazing process depends on the type of material, i.e. foil or paste. If a paste is used, enough material is applied so that the surface that is to be brazed is covered. Typically, the thickness is between 20 and 200 pm, preferably between 50 and 100 pm.

The parts are then placed in a furnace with an inert or reducing environment, i.e. with minimum amount of oxygen. Preferably, the brazing temperature in the furnace is between 600 and 780°C, preferably between 650 and 750°C more preferably between 730 and 740°C. The time the parts are subjected to the elevated temperature is between 1 and 60 minutes, preferably between 5 and 15 minutes. If the time at elevated temperature is shorter, there is not enough time for the braze joint to form and the Ti to react to reach the desired strength of braze joint. If the time at elevated temperature is longer, the Ti- containing, brittle reaction zone will grow uncontrolled, which negatively influences the joint properties, e.g. shear strength.

The brazing suitably takes place in vacuum or with the presence of Argon at low partial pressure. By vacuum is herein meant that the pressure in the furnace is below 5xl0 ⁴ mbar, preferably below 5xl0 ⁵ mbar. If argon is present, the argon pressure is below lxlO ² mbar.

During the brazing in the furnace, a clamping force might be applied to further enhance the brazing. By clamping force is herein meant that the steel part and the cemented carbide part is pressed against each other so that a force is applied, preferably by placing external weights in the carbide part. The force that will act on the braze joint by the weight of the cemented carbide part or maraging steel part, depending on which part that is on top of the other, is not included in these values.

In one embodiment, a clamping force between 0.5 and 10 MPa, preferably between 2 and 8 MPa is applied.

In one embodiment of the present invention, no clamping force is applied. After brazing, the parts are subjected to an ageing step by subjecting the parts to an elevated ageing temperature of between 300 and 600°C, preferably between 350 and 500°C and most preferably between 400 and 440°C, for a time of between 5 minutes and 12 hours, preferably between 3 and 6 hours.

Suitably the heating rate up to the ageing temperature is between 1 to 50 °C/min, preferably is between 5 to 10 °C/min. Suitably the cooling rate from the ageing temperature down to a temperature at least below the solidus temperature of the braze material, preferably below 300°C, is between 1 to 50 °C/min, preferably is between 5 to 10 °C/min.

The brazing furnace used according the present invention can be any furnace that can provide such well controlled conditions with regard to a vacuum, heating and cooling rate etc. as has been described above. The brazing and ageing steps can either be done in the same furnace or in two separate furnaces.

It is common that the steel part will be subjected to a machining operation like e.g. threading, grinding etc. To be able to machine the steel part, the hardness cannot be too high, and depending on what type of maraging steel grade that is chosen, the ageing step can be done either before or after the machining of the steel part, in order to achieve the desired hardness and wear resistance in the final tool.

In one embodiment of the present invention, the ageing takes place directly after the brazing step and any machining of the steel like e.g. threading, is performed onto the already aged maraging steel, i.e. after the ageing step.

In another embodiment of the present invention, the ageing takes place after any machining of the steel like e.g. threading etc.

Figures

Figure 1 shows s schematic image of the tool where A is the steel part, B is the braze joint, C is the cemented carbide support and D is the superhard compact.

Figure 2 shows a SEM image of the interface between the cemented carbide part and the braze material of Invention 1 in Example 1 at a magnification of 10000.

Figure 3 shows a SEM image of the interface between the cemented carbide part and the braze material of Invention 1 in Example 1 at a magnification of 40000.

Figure 4 shows the EDS mapping of the element Ti at the interface between the cemented carbide part and the braze material of Invention 1 in Example 1 at a magnification of 10000.

Figure 5 shows the hardness profile of a tool where induction heating has been used. A is the steel part, B is the braze joint and C is the cemented carbide. Figure 6 shows a schematic drawing of the shear testing device where 1 is the steel part and 2 is the cemented carbide part.

Figure 7-9 showing different parts of the braze joint from invention 1 where A is the cemented carbide part and B is the maraging steel part.

Figure 10 shows the steps for the claimed method where A is providing a cemented carbide part, B is providing a maraging steel part, C is placing a braze material between the cemented carbide part and the maraging steel part. D is the brazing step and E is the ageing step.

Figure 11 shows an example of a brazed tool where A is the cemented carbide supported PCD part and B is the maraging steel part.

Figure 12 shows the pattern of the hardness measurements in the cross section of the steel part from Invention 2 in Example 1.

Figure IB shows the results of the hardness measurements done on Invention 2 in Example 1.

