Hard alloy, method for producing the hard alloy, and method for testing

Abstract

The invention relates to a cobalt-based hard alloy which is characterised by a relatively homogeneous microstructure comprising fine carbides, in particular fine M7C3 carbides. The invention also relates to a method for producing the alloy according to the invention and to a test method for assessment and quality assurance of components produced from the alloy.

Description

[0001] Hard alloy, methods for producing the hard alloy and methods for testing

[0002] Description

[0003] The invention relates to a cobalt-based hard alloy. It is characterized by a relatively homogeneous microstructure with fine carbides, in particular fine MyCs carbides. Furthermore, the invention relates to a method for producing the alloy according to the invention and to a testing method for evaluating and ensuring the quality of components produced from the alloy.

[0004] Cobalt-based alloys, such as those marketed under the commercial name "Stellite™," are widely known. For example, the alloy Stellite™ 20 exhibits high resistance to wear and corrosion, which is maintained even at high temperatures. Therefore, it is used in a wide range of industries, including food processing, aerospace, and petrochemicals.

[0005] The positive properties of Stellite™ 20, however, present significant challenges in the production and machining of the alloy. Its high wear resistance results in high wear of the cutting tools used for machining the alloy, as well as reduced machining speeds. Furthermore, achieving a homogeneous distribution of the alloying elements and a fine microstructure with small grain sizes proves difficult during alloy production. This ultimately affects the alloy's mechanical properties.

[0006] The microstructure is largely determined by the temperatures and cooling conditions during casting, as well as by the chemical composition of the alloy. Among other things, carbides, compounds of an element (M) and carbon (C) with the general formula M, can be involved. x C yThe distribution of alloying elements during solidification follows the Scheil solidification mechanism, allowing conclusions to be drawn about the solidification process, particularly in casting processes such as centrifugal casting or investment casting. For example, the Scheil solidification mechanism reveals that, in addition to carbides of the form M23C6, MC, M12C, M3C, or MeC, primary carbides of the form M7C3 are formed in the conventionally cast alloy Stellite™ 20. The MyCs carbides are coarse, needle-shaped carbides where stress concentrations or stress peaks occur. They thus promote the formation of microcracks in the solidified alloy and can therefore be considered points of failure.

[0007] The object of the present invention is therefore to provide a cobalt-based alloy with high resistance to wear and corrosion, wherein the manufacturability and machinability of the alloy is improved compared to conventional cobalt-based alloys.

[0008] The problem is solved by the cobalt-based alloy according to the invention with an adapted composition. It was found that with this specifically chosen composition, an alloy can be produced that is characterized above all by a relatively fine microstructure compared to conventional cobalt-based alloys, in particular the alloy Stellite™ 20. The microstructure of the alloy according to the invention is characterized by a more uniform distribution of the alloying elements and fine grain sizes. In particular, the MyCs carbides in the alloy according to the invention are distributed comparatively finely and homogeneously within the alloy. It was also found that, compared to Stellite™ 20, fewer chippings occur during both machining and use of the solidified alloy according to the invention.It has been shown that these positive properties are achieved primarily when the alloy according to the invention is produced using the manufacturing process according to the invention.

[0009] The alloy according to the invention comprises at least the elements cobalt (Co), carbon (C), chromium (Cr), tungsten (W), silicon (Si), and manganese (Mn). Further alloying elements may include iron (Fe), nickel (Ni), and molybdenum (Mo). The alloy may also contain trace elements. These trace elements can be introduced into the alloy, for example, via the raw materials in which they are present as impurities.

[0010] The proportions of the individual alloying elements in the solidified alloy are given below in weight percent (wt%), where the percentage indicates the mass fraction of the respective alloying element relative to the total mass of the solidified alloy.

[0011] According to the invention, the alloy is composed of cobalt with a proportion of 40 wt.% to 60 wt.%, carbon with a proportion of 1.7 wt.% to 2.0 wt.%, chromium with a proportion of 28.0 wt.% to 32.0 wt.%, tungsten with a proportion of 14.0 wt.% to 17.0 wt.%, silicon with a proportion of 1.0 wt.% to 1.5 wt.%, manganese with a proportion of 0.8 wt.% to 1.2 wt.%, iron with a proportion of 0.5 wt.% to 2.5 wt.%, nickel with a proportion of 0.5 wt.% to 2.5 wt.% and molybdenum with a proportion of 0.0 wt.% to 1.0 wt.%, wherein the total proportion of trace elements is a maximum of 1.5 wt.%.

