High-temperature forming tools

Abstract

The present invention relates to a high temperature forming tool (1), at least part of which consists of a molybdenum-based alloy having a molybdenum content of ≥ 90 wt.%, the molybdenum-based alloy being in a compacted and sintered state and having a thermal shock resistance of at least 250 K in the compacted and sintered state, where the thermal shock resistance is R eH is defined as the quotient of / (α·E), and R eH is the yield point at room temperature in MPa, α is the coefficient of thermal expansion in 1 / K, and E is the elastic modulus in MPa.

Description

[Technical Field]

[0001] The present invention , high hot forming tools 1. A high temperature forming tool comprising at least a portion of a molybdenum-based alloy having a molybdenum content of ≥ 90 wt. % in a compacted and sintered state, the molybdenum-based alloy having a thermal shock resistance of at least 250 K in the compacted and sintered state, wherein the thermal shock resistance is defined as the quotient ReH / (α·E), where ReH is the yield point at room temperature in MPa, α is the coefficient of thermal expansion in 1 / K, and E is the modulus of elasticity in MPa. , relates to a method for making high temperature forming tools and their use.

[0002] In the context of this application, a high temperature forming tool refers to a forming tool for shaping high strength materials such as, for example, high alloy heat resistant steels. This forming is generally carried out at temperatures above 1000°C, which is referred to herein as high temperature.

[0003] In particular, high temperature forming tools for this application include: Piercing plugs used to produce seamless pipes, stamping dies used, for example, in flow forming (Fliesspressen), and dies used, for example, in metal extrusion (Strangpressen).

[0004] When a piercing plug is used, typically in a tilt rolling process such as the Mannesmann process, a heated billet is pulled over the piercing plug. In this case, the perforation plug both widens and smooths the inner diameter. The resulting thick-walled hollow semi-finished product is then drawn in a subsequent rolling process to form the finished tube.

[0005] In metal flow forming, the die presses the material of the workpiece, while in reverse flow forming the shell of the die also forms the contour of the workpiece being produced. When producing a profile by extrusion, the material is forced through a shaping die to create a shape.

[0006] The loads on these hot forming tools are similar. The demands on the material of the hot forming tools are, in particular, high heat resistance and resistance to thermal and corrosive attack.

[0007] Depending on the material of the workpiece to be formed, the blank (block stock in the case of pipe production) is then heated to temperatures of 1300°C for forming. Especially when working with high alloy steels, the forming operation (piercing the block stock in the case of pipe production) requires high forces, even with high preheating temperatures.

[0008] Additionally, the rubbing and forming operations place high mechanical, thermal and frictional / corrosive loads on the hot forming tools.

[0009] Even hot forming tools made from heat resistant steels must be re-cooled between forming operations to avoid exceeding the allowable service temperature of the hot forming tool material and to ensure sufficient strength of the hot forming tool during use.

[0010] Therefore, the use of high temperature forming tools made from more heat resistant materials is a general goal in the field. Thus, in addition to high alloy steels, molybdenum-based alloys have also been proposed for making high temperature forming tools.

[0011] Below, material requirements for high temperature forming tools are discussed in more detail using pierce plugs as an example, although the discussion and conclusions are also applicable to other high temperature forming tools.

[0012] Patent Document 1 describes, by way of example, a drilling plug and a drilling rod made of a molybdenum material having a molybdenum content of 75% by weight or more, preferably 80% by weight or more, more preferably 85% by weight or more, and particularly preferably 90% by weight or more.More preferably, the molybdenum material proposed therein has a titanium content of 0.5% by weight or more, a zirconium content of 0.08% by weight or more, and a carbon content of 0.01 to 0.04% by weight. This corresponds to the alloy specification for the molybdenum alloy known as "TZM." Compared to pure molybdenum, TZM is stronger, has a higher recrystallization temperature, and also has higher creep strength. The higher heat resistance of the proposed molybdenum alloy allows for a larger number of drilling operations to be performed without the need to cool the drill plug between operations. As a result, cycle times can be further reduced, i.e., more drilling operations can be performed in a given period of time.

[0013] Experiments by the applicant have shown that further improvement of the heat resistance, as proposed in the patent application WO 2007 / 024990, would allow for higher operating temperatures of the piercing plug, but would be detrimental to product quality. In particular, the inner surface of the tubes produced in this way could be thermally damaged. The same is true for other high-temperature forming tools such as stamping dies. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] German Patent Invention No. 102007037736B4 Summary of the Invention [Problem to be solved by the invention]

[0015] SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved high temperature forming tool, which in particular should be economical to manufacture.

[0016] This problem is solved by a high-temperature forming tool having the features of claim 1.

