RHENIUM-TUNGSTEN WIRE ROD AND THERMOCOUPLE USING THE SAME

Abstract

The rhenium tungsten wire rod according to an embodiment is a wire rod made of a tungsten alloy containing rhenium, wherein a rhenium content is less than 30 wt % in any measurement area of a wire rod body where a unit area is 1 m in diameter.

Claims

1. A rhenium-tungsten wire rod which is a wire rod made of a tungsten alloy containing rhenium, the rhenium-tungsten wire rod having a rhenium content of less than 30 wt % in any measurement area of a wire rod body where a unit area is 1 m in diameter.

2. The rhenium-tungsten wire rod according to claim 1, wherein the rhenium content has a coefficient of variation of 0.10 or less in semi-quantitative analysis using EPMA.

3. The rhenium-tungsten wire rod according to claim 1, wherein the rhenium content is 1 wt % or more and less than 30 wt %.

4. The rhenium-tungsten wire rod according to claim 2, wherein the rhenium content is 1 wt % or more and less than 30 wt %.

5. The rhenium-tungsten wire rod according to claim 1, wherein the rhenium content is 2 wt % to 28 wt %.

6. The rhenium-tungsten wire rod according to claim 2, wherein the rhenium content is 2 wt % to 28 wt %.

7. The rhenium-tungsten wire rod according to claim 1, wherein the tungsten alloy contains 30 wtppm to 90 wtppm of potassium (K).

8. The rhenium-tungsten wire rod according to claim 1, wherein the wire rod has a diameter of 0.1 mm to 5.0 mm.

9. The rhenium-tungsten wire rod according to claim 8, wherein the wire rod has a standard deviation of tensile strength of 35 N/mm.sup.2 or less.

10. A thermocouple which comprises the rhenium-tungsten wire rod according to any one of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a schematic diagram of a temperature measuring system using a thermocouple.

[0012] FIG. 2 is a table showing the types of thermocouples.

[0013] FIG. 3 is a diagram showing inclusions (a phases) present in a ReW matrix and results of semi-quantitative analyses (an example of a conventional material).

[0014] FIG. 4 is a schematic diagram of a radial cross-section and a surface layer portion of a sample.

[0015] FIG. 5 is a schematic diagram of Re-content measurement points.

[0016] FIG. 6 is an explanatory diagram of a particle size distribution.

DETAILED DESCRIPTION

[0017] The method described in Patent Literature 1 (Japanese Patent No. 4256126) aims to suppress the presence of a segregated phase of a a phase to a level where no breaking of a wire occurs during drawing by refining the region where the segregated phase of the a phase is present to a predetermined size or smaller and dispersing the segregated phase of the a phase in a wide range instead of the state in which the segregated phase of the a phase is unevenly distributed in a specific region, and allows the presence of a fine segregated phase of a a phase. However, even if the segregated phase of the a phase is homogeneously present, a change in the abundance ratio in a certain volume will likely cause variation in the amount of Re (a change in the material quality) in an axial cross-section or a radial cross-section and generate an inhomogeneous portion.

[0018] FIG. 3 shows, as an example, the amount of Re in a W matrix in the case where a segregated phase of a a phase is present in a 26% Re-W wire. The results of semi-quantitative analyses according to EPMA (acceleration voltage: 15.0 kV, irradiation current: 5.0E.sup.8A, beam diameter: 1 m or less) also demonstrate that the a phase is an inclusion. Since the a phase is harder than the matrix, it is present in the form of an inclusion in this manner. The amount of Re in the matrix varies in the vicinity of the a phase. As described herein, the presence of the a phase causes variation (inhomogeneity) in the amount of Re in the surrounding portion of the a phase. [0019] To solve the above problem, a rhenium-tungsten wire rod according to an embodiment is a wire rod made of a tungsten alloy containing rhenium, wherein the rhenium content is less than 30 wt % in any measurement area of the wire rod body where the unit area is 1 m in diameter. [0020] A rhenium-tungsten wire rod of an embodiment will be described below with reference to the drawings. Hereinafter, the rhenium-tungsten wire rod may be referred to as a ReW wire rod. The drawings are schematic, and a ratio of dimensions of each part and the like are not limited to those shown in the drawings.