[0012] In another embodiment, the alloy comprises cobalt with a proportion of 45 wt.% to 55 wt.%, carbon with a proportion of 1.8 wt.% to 1.9 wt.%, chromium with a proportion of 29.5 wt.% to 31.0 wt.%, tungsten with a proportion of 15.0 wt.% to 16.0 wt.%, silicon with a proportion of 1.1 wt.% to 1.4 wt.%, manganese with a proportion of 0.9 wt.% to 1.1 wt.%, iron with a proportion of 0.0 wt.% to 2.5 wt.%, nickel with a proportion of 0.0 wt.% to 2.5 wt.% and molybdenum with a proportion of 0.0 wt.% to 0.5 wt.%, wherein the proportion of trace elements is a maximum of 1.0 wt.%.

[0013] A transition was observed in which MyCs carbides solidify directly from the liquid phase as primary components and later transform into secondary carbides from the liquid phase after the solidification of the hexagonally close-packed cobalt solid solution (HCP cobalt solid solution). This transition is particularly dependent on the carbon content, with the critical carbon content in turn depending on the chromium and tungsten contents in the system. In this regard, the empirical equation w(C) = 3.5 wt% - 0.2 w(W) + 0.05 w(Cr) was derived for the equilibrium state between the proportions w in weight percent of carbon w(C), chromium w(Cr), and tungsten w(W). It was found that in the Scheil solidification mechanism, with a carbon content w(C) between 1.8 wt.% and 1.9 wt.%, a tungsten content w(W) between 15.0 wt.% and 16.0 wt.%, and a chromium content w(Cr) between 29.0 wt.% and 31.0 wt.%,-% the direct formation of HCP-cobalt mixed crystals from the liquid phase is promoted, whereby the MyCs carbides solidify secondarily and their growth is suppressed, resulting in a finer and more homogeneous distribution of the MyCs carbides.

[0014] In another embodiment, the alloy is composed of carbon with a proportion of 1.85 wt.%, chromium with a proportion of 30.0 wt.%, tungsten with a proportion of 15.5 wt.%, silicon with a proportion of 1.25 wt.%, manganese with a proportion of 1.0 wt.%, iron with a proportion of up to 1.5 wt.% and nickel with a proportion of up to 1.5 wt.%, wherein the total proportion of trace elements is a maximum of 1.0 wt.% and the alloy contains cobalt as the remainder.

[0015] In another embodiment, the proportion of a single trace element in the alloy is a maximum of up to 1 wt.%, preferably up to 0.8 wt.% and particularly preferably up to 0.5 wt.%.

[0016] In small quantities, the trace elements hardly affect the special properties of the alloy, or only to a tolerable extent. However, a high proportion of trace elements should be avoided. Therefore, the sum of the proportions of all trace elements in the alloy is preferably a maximum of 1.5 wt.%, but particularly preferably a maximum of 1.0 wt.%.

[0017] In one embodiment, phosphorus and / or sulfur may be present in the alloy as trace elements. Large amounts of both phosphorus and sulfur negatively affect the castability and high-temperature properties of the alloy. Therefore, the phosphorus content in the alloy is preferably a maximum of 0.1 wt.%, more preferably a maximum of 0.06 wt.%, and particularly preferably a maximum of 0.03 wt.%. The sulfur content in the alloy is also preferably a maximum of 0.1 wt.%, more preferably a maximum of 0.06 wt.%, and particularly preferably a maximum of 0.03 wt.%.

[0018] In another embodiment, titanium, tantalum, and / or hafnium can be present in the alloy as trace elements. Large proportions of these elements can promote the formation of carbides of types MC, MeC, or M12C. Therefore, the proportion of these elements in the alloy is preferably a maximum of 0.4 wt.% each, more preferably a maximum of 0.2 wt.% each, and particularly preferably a maximum of 0.1 wt.% each.