[0017] Therefore, hot forming tools should have a molybdenum content ≧90% by weight It is proposed that the molybdenum-based alloy be at least partially composed of a molybdenum-based alloy of the present invention, wherein the molybdenum-based alloy is in a pressed and sintered state and has a thermal shock resistance of at least 250 K in the pressed and sintered state.

number

[0018] Yield point R eH is determined by tensile testing according to DIN standard EN ISO 6892-1. The elastic modulus E is determined according to DIN EN ISO 6892-1, Annex G. The thermal expansion coefficient α is determined by dilatometer measurements.

[0019] The molybdenum-based alloy thus characterized forms the substrate for the high temperature forming tool. Advantageously, the high temperature forming tool is made exclusively of this molybdenum-based alloy. Composite materials in which other materials are partially present are also contemplated. Furthermore, it is of course possible for coatings to be formed on the hot forming tool.

[0020] Molybdenum-based alloys are produced by powder metallurgy (abbreviation: "powder metallurgical" molybdenum-based alloys) and therefore have a sintered microstructure. The sintered microstructure is substantially different from the cast microstructure, as readily discernible by those skilled in the art. The sintered microstructure, particularly that of molybdenum-based alloys, is characterized by, among other things, a finer and more uniform grain structure compared to the cast microstructure. Generally, the cast microstructure has fewer porosities than the sintered microstructure. Compared to the porosity in the cast microstructure, the porosity in the sintered microstructure is more uniformly distributed.

[0021] Generally, the chemical homogeneity is also better in powder metallurgical materials than in materials produced by melt metallurgy. In particular in the case of refractory metals, the powder metallurgical process is also more economical. This is because, among other things, sintering is carried out at temperatures well below the melting point. The yield point R eHIf it is not possible to obtain the yield point R, a 0.2% offset is used as a substitute variable. p0.2 The 0.2% offset yield point (i.e. the elongation with a plastic deformation of 0.2%) can be determined by a tensile test according to DIN EN ISO 6892-1. The unit of thermal shock resistance defined in this way is the Kelvin [K], which can be interpreted as the temperature difference that the material can withstand without damage. Damage here is measured as exceeding the yield point. In other words, a temperature difference above this value will result in permanent plastic deformation of the material. It is even more advantageous if the thermal shock resistance is above 260 K or even above 275 K. In this case, the material can withstand even larger temperature gradients.

[0022] High-temperature forming tools having the features according to the invention have very advantageous technical properties. For example, they can be cooled particularly rapidly without being damaged. It has been found that the suitability of high-temperature forming tools for strong cooling is in fact important when users want to achieve short cycle times between forming operations.

[0023] According to the prior art, hot forming tools are manufactured from semi-finished products obtained by rolling or forging, and the microstructure obtained in the prior art is a formed microstructure.

[0024] In contrast, the molybdenum-based alloy according to the present invention is in a compacted and sintered state. A microstructural state characterized as compacted and sintered exists when the material has not undergone substantial, and in particular no, plastic deformation. By "substantially" undeformed, we mean that no deformation involving significant shape and / or cross-sectional changes has been applied. For example, slight surface deformations, such as those caused by skin or sizing passes, smooth rolling, shot blasting, etc., are not considered to be significant deformations that alter the shape and / or change the cross section. The advantage of this is, inter alia, that economical manufacturing is possible, since extensive molding and possibly subsequent machining can be avoided.

[0025] It is preferred that the base material of the hot forming tool, i.e., a molybdenum-based alloy, has a relative density between 90% and 97%, in other words, a porosity between 3% and 10%. Relative density is expressed as the ratio of the actual density of the substance under consideration divided by the nominal density of the corresponding material. For pure molybdenum, the nominal density is 10.22 g / cm. 3 The density of a molybdenum object is 9.2 g / cm 3 If only the porosity is 10%, the relative density is about 90%. Particularly preferably, the relative density is 91% to 96%, and more preferably 94%±1%. The buoyancy method is used to determine the relative density.

[0026] Thus, the substantially undeformed state is characterized by the presence of porosity, as opposed to a state deformed by rolling or forging, etc., where the density is generally about 100%. Particularly advantageous for the intended application is the grain growth inhibition effect of the pores, resulting in no or only minor microstructural coarsening during high temperature use, which can adversely affect important mechanical parameters relevant to the application. While the usual approach in the prior art is to establish the highest possible density, the present invention takes a different approach and allows for porosity.

[0027] With regard to the formation of the microstructure, it can be stated in particular that the compacted and sintered state is free of deformation texture, which refers to the preferential crystallographic orientation of the grains caused by deformation. Deformation structures can be detected on metallographic sections, for example by EBSD measurements (electron backscatter diffraction). Alternatively or additionally, the compacted and sintered microstructural state can be expressed in terms of grain aspect ratio (GAR). The grain aspect ratio can be expressed as a GAR value, which indicates the ratio of grain length to grain width. A grain aspect ratio greater than 1 means that for that grain, the longitudinal elongation is greater than the lateral elongation. In other words, the grain is elongated.