[0021] FIG. 4 shows an example of a radial cross-section of a sample taken from a ReW wire rod. In the radial cross-section, the body of the rhenium-tungsten wire rod is indicated by C. FIG. 4 also shows an enlarged view of a portion, indicated by A, of the outer periphery of the rhenium-tungsten wire rod in the radial cross-section. As shown in the enlarged view of FIG. 4, a surface mixture layer B is formed on the outer periphery of the rhenium-tungsten wire rod body C. The surface mixture layer B contains W, O, and C as constituent elements. The diameter of the wire rod is preferably 0.1 mm to 5.0 mm so that the wire rod can be used for various thermocouples or used as a wire for drawing processing. In the case of using the wire rod for thermocouples, for example, the wire diameter being less than 0.1 mm is likely to cause wire breakage due to evaporation consumption during use at a high temperature, resulting in short life. If the wire diameter is more than 5.0 mm, the temperature of an object cannot be accurately measured due to the heat capacity of the thermocouples themselves. A more preferred range of the wire diameter is from 0.2 mm to 3.5 mm. The ReW wire rod is processed into a diameter of 0.1 mm to 5.0 mm through a swaging (SW) step and a subsequent drawing (DW) step. The wire rod after SW or DW has a mixture layer on its surface. The mixture contains W, O, and C as constituent elements, and is removed through, for example, an electrolytic process when the wire rod is commercialized. The body part excluding this mixture is used as a sample. Although the sampling position is discretionary, it is desirable to sample two or more positions apart from each other within a single wire rod in consideration of the yield of the product and in order to evaluate variation. Within a single ReW wire rod, the front and rear ends have a portion where the conditions become unstable, for example, at the time when a DW apparatus is started and stopped. This portion is not included in the sampling. The length of the unstable portion varies depending on the layout and size of the apparatus.

[0022] The measurement may be performed on any cross-section of a sample, but preferably is performed on a radial cross-section of a sample, as shown in FIG. 4, from the viewpoint of the ease of processing the sample. Embedding a sample in a resin, and polishing and etching it as necessary makes it easy to observe the sample. For the obtained measurement surface, the amount of Re in a region having a diameter of 1 m is quantified using an electron probe microanalyzer (EPMA) at a total of 17 points, which are intersections (16 points) of four equally spaced concentric circles 2 to 5 with the X-axis and the Y-axis and a center 1, in the radial cross-section of the rhenium-tungsten wire rod body C, as shown in FIG. 5, for example. The measurement position described is an example, and the measurement may be performed at any position; However, the above-described position is preferred in order to measure the entire cross-section evenly. In addition, the radial cross-section to be measured is not a single section of the wire rod discretionarily selected but two or more discretionarily selected sections of the wire rod apart from each other.

[0023] In any measurement area of the ReW wire rod body of the embodiment, where the unit area is 1 m in diameter, the content of rhenium is less than 30 wt %. A Re content of 30 wt % or more exceeds an average adding amount. This indicates that Re or W is not sufficiently diffused in the sintering step and that the Re content varies in the axial cross-sectional direction and the radial cross-sectional direction. The variation in the Re content is a cause of inhomogeneity, and may cause the thermoelectromotive force to vary between positions in the ReW wire rod.

[0024] Next, an average value (Ave), a standard deviation (Sd), and a coefficient of variation (CV) calculated according to Sd/Ave are obtained for the data of the amount of Re obtained. The CV indicates a ratio of the magnitude of variation in data to the average, allowing comparison of the variation regardless of whether the Re ratio of the ReW wire rod is high or low.

[0025] The CV of the rhenium content in the ReW wire rod of the embodiment is preferably 0.10 or less. It is more preferably 0.05 or less. The CV being larger than 0.10 indicates, for example, that there is variation in the Re content in the axial cross-sectional direction or the radial cross-sectional direction even if there is no segregated phase of the a phase. The variation in the Re content is a cause of inhomogeneity, and may cause the thermoelectromotive force to vary between positions in the ReW wire rod.