[0019] In another embodiment, aluminum can be present as a trace element in the alloy. Aluminum can be introduced as a deoxidizing agent via the slag removal powder to react with the gases in the melt. On the other hand, aluminum can impair castability in air. Therefore, the proportion of aluminum in the alloy is preferably a maximum of 0.75 wt.%, more preferably 0.5 wt.%, and particularly preferably 0.25 wt.%.

[0020] In one embodiment, boron can be present as a trace element in the alloy. Boron can improve the high-temperature properties and fracture toughness. Furthermore, boron can act as a grain refiner in Co-Cr-W systems. However, boron can also negatively affect the machinability of the alloy. Therefore, the proportion of boron in the alloy is preferably a maximum of 0.4 wt.%, more preferably a maximum of 0.2 wt.%, and particularly preferably a maximum of 0.1 wt.%.

[0021] For example, in a preferred embodiment, the trace elements titanium, tantalum, hafnium, phosphorus, sulfur, aluminum, and boron can be present in the alloy. The proportion of titanium, tantalum, and hafnium is a maximum of 0.1 wt.% each, the proportion of phosphorus and sulfur a maximum of 0.03 wt.% each, the proportion of aluminum a maximum of 0.25 wt.%, and the proportion of boron a maximum of 0.1 wt.%.

[0022] It is also conceivable that in one embodiment the proportion of titanium, tantalum, and hafnium is a maximum of 0.4 wt.% each, the proportion of phosphorus and sulfur is a maximum of 0.1 wt.% each, the proportion of aluminum is a maximum of 0.75 wt.%, and the proportion of boron is a maximum of 0.4 wt.%. It is also conceivable that in another embodiment the proportion of titanium, tantalum, and hafnium is a maximum of 0.4 wt.% each, the proportion of phosphorus and sulfur is a maximum of 0.03 wt.% each, the proportion of aluminum is a maximum of 0.75 wt.%, and the proportion of boron is a maximum of 0.4 wt.%.

[0023] The alloy preferably has a melting range of 1100 °C to 1450 °C, more preferably of 1150 °C to 1400 °C and particularly preferably of 1220 °C to 1340 °C.

[0024] To evaluate the microstructure of the alloy, the sizes of the MyCs carbides present within it are determined, among other things. This is done according to standard test methods for determining average grain sizes according to ASTM 112-13(2021).

[0025] It was found that, with the adapted composition of the alloy according to the invention, the direct formation of HCP-cobalt solid solutions and M7C3 as a secondary phase is promoted. This is accompanied by the suppression of MyCs carbide growth. Compared to the conventional Stellite™ 20 alloy, the alloy according to the invention thus contains finer and more homogeneously distributed MyCs carbides. The MyCs carbides in the alloy according to the invention have a longitudinal size of less than 250 pm, preferably less than 200 pm, and particularly preferably less than 100 pm.

[0026] The alloy according to the invention is preferably produced by the casting process according to the invention. The Scheil solidification mechanism stabilizes the face-centered cubic (FCC) structure. In contrast, under equilibrium conditions, the alloy solidifies in a high-carbon polycarbonate (HCP) structure. This enhances the direct formation of HCP-cobalt solid solutions during subsequent annealing and further suppresses the growth of MyCs carbides.

[0027] The fact that the alloy has a finer and more homogeneous microstructure, and in particular smaller and more homogeneously distributed MyCs carbides, results in increased ductility and strength compared to Stellite™ 20. This makes the alloy according to the invention less brittle, which has a positive effect on its formability.

[0028] In one embodiment, the alloy can be heat-treated. Annealing the alloy leads to a phase transformation of the face-centered cubic (FCC) lattice structure to an HCP lattice structure. This increases the hardness of the alloy. Preferably, the alloy according to the invention has a hardness of 52 HRC to 62 HRC after annealing, and more preferably 54 HRC to 60 HRC, with a hardness of 56 being particularly preferred.

