Method for joining a pair of metal wires including platinum wires

Abstract

To provide a method for joining heat-resistant platinum wires by welding while preventing their strength from deteriorating.SOLUTION: The present invention provides a method for joining a pair of metal wires that are a combination of platinum wires or a platinum wire and an alloy wire. At least one platinum wire of the pair of metal wires is a heat-resistant platinum wire comprising oxygen of 0.020-0.20 at% and nitrogen of less than 0.014 at% with the balance being platinum and unavoidable impurities. The end faces of the pair of two metal wires are butted against each other. While the butted end faces are pressurized, a current is flowed thereto for joining together.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for joining platinum materials having excellent heat resistance, particularly heat-resistant platinum wires useful as constituent materials for thermocouples, resistors, etc. used at high temperatures. [Background technology]

[0002] Platinum is used in a wide range of industrial fields as a constituent material for thermocouples, resistors, etc. that are used at high temperatures. For example, platinum-based thermocouples include the R thermocouple, which has a Pt-13wt%Rh alloy for the positive electrode and a platinum for the negative electrode, and the S thermocouple, which has a Pt-10wt%Rh alloy for the positive electrode and a platinum for the negative electrode. Platinum is also widely used in resistors due to its oxidation resistance.

[0003] Thermocouples and resistors are used at high temperatures, so if they are used above the recrystallization temperature, the crystal grains grow, and if a single crystal grain appears in the cross section of the wire, grain boundary fracture or fracture from the slip plane becomes more likely to occur. Furthermore, in thermocouples, due to the difference in mechanical strength between Pt-Rh alloys and platinum at high temperatures, platinum often breaks first, posing the problem of shortening the lifespan of the thermocouple due to the breakage of platinum, which is the negative electrode.

[0004] Oxide dispersion strengthened platinum technology has been developed to improve the strength of platinum materials with the aim of extending the life of thermocouples. Patent Document 1 describes a technology in which zirconia oxide is dispersed in platinum to improve the strength of platinum wire with the aim of extending the life of thermocouples. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5308499 Summary of the Invention [Problem to be solved by the invention]

[0006] Generally, adding other metal elements or their oxides to platinum to improve its strength at high temperatures tends to cause a deviation in electromotive force, so it is preferable to keep the amount of these additives to a minimum. Therefore, there is a need for a new heat-resistant platinum that has higher high-temperature strength than platinum and reduces the deviation in electromotive force.

[0007] When used with wires, they are often joined together, as in the case of thermocouples. However, in the case of thermocouples, for example, depending on the object of temperature measurement, it is necessary to make the shape of the joint as small as possible.

[0008] Welding is preferred to reduce the size of the joint, but the method of melting and joining with an oxyhydrogen burner reduces strength, so there is a need for a welding method that minimizes the reduction in strength. An object of the present invention is to provide a welding method for joining heat-resistant platinum wires that suppresses a decrease in strength. [Means for solving the problem]

[0009] The inventors first heat-treated platinum powder in an atmosphere containing hydrogen to remove the nitrogen and oxygen adsorbed and / or occluded in the powder, and then sintered it in an atmosphere containing oxygen to remove the hydrogen, resulting in a sintered body with a predetermined amount of oxygen introduced into the platinum powder surface.Then, by forging and wiredrawing, heat-resistant platinum was obtained.

[0010] When platinum wire produced by the method described above is used for thermocouples or resistance wire, it may be used by joining wires such as PtRh alloys, platinum wires themselves, or platinum produced by melting. Normally, joining wires together is done by welding using an oxyhydrogen burner, but when joining platinum wires whose lifespan has been improved by sintering, there is a problem that the strength of the joint decreases when joined by welding, so the following joining method was considered.

[0011] The inventors have discovered a method for joining a pair of platinum wires, or a combination of a platinum wire and an alloy wire such as a PtRh alloy, in which at least one platinum wire of the pair of metal wires is a heat-resistant platinum wire containing 0.020 to 0.20 at% oxygen, less than 0.014 at% nitrogen, and the remainder being platinum and unavoidable impurities, by butting the end faces of the pair of metal wires together, applying pressure to the butted surfaces while passing an electric current, melting the vicinity of the butted surfaces by resistance heat, and applying pressure to extrude the molten part to the outside, minimizing the molten part remaining within the wire diameter, thereby joining the wires together. This suppresses a decrease in strength and solves the above-mentioned problems, and has completed the present invention.

