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Vacuum brazing process of nickel-based mixed brazing filler metal

Vacuum brazing of 316L stainless steel with nickel-based mixed brazing filler metal. Technological test research was carried out on it, and by means of metallographic imaging microscope, scanning electron microscope, energy spectrum analysis and microhardness analysis and other modern testing methods, the process research was carried out on the influence of brazing welding seam by welding process parameters. The research results show that the most suitable formula for 316L stainless steel nickel-based mixed brazing filler metal is BNi-2+40%BNi-5, and the best vacuum brazing process parameters are: heating temperature of 1140℃, heat preservation for 10min, and the brazing assembly room image. is 30um. The obtained braze structure has a large amount of solid solution and almost no compounds.

1 Sample preparation and test method

In this test, the base metal is 316L stainless steel, its composition is 00Cr17Ni14Mo2, and the structure is plate-like. The test brazing filler metals are nickel-based mixed brazing filler metals of BNi-2 and BNi-5, and their chemical compositions are listed in Table 1.

The preparation process of the sample is as follows: first, the sample is polished and cleaned, then the two samples are butted in the form of sheet metal lap joint, and Ni foils of different thicknesses are added at both ends of the brazing seam to adjust the gap of the brazing seam and fix it. . Add the brazing filler metal and fix the brazing filler metal with butyl acetate as a binder. Finally, the samples were put into a vacuum brazing furnace for vacuum brazing.

A total of 4 kinds of nickel-based mixed brazing filler metals were prepared in this test. The formulas were pure BNi2, BNi2+20%BNi5, BNi2+30%BNi5, BNi2+40%BNi5. The heating process of vacuum brazing is shown in the figure below, and the cooling method is cooling with the furnace.

After the vacuum brazing is completed, the sample is cut from the middle along the flow direction of the solder, so that the morphology and structure of the entire weld can be seen. The etching solution is the most commonly used FeCls etching solution. After the corrosion is completed, the metallographic analysis of the weld structure is carried out through a metallographic microscope. Scanning electron microscopy and energy dispersive analysis were performed to further understand the elemental composition of the weld.

2.1 Influence of brazing temperature on brazing seam

Figure 2 shows the brazing seam structure of vacuum brazing stainless steel at different temperatures with a gap value of 140um. The brazing temperature of Fig. 2(a) is 1050°C. By observing the metallographic diagram, it can be found that there are many defects in the weld, and the filling effect of the solder is not good. In the samples No. 5 and No. 7, the brazing filler metal was well filled, and the heating temperatures were 1100°C and 1140°C, respectively. This shows that when vacuum brazing stainless steel, if the brazing gap is large, the heating temperature should not be too low, otherwise the brazing filler metal will not be easily melted and the filler filler effect will not be good. By observing the metallographic photographs of the welds of No. 5 and No. 7 samples, it is found that the phenomenon of grain and grain boundary diffusion is more obvious with the increase of temperature, which indicates that the increase of brazing temperature is helpful for the diffusion process. It shows that the brazing temperature should not be too low, and it is more appropriate to set it as 1100°C and 1140°C.

2.2 Influence of holding time on brazing seam

The No. 4 and No. 6 samples in the test were at the same brazing temperature, respectively using 5min and 30min holding time. The photo of the metallographic structure of the brazing seam is shown in Figure 3. It can be seen from Fig. 3 that the composite phase structure in the brazing seam in Fig. 3(a) is significantly less than that in Fig. 3(b). The element composition at the center of the brazing joint was further analyzed by scanning electron microscope and energy spectrum analysis, and it was found that the content of Si element in No. 4 sample was more than that of No. 6 sample. In Ni-based solders, B and Si elements are mainly responsible for the brittle compound phase. This shows that the sufficient diffusion of B and Si elements is beneficial to reduce the formation of brittle compound phases in the brazing joint. It shows that the longer the holding time is in the vacuum brazing process, the more sufficient the diffusion of the brazing material to the base metal is.

2.3 Influence of brazing joint gap on brazing seam

The gap of the brazed joint determines the diffusion distance of the solder elements, which has a great influence on the mechanical properties of the brazed joint. Especially for nickel-based solders, the size of the brazing gap has a great influence on the formation of brittle phases in the brazing seam. When the brazing gap is large, the diffusion distance is too large, and the diffusion cannot be fully carried out. When the brazing gap is small, the content of these elements in the brazing seam is small, and the diffusion distance is short, which can be fully diffused in a short time, and the nickel-based solid solution structure is formed when the brazing filler metal solidifies. Through the test, it is known that the brazing joint gap of 30um is the best.

3 Conclusions

(1) When using nickel-based solder for vacuum brazing of 316L stainless steel, adding a certain content of BNi5 to the solder can limit the excessive flow of the solder, improve the ability to fill large gaps, and reduce the The role of small dissolution.

(2) When brazing a large gap, if the brazing temperature is too low, the solder is not easily melted and the filling effect of the solder is not good. The increase of brazing temperature helps to reduce the compound phase in the brazing seam, but at the same time may also cause the grain growth of the base metal. The smaller the brazing gap is, the less likely the brittle compound phase will be generated, and the more uniform the structure will be. However, if the gap is too small, the brazing filler metal is not easy to fill the gap or even the gap is not filled, which may become the source of welding cracks. Therefore, the control of the brazing gap is very important.

(3) Through the comparison of the brazing seam structure of each sample, it is found that the best brazing seam structure and performance is the No. 8 sample. Its process parameters are: brazing temperature 1140 ℃, holding time 10min, assembly gap 30um, mixed brazing The content of BNi-5 in the material is 40%. The brazing joint structure obtained by this sample has a large amount of solid solution, almost no precipitated compound phase and moderate hardness.

Process technology must be matched with excellent equipment to play a role, and the brazing furnace selected in vacuum brazing is very important. The RHVB vacuum high temperature brazing furnace produced by SIMUWU is an excellent product for handling such processes. With its excellent temperature control accuracy and temperature uniformity, the workpiece can be brazed evenly and evenly, so as to achieve high-level brazing standards, reduce scrap rates, increase production efficiency and reduce costs.