[0029] The alloy according to the invention preferably has a density between 7.7 g / cm³. 3 and 9.9 g / cm³ 3 , preferably between 8.0 g / cm³ 3 and 9.5 g / cm³ 3 and especially preferably between 8.5 g / cm² 3 and 9 g / cm² 3 on.

[0030] The melting range of the alloy according to the invention preferably extends from 1260 °C (solid temperature) to 1300 °C (liquidus temperature) and particularly preferably from 1220 °C to 1340 °C.

[0031] The alloy according to the invention is preferably used as a material for folding rollers. It is also conceivable to use the alloy according to the invention as a material for other components. The metallic hard alloy according to the invention, based on cobalt, is preferably used as a folding roller. Preferably, the folding roller consists at least partially or completely of the alloy according to the invention.

[0032] Furthermore, the invention relates to a method for producing alloys, preferably the alloy according to the invention.The process comprises the following steps: a) charging a furnace with raw materials, wherein the raw materials are selected such that the solidified alloy contains at least the elements according to at least one of the aforementioned compositions for the alloy according to the invention; b) melting the raw materials by heating them to a casting temperature in a preferred range of 1530 °C to 1600 °C, and in a further preferred range of 1530 °C to 1700 °C; c) pouring the melt into a mold; d) cooling the mold, including the melt contained in the mold, in air at a cooling rate in a preferred range of 50 °C / min to 200 °C / min, and in a further preferred range of 100 °C / min to 200 °C / min, to a temperature of 800 °C; e) demolding the alloy; f) free cooling at a preferred cooling rate of 20 °C / min to 50 °C / min. The process is based on the investment casting or centrifugal casting processes.However, it may also be based on another casting process known from the prior art. Therefore, those skilled in the art will be familiar with individual process steps and procedures typical of known casting processes, which will not be described further here. For example, those skilled in the art know how the desired composition of the solidified alloy can be achieved by adding the starting materials in pure and / or mixed form.

[0033] When charging the furnace, cobalt is preferably added first, followed by chromium (it is conceivable that the chromium is already pre-alloyed with carbon), then tungsten, then silicon, then manganese, and finally carbon. It is also conceivable that, in the charging sequence, the return from a previous casting process of the alloy is added to the furnace before the addition of cobalt.

[0034] It is conceivable that melting furnaces known as furnaces, such as induction furnaces, electric arc furnaces or resistance furnaces, could be used.

[0035] The starting materials, also referred to here as raw materials, can contain the required elements in pure form and / or mixtures. In the case of raw materials existing as mixtures of different elements, the composition of the raw materials can be calculated by a batch optimization.

[0036] The melting of the raw materials and the pouring of the molten metal can take place in air. However, it is also conceivable that the melting and / or pouring is carried out under a protective gas atmosphere or under vacuum. The molten metal is preferably heated to a pouring temperature of 1530 °C to 1600 °C. Preferably, the molten metal is maintained at this pouring temperature throughout the entire pouring process. However, it is particularly preferred that the molten metal is maintained at this pouring temperature for at least 10 minutes and no longer than 15 minutes to achieve a homogeneous distribution. It is also conceivable that the molten metal is superheated when tungsten and chromium are added. Preferably, the temperature of the molten metal is increased by an additional 100 °C during superheating. The molten metal can then be poured. The molds commonly used for casting processes are employed.This allows for the use of both permanent molds, such as reusable ingot molds, and so-called expendable molds, which are only usable once. Optionally, coating materials can be applied to the molds to smooth the surface of the molded part.

[0037] Cooling of the melt or the mold, including the melt contained within it, is preferably carried out in air. Cooling rates of 50 °C / min to 200 °C / min are preferred. However, it is also conceivable that cooling takes place in a furnace or with active cooling of the mold. Preferably, the melt or the solidified melt is continuously cooled to a temperature of 800 °C before the alloy is demolded. The demolded alloy can then cool to ambient temperature in air.

[0038] Pouring the molten metal into the mold can take place directly from the furnace. In another embodiment, however, it is also conceivable that the molten metal is first poured into a ladle and then transferred from there into the mold. Preferably, the ladle is preheated to a temperature of 1000 °C to 1200 °C. This prevents premature solidification of the molten metal.

