Process method for realizing direct connection of tungsten and steel

Abstract

The invention discloses a process method for realizing direct connection of tungsten and steel, which belongs to the technical field of connection of dissimilar materials, and compared with an indirect connection method with a transition layer, ferrite forming elements are adopted as brazing filler metal or a functional diffusion layer to promote automatic formation of ferrite on a tungsten and steel interface, so that the direct connection of the tungsten and the steel is realized. Therefore, slow release and high-strength connection of tungsten and steel interface stress are achieved. The brazing filler metal has the beneficial effects that the functional diffusion layer can be dissolved in steel in a solid mode, formation of a ferrite slow-release stress layer with a certain width is promoted, introduction of other elements except tungsten and steel base metal is reduced, the brazing filler metal can serve as the slow-release stress layer, formation of the ferrite slow-release stress layer can also be promoted, and therefore the brazing filler metal can serve as the slow-release stress layer. Therefore, direct connection of tungsten and steel is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of dissimilar material connection, and in particular relates to a process method for realizing direct connection of tungsten and steel. Background Art

[0002] Fusion reactors rely on deuterium-tritium reactions to achieve discharge, where deuterium can be extracted from seawater, while tritium can only be produced by fission reactors at present, and tritium has a short half-life, and the cost of preparation, transportation and storage is high. Therefore, future fusion reactors need to achieve self-circulation of tritium, that is, to achieve tritium self-sustaining. Tritium can be produced by the reaction of neutrons with lithium-rich materials. Therefore, by designing certain components to contain lithium-rich materials, they can react with deuterium-tritium fusion neutrons to produce tritium. Then, by collecting and extracting tritium, tritium self-sustaining is achieved. Among them, the component that carries lithium-rich materials and realizes the function of tritium production is the breeder blanket. In addition to producing tritium, the breeder blanket is directly facing the plasma, so it also has the functions of removing nuclear heat and shielding radiation. Under the combined action of heat and particle erosion, the performance requirements of the blanket components are often high. This is especially true for the first wall composed of plasma-facing material tungsten and structural material ferrite / martensitic steel (RAFM). Due to the special functional requirements of the cladding, tungsten needs to be connected to steel by metallurgical bonding to improve heat transfer capacity and resistance to radiation and particle erosion. However, tungsten and steel are dissimilar materials, and the thermal properties between the two are quite different. For example, the melting point of tungsten is 3400℃, while the melting point of steel is 1500℃. The thermal expansion coefficient of tungsten is 4×10 -6 / K, while the thermal expansion coefficient of steel is 12×10 -6 / K. When tungsten and steel are welded at high temperatures, brittle intermetallic compounds of Fe and W are also easily generated. Therefore, the difficulties in the connection between tungsten and steel are currently concentrated in two aspects. One is how to reduce the welding residual stress caused by the difference in thermal expansion coefficient between tungsten and steel. The other is how to avoid or reduce the formation of brittle intermetallic compounds between tungsten and steel. The use of transition layer metal to indirectly connect tungsten and steel is currently a topic of much research, but the transition layer metal needs to consider some special requirements of fusion reactor materials, such as hydrogen embrittlement resistance, low activation performance, and neutron irradiation resistance. It is difficult for pure metals to meet the requirements of fusion reactor materials while meeting weldability. For example, metals such as titanium, vanadium, and tantalum can achieve good bonding between tungsten and steel. At the same time, these materials have low activation, but high hydrogen embrittlement sensitivity. Although nickel can achieve good metallurgical bonding with both steel and tungsten, nickel is a highly radioactive element and is not desirable in fusion reactors. Tungsten and steel gradient transition layer is a potential tungsten and steel transition layer material. This material does not add metals other than tungsten and steel, and achieves a gradient transition of stress. However, the preparation process of tungsten steel gradient transition layer is complicated. To this end, we propose a direct connection method that forms a ferrite transition layer at the interface between tungsten and steel. The idea of ​​this method is to form a ferrite layer on the surface of the steel, and use the softer properties of ferrite to act as a stress relief layer, thereby achieving direct connection between tungsten and steel. In order to achieve this technical goal, it is necessary to consider how the ferrite phase is formed. There are mainly two types of methods. The first type is to ferritize the surface of the steel to be welded before welding tungsten and steel, such as decarburization, deposition of ferrite-forming elements and solid solution treatment. After the ferrite is formed, it is solid-phase diffusion-bonded with tungsten. The second type is to use a brazing material that promotes the formation of ferrite to achieve the formation of ferrite and direct connection of tungsten and steel during the brazing process. The second method has been pre-tested and confirmed to achieve a close connection between tungsten and steel.