[0028] For hot forming tools, and more precisely for the molybdenum-based alloys forming the hot forming tools, it is particularly preferred to have an average grain aspect ratio with a GAR value of less than 1.5, in particular less than 1.2. A grain aspect ratio with a GAR value of 1 indicates equal grain elongation in the longitudinal and transverse directions. It is particularly preferred that in the hot forming tool there be a grain aspect ratio with a GAR value of 1±10%, more preferably a GAR value of 1±5%. Forming, such as by forging, will typically result in grain aspect ratios with GAR values ​​greater than 1.5. The GAR value is determined by image analysis of metallographic samples, where the average grain length and average grain width are determined, and the GAR value is obtained as the quotient of the average grain length divided by the average grain width. To determine the average grain length and average grain width, respectively, an evaluation of at least 10 grains is preferred. The grain length is considered to be the longitudinal extent of the grain, and the grain width is considered to be the extent of the grain transverse to the longitudinal direction.

[0029] The advantages of the compacted and sintered state are, in particular, isotropic microstructural properties and economical manufacturability. Isotropic structural properties mean that, in contrast to a deformation microstructure, the structure of the hot forming tool according to the invention has substantially the same properties in all spatial directions. This is particularly important for mechanical and thermophysical properties. Furthermore, the manufacture of hot forming tools in a compressed and sintered state has advantages over manufacture by deformation, for example by forging. The basic shape of the hot forming tool can already be achieved in a powder press, which is also particularly easy to process. Since no or little post-processing is required after sintering, the hot-forming tool can be produced in a final or quasi-final shape. The compacted and sintered state refers to the microstructural state that occurs during production, especially by compaction and sintering, but can also occur, for example, during production by hot isostatic pressing (HIP) or hot pressing. In powder metallurgy, compacted and sintered (abbreviated "p / s") is the term used when a part is produced by pressing a powder or powder mixture to obtain a green body, which is then sintered, especially without compaction. The compaction of the powder can be carried out, for example, in a mold or in a rubber hose, for example, cold isostatically. This is the simplest and most advantageous method for establishing the compacted and sintered state of this type of hot-forming tool.

[0030] In contrast to the conventional approach of optimizing hot forming tools for high temperature strength, in other words maximizing tensile strength at high temperatures, the present invention pursues a different path, because even if the hot forming tool can withstand particularly high service temperatures, this method is uneconomical if the workpieces (e.g., tubes or profiles) produced are damaged during production, as has been shown by extensive technical testing by the applicant.

[0031] The present invention is based on the finding that intensive cooling of the hot forming tools is essential for the economical implementation of the method. The Applicant's surprising starting point is that the decisive parameter for the economical exploitation of the advantages of molybdenum-based alloys is their ability to withstand temperature differences without damage, and not, for example, a further increase in their high-temperature strength and / or in their use temperature.

[0032] The thermal shock resistance of the molybdenum-based alloys used above 250 K allows for powerful cooling of the hot forming tools during or between forming operations without damage, and in the case of plug piercing, powerful cooling can be achieved between and / or during the piercing operations. In this way, the fundamentally advantageous property of the molybdenum-based alloys, namely their high high-temperature strength, can also be utilized to technical and economic advantage.

[0033] The hot forming tool preferably consists exclusively of a molybdenum-based alloy with the characteristics defined above. It is also conceivable that only a portion of the hot forming tool, for example in the outer portion, is made of the molybdenum-based alloy defined above, while the inner portion is provided with a conventional molybdenum alloy.

[0034] Thermal shock resistance is given by the above quotient including the yield point. Therefore, the yield point is only one of several parameters. Molybdenum-based alloys have a yield point R of at least 400 MPa at room temperature. eH This development is aimed at achieving a high level of yield point R at room temperature. eH Highlight the advantages of

[0035] Yield point R eH If it is not possible to use a 0.2% offset yield point, a 0.2% offset yield point can be used instead. Furthermore, it has proven highly favorable for the use of hot forming tools if they are constructed from materials that have an elongation at break (generally designated by the symbol "A") of at least 8%, preferably greater than 10%, and more preferably greater than 15% in a tensile test at room temperature. The elongation at break A is determined in a tensile test according to the DIN EN ISO 6892-1 standard. This property means that after a hot forming tool is subjected to plastic strain during use, it still has a margin before it fails by fracture. It is therefore preferred that the molybdenum-based alloy according to the invention in the compacted and sintered state has an elongation at break of at least 8%, preferably greater than 10%, more preferably greater than 15%. This makes the advantages of the compacted and sintered state, such as the isotropic advantage, more apparent and more effective.

[0036] Furthermore, for technical properties, the fracture toughness at room temperature of the base material of the hot forming tool, i.e., the molybdenum-based alloy, K IC is 10 MPa m 1 / 2 It is advantageous if it is more than this. Fracture toughness K IC The fracture toughness K represents the ability of a material to withstand mechanical loads under crack stress, i.e., after it has been previously damaged. IC is determined in accordance with ASTM E399.