Example 1 (Invention)

A steel part made of maraging steel 1.2709 was provided together with a cemented carbide part with a composition of 10 wt% Co, 1 wt% other carbides and the remaining WC.

The braze material was provided in the form of a paste (TB-629) from Tokyo Braze which was applied in a thin layer but in an amount enough to cover the surface to be brazed. The braze material had a composition of 58-62 wt% Ag, 22-26 wt% Cu, 1.5-2.5 wt% Ti, and 13-15 wt% In. The solidus temperature is ca. 620°C, the liquidus temperature is ca. 720°C.

The paste was placed between the maraging steel part and the cemented carbide part so that both pieces were in contact with the paste. The assembled joining pieces were then placed into a Torvac vacuum furnace where the temperature was first increased to 350°C at a rate of 20°C/min. The debinding of the braze material at a temperature of 350°C was kept for 10 minutes after which the pieces were heated up to 600°C at a rate of 20°C/min. The heating through temperature 600 °C was kept for 10 min after which the pieces were heated up to 740°C at a rate of 5°C/min. The brazing temperature 740°C was kept for 20 minutes after which the pieces were cooled down to 300°C at a rate of 5°C/min. After 300°C it was free cooling.

After the brazing step, the brazed pieces were subjected to an ageing process to increase the hardness of the maraging steel. Three different ageing temperatures were tested and one brazed piece was not subjected to an ageing process for comparison. The pieces were placed in a furnace where the temperature was increased to the ageing temperature at a rate of 5°C/min. The different aging temperatures applied were 355, 440 and 490 °C. The ageing temperature was kept for 3h after which the pieces were cooled down to 300°C at a rate of 5°C/min. After 300°C it was free cooling.

Table 1

Example 2 (Comparative)

A steel part made of steel 1.6582 (34CrNiMo6) was provided together with a cemented carbide part with a composition of 10 wt% Co and 0.4 wt% Cr and the remaining WC.

The braze material was Ag49Zn23Cul6Mn7.5Ni4.5 in the form of a wire which was applied as a ring with a diameter of 1-2 mm.

The pieces were joined by induction heating using a coil by rapidly heating the braze joint to 700°C and hold for 15 s, after which the powder is turned off and the tool is allowed to cool to room temperature. In figure 5 the hardness values of the steel part are shown where the measuring points is placed along a line from a distance from the braze joint in the steel part and over the braze joint to the cemented carbide part.

The sample are herein denoted Comparative 2.

Example 3 (Comparative)

A steel part made of the carbon-hardening hot-work steel 1.2344 was provided together with a cemented carbide part with a composition of 10 wt% Co, 1 wt% other carbides and the remaining WC. The hot-work steel component was in a pre-hardened condition. The steel was quenched from 1060°C with ISh in a vacuum furnace and subsequently relaxed three times at 200 °C for 10 minutes. The mean hardness value of the quenched 1.2344 part was 582 HV1 with a standard deviation of 66 HV1.

The braze material 1 was provided in the form of a paste. Braze material 2 was provided in the form of a foil with a thickness of 100 pm. The braze material 1 had a composition of 60.0 wt% Ag, 24.0 wt% Cu, 14.0 wt% In, and 2.0 wt% Ti. The solidus temperature is ca. 620°C, the liquidus temperature is ca. 720 °C. The brazing material 2 had a composition of 59.0 wt% Ag, 27.25 wt% Cu, 12.5 wt% In, and 1,25 wt% Ti. The solidus temperature is ca. 605°C, the liquidus temperature is ca. 715°C.

The braze material was placed between the maraging steel part and the cemented carbide part so that both pieces were in contact with the braze material. The assembled joining pieces were placed into the furnace where the temperature was first increased to 500°C at a rate of 20°C/min and hold for 5 minutes. From 500°C the temperature was then increase by a rate of 50°C/min to the brazing temperature T _Br azin _g , which differed between 685°C (braze material 1) and 715°C (braze material 2). Te _n zi _ng was kept for a dwell time of 4 min, after which the pieces were cooled down to room temperature by free cooling.

The hardness was measured and the results are shown in Table 2.

The two samples 685°C (braze material 1) and 715°C (braze material 2) are herein denoted Comparative 3 and 4, respectively.

Shear strength values were not determined.

Example 4 (Comparative)

A steel part made of the carbon-hardening hot-work steel 1.2344 was provided together with a cemented carbide part with a composition of 10 wt% Co, 1 wt% other carbides and the remaining WC.

The braze material was provided in the form of a foil with a thickness of 100 pm.

The braze metal had a composition of 100.0 wt% Cu. The melting temperature is 1085°C.