[0003] The implementation of this method will provide a new idea and technical route for the connection of tungsten and steel, and can be applied to engineering practice. It is a technical method with great engineering application value. Summary of the invention

[0004] The present invention is to solve the problem of connecting tungsten and steel. By adding a small amount of ferrite-forming elements on the surface of steel, tungsten and steel are promoted to form a transition layer that relieves stress at the interface, or ferrite is directly used to form a brazing material to achieve the formation of a stress-relieving transition layer during welding. Compared with the indirect connection method of adding a transition layer, the benefits of this method are very obvious, reducing the introduction of other elements except tungsten and steel. At the same time, the introduction of ferrite-forming elements inhibits the formation of brittle intermetallic compounds to a certain extent. It is a method for connecting tungsten and steel in fusion reactors with great application prospects.

[0005] The present invention is achieved through the following technical solutions: A process for directly connecting tungsten and steel, comprising the following process steps: Step 1: Prepare the required tungsten and steel materials by machining, grind and polish the surfaces of these materials, clean them, and vacuum package them for use.

[0006] Step 2: Use one of the following two methods to directly connect tungsten and steel: 1. Form a ferrite layer on the steel surface by pretreatment and then diffuse weld it with tungsten. 2. Use brazing with a brazing material that promotes ferrite formation to directly connect tungsten and steel.

[0007] Step 3: Perform post-weld heat treatment to restore the properties of the steel.

[0008] In the above technical solution, the steel is low activation ferrite / martensite (RAFM) steel or other low activation steel.

[0009] In the above technical solution, in step 2, the pretreatment method includes surface decarburization, magnetron sputtering, electroplating or other thin film deposition methods. Surface decarburization can be performed by air decarburization, salt bath decarburization, gas decarburization or other decarburization methods. A ferrite layer can be formed immediately after decarburization. After the film is prepared by magnetron sputtering, the film needs to be vacuum annealed to dissolve the film in the steel matrix to form ferrite. The thickness of the formed ferrite layer is less than 100 μm.

[0010] In the above technical solution, in step 2, the pre-treatment method pre-deposit thin film materials are ferrite forming elements, such as chromium, vanadium, copper, germanium and other elements. The vacuum degree during vacuum annealing should be greater than 1×10 -2 Pa, the annealing temperature is usually 0.6-0.95 times the melting point of the deposited element, and the annealing time is related to the material, thickness and required ferrite thickness of the film, and is about 1-10h.

[0011] In the above technical solution, in step 2, the diffusion welding includes solid phase diffusion welding techniques such as hot isostatic pressing diffusion welding, vacuum hot pressing diffusion welding or spark plasma sintering. The welding process of tungsten and steel is optimized by controlling the welding temperature, holding time and pressurization pressure. The welding temperature is usually 0.6-0.8 times the melting point of steel, the holding time is 1-6h, and the pressure is 10-200MPa.

[0012] In the above technical solution, in step 2, the solder that promotes ferrite formation includes pure copper, pure germanium and its alloys such as germanium-copper alloy, and the brazing is performed by vacuum brazing. The welding temperature is usually 30-50°C higher than the liquidus temperature of the solder, and the welding time is 0.03-6h.

[0013] In the above technical solution, in step 3, the post-weld heat treatment is to restore the steel properties. For RAFM steel, if the welding temperature is higher than 850°C, normalizing at 1050°C and tempering at 750±10°C are required. If the welding temperature is lower than 850°C, tempering is required. The holding time is related to the wall thickness, usually 1-2 hours.

[0014] The beneficial effects of the present invention are: The present invention discloses a process method for directly connecting tungsten and steel. In the prior art, indirect connection of tungsten and steel inevitably introduces other elements besides the two. Since the transition layer needs to meet a series of requirements of fusion reactor materials such as stress relief, weldability, hydrogen embrittlement, and radiation resistance at the same time, the conditions are extremely harsh. Therefore, there is no transition layer material that fully meets the above requirements. Therefore, by introducing a small amount of ferrite-forming elements, making them completely dissolved in the steel surface to form a transition layer for stress relief, it is a process method for minimizing the introduction of elements and realizing the connection of tungsten and steel, and it is a very potential tungsten and steel connection method. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Schematic diagram of four technical routes to achieve direct connection between tungsten and steel.

[0016] Figure 2 This is a scanning electron microscope image of the direct connection joint of tungsten and steel obtained using technical route four.

[0017] Among them: 1-steel, 2-functional diffusion layer, 3-ferrite layer, 4-pure tungsten, 5-carbon atoms on the surface of steel, 6-ferrite forming brazing material. DETAILED DESCRIPTION

[0018] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. However, the following embodiments are limited to explaining the present invention, and the protection scope of the present invention should include the entire contents of the claims, and through the description of the following embodiments, those skilled in the art can fully implement the entire contents of the claims of the present invention.

[0019] Example There are four technical routes to achieve direct connection between tungsten and steel: Figure 1 As shown, technical route 1: a functional diffusion layer film 2 is plated on the surface of steel 1, and then a ferrite layer 3 is obtained through solid solution treatment, and then a welding method is used to connect it with pure tungsten 4.