[0037] Tests by the Applicant have shown that this mechanical parameter is equally important for the rough service conditions of hot forming tools: in particular for hot forming tools that are often subjected to sudden and / or impact loads and are subject to strong re-cooling, a sufficiently high fracture toughness at room temperature is important.

[0038] Preferably, the molybdenum-based alloy has a ductile-brittle transition temperature of ≦60° C., as determined by bending tests. More preferably, the ductile-brittle transition temperature is ≦50°C, especially ≦40°C. The ductile-to-brittle transition temperature (DBTT) indicates the transition of the fracture mechanism in a material from a fracture behavior at fracture with low energy absorption and / or elongation at break (i.e., brittle behavior of the material) to a fracture event with high energy absorption and / or elongation at break. Therefore, a low ductile-brittle transition temperature means that the material exhibits good ductile behavior even at low temperatures.

[0039] It has been found to be particularly advantageous when the hot forming tool has a ductile-brittle transition temperature of ≦60° C., more preferably ≦50° C., and especially ≦40° C. In that case, the hot forming tool can be used even after uncontrolled and / or prolonged water cooling without a significantly increased risk of fracture compared to the preheated state. This is both technically and economically important, since the cooling does not have to be monitored or adjusted or controlled in a complex manner. The ductile-brittle transition temperature is determined by performing three-point bending tests on test specimens at different temperatures. In this application, the bending of the specimen during fracture at a bending angle of 20° is used as the definition of the ductile-brittle transition temperature. In other words, the base material proposed for this high-temperature forming tool achieves a bending angle of at least 20° at 60°C.

[0040] Research by the applicant has revealed that the desired mechanical and thermophysical characteristics are achieved, for example, by a molybdenum-based alloy having a molybdenum content of ≥ 99.0 wt. %, a boron content "B" of ≥ 3 ppm by weight (ppm "by weight", i.e., ppm on a weight basis), and a carbon content "C" of ≥ 3 ppm by weight.

[0041] Research by the applicant has shown that minor doping with boron and carbon in the amounts mentioned above achieves high thermal shock resistance in the compacted and sintered state according to the invention. Molybdenum-based alloys having a molybdenum content of ≥ 99.0 wt. %, a boron content "B" of ≥ 3 wt. ppm, and a carbon content "C" of ≥ 3 wt. ppm exhibit significantly increased ductility and a high offset yield point Rp compared to conventional powder metallurgical pure molybdenum (Mo). 0.2 It has. This is especially true in comparison with conventional molybdenum that is undeformed and / or in a (fully or partially) recrystallized state.

[0042] More preferably, the molybdenum-based alloy has a molybdenum content of ≥ 99.93% by weight, a boron content "B" of ≥ 3 ppm by weight and a carbon content "C" of ≥ 3 ppm by weight.

[0043] More preferably, the total carbon and boron content "BuC" is in the range of 15 ppm by weight ≦ "BuC" ≦ 50 ppm by weight, particularly 25 ppm by weight ≦ "BuC" ≦ 40 ppm by weight, and the oxygen content "O" is in the range of 3 ppm by weight ≦ "O" ≦ 20 ppm by weight.

[0044] More preferably, the molybdenum-based alloy has a molybdenum content of ≥ 99.93 wt.-%, a boron content "B" of ≥ 3 wt. ppm and a carbon content "C" of ≥ 3 wt. ppm, the combined carbon and boron content (i.e., total) "BuC" being in the range of 15 wt. ppm < "BuC" < 50 wt. ppm, in particular in the range of 25 wt. ppm < "BuC" < 40 wt. ppm, and the oxygen content "O" being in the range of 3 wt. ppm < "O" < 20 wt. ppm. In particular, the maximum content of tungsten (W) is ≦330 ppm by weight. In particular, the maximum content of other impurities is ≦300 ppm by weight. This reflects the fact that more precise control of the chemical composition is advantageous for the development of desirable mechanical and technological properties.

[0045] The grain boundary strength of molybdenum is reduced by the segregation of oxygen and possibly other elements such as nitrogen and phosphorus in the grain boundary regions. Without needing to confirm with metallurgical explanation, it is presumed that the excellent properties of the proposed molybdenum-based alloy, including high ductility and high strength, are established by the boron (B) and carbon (C) contents, and more preferably, by the relatively low oxygen (O) content.

[0046] Furthermore, a combination of a low maximum content of other impurities and a low maximum content of tungsten (W) is also preferred.

[0047] It has been found that low carbon and boron contents alone significantly increase the grain boundary strength and also favorably influence the flow properties of the material (responsible for high ductility) when the oxygen content is low and at the same time the contents of other impurities (and W) are below the above mentioned limits. In particular, this carbon content allows the oxygen content in molybdenum-based alloys to be kept low.