[0039] In another embodiment, the mold is preheated to a temperature of 1050 °C before the molten metal is poured into it. This is intended to prevent premature solidification of the molten metal. To prevent defects, the molten metal should only solidify once the mold is completely filled.

[0040] In another embodiment, the mold, preferably the die, is preheated to a temperature of 240 °C before the molten metal is poured into it.

[0041] At least in cases where the process is based on centrifugal casting, the mold preferably has a rotationally symmetrical shape. Accordingly, the mold is set into rotation, causing centrifugal forces to act on the molten metal. These forces, preferably g-forces of 140 g to 160 g (g: acceleration due to gravity), act on the molten metal at the inner diameter of the mold. This is typically achieved at a minimum rotational speed of 3000 rpm. The centrifugal force presses the molten metal against the inner wall of the mold, thereby reducing the formation of pores and cavities. In a further embodiment, the process includes additional steps for the post-treatment of the alloy. These additional steps are preferably carried out after demolding the solidified alloy and before allowing it to cool freely. However, it is also conceivable that these steps take place after the alloy has cooled freely.In the latter case, the alloy may need to cool down to ambient temperature again after post-treatment. Furthermore, it is conceivable that the subsequent post-treatment steps take place immediately before demolding the alloy, meaning that the solidified alloy is still in the mold.

[0042] These further steps for the post-treatment of the alloy include e1. Homogenizing the component at a temperature of 900 °C for a duration of two hours; e2. Cooling the component in the oven with a cooling rate of less than 50 °C / h.

[0043] To homogenize the component or alloy, it is heated again to 900 °C. The alloy is then held at this temperature for a period of two hours.

[0044] When the component is cooling in the oven, the oven is preferably closed. However, it is also conceivable that the oven is open for cooling the component.

[0045] Furthermore, the invention relates to a method for testing the durability of components. In particular, it aims to provide information about the durability of crimping rolls for the food industry. The method specifically aims to evaluate the mechanical properties of the alloy according to the invention. The test results should also allow conclusions to be drawn about the microstructure and machinability of the alloy. In this respect, the measurement results correlate with the fineness of the microstructure, such that a finer microstructure of the alloy is present when greater forces are applied until the failure of the test piece (ring) described below. The finer microstructure is also associated with improved machinability, which manifests itself in a reduction of chipping during machining of the components. Thus, the test method enables a comprehensive evaluation of the suitability of the alloy for specific applications.The test method is a modification of the ring folding test according to standards ASTM A 370 and ASTM B939. Therefore, the test method is a compression test. The method according to the invention comprises at least the following steps: a) providing a ring made of the alloy according to the invention, wherein the ring has an inner diameter of 38 mm, an outer diameter of 53 mm, and a width of 10 mm; b) placing the ring between two disks made of Stellite™ 3, each with a diameter of 50 mm, a hardness of 54 HRC, and a center roughness of [value missing in original text].

[0046] Ra = 0.05 pm, with the disks resting opposite each other on the lateral surface of the ring; c) Moving the two disks towards each other at a speed of 0.1 mm / min until the ring fails; d) Determining the force at which the ring fails.

[0047] The ring has the circular geometry of a hollow cylinder. The inner and outer surfaces are arranged concentrically and parallel to each other. Preferably, the outer diameter of the ring is 53 mm ± 0.06 mm and the inner diameter is 38 mm ± 0.05 mm. The two base surfaces of the ring are parallel and spaced 10 mm apart, and are perpendicular to the axis of the hollow cylinder. The edges between the two base surfaces and the inner and outer surfaces can be chamfered, preferably with a chamfer angle of 45° and a depth of 0.03 mm.

[0048] The two discs are made from the alloy known commercially as Stellite™ 3. Each disc has a cylindrical geometry with base surfaces parallel to each other and perpendicular to the cylinder's longitudinal axis. The cylinders / discs are 10 mm to 15 mm high and have a diameter of 50 mm. The discs have a hardness of 54 HRC and a surface roughness of Ra = 0.05 pm.