[0012] That is, the present invention provides: A method for joining a pair of metal wires, which are platinum wires or a combination of platinum wires and alloy wires, At least one of the platinum wires in the pair of metal wires is a heat-resistant platinum wire containing 0.020 to 0.20 at % oxygen, less than 0.014 at % nitrogen, and the remainder being platinum and unavoidable impurities; The ends of two paired metal wires are butted together, This is a method for joining a pair of metal wires, characterized in that the end faces of the wires are joined by applying pressure to them while passing an electric current through them.

[0013] In the pair of metal wires having the above-mentioned configuration, the method for producing platinum of the heat-resistant platinum wire containing 0.020 to 0.20 at % of oxygen, less than 0.014 at % of nitrogen, and the remainder being platinum and unavoidable impurities, The method may include a step of filling a container with platinum powder, heat treating it at 900°C to 1200°C in an atmosphere containing hydrogen, and then sintering it at 1200°C to 1500°C in an atmosphere containing oxygen, heating the sintered body to 800°C to 1100°C and hot forging it, and then wiredrawing it. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a welding method for joining heat-resistant platinum wires, which suppresses a decrease in strength. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a photograph of the structure of a prototype heat-resistant platinum wire (Comparative Example 2) after annealing before welding. [Figure 2] 1 is a photograph of the weld structure of Example 1. [Figure 3] 1 is a photograph of the weld structure of Example 2. [Figure 4] 1 is a photograph of the weld structure of Comparative Example 1. [Figure 5] 1 is a photograph of the weld structure of Comparative Example 3. [Figure 6] 1 is a graph showing creep test results. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the present invention will be described in detail.

[0017] The present invention provides a method for joining a pair of platinum wires or a pair of platinum wires and alloy wires, in which at least one platinum wire is a heat-resistant platinum wire containing 0.020 to 0.20 at % oxygen, less than 0.014 at % nitrogen, and the remainder being platinum and unavoidable impurities.

[0018] The heat-resistant platinum wire can be manufactured, for example, as follows.

[0019] First, platinum powder is filled into a container and heat treated at 900°C to 1200°C in an atmosphere containing hydrogen, then sintered at 1200°C to 1500°C in an atmosphere containing oxygen. The sintered body is then heated to 800°C to 1100°C and hot forged, after which wire drawing and heat treatment are repeated to produce a heat-resistant platinum wire.

[0020] The above manufacturing method makes it possible to manufacture a heat-resistant platinum wire characterized by containing 0.020 to 0.20 at % oxygen, less than 0.014 at % nitrogen, and the remainder being platinum and unavoidable impurities.

[0021] The present invention involves butting the end faces of two metal wires that make up a pair together and joining them by applying pressure to the butted end faces and passing an electric current through them.

[0022] The metal pair combinations are as follows: Combination of the heat-resistant platinum wires we made, A combination of heat-resistant platinum wire and platinum wire made by melting. The combination of heat-resistant platinum wire and alloy wire is used. For example, PtRh alloy wire such as Pt-13 wt% Rh wire can be used as the alloy wire.

[0023] First, the end faces of two metal wires that make up a pair (a heat-resistant platinum wire and a metal wire to be joined thereto) are cut, for example, perpendicular to the central axis of the metal wires.

[0024] The two metal wires are fixed to the clamping part of the welding machine, butted together, and pressed with a pressure device.

[0025] Electricity is passed through two butted metal wires, and the heat generated by resistance (Joule heat) is used to weld them.

[0026] The power applied to the welding machine can be either AC or DC.

[0027] For example, 0.5KVA to 2.5KVA / mm 2 , (500~2500(W / mm 2 )) AC current of 1.0KVA~2.0KVA / mm 2 In the case of direct current, the product of voltage and current is preferably 500 to 2500 (W / mm 2 For example, if the voltage is 10 V, the current is 50 to 250 A / mm 2 This becomes:

[0028] The pressure may be such that the end faces of the two metal wires do not separate while electricity is being applied. For example, it may be 1 to 40 kgf / mm 2 , (10 to 400 MPa). 2 to 20 kgf / mm 2, (20 to 200 MPa) is preferred.

[0029] The following welding method can also be used: A two-stage pressure (double upset) method is used in which a small force (primary pressure) is applied to the edges of the metals to be welded, and then a powerful upset force (secondary pressure) is applied instantaneously once the materials have sufficiently melted.