[0048] At the low oxygen, other impurity and W contents proposed herein, the combination of a low boron content and a relatively low carbon content is sufficient to achieve the desired high thermal shock resistance and high ductility and strength values.

[0049] It should be understood that normal impurities may be present in the chemical compositions set forth in this application. When the total does not add up to 100%, the difference is made up of normal impurities.

[0050] The ratios of the various elements are determined by chemical analysis, in particular the ratios of most metal elements (e.g., Al, Hf, Ti, K, Zr, etc.) are determined by ICP-MS (Inductively Coupled Plasma Mass Spectrometry), the ratio of boron is determined by ICP-MS (Inductively Coupled Plasma Mass Spectrometry), the ratio of carbon is determined by combustion analysis, and the ratio of oxygen is determined by high-temperature extraction analysis (carrier gas high-temperature extraction).

[0051] According to an advantageous development, the boron content and carbon content are each ≧5 ppm by weight. Conventional analytical methods are generally able to provide certified content data for boron and carbon above 5 ppm by weight. With regard to low boron and carbon contents, boron and carbon each with a content of less than 5 ppm by weight are also clearly detectable (at least as long as the respective content is ≧2 ppm by weight), and their contents can be determined quantitatively, but it should be noted that contents in this range—depending on the analytical method—may no longer be reported as certified values.

[0052] According to one development, the total carbon and boron content "BuC" is in the range 25 ppm by weight≦"BuC"≦40 ppm by weight. According to one development, the boron content "B" is in the range 5 ppm by weight≦"B"≦45 ppm by weight, more preferably in the range 10 ppm by weight≦"B"≦40 ppm by weight. According to one development, the carbon content "C" is in the range 5 ppm by weight≦"C"≦30 ppm by weight, more preferably in the range 15 ppm by weight≦"C"≦20 ppm by weight.

[0053] In these developments, and especially in the case of the narrower range expressions, the two elements (B, C) are contained in the molybdenum-based alloy in high and sufficient amounts so that their beneficial interaction is clearly detectable, while at the same time the carbon and boron contents do not act against each other. In particular, the effect of carbon is to keep the oxygen content in the molybdenum-based alloy low, and the effect of boron is to allow a sufficiently low carbon content while achieving high ductility and high strength.

[0054] According to one development, the oxygen content "O" is in the range 5 ppm by weight≦"O"≦15 ppm by weight. According to previous knowledge, oxygen collects (segregates) in the grain boundary regions, leading to a decrease in the grain boundary strength. Therefore, an overall low oxygen content is advantageous. Such a low oxygen content is advantageous when the material has a low oxygen content (for example ≦600 ppm by weight, in particular ≦500 ppm by weight). ppm ), by sintering under vacuum, under an inert gas (e.g., argon), or preferably in a reducing atmosphere (e.g., in a hydrogen atmosphere or in an H2 partial pressure atmosphere), and by providing a sufficient carbon content in the starting powder.

[0055] According to one development, the maximum content of impurities from zirconium (Zr), hafnium (Hf), titanium (Ti), vanadium (V), and aluminum (Al) is 50 ppm by weight in total. In this case, the content of each element in this group (Zr, Hf, Ti, V, Al) is preferably ≦15 ppm by weight each. According to one development, the maximum content of impurities from silicon (Si), rhenium (Re), and potassium (K) is ≦20 ppm by weight in total. In this case, the content of each element in this group (Si, Re, K) is preferably ≦10 ppm by weight each, in particular ≦8 ppm by weight each. Potassium has the effect of reducing grain boundary strength, so its content should be as low as possible. Zr, Hf, Ti, Si, and Al are oxide formers and can, in principle, be used to prevent oxygen accumulation in the grain boundary region by bonding with oxygen (oxygen getters), thereby increasing grain boundary strength. However, in part, they are suspected to reduce ductility, especially when they are present in relatively large amounts. Re and V have a ductility-enhancing effect, i.e., they could essentially be used to increase ductility. However, the addition of these additives (elements / compounds) means that they may have a destructive effect under certain conditions of use.

[0056] According to one further development, the molybdenum-based alloy has a total molybdenum and tungsten content of ≥ 99.97 wt. A tungsten content of ≤ 330 wt. ppm is not problematic for the above-mentioned applications and is generally already determined by Mo production and powder manufacturing. The molybdenum content of this molybdenum-based alloy is in particular ≥ 99.97 wt. %, i.e., the molybdenum-based alloy consists mostly of molybdenum.

[0057] According to one development, the carbon and boron are present in dissolved form (i.e. they do not form a separate phase) for at least 70% by weight of the total carbon and boron content. Studies on the proposed molybdenum-based alloys have shown that a small proportion of boron may be present as the Mo2B phase, but this phase is not problematic in small amounts. If carbon and boron are present in solution at least in high contents (e.g., ≥ 70 wt. %, in particular ≥ 90 wt. %), they segregate to the grain boundaries and can achieve the effects described above to a particularly high extent. It is preferred that the above-mentioned limit values ​​are also observed for each of the elements B and C individually.

