CuMnNi brazing filler metal vacuum brazing 40Cr steel and YG8 cemented carbide

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1.Materials and methods

The dimensions of 40Cr steel and YG8 cemented carbide used are: 5mmx16mmx15.5mm (5mmx16mm surface is coal junction surface, 16mmx15.5mm surface is spreading surface); Ni sheet is used as the middle layer, and the thickness is 0.05mm, 0.1mm, 0.2mm , 0.3mm and 0.4mm; w (Mm)= (23.00~31.00)%, w (Ni)=(2.00~4.00)%, w (Cu) remainder in the self-made solder CuMnNi.

Before welding, place the YG8 cemented carbide on the cast iron block coated with diamond (w3.5) grinding paste and grind it until it is bright and free of contamination. (w40) on sandpaper; solder on sandpaper (w150)

Sand to remove oxide layer and make surface clean. Then, put the 40Cr steel YC8 cemented carbide and the brazing filler metal into acetone for cleaning to remove the oil stains on the surface. Finally, the brazing filler metal is placed between cemented carbide and 40Cr steel, and two pieces of heat-resistant steel are used as loading blocks to stabilize it. The vacuum brazing test was carried out in a vacuum sintering furnace. During the heating and cooling process, the vacuum degree was maintained at about 5 xI0-2Pa.

The measured melting temperature range of the solder is 920C~950C, which is basically consistent with the melting point found by the CuMnNi ternary phase diagram. Therefore, it is determined that the vacuum brazing temperature of this test is 980C~1080C, and there are 6 vacuum brazing temperature test points every 20C. The spreadability test of the brazing filler metal (about 0.15g) and the base metal was carried out at each temperature, and the corresponding wetting angle was measured with a goniometer.

The heating speed is an important parameter in the vacuum brazing process. If the heating speed is too fast, the vacuum degree drops sharply, and it is easy to oxidize the cemented carbide and the brazing filler metal. In this test, the heating method is: from room temperature to 800C at a rate of 10C/min, and keeping the temperature for 30 minutes. Then rise to the set temperature at a rate of 9C/min, hold for 10 minutes, and then cool with the furnace.

After the vacuum brazing is completed, the sample line is cut into rectangular blocks of 5mmx5mmx32mm, and a three-point bending strength test is performed. At the same time, the sample was polished with sandpaper and finely ground with abrasive paste. Finally, the cemented carbide was corroded with the corrosive solution mixed with K3Fe(CN)6 and NaOH solution, and the steel was corroded with 3%-5% nitric acid alcohol to prepare the microstructure. for analysis.

2.Analysis and discussion of the results

2.1 Influence of brazing temperature on the strength of brazed joints

The wetting angles of the solder on the two base metals at different temperatures are shown in Table 1. It can be seen from the table that the wetting angle of the solder to the base metal at each temperature is less than 15°, and has good wettability; as the temperature increases, the wetting angle decreases and the wettability increases; The wettability is better than that of YC8 cemented carbide.

Table 2 shows the three-point bending test results of the brazed joints obtained at different welding temperatures (adding 0.2 mm of Ni interlayer). It can be seen from the table that the flexural strength of the joint first increases and then decreases with the increase of temperature, and the strength of the brazed joint at 1040C reaches the highest 665MPa. It is observed that the fracture is mostly on the cemented carbide side near the brazing seam. This is due to the large difference in the coefficient of linear expansion between cemented carbide and 40Cr steel. During the cooling process of the joint, due to the larger expansion coefficient of the steel and the faster shrinkage, a larger residual tensile stress is formed on the cemented carbide side. To.

Combining Tables 1 and 2, it is determined that the optimum brazing temperature for this test is 1040C. At the interface between 40Cr steel and brazing seam, the interaction between the brazing filler metal and the base metal is obvious, forming a clear and uniform diffusion reaction zone. The brazing seam has an obvious interface on the cemented carbide side, indicating that the bonding between the brazing filler metal and YC8 is not as good as that of 40Cr steel, which is consistent with the results of the spreadability test. The interface between the Ni intermediate layer and the solder is blurred, indicating that the two are well combined.

Since Fe, Co, and Ni are close to each other in the periodic table, they are easy to form substitutional solid solutions. Relevant data easily show that these three elements can form single-phase solid solution in a wide range. It can be inferred that the solid solution phase region with a certain width formed at the interface between 40Cr steel and the brazing filler metal is Fe-Co-Ni-based single-phase solid solution. The single-phase solid solution has high strength and good plasticity, which is beneficial to improve the strength of the brazed joint.

2.2 Influence of the thickness of the intermediate layer on the microstructure and properties of the brazed joint

Table 3 shows the test results of three-point bending at 1040C and different intermediate layer thicknesses. It can be seen that when the thickness of the Ni intermediate layer is 0.2 mm, the maximum bending strength of the brazed joint is 660 MPa. If the thickness of the intermediate layer is too small, it is beneficial to the diffusion of elements and can form a good metallurgical bond, but the release of the intermediate layer to the joint stress is poor, and the joint performance is poor; as the thickness of the intermediate layer increases, its ability to release the joint stress It increases, but the capillary action decreases, and the solder is easily lost during brazing, and it is difficult to fill the gap between the welds; and the excessive thickness of the intermediate layer affects the long-range diffusion of elements such as Fe and Co, thereby affecting the performance of the joint.

3.Conclusion

(1) CuMnNi solder has good wettability on both 40Cr and YG8, but the wettability on 40Cr is better. As the temperature increases, the wetting angle decreases and the wettability increases.

(2) The joint strength first increases and then decreases with the increase of vacuum brazing temperature. When the vacuum brazing temperature is 1040C (the thickness of the Ni intermediate layer is 0.2mm), the optimal brazing joint can be obtained, and the bending strength of the joint can reach 660MPa. Due to the long-range diffusion of Fe and Co, the formation of Fe-Co-Ni-based single-phase solid solution at the brazing interface is an important reason for the improvement of the strength of the welded joint.

(3) The thickness of the intermediate layer has an important influence on the strength of the brazed joint. If the thickness is too small, the ability of the middle layer to release the stress of the joint is poor; if the thickness is too large, the long-range diffusion of elements such