The foil was placed between the maraging steel part and the cemented carbide part and assembled joining pieces were placed into the furnace where the temperature was first increased to 650 °C at a rate of 20°C/min and hold for 5 minutes. From 650 °C the temperature was then increase by a rate of 10 K/min to the brazing temperature T _Br azing, which was 1100 °C. T _Br azing was kept for a dwell time of 15 min, after which the pieces were cooled down to 850 °C with a cooling rate of 50 K/min. From 850 °C, the specimens were ISh- quenched with an overpressure of 2 bars and a fan frequency of 2500 min ^-1 .

Subsequently, the cemented carbide-steel joint with the carbon-hardening hot- work steel 1.2344 was aged at 630 °C for 2 h two times.

The sample will herein be denoted Comparative 5.

Example 5 (Comparative)

A steel part made of the carbon-hardening cold-work steel 1.2714 was provided together with a cemented carbide part with a composition of 10 wt% Co, 1 wt% other carbides and the remaining WC.

The braze metal was provided in the form of a foil with a thickness of 100 pm. The braze material 1 had a composition of 100.0 wt% Cu. The melting temperature is 1085 °C.

The foil was placed between the maraging steel part and the cemented carbide part and assembled joining pieces were placed into the furnace where the temperature was first increased to 650°C at a rate of 20°C/min and hold for 5 minutes. From 650 °C the temperature was then increase by a rate of 10 K/min to the brazing temperature T _Bra zing, which was 1100°C. Te _ra zi _ng was kept for a dwell time of 15 min. After the dwell time, free cooling was initiated until room temperature.

Subsequently, the cemented carbide-steel joint with the carbon-hardening cold- work steel 1.2714 part was heated to a temperature of 850°C for 10 min by torch and then quenched to room temperature in oil. After that, tension relaxing was conducted in a vacuum furnace at 200°C for 2 hours.

Subsequently, the cemented carbide-steel joint with the carbon-hardening cold- work steel 1.2714 part was aged at 500°C for 2 h.

The sample will herein after be denoted Comparative 6.

Example 6

The assembled joining pieces were evaluated by measuring the shear strength of the brazed joint, the hardness of the maraging steel part and the TiC layer and the TiC accumulation layer in the braze joint where applicable.

In order to assess the joint strength properties, the samples were shear tested using a shear device set-up as shown in figure 6 where 1 is the steel part in the shape of a steel cylinder (0=20 mm, h=5 mm) and 2 is the cemented carbide part in the shape of a cemented carbide cylinder (0=10 mm, h=5 mm). The steel cylinder is positioned in the gap of the shear strength test device and therefore can only be moved in loading direction. A notch, which was eroded into the surface of the device, holds the joined parts in the right position and guarantees an evenly distributed force induction into the braze joint. The applied force was constantly increased until the braze joint failed and the cemented carbide cylinder sheared off. The ultimate shear strength was then calculated by the quotient of the maximal measured force and the initial joining surface (A=78,5 mm ² ). The braze material was not removed before the determination of brazed joint shear strength.

The hardness of the maraging 1.2709 steel parts was measured by a Vickers hardness tester on a cross section of the maraging steel part, applying a load of 1 kgf (kilogram force) and a loading time of 15 s. A pattern of 3 x 6 indents covering the complete profile (ca. 20 x 5 mm ² ) of the maraging steel 1.2709 part in the cross-section was applied, see e.g. Figures 12 and 13.

To analyze the interface between the braze joint and the cemented carbide on Invention 1-3, SEM-EDS technique was used. The SEM used was a Jeol JSM-7001F which is a high-resolution Field Emission Scanning Electron Microscopy (FE-SEM) with a thermal Field emission cathode (Schottky). The thickness of the TiC layer in the braze joints of Invention 1- 3 was measured on a SEM image at a magnification of 10000. The TiC layer was identified by visual appearance in the back scattered electron mode. In Figure 3, a SEM image of Invention 3 is shown where the TiC layer is clearly visible. The values of the thickness of the TiC layer given in Table 2 is an average of 3 measurements, all taken in the middle of the braze joint, i.e. far from the edges.

Using EDS, the accumulation of Ti can be identified and measured as a Ti- accumulation layer. By accumulation layer is herein meant the thickness of the accumulation estimated from an EDS scan. The values in Table 2 is an estimation from visual inspection of the EDS scan and is therefore given as an interval.

The thickness of the TiC layer in Table 2 is an average of 3 measurements all taken in the middle of the braze joint, i.e. far from the edges on a SEM image at 10000 magnification. In Figure 4, the accumulation of Ti is shown using EDS.

Table 2

*No Ti in braze material