[0026] The amount of Re contained in the ReW wire rod of the embodiment is preferably 1 wt % or more and less than 30 wt %, and more preferably 2 wt % to 28 wt %. The amount of Re is a value obtained by performing analysis according to inductively coupled plasma optical emission spectrometry (ICP-OES) suitable for evaluation of constituent elements, not according to inductively coupled plasma mass spectrometry (ICP-MS) suitable for evaluation of trace impurities. Re improves the elongation of W at high temperatures and enhances the workability. The strength is also increased by solid solution strengthening. However, if the content is less than 1 wt %, the effects are insufficient. For example, if the ReW wire rod is used as a material for a probe pin, the amount of deformation of the completed probe pin increases with the frequency of use and failure in contact occurs, degrading the inspection accuracy of a semiconductor. If the Re content is more than about 28 wt %, it exceeds the limit of solid solubility with W, thus generating a segregated phase of a a phase and likely generating an inhomogeneous portion in the wire rod. The generation of an inhomogeneous portion causes variation in thermoelectromotive force and strength. By setting the amount of Re to 1 wt % or more and less than 30 wt % or setting the amount of Re to 2 wt % to 28 wt %, it is possible to manufacture, with high yield, thermocouples including a plus-side conductor and a minus-side conductor (the plus-side conductor referring to a positive side conductor and the minus-side conductor referring to a negative side conductor) that include the embodiment for the constituting material and ReW wires for probe pins including the embodiment for the constituting material while securing thermoelectromotive force properties (stability) and mechanical properties (strength and wear resistance).

[0027] The ReW wire rod of the embodiment may contain 30 wtppm to 90 wtppm of K as a dopant. Containing K improves tensile strength and creep strength at high temperatures due to the doping effects. If the K content is smaller than 30 wtppm, the doping effects become insufficient. If the K content exceeds 90 wtppm, the workability is decreased, likely causing a large decrease in the yield. By containing K as a dopant in a range of 30 wtppm to 90 wtppm, it is possible to manufacture, with high yield, thermocouples including a plus-side conductor and a minus-side conductor each including the embodiment for the constituting material and ReW wires for electronic tube heaters including the embodiment for the constituting material while securing high-temperature properties (prevention of wire breakage and deformation during high-temperature use).

[0028] The ReW wire rod of the embodiment can have a tensile strength standard deviation of 35 N/mm.sup.2 or less. Since the processing stability of the ReW wire rod can be improved by suppressing variation in tensile strength, an improved yield of products (such as thermocouples, probe pins, and medical needles) which include the ReW wire rod can be expected. Also, if the ReW wire rod is used as a material of medical needles due to its stable tensile strength, the quality of the medical needles is improved. If the ReW wire rod of the embodiment has a tensile strength standard deviation of 35 N/mm.sup.2 or less and a wire rod diameter of 0.1 mm to 5.0 mm, it can achieve more excellent processing stability.

[0029] Tensile strength is measured using a universal tension and compression tester. The load varies depending on the wire diameter; thus, for the universal tension and compression tester, the load cell may be replaced or different devices may be used according to the wire diameter. For example, AG-I 5 kN manufactured by SHIMADZU or LTS 500N manufactured by Minebea Inc. may be used. A test piece is chucked by a flat plate via non-slip sandpaper, and both ends thereof are fixed to the device. Setting the gage length to 50 mm, the tensile test is conducted at a rate of 10 mm/min. If there is no broken portion between the gage marks, measurement is performed again.

[0030] According to the embodiment described above, it is possible to realize a ReW wire rod which has no variation in the material quality (i.e., no inhomogeneous portion) and greatly contributes to improvement of the stability of the thermoelectromotive force, and the ReW wire rod can be applied to thermocouples for high-temperature use. The ReW wire rod can also be applied to probe pins. The ReW wire rod is not limited to having a circular cross-section, and may have a cross-section having a shape other than a circular shape, such as an elliptical shape or a polygonal shape.

[0031] Next, a method of manufacturing the ReW wire rod according to the embodiment will be described. The manufacturing method is not particularly limited, and examples thereof include the following.

[0032] A W powder and a Re powder are mixed so that the Re content will be 1 wt % or more and less than 30 wt %. The mixing method is not particularly limited; however, forming powders into slurry using water or an alcohol solution and mixing them is particularly preferred since powders with good dispersibility can be obtained. Also, in order to ensure the homogeneity of the powder lots, it is more preferable to dry the slurry and then collectively stir the same powder lots in a dry state.

[0033] The Re powder to be mixed preferably has an average particle size of less than 8 m. The particle size distribution preferably has an SD value of less than 11 pam. FIG. 6 is an explanatory diagram of the particle size distribution. The horizontal axis represents particle size (m), the left vertical axis represents frequency (%), and the right vertical axis represents accumulation (%). The SD value is a value determined by

[0034] SD=(d (84%)d (16%))/2, where d (84%) is a particle size at 84% accumulation and d (16%) is a particle size at 16% accumulation, and serves as a measure of the distribution width of the measured particle sizes. The particle size distribution is measured by a laser diffraction method. The amount of powder used for single measurement is the amount recommended for the measuring device. In general, 0.02 g is recommended. The measurement sample is sufficiently stirred before measurement and then weighed.