[0058] The high thermal shock resistance characteristic of molybdenum-based alloys in the compacted and sintered state can be achieved by various combinations of trace doping elements, as described above, with carbon and boron as examples. Along the lines of obtaining high ductility through high grain boundary strength, trace doping elements other than carbon and boron, as well as combinations of these trace doping elements, are also possible. Therefore, the present invention is not necessarily limited to molybdenum-based alloys with the alloying strategies considered based on carbon and boron as trace doping elements. Alternative alloying strategies would be, for example, ductilization with rhenium.

[0059] In selected examples, the molybdenum-based alloys forming the hot forming tools exhibited the following typical material property values ​​at room temperature for the pressed and sintered, i.e., unformed, material: [Table 1] Here, the density of the molybdenum-based alloy forming the hot forming tool is about 9.4 g / cm 3 This gives a density of molybdenum of 10.2 g / cm 3 This corresponds to a relative density of approximately 92%. The carbon and boron contents were each approximately 15 μg / g. The molybdenum content was approximately 99.97% by weight. Typical impurities added together make up the total of 100%. The elastic modulus is proportional to the relative density and was determined to be approximately 305,000 MPa. The thermal expansion coefficient α of this molybdenum-based alloy is 5.2 × 10 -6 [K-1 ] was. The thermal shock resistance of the selected example was determined to be about 252K, defined as the quotient of the following formula:

number

[0060] This hot forming tool is in particular configured as a drill plug. Tests by the Applicant have shown that the properties of the molybdenum-based alloy defined above are particularly advantageous when applied to drill plugs.

[0061] Furthermore, protection is claimed for the use of a hot forming tool as claimed in any of the preceding claims, in particular for producing tubes or profiles made of high strength metal, in particular high alloy steel. The use of forming tools having the above characteristics has proven particularly useful in industrial pipe manufacturing. The property profile according to the invention is advantageous, in particular in the case of piercing high-alloy steels in tilt rolling processes. The use of a die according to any one of claims 1 to 3 is particularly advantageous when producing profiles by extrusion. The use of a die according to any one of the preceding claims is particularly advantageous when producing profiles by extrusion, since the property profile remains valid even in these hot forming operations. When the hot forming tool according to the invention is used, the user can particularly benefit from the robustness and economy of this hot forming tool.

[0062] Additionally, protection is claimed for a method of manufacturing high temperature forming tools. Industrial-scale molybdenum-based alloys are typically processed into components by powder metallurgical processes, as melt metallurgy is generally impractical and / or uneconomical for refractory metals. Typically, a powder or powder mixture is compressed into a green body, then sintered, and then formed into a semi-finished product by rolling, forging, etc. Unlike this conventional manufacturing process, the manufacture of the hot forming tool according to the present invention is carried out without or almost without plastic deformation.

[0063] The method for manufacturing a high temperature forming tool is characterized by the following steps: a) Compressing a powder mixture of molybdenum powder and powder containing boron and carbon into a green body (G) Step to obtain; b) optionally processing the green body to high temperature forming tools Steps to get closer to the final shape; c) sintering the green body in an oxidizing protective atmosphere at a temperature ranging from 1600°C to 2200°C with a residence time of at least 45 minutes to obtain a sintered blank of a high-temperature forming tool; d) optionally, the sintered blank (R) to produce a finished hot forming tool, here a perforated plug. The advantages explained above in connection with the molybdenum-based alloy are likewise obtained by the above method for producing high-temperature forming tools, and furthermore the developments explained above are also applicable in this method.

[0064] For the production of the molybdenum-based alloys lightly doped with boron and / or carbon, the powder containing boron and carbon can be a molybdenum powder having a corresponding boron and / or carbon content. What is important here is that the starting powder used for compacting the green bodies contains sufficient amounts of boron and carbon, and that these additives are distributed as uniformly and finely as possible in the starting powder.

[0065] In particular, the sintering step involves a heat treatment in the temperature range of 1800° C. to 2100° C. for a residence time of 45 minutes to 12 hours (h), preferably 1 to 5 hours. In particular, the sintering step is carried out under reduced pressure, in an inert gas (e.g., argon), or preferably in a reducing atmosphere (in particular in a hydrogen atmosphere or in an H2 partial pressure atmosphere).

[0066] As already explained, the production of high-temperature forming tools having the properties according to the invention, such as thermal shock resistance in the compacted and sintered state, is not necessarily limited to molybdenum-based alloys with the above-mentioned alloying strategy based on the trace doping elements carbon and boron. Rather, the method claims show a particularly advantageous and economical route. Trace doping elements other than carbon and boron and combinations of these trace doping elements or different alloying strategies are also possible along the lines of obtaining high ductility through high grain boundary strength.