[0030] Furthermore, the following welding method can also be used: In this method, electric current is passed through the metals to be welded while the end faces of the metals are butted together, and after the contact area has melted and scattered, sparks are generated and the contact surfaces (welding surfaces) are sufficiently heated, and a strong pressure is applied to join them.

[0031] The mechanism that suppresses the decrease in the welding strength of heat-resistant platinum wire is assumed to be as follows.

[0032] The heat-resistant platinum wire produced has suppressed platinum grain growth when used under high-temperature conditions, making it less susceptible to grain boundary fracture and fracture from the slip surface, and improving high-temperature strength. On the other hand, when heat-resistant platinum wire is welded by a method such as using an oxyhydrogen burner, the fused portion has no difference in properties from platinum produced by melting, and the strength decreases. However, when heat-resistant platinum wires are butted together at their end faces, and a current is passed through the butted ends while applying pressure to weld them using the resulting resistance heat, the resistance heat melts the area near the butted surfaces, and the application of pressure forces the molten area outward, minimizing the molten area remaining within the wire diameter and joining the wires. This prevents a decrease in strength. Furthermore, the extruded molten platinum covers the joint, increasing its cross-sectional area and contributing to improved weld strength at the joint. As a result, compared to regular welding, fracture at the joint is less likely to occur and a decrease in strength is suppressed. [Example]

[0033] The present invention will be described in the following examples, but is not limited to these embodiments.

[0034] Heat-resistant platinum wire and Pt-13 wt% Rh wire were prepared as follows.

[0035] 350 g of high-purity platinum powder (99.995% or higher platinum purity) was prepared and pressurelessly packed into an alumina container. The platinum powder was heat-treated in a hydrogen atmosphere at 1000°C for 4 hours, and then sintered in air at 1450°C for 1 hour. The sintered body was heated to 1000°C and hot-forged to form a platinum ingot into a rod shape. The platinum ingot was heat-treated at 1000°C for 30 minutes, processed using a grooved roll, and heat-treated again at 1000°C for 30 minutes. It was then drawn to a diameter of 0.5 mm using a die wiredrawing method to produce a heat-resistant platinum wire with a diameter of 0.5 mm.

[0036] The specified amounts of Pt and Rh were weighed and melted in an Ar atmosphere in a high-frequency melting furnace. The molten metal was poured into a copper mold to produce an ingot. The produced ingot was processed into a rod-shaped ingot by hot forging, and then repeatedly heat-treated, grooved-rolled, and die-drawn to a diameter of 0.5 mm to produce a Pt-13wt%Rh wire with a diameter of 0.5 mm.

[0037] (Gas Analysis) Gas analysis of the fabricated platinum wire was performed using an oxygen, nitrogen, and hydrogen analyzer manufactured by LECO. The results of the oxygen and nitrogen analysis are shown in Table 1.

[0038] [Table 1]

[0039] Example 1 The heat-resistant platinum wires (φ0.5 mm) prepared in the above process were welded together. For welding, a micro butt welding machine BMS-0.1 (manufactured by Nippon Welding Machine Co., Ltd.) was used. The maximum applied power (AC) of the device was 0.5 KVA, and the frequency was 50 Hz.

[0040] Two heat-resistant platinum wires were fixed to the clamping part of the micro butt welding machine with a manual lever, and the two platinum wires were butted together. The butt pressure of the wires was applied by manually clamping them with a spring-type pressure mechanism.

[0041] Heat-resistant platinum wires are pressed together as described above, and the wires are heated to approximately 0.3KVA (300W) (1.5KVA / mm 2 ) and an AC current was passed through for 1 second. The wires were melted and joined.

[0042] Example 2 The heat-resistant platinum wire (φ0.5 mm) and Pt-13 wt% Rh wire (φ0.5 mm) prepared by the above process were welded together. For welding, a micro butt welding machine BMS-0.1 (manufactured by Nippon Welding Machine Co., Ltd.) was used. The maximum applied power (AC) of the device was 0.5 KVA, and the frequency was 50 Hz.

[0043] The heat-resistant platinum wire and the Pt-13wt%Rh wire were fixed to the clamp of the micro butt welding machine with a manual lever, and the two platinum wires were butted together. The butt pressure of the wires was applied by manually clamping them with a spring-type pressure mechanism.