[0049] By placing the ring between the two Stellite™ 3 discs, with the discs resting diametrically on the ring's outer surface, radial pressure is exerted on the ring's outer surface during the test. Because the two discs rest on the outer surface of the ring, a small contact area is created between the discs and the ring, initially resembling a line. This results in minimal friction between the discs and the ring, thus positively impacting the accuracy of the test procedure.

[0050] The two discs made of Stellite™ 3 serve, on the one hand, to ensure stability during testing and, on the other hand, to increase reproducibility by minimizing deformation effects between the ring and the actual specimen holder. It is also conceivable that the specimen holder itself is made of Stellite™ 3, thus taking over the function of the two discs and replacing them.

[0051] It is conceivable that the two disks are initially moved towards each other at a relatively high speed of 0.1 mm / min until both disks come into contact with the outer surface of the ring.

[0052] If the two discs are in contact with the ring, they are moved towards each other at a speed of 0.1 mm / min until the ring fails. Failure is defined here as the ring breaking due to crack propagation. The test force required for this is an indicator of the material's strength and microstructure quality.

[0053] Choosing such a traverse speed has the advantage of minimizing the effect of work hardening of the rings, leading to more accurate and reliable results.

[0054] In a further embodiment, the test procedure optionally includes at least one of the following steps: e) regrinding the discs to a mean roughness value of Ra = 0.05 pm if deformation marks are detected on one of the discs; f) moving the bearing surface of the rings on the discs between different test runs.

[0055] During regrinding, the discs are ground with a 1200 grit abrasive. This ensures that a specific surface roughness of Ra = 0.05 pm is restored. Regrinding should be performed regularly, or at the latest after any signs of deformation are detected on the discs. To reduce the frequency of regrinding and thus optimize the testing procedure in terms of time, the positioning of the rings on the discs should be strategically varied during each test run. This approach ensures a uniform load distribution across the disc surfaces and minimizes the need for frequent regrinding.

[0056] Further advantages, objectives, and features of the present invention are explained with reference to the accompanying figures. Similar components may have the same reference numerals in the different embodiments.

[0057] The figures show:

[0058] Fig. 1 Phase diagram of Stellite™ 20 at a Cr content of 32 wt.% and W of 15 wt.%

[0059] Fig. 2 Phase diagram of Stellite™ 20 at a Cr content of 32 wt.% and W of 17 wt.%

[0060] Fig. 3 Phase diagram of Stellite™ 20 at a Cr content of 33 wt.% and W of 17 wt.%

[0061] Fig. 4 Phase diagram of Stellite™ 20 at a Cr content of 33 wt.% and W of 15 wt.%

[0062] Fig. 5a, b Micrograph of a sample of the alloy according to the invention

[0063] Fig. 6 Micrograph of a Stellite™ 20 sample

[0064] Fig. 7 Test setup of the test procedure

[0065] Fig. 8 Image of the fracture surface of a sample made from the alloy according to the invention

[0066] Fig. 9 Image of the fracture surface of a Stellite™ 20 sample

[0067] Figures 1-4 show phase diagrams of the conventional alloy Stellite™ 20 for a carbon range of 0.5 wt% to 2.5 wt% and varying chromium and tungsten contents. The phase diagrams show that, depending on the carbon content, MyCs carbides solidify directly from the liquid phase as primary units and subsequently transform into secondary carbides after hexagonally close-packed (HCP) cobalt solid solutions solidify from the melt via the equilibrium solidification mechanism. The phase diagrams also demonstrate that the range of carbon contents at which these transitions occur is significantly dependent on the mass ratio of chromium to tungsten.

[0068] Figures 5a, 5b, and 6 show metallographic cross-sections of samples made from the alloy according to the invention (Figs. 5a, 5b; each the same sample at different magnifications) and from the conventional alloy Stellite™ 20 (Fig. 6). A comparison of Figures 5a and 6 reveals that the sample produced from the alloy according to the invention is characterized by a finer and more homogeneous microstructure without massive primary carbides compared to the Stellite™ 20 sample. Thus, the alloying elements are more uniformly distributed in the sample produced from the alloy according to the invention. This results in altered mechanical properties.

