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High Temperature Vacuum Brazing of Niobium Alloy and Stainless Steel

In the nuclear industry, many welding problems between dissimilar metals and refractory metals are often encountered, and the connection between niobium alloys and stainless steel is one of them. Due to the large difference in chemical composition, melting point, linear expansion coefficient and other physical properties of niobium alloy and stainless steel, the weldability of the two is poor, and the quality of the joint after welding is difficult to guarantee. Therefore, the welding of niobium alloy and stainless steel has always been a welding method. technical difficulties in.

It is pointed out in the literature that when niobium alloy and stainless steel are welded and brazed, in order to make the welded joint have the best mechanical properties, the heating temperature of the niobium alloy should not be lower than 80% of the temperature of the molten stainless steel in the welding pool, while the molten stainless steel and The contact time of the niobium alloy shall not be less than 0.8 s or exceed 1.1-1.3 s, otherwise, the welded joint cannot be formed, or brittle metal compounds will be formed in the weld. Obviously this requirement is extremely harsh on the control of the welding process. Relevant personnel have studied the microstructure and properties of Nb-1Zr alloy and 1Cr18Ni9 stainless steel vacuum electron beam fusion-brazing joints. The study found that the interdiffusion layer formed at the connection interface contains a large number of strip-shaped precipitates, and the precipitates are in the form of Full-sheet layered structure.

This work studies the high-temperature vacuum brazing connection method for two base metals, Nb-1Zr and 1Cr18Ni9Ti. The melting point and linear expansion coefficient of niobium alloy and stainless steel are significantly different, and the fusion-brazing connection method is very complicated. In addition, the practical welding structure has certain limitations on the implementation of fusion-brazing, which makes high-temperature vacuum brazing The method of welding can be applied. During the high-temperature vacuum brazing process, the liquid solder will interact with the base metal during capillary filling, such as dissolution and diffusion, causing a series of changes in the composition of the brazing joint, the performance of the brazed joint, and the remelting temperature. The parameters of the brazing process, such as the roughness and cleanliness of the surface of the part, the degree of vacuum, the gap between the weld seam, the brazing humidity, the holding time, etc., not only affect the fluidity and wet strength of the solder, but also determine the relationship between the solder and the base material. The metallurgical reaction between the materials affects the performance of the joint. Therefore, it is of great significance to determine reasonable process specifications and control these specification parameters more accurately to ensure high-quality high-temperature vacuum brazed joints.

1 High temperature vacuum brazing joint form, application conditions and performance requirements

The base material of the brazing joint is Nb-1Zr alloy and 1Cr18Ni9Ti stainless steel. The composition of the two is very different, and the structure and performance are also very different. The melting point of 1Cr18Ni9Ti is more than 1000 ℃ lower than that of Nb-1Zr, and the linear expansion coefficient is about that of Nb-1Zr. 2 to 3 times, the specific heat capacity is about 2 times that of Nb-1Zr, and the thermal conductivity is only about 1/3 of that of Nb-1Zr. These differences make it very difficult to join Nb-1Zr and 1Cr18Ni9Ti by welding. Relatively speaking, the method of high temperature vacuum brazing has more advantages.

According to application requirements, the welded joint is in the form of pipe joint socket, the inner layer is 1Cr18Ni9Ti, and the outer layer is Nb-1Zr alloy. Due to the large difference in the linear expansion coefficients of the two base metals, for this joint form, a large thermal stress will be generated during the brazing process, and there is no free end to release, and the interior of the joint is in a state of tensile stress after cooling.

When the brazed joint is working, the interior is in a vacuum state, there is strong neutron radiation in the working environment, and the working temperature is 600~700°C, which will be accompanied by certain thermal shock conditions. The joints after brazing are required to have strict vacuum sealing performance, thermal shock resistance and neutron radiation resistance.

2 High temperature vacuum brazing process test

2.1 Vacuum solder

Nickel-based solder BNi-5 is selected as the solder for welding Nb-1Zr alloy and 1Cr18Ni9Ti stainless steel. The specific composition of BNi-5 solder is: Cr, 18.5%~19.5%; Si, 9.75%~10.75%; C, 0.10% ;Others, 0.5%; The balance is Ni, which is formed by adding nickel as the matrix, adding the element Si that lowers the melting point and the element Cr that improves the thermal strength, anti-oxidation and corrosion resistance. The solidus temperature of the solder is 1079 ℃, the liquidus temperature is 1 135 ℃, and it is composed of Ni+Ni5Si2+Cr3Ni5Si2, but the amount of Cr3Ni5Si2 phase is extremely small.