[0044] The heat-resistant platinum wire and Pt-13wt%Rh wire were pressurized as described above, and the pressure was approximately 0.3KVA (300W) (1.5KVA / mm 2 An AC current was passed through the wires at 1000 V for 1 second. The wires were melted and joined.

[0045] (Comparative Example 1) Heat-resistant platinum wires (φ0.5 mm) were brought into contact with each other so that they overlapped, and the contacting parts were melted and joined using an oxyhydrogen burner.

[0046] (Comparative Example 2) An unwelded heat-resistant platinum wire was used as a comparative sample.

[0047] (Comparative Example 3) A heat-resistant platinum wire (φ0.5 mm) and a Pt-13 wt% Rh wire (φ0.5 mm) were brought into contact with each other so that they overlapped, and the contacting area was melted and joined using an oxyhydrogen burner.

[0048] (Observation of weld structure) The structures of the welded parts of Examples 1 and 2 and Comparative Examples 1 to 3 were observed.

[0049] (Comparative Example 2) A photograph of the structure of the prototype heat-resistant platinum wire after annealing before welding is shown in Figure 1. The structure maintains a large aspect ratio that is long in the wire drawing direction.

[0050] Example 1 A photograph of the weld structure of Example 1 is shown in Figure 2. The structure of the weld created using a micro butt welding machine remains the same as before welding. The remaining molten zone is limited to the vicinity of the butt surface (within a range of approximately 130 μm).

[0051] Example 2 A photograph of the weld structure of Example 2 is shown in Figure 3. The structure of the weld, which was made using a micro-butt welding machine, shows that the Pt-13wt%Rh wire has coarsened crystal grains at the weld interface between the Pt-13wt%Rh and heat-resistant platinum wire, and while the heat-resistant platinum wire side has slightly collapsed from its pre-weld structure, no coarsening of the structure or structural changes such as equiaxed crystals have occurred. No remaining molten zone was observed.

[0052] (Comparative Example 1) FIG. 4 shows a photograph of the structure of the welded joint between heat-resistant platinum wires in Comparative Example 1, which was welded using an oxyhydrogen burner. The left side of the photo shows a molten area of ​​1mm or more, and to the right of the welded area is an unmolten area.

[0053] (Comparative Example 3) Figure 5 shows a microstructure photograph of the welded joint made with an oxyhydrogen burner using the heat-resistant platinum wire and Pt-13 wt% Rh wire of Comparative Example 3. Unlike welding using a micro-butt welder, the fusion zone was 1 mm or thicker, the structure collapsed from the fusion zone, and equiaxed crystals were formed in the fusion zone.

[0054] (Creep test) Creep tests were conducted on the welded heat-resistant platinum wires of Example 1 and Comparative Example 1, and the unwelded heat-resistant platinum wire of Comparative Example 2. The tests were conducted in air at 1300°C. For comparison, the creep results of a platinum plate prepared by melting are also shown as a reference example.

[0055] The creep test results are shown in Figure 6. The creep strength of Example 1 is almost the same as that of the unwelded Comparative Example 2, and the decrease in strength is suppressed. On the other hand, the strength of Comparative Example 1, which was welded with an oxyhydrogen burner, is decreased compared to Example 1.

Claims

1. A method for joining a pair of metal wires, which are platinum wires or a combination of platinum wires and alloy wires, At least one of the platinum wires in the pair of metal wires is a heat-resistant platinum wire containing 0.020 to 0.20 at % oxygen, less than 0.014 at % nitrogen, and the remainder being platinum and unavoidable impurities; The ends of two paired metal wires are butted together, A method for joining a pair of metal wires, characterized in that pressure is applied to the butted end faces while an electric current is passed through them to join them by resistance heat, and during this joining, the melted part created by the resistance heat is pushed outward by the pressure.

2. A method for manufacturing platinum for a heat-resistant platinum wire in a pair of thermocouples manufactured by the joining method according to claim 1, the heat-resistant platinum wire containing 0.020 to 0.20 at % oxygen, less than 0.014 at % nitrogen, and the remainder being platinum and unavoidable impurities, The process involves filling a container with platinum powder, heat treating it at 900 to 1200°C in a hydrogen-containing atmosphere, sintering it at 1200 to 1500°C in an oxygen-containing atmosphere, and then heating the sintered body to 800 to 1100°C, hot forging it, and then drawing it into wire. A method for producing a heat-resistant platinum wire.

Images