[0020] Technical route 2: A functional diffusion layer film 2 is plated on the surface of steel 1 and directly welded to pure tungsten 4 to achieve connection.

[0021] Technical route three: Steel 1 is decarburized (forming carbon atoms 5 on the steel surface) to obtain a ferrite layer 3, which is then directly welded with pure tungsten 4 to achieve a connection.

[0022] Technical route 4: directly using ferrite to form brazing filler metal 6 to achieve direct connection between tungsten and steel.

[0023] By comparing the above technologies, this embodiment selects technical route 4, that is, using ferrite to form brazing filler metal vacuum brazing method to realize direct connection between tungsten and steel, and the steps are as follows: 1) First, RAFM steel (Institute of Metal Research, Chinese Academy of Sciences) and pure tungsten sheet (Beijing Antai Zhongke Metal Materials Co., Ltd.) were surface polished to a roughness less than Ra0.8 and cleaned with alcohol.

[0024] 2) Remove the surface oxide scale of 0.05mm Ge-12Cu wt% (Shenyang Nonferrous Metals Research Institute) solder and clean it with alcohol.

[0025] 3) Place the brazing material between the pure tungsten sheet and the RAFM steel for assembly and place in a vacuum brazing furnace.

[0026] 4) Set the welding temperature to 1040℃, the holding time to 10min, and the background vacuum degree to less than 10 -2 Pa, heating rate 10℃ / min, furnace cooling.

[0027] 5) After cooling to room temperature, set the brazing furnace temperature to 750°C, keep it at this temperature for 1 hour, then cool it down in the furnace, with a heating rate of 10°C / min.

[0028] 6) Take it out after cooling to achieve the connection between tungsten and steel.

[0029] The joint directly connected between tungsten and steel obtained in the above embodiment was tested by scanning electron microscope, and the scanning electron microscope image obtained is as follows: Figure 2 As shown in the figure, it can be concluded that, compared with the indirect connection method of adding a transition layer, the method in this embodiment reduces the introduction of elements other than tungsten and steel. At the same time, the introduction of ferrite-forming elements inhibits the formation of brittle intermetallic compounds to a certain extent.

[0030] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions only describe the principles of the present invention. The present invention may be subject to various changes and improvements without departing from the spirit and scope of the present invention. These changes and improvements fall within the scope of the present invention to be protected. The scope of protection claimed by the present invention is defined by the attached claims and their equivalents.

Claims

1. A process for directly connecting tungsten and steel, characterized in that: Step 1: Prepare the required tungsten and steel materials by machining, grind and polish the surfaces of these materials, clean them, and vacuum-pack them for standby use; Step 2: directly connect tungsten and steel by one of the following two methods:

1. form a ferrite layer on the steel surface by pretreatment and then diffuse weld it with tungsten; 2. use brazing with a brazing material that promotes ferrite formation to directly connect tungsten and steel; Step 3: Perform post-weld heat treatment to restore the properties of the steel.

2. A process for achieving direct connection between tungsten and steel according to claim 1, characterized in that: The steel is low-activation ferrite / martensitic steel or other low-activation steel.

3. The process for achieving direct connection between tungsten and steel according to claim 1, characterized in that: The pretreatment method includes surface decarburization, magnetron sputtering, electroplating or other thin film deposition methods, wherein the surface decarburization adopts air decarburization, salt bath decarburization or gas decarburization. After the film is prepared by magnetron sputtering, the film is subjected to vacuum annealing treatment to make the film dissolve in the steel matrix to form ferrite, and the thickness of the formed ferrite layer is less than 100μm.

4. The process for achieving direct connection between tungsten and steel according to claim 1, characterized in that: The pre-deposited thin film material in the pre-treatment method is a ferrite forming element, and the vacuum degree during vacuum annealing is greater than 1×10 -2 Pa, the annealing temperature is 0.6-0.95 times the melting point of the deposited element, and the annealing time is 1-10h.

5. The process for achieving direct connection between tungsten and steel according to claim 1, characterized in that: The diffusion welding is hot isostatic pressing diffusion welding, vacuum hot pressing diffusion welding or discharge plasma sintering, the welding temperature is 0.6-0.8 times the melting point of steel, the insulation time is 1-6h, and the pressure is 10-200MPa.

6. The process for achieving direct connection between tungsten and steel according to claim 1, characterized in that: The solder for promoting ferrite formation includes pure copper, pure germanium and alloys thereof. The brazing adopts vacuum brazing, the welding temperature is 30-50°C higher than the liquidus temperature of the solder, and the welding time is 0.03-6h.

7. A process for achieving direct connection between tungsten and steel according to claim 2, characterized in that: Post-weld heat treatment is used to restore steel properties. For low-activated ferrite / martensitic steel, if the welding temperature is higher than 850°C, normalizing at 1050°C and tempering at 750±10°C are required. If the welding temperature is lower than 850°C, tempering is required with a holding time of 1-2h.

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