[0069] Reference numeral 1 here denotes MyCs carbides, which occur as needle-like structures. These are strongly pronounced in the sample shown in Figure 6 and exhibit exemplary lengths of approximately 178 pm to 592 pm. Such massive carbides affect the mechanical properties of the alloy, since stress concentrations occur at these needle-like structures, leading to the formation of microcracks and thus negatively impacting the fracture toughness. In contrast, the sample of the alloy according to the invention shown in Figure 5b exhibits MyCs carbides with grain lengths of approximately 44 pm to 60 pm along their longest direction of extension.

[0070] Figure 7 shows the test setup for testing the durability of folded rolls. Reference numerals 2a and 2b denote the lower and upper sample holders of the testing machine. The testing machine is designed to move the two sample holders 2a and 2b towards each other at a speed of 0.1 mm / min. Preferably, the lower sample holder 2a is stationary, and the upper sample holder 2b is moved towards the lower sample holder 2b in direction R. The testing machine measures the force required to move the sample holders.

[0071] Reference numerals 3a and 3b identify the Stellite™ 3 discs. Each of these discs is arranged flat against a specimen holder of the testing machine. The ring represents the specimen and is preferably made of the alloy according to the invention. The ring, or specimen, is identified by reference numeral 4 and is located between the two Stellite™ 3 discs, with the discs arranged diametrically opposite each other with respect to the ring or specimen. In the embodiment shown in Figure 7, the two discs are already in contact with the ring. The two specimen holders of the testing machine, and thus the two discs, can therefore be moved towards each other for testing. The testing machine determines the force required for this movement and plots it against the distance traveled. In this way, the force at which the ring fails is determined.

[0072] The following table lists results from the test method according to the invention for various samples (rings). For this purpose, the rings were produced from different tubes. Several rings could be manufactured from each tube. Thus, one tube can be considered a batch with which several tests could be carried out. Batches 1 to 5 were produced from the conventional alloy Stellite™ 20, and batches 6 and 7 from the alloy according to the invention. The average force to failure of the specimens produced from the conventional Stellite™ 20 alloy ranges from 10.3 kN ± 0.2 kN to 12.2 kN ± 0.1 kN, and for the specimens produced from the alloy according to the invention, from 13.5 kN ± 1.2 kN to 15.9 kN ± 0.8 kN. The results therefore indicate an improvement in the mechanical properties of the components produced from the alloy according to the invention with respect to radial force application.

[0073] Figures 8 and 9 show fracture edges of specimens tested using the inventive test method. Figure 8 shows the fracture edge of a specimen produced from the alloy according to the invention. In contrast to the Stellite™ 20 specimen (Fig. 9), this specimen has two areas, 6 and 7. Area 7 indicates brittle fracture, whereas area 6 indicates flexural fracture. Flexural fracture initially occurs after plastic deformation. Subsequently, during brittle fracture, a sudden material failure occurs.

[0074] Brittle fracture occurs immediately in the Stellite™ 20 sample. This is evident in Figure 9, where fracture surface 5 exhibits only one area. Also striking are the massive MyCs carbides, which appear as dark, needle-shaped structures and are identified by reference numeral 1.

[0075] The occurrence of a deformation fracture in Figure 8 shows that the alloy according to the invention exhibits increased ductility compared to Stellite™ 20 (Figure 9). As a result, the alloy according to the invention is less brittle, which has a positive effect on its formability.

[0076] The applicant reserves the right to claim all features disclosed in the application documents as essential to the invention, provided they are novel individually or in combination compared to the prior art. It is further noted that the individual figures also describe features which may be advantageous on their own. A person skilled in the art will immediately recognize that a particular feature described in a figure may be advantageous even without incorporating other features from that figure. Furthermore, a person skilled in the art will recognize that advantages may also arise from a combination of several features shown in individual or different figures.