2.2 Determination of vacuum brazing process specification

Brazing temperature and holding time are the most important parameters in the brazing process. The main basis for determining the brazing temperature is the melting point of the selected solder. Usually, the brazing temperature is 25-60°C higher than the solder liquidus temperature. The higher the temperature during brazing, the smaller the surface tension of the liquid solder, the better the wetting effect, and the stronger the seam filling ability. However, if the temperature is too high, it may cause excessive growth, overheating, and overburning of the base metal grains, and when the brazing filler metal spreadability is too strong, it is easy to cause the loss of the brazing filler metal, it is difficult to fill the brazing joint and form a good fillet, and it is also easy to cause corrosion of the base metal. The main basis for determining the holding time is the characteristics of the interaction between the solder and the base metal, and factors such as the size of the weldment, the gap between the brazing seam and the placement of the solder should also be considered. Brazing temperature and holding time cannot be determined in isolation, and they have a certain complementary relationship.

Heating and cooling rates also have a certain influence on the quality of brazed joints. Under the premise of ensuring uniform heating, the heating rate should be increased as much as possible. Generally, the temperature can be raised slowly in the range of room temperature to 900 °C, and the temperature can be kept at 900 °C for a period of time. In this way, uniform heating can be realized and the vacuum degree of brazing can be improved. In the range from the soaking temperature to the brazing temperature, the temperature should be raised as quickly as possible to avoid premature alloying with the base metal before the brazing filler metal spreads, or even corrosion. During cooling, the cooling rate should be controlled reasonably, so that the post-weld heat treatment and post-weld cooling can be completed at the same time. Generally, in the range of brazing temperature to 750 °C, in order to avoid segregation of the solder, rapid cooling should be performed, while slow cooling within the range of 750 °C to room temperature can effectively prevent welding deformation.

2.3 Vacuum brazing in furnace

Brazing is carried out in a vacuum furnace. The vacuum degree is better than 8×10-3Pa at high temperature. The tantalum heating element is heated by high-frequency induction heating equipment. The heating element heats the brazing joint and solder through thermal radiation to achieve the purpose of joint welding.

Using BNi-5 brazing filler metal, a series of process parameters were selected to carry out the high-temperature vacuum brazing process test of Nb-1Zr and 1Cr18Ni9Ti furnace. After welding, perform performance inspection and microscopic analysis on the welded joints. According to inspection and analysis results, optimize parameters and improve brazing process.

3 Conclusion

Through the high-temperature vacuum brazing process test of B-Ni5 brazing Nb-1Zr and 1Cr18Ni9Ti, and the performance inspection and analysis of the brazed joint after welding, the following improvement plan for the high-temperature vacuum brazing process is proposed.

1) When B-Ni5 is used to braze Nb-1Zr and 1Cr18N9Ti, the brazing temperature is 1170-1190 °C, and the holding time is controlled at 2-10 min according to the structure and size of the joint. Under this condition, good results can be obtained. The vacuum sealing performance, thermal shock resistance and bearing capacity of the brazed joint can be guaranteed, and the brazed joint meets the requirements of the working conditions.

2) When B-Ni5 is used to braze Nb-1Zr and 1Cr18Ni9Ti, there is a 10-15 μm thick Nb, Ni, Si, Cr interdiffusion layer in the weld near the Nb-1Zr side. In terms of mechanical properties, this interdiffusion layer is the weak link of the brazing joint, and the brazing process should be controlled as much as possible to suppress the formation of the diffusion layer.

3) Due to the great difference between the linear expansion coefficients of Nb-1Zr and 1Cr18Ni9Ti, there is a large thermal stress in the pipe joints after brazing, which makes the vacuum sealing of the joints more difficult. If the actual conditions permit, the joint should be designed in the form of niobium inner layer and stainless steel outer layer, which can improve the stress state of the joint after brazing.