[0067] Further advantages and suitability of the invention will become apparent from the following description of an embodiment with reference to the accompanying drawings. [Brief explanation of the drawings]

[0068] [Figure 1] 1 is a perspective view of an embodiment of a hot forming tool (e.g., a piercing plug); [Figure 2] Side view of the perforation plug [Figure 3] Cross section of a perforation plug [Figure 4a] Another embodiment of a hot forming tool (e.g., die) [Figure 4b] Another embodiment of a hot forming tool (e.g., die) [Figure 5a] Other examples of hot forming tools (e.g., stamping dies) [Figure 5b] Other examples of hot forming tools (e.g., stamping dies) [Figure 6] Schematic diagram of the manufacturing process for hot forming tools using a perforated plug as an example [Figure 7] Diagram of ductile-brittle transition temperature (horizontal axis: temperature [℃], vertical axis: bending angle [°]) [Figure 8]Scanning electron microscope image of Mo material using conventional technology [Figure 9] Scanning electron micrograph of a molybdenum-based alloy for hot forming tools according to the present invention DETAILED DESCRIPTION OF THE INVENTION

[0069] 1 shows diagrammatically a hot forming tool according to the invention, in this example formed as a drilling plug 1. This drilling plug 1 has a leading portion 2 and a trailing portion 3. In the trailing portion 3, the drilling plug 1 is typically held by a plug rod (not shown), for which a receptacle is formed.

[0070] The same is evident from Fig. 2, which shows a side view of the drill plug 1. In this embodiment, the drill plug 1 is made rotationally symmetrical about an axis of symmetry L.

[0071] 3 shows a cross section of the drilled plug 1. Here, an optional structure 4 for cooling and / or instrumentation of the drilled plug 1 is shown. In this example, the structure 4 is made as a drilled hole.

[0072] Figures 4a and 4b show views of another embodiment of a hot forming tool of the invention, here by way of example a die 1 for forming metals, Figure 4a showing a perspective view and Figure 4b showing a cross-sectional view. Dies of the kind shown here are used, for example, in the extrusion of high alloy steels. Naturally, the die 1 can have different shapes, in particular different cross-sectional shapes.

[0073] Figures 5a and 5b show views of another embodiment of the hot forming tool of the invention, here by way of example showing a die 1 for forming metal. Figure 5a shows a perspective view and Figure 5b shows a cross-sectional view. A structure 4 for introducing a cooling medium can be provided. In this example, the structure 4 is also configured as a receptacle. A die of the type shown here is used, for example, in the reverse extrusion of high alloy steels. Naturally, the die can have a shape different from that shown here.

[0074] 6 shows a schematic representation of the manufacturing process of a hot forming tool according to the invention for the example of a drilled plug 1. In step a), a powder mixture of molybdenum powder and powders containing boron and carbon is compressed to obtain a green body G. Optional step b) represents the processing of the green body G to approximate the final shape of the drilled plug 1 . In step c), the green body G is sintered to obtain a sintered blank R of the drill plug 1 . After sintering, in step d), drill plugs 1 are obtained from the sintered blank R. If desired, machining of the sintered blank R may be carried out.

[0075] FIG. 7 is a diagram of the ductile-brittle transition temperatures of various materials that are essentially candidates for hot forming tooling. The plotted variables are temperature [°C] on the horizontal axis and bending angle [°] of the three-point bending sample on the vertical axis. The plotted variables are the temperature (°C) on the horizontal axis versus the bending angle (°) of the three-point bend specimen on the vertical axis. The bending angle indicates how much plastic bending the specimen underwent before fracture initiation. In this figure, the curve on the right (dotted line, symbol "Mo") shows the typical course of fracture properties of pure molybdenum in the compacted and sintered state, and it can be seen that this material only exhibits significant ductility above approximately ≥ 140°C. Slightly more advantageous is the course of the middle curve (dashed line, symbol "TZM"), which shows the course of the ductile-brittle transition of TZM in the pressed and sintered state. This course is slightly shifted towards lower temperatures, which characterizes the somewhat better properties. The two courses on the right ("Mo" and "TZM") correspond to the prior art. The curve on the left (solid line, symbol "MoB15") shows a typical course of the ductile-brittle transition for a molybdenum-based alloy having a molybdenum content of ≥ 99.0 wt. %, a boron content "B" of ≥ 3 wt. ppm, and a carbon content "C" of ≥ 3 wt. ppm, which has been proposed as being particularly preferred for high-temperature forming tools. These advantages are realized in one development when the base material of the hot forming tool has a ductile-brittle transition temperature of ≦60° C. In the example shown here, the ductile-brittle transition temperature, defined by plastic bending with a bending angle of 20°, is significantly lower than 60° C., at about 30° C.