[0035] The W powder is a pure W powder excluding inevitable impurities or a doped W powder containing K in an amount that takes into consideration the yield up to the wire material. The W powder preferably has an average particle size of less than 16 m. The particle size distribution preferably has an SD value of less than 13 pam. If the average particle size and the particle size distribution of each of the Re powder and the W powder are equal to or more than the above numerical values, the diffusion distance of the Re atoms or W atoms for obtaining homogeneity increases, facilitating generation of a a phase.

[0036] A ratio of the Re average particle size to the W average particle size is preferably 0.4 to 2.0. If the ratio of the Re average particle size to the W average particle size is smaller than 0.4 or larger than 2.0, the diffusion distance of the Re atoms to the center of the W particles or the diffusion distance of the W atoms to the center of the Re particles becomes large, likely facilitating generation of a a phase.

[0037] Next, the mixed powder is put into a predetermined mold and press-molded. The pressing force at this time is preferably 150 MPa or more. The molded body may be subjected to presintering at 1200 to 1400 C. in a hydrogen furnace for the sake of easy handling. The molded body obtained is sintered in a hydrogen atmosphere, an inert gas atmosphere such as argon, or a vacuum. The sintering temperature is preferably 2500 C. or higher. If the temperature is lower than 2500 C., diffusion of the Re atoms and the W atoms does not proceed sufficiently during sintering. The upper limit of the sintering temperature is 3400 C. (the melting point of W is 3422 C. or less).

[0038] The relative density of the sintered body is preferably 90% or more. The relative density after sintering is a relative density (%) with respect to the true density, and the relative density (%) with respect to the true density is represented by [sintered body density/true density]100%. For example, in a single sintered body, a ratio of the density of the lowest portion such as the lower end during electric sintering to the average density of the entire sintered body is preferably 0.98 or more. The variation in the Re content can be suppressed by setting the relative density of the sintered body to 90% or more and the ratio of the density of the lowest portion to the average density of the entire sintered body to 0.98 or more.

[0039] First SW processing is performed on the sintered body obtained in the main sintering step. The first SW processing is preferably performed at a heating temperature of 1300 to 1600 C. The reduction rate of the cross-sectional area (area reduction rate) at which processing is performed by single heat treatment (single heating) is preferably 5 to 15%.

[0040] Instead of the first SW processing, rolling processing may be performed. The rolling processing is preferably performed at a heating temperature of 1200 to 1600 C. The area reduction rate in single heating is preferably 40 to 75%. A two-way roller rolling mill, a four-way roller rolling mill, a die roll rolling mill, or the like can be used as a rolling mill. The rolling processing can greatly enhance the production efficiency. The first SW processing and the rolling processing may be combined.

[0041] Second SW processing is performed on the sintered body (ReW rod) which has been subjected to the first SW processing, the rolling processing, or the combined processing. The second SW processing is preferably performed at a heating temperature of 1200 to 1500 C. The area reduction rate in single heating is preferably about 5 to 20%.

[0042] The ReW rod which has been subjected to the second SW step is then subjected to recrystallization treatment. The recrystallization treatment can be performed at a treatment temperature of 1800 to 2600 C. and using, for example, a high-frequency heating apparatus in a hydrogen atmosphere, an inert gas atmosphere such as argon, or a vacuum.

[0043] The ReW rod which has been subjected to the recrystallization treatment is then subjected to third SW processing. The third SW processing is preferably performed at a heating temperature of 1200 to 1500 C. The area reduction rate in single heating is preferably about 10 to 30%. The third SW processing is performed until the ReW rod has a diameter at which drawing processing can be performed (preferably a diameter of 2 to 4 mm).

[0044] In order to enable drawing (DW) processing to be performed smoothly, the ReW rod which has been subjected to the third SW processing is then subjected to DW processing in which a process of applying a lubricant to the surface, a process of drying the lubricant and performing heating at a temperature that allows the processing to be performed, and a process of drawing using a drawing die are repeated. It is desirable to use a C-based lubricant excellent in heat resistance as the lubricant. The processing temperature is preferably 1100 C. or less. The processing temperature is set accordance to the wire diameter to be obtained by DW. The area reduction rate per die is preferably 10 to 35%. If necessary, an annealing step or a surface polishing step (e.g., an electrolytic step) may be added during the DW step.