[0077] Reference symbol list

[0078] 1 MyCs carbides

[0079] 2a, 2b upper and lower sample holder of the testing machine

[0080] 3a, 3b upper and lower Stellite™ 3-disc

[0081] 4 rings / sample

[0082] 5 Fracture surface

[0083] 6. Area of ​​deformation fracture

[0084] 7 Area of ​​brittle fracture

[0085] R Direction of movement of the upper sample holder during testing

Claims

Hard alloy, methods for producing the hard alloy and methods for testing Patent claims 1) Co-based metallic hard alloy with the following composition: Co between 40.0 wt.% and 60.0 wt.% C between 1.7 wt.% and 2.0 wt.% Cr between 28.0 wt.% and 32.0 wt.% W between 14.0 wt.% and 17.0 wt.% Si between 1.0 wt.% and 1.5 wt.% Mn between 0.8 wt.% and 1.2 wt.% Fe between 0.5 wt.% and 2.5 wt.% Ni between 0.5 wt.% and 2.5 wt.% Mo up to 1.0 wt.% Trace elements cumulatively up to 1.5 wt.%. 2) Metallic hard alloy according to claim 1, wherein the proportion of each individual trace element in the alloy is a maximum of up to 1 wt.%, preferably up to 0.8 wt.% and particularly preferably up to 0.5 wt.%. 3) Metallic hard alloy according to claim 1 or 2, wherein the alloy contains as trace elements one, several or all of the following elements: Ti up to 0.1 wt.% Ta up to 0.1 wt.% Hf up to 0.1 wt.% P up to 0.03 wt.% S up to 0.03 wt.% AI up to 0.25 wt.% B up to 0.1 wt.%. 4) Metallic hard alloy according to one of the preceding claims, wherein the melting range extends from 1100 °C to 1450 °C, preferably from 1150 °C to 1400 °C and particularly preferably from 1220 °C to 1340 °C. 5) Metallic hard alloy according to any of the preceding claims, wherein the size of the MyCs carbides in the alloy in the longitudinal direction is less than 250 pm, preferably less than 200 pm and particularly preferably less than 100 pm. 6) Metallic hard alloy according to any of the preceding claims, wherein the hardness is between 48 HRC and 60 HRC, preferably between 50 HRC and 58 HRC and particularly preferably between 52 HRC and 56 HRC. 7) A method for producing a hard alloy according to any one of the preceding claims, comprising the following steps: a. Charging a furnace with raw materials, wherein the raw materials comprise at least the elements in the composition according to any one of claims 1 to 6; b. Melting the raw materials by heating to a casting temperature in the range of 1530 °C to 1600 °C; c. Pouring the melt into a mold; d. Cooling the mold, including the melt contained in the mold, in air at a cooling rate in the range of 50 °C / min to 200 °C / min to a temperature of 800 °C; e. Demolding the hard alloy; f. Free cooling at a cooling rate of 20 °C / min to 50 °C / min. 8) Method according to claim 7, wherein, when pouring the melt, the melt is fed to the mold via a casting ladle heated to 1000 °C to 1200 °C. 9) Method according to one of claims 7 or 8, wherein the mold is preheated to a temperature of 1050 °C before the melt is poured into this mold. 10) Method according to one of claims 7-9, wherein the mold rotates, wherein g-forces of 140 g to 160 g are present at the inner diameter of the mold. 11) Method according to any one of claims 7 to 10, wherein the method after step e. and before step f. comprises the following post-treatment steps of the alloy: e1. Homogenizing the component at a temperature of 900 °C for a duration of two hours; e2. Cooling the component in the furnace with a cooling rate of less than 50 °C / h. 12) Method for testing the durability of folding rolls for the food industry by a compression test, the method comprising the following steps: a) providing a ring made of the hard alloy according to any one of claims 1-6, wherein the ring has an inner diameter of 38 mm, an outer diameter of 53 mm and a width of 10 mm; b) placing the ring between two disks made of Stellite™ 3, each with a diameter of 50 mm, a hardness of 54 HRC and a center roughness of Ra = 0.05 pm, with the disks resting opposite each other on the lateral surface of the ring; c) Moving the two disks towards each other at a speed of 0.1 mm / min until the ring fails; d) Determining the force at which the ring fails. 13) Method according to claim 12, which optionally includes at least one of the following steps: e) Re-grinding the discs to a mean roughness value of Ra = 0.05 pm when deformation marks are detected on one of the discs; f) Shifting the bearing surface of the rings on the discs between different test runs.

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