[0076] Furthermore, an auxiliary line is drawn at a bending angle of 20°. The bending of the specimen at fracture at a bending angle of 20° is used in the context of this application as a criterion for the ductile-brittle transition temperature. For technical purposes, a plastic bending of ≥ 20° can be assumed to be a ductile material behavior. The test parameters adopted for the three-point bending test were: test load 20 N [Newton], test speed 10 mm / min, and support span 20 mm. The radius of the support roller was 1.5 mm, the same as the radius of the die. The dimensions of the specimen were 6 x 6 x 35 mm.

[0077] Figure 8 shows a scanning electron microscope photograph of a molybdenum material according to the prior art. The molybdenum material is in a recrystallized state. The photograph shows the fracture surface of a tensile specimen tested at room temperature. A notable feature is the presence of so-called intercrystalline fractures, i.e., fractures due to material separation mainly along grain boundaries. One of the grain boundary separations is indicated by the inset arrow. In the presence of such intergranular fractures, the ductility is determined by the grain boundary strength.

[0078] 9 shows a fracture surface of a molybdenum-based alloy that is proposed to be suitable and preferred for manufacturing the drilled plug of the present invention. This alloying strategy is based on improving grain boundary strength, which is achieved in particular when the molybdenum-based alloy has a molybdenum content of ≥ 99.0 wt. %, a boron content "B" of ≥ 3 wt. ppm, and a carbon content "C" of ≥ 3 wt. ppm. The fracture phenomenon here is transcrystalline, meaning that the fracture runs across the grains. This fracture phenomenon is essentially due to increased grain boundary strength, which, macroscopically, is associated with essentially higher ductility.

Claims

1. 1. A high temperature forming tool (1), at least a portion of which consists of a molybdenum-based alloy having a molybdenum content of ≥ 90 wt. % and which is in a compacted and sintered state and which has a thermal shock resistance of at least 250 K in the compacted and sintered state, wherein the thermal shock resistance is defined as the quotient ReH / (α·E), where ReH is the yield point at room temperature in MPa, α is the coefficient of thermal expansion in 1 / K, and E is the modulus of elasticity in MPa.

2. 2. The high temperature forming tool (1) according to claim 1, characterized in that the molybdenum-based alloy has a yield point ReH at room temperature of at least 400 MPa.

3. A hot forming tool (1) according to claim 1 or 2, characterized in that the molybdenum-based alloy has a relative density of 90% to 97%.

4. A high temperature forming tool (1) according to any one of claims 1 to 3, characterized in that the molybdenum based alloy has an elongation at break at room temperature of at least 8%.

5. A high-temperature forming tool (1) according to any one of claims 1 to 4, characterized in that the molybdenum-based alloy has a fracture toughness KIC at room temperature of 10 MPa·m½ or more.

6. A high temperature forming tool (1) according to any one of claims 1 to 5, characterized in that the molybdenum based alloy has a ductile-brittle transition temperature of ≦60°C, determined in a bending test.

7. 7. The high-temperature forming tool (1) according to any one of claims 1 to 6, characterized in that the molybdenum-based alloy has a molybdenum content of ≥ 99.0% by weight, a boron content "B" of ≥ 3 ppm by weight and a carbon content "C" of ≥ 3 ppm by weight.

8. 8. The high-temperature forming tool (1) according to claim 7, characterized in that the molybdenum-based alloy has a molybdenum content of ≥ 99.93 wt.-%, a boron content "B" of ≥ 3 ppm by weight and a carbon content "C" of ≥ 3 ppm by weight, the total carbon and boron content "BuC" being in the range 15 ppm by weight ≤ "BuC" ≤ 50 ppm by weight.

9. 9. The hot forming tool (1) according to claim 7 or 8, characterized in that the oxygen content "O" is in the range 3 ppm by weight≦"O"≦20 ppm by weight.

10. 10. The high-temperature forming tool (1) according to any one of claims 7 to 9, characterized in that the molybdenum-based alloy has a molybdenum content of ≥ 99.93 wt.-%, a boron content "B" of ≥ 3 ppm by weight and a carbon content "C" of ≥ 3 ppm by weight, the total carbon and boron content "BuC" being in the range 15 ppm by weight ≤ "BuC" ≤ 50 ppm by weight and the oxygen content "O" being in the range 3 ppm by weight ≤ "O" ≤ 20 ppm by weight.

11. 11. The hot forming tool (1) according to any one of claims 1 to 10, characterized in that the molybdenum based alloy has an average grain aspect ratio, taken as the grain length / grain width quotient and expressed as a GAR value, of less than 1.

5.

12. A high temperature forming tool (1) according to any one of claims 1 to 11, characterized in that the high temperature forming tool (1) is made entirely from the molybdenum-based alloy.

13. A hot forming tool (1) according to any one of claims 1 to 12, characterized in that at least one structure (4) for introducing a cooling medium is formed in the hot forming tool (1).

14. Use of a hot forming tool (1) according to any one of claims 1 to 13 for producing tubes or profiles.

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