[0045] Necessary steps such as heat treatment and surface polishing are additionally performed on an appropriate amount of ReW wire rod after SW or DW, so that the resultant can be used as a material for thermocouples. Thereafter, thermocouples are manufactured with predetermined combinations of materials.

EXAMPLES

[0046] In Examples 1 to 4, sintered bodies were produced under the processing conditions described above. In Example 5 and Comparative Example 1, sintered bodies were produced under the conventional sizes of a Re powder and a W powder. In Example 6 and Comparative Example 2, sintered bodies were produced under the conventional condition of the size of a W powder. Table 1 shows the analytical results of each example. The analysis of Re and K was performed not by inductively coupled plasma mass spectrometry (ICP-MS) but by inductively coupled plasma optical emission spectrometry (ICP-OES). The lower detection limit of K is 5 wtppm, and the case where the analytical value was lower than 5 wtppm because K was not added is indicated by .

TABLE-US-00001 TABLE 1 Re Powder W Powder Re Average Average Average Diameter/ Relative Re K Diameter SD Value Diameter SD Value W Average Density Density (wt %) (wtppm) (m) (m) (m) (m) Diameter (%) Ratio Example 1 3 60 5.7 7.4 14.2 9.0 0.40 92 0.98 Example 2 5 60 5.7 7.4 14.2 9.0 0.40 92 0.98 Example 3 5 5.7 7.4 3.2 6.3 1.78 92 0.98 Example 4 26 5.7 7.4 3.2 6.3 1.78 97 0.99 Example 5 3 60 11.0 13.0 35.0 13.2 0.31 90 0.95 Example 6 5 60 5.7 7.4 35.0 13.2 0.16 91 0.95 Comparative 5 60 11.0 13.0 35.0 13.2 0.31 88 0.94 Example 1 Comparative 26 5.7 7.4 17.0 11.0 0.34 89 0.96 Example 2

[0047] Each of the sintered bodies was processed to have a diameter of 0.5 mm in the above-described processing steps. The sintered bodies of Examples 2 and 4 and Comparative Example 2 were separately processed to have a diameter of 5.0 mm and to a diameter of 0.1 mm. After completion of processing, samples were taken from both ends of each wire rod by the above-described method. At 34 points (17 points2 samples) in total per size, the Re content in a 1 m-diameter region was analyzed using EPMA (JXA-8100 manufactured by JEOL Ltd., magnification: 1000 times, acceleration voltage: 15.0 kV, irradiation current: 5.0E.sup.8A). The CV was then calculated from the analytical values. Table 2 shows the results of evaluation. The Re content is indicated by when the content is less than 30 wt % at all measurement points, and indicated by when the content is 30 wt % or more at one or more points. The diagonal lines in the table indicate the sizes for which no samples were made. Also, the case where the analytical value was lower than 5 wtppm because K was not added is indicated by .

TABLE-US-00002 TABLE 2 Re K Diameter 0.1 mm Diameter 0.5 mm Diameter 5 mm (wt %) (wtppm) Re Content CV Re Content CV Re Content CV Example 1 3 60 0.05 Example 2 5 60 0.05 0.05 0.05 Example 3 5 0.02 Example 4 26 0.02 0.02 0.02 Example 5 3 60 0.15 Example 6 5 60 0.13 Comparative 5 60 x 0.15 x 0.15 x 0.15 Example 1 Comparative 26 x 0.13 x 0.13 x 0.13 Example 2

[0048] Next, using the wire rods that have been processed to have a diameter of 0.5 mm, prototypes of thermocouples were made through predetermined processes, with the combinations of the wire rods of prototypes 1 to 8 shown in Table 3. Relating to prototype 2, a prototype having a diameter of 0.1 mm (prototype 2-2) and a prototype having a diameter of 5 mm (prototype 2-3) were also made. As the materials of each prototype, two wire rods were sampled from both ends of the wire rods. Four thermocouples made of respective combinations were produced so that the combinations of the positions did not overlap each other (i.e., so that the combinations were front-front, front-rear, rear-front, rear-rear). Herein, one end of a wire rod is the front and the other end of the same wire rod is the rear. For example, front-front indicates a combination of the front of one of the wire rods and the front of the other of the wire rods. For each prototype, the thermocouple was put into an electric furnace together with a calibrated platinum-rhodium thermocouple, and thermoelectromotive force was measured by the system shown in FIG. 1 at the temperature of 1600 C. by the platinum-rhodium thermocouple to calculate a temperature (JISC1602). The temperature measurement system shown in FIG. 1 includes a positive + side conductor and a negative side conductor of a thermocouple, a temperature measuring junction, a reference junction, a measuring device, and a compensation conductor. The temperature measuring junction is formed by welding a distal end of the positive side conductor of the thermocouple and a distal end of the negative side conductor of the thermocouple. The positive side conductor of the thermocouple and the negative side conductor of the thermocouple are each connected to the reference junction. The reference junction and the measuring device are connected by the compensation conductor. Table 3 shows the difference between the maximum temperature and the minimum temperature (Max-Min) obtained by using each prototype. As is apparent from the table, in the ReW wire rod according to the embodiment, the variation in Re of the ReW wire rod body was suppressed, and the variation in temperature of the thermocouple including the same wire rod was suppressed. In contrast, in the comparative examples, the variation in Re was not suppressed, and the variation in temperature of the thermocouples using the same wire rods was large. Thus, the yield of thermocouples is greatly improved if the embodiment is employed.

TABLE-US-00003 TABLE 3 Re K Re Combination in Thermocouple (wt %) (wtppm) Content CV Prototype 1 Prototype 2 Prototype 3 Prototype 4 Prototype 5 Prototype 6 Example 1 3 60 0.05 .circle-solid. Example 2 5 60 0.05 .circle-solid. Example 3 5 0.02 .circle-solid. Example 4 26 0.02 .circle-solid. .circle-solid. .circle-solid. Example 5 3 60 0.15 .circle-solid. Example 6 5 60 0.13 .circle-solid. Comparative 5 60 X 0.15 .circle-solid. Example 1 Comparative 26 X 0.13 .circle-solid. .circle-solid. .circle-solid. Example 2 Max-Min ( C.) 1 1 1 32 22 25 Combination in Thermocouple Re K Re Prototype Prototype (wt %) (wtppm) Content CV Prototype 7 Prototype 8 Prototype 9 2-2 2-3 Example 1 3 60 0.05 Example 2 5 60 0.05 .circle-solid. .circle-solid. .circle-solid. Example 3 5 0.02 Example 4 26 0.02 .circle-solid. .circle-solid. .circle-solid. .circle-solid. Example 5 3 60 0.15 .circle-solid. Example 6 5 60 0.13 .circle-solid. Comparative 5 60 X 0.15 Example 1 Comparative 26 X 0.13 .circle-solid. Example 2 Max-Min ( C.) 15 15 11 1 1

[0049] In addition, the tensile strength was compared between Example 4 having a diameter of 0.5 mm and Comparative Example 2 having a diameter of 0.5 mm. For the tensile piece, twenty samples were taken evenly from the entire length. A universal tension and compression tester (AG-I 5 kN manufactured by SHIMADZU) was used to conduct the test. The test piece was chucked by a flat plate via non-slip sandpaper, and both ends thereof were fixed to the device. Setting the gage length to 50 mm, the tensile test was conducted at a rate of 10 mm/min. Table 4 shows the results of the test. Although there is no difference in the average value of the tensile strength between Example 4 and Comparative Example 2, the standard deviation indicating variation is very small in Example 4 as compared with Comparative Example 2. Therefore, the stability of conditions when the example is processed as a material is greatly improved, which contributes to improvement of the yield. In the other examples as well, the standard deviation of the tensile strength was 35 N/mm.sup.2 or less.

[0050] The stability of conditions when the example is processed as a material is greatly improved, which contributes to improvement of the yield. If medical needles are manufactured by cutting this wire rod into a plurality of pieces, needles having a stable tensile strength can be obtained.

REFERENCE SIGNS LIST

[0051] A. Outer periphery of rhenium-tungsten wire rod [0052] B. Surface mixture layer [0053] C. Rhenium-tungsten wire rod body [0054] 1. Center of radial cross-section [0055] 2, 3, 4, 5. Concentric circle in radial cross-section [0056] X, Y. X-axis and Y-axis of radial cross-section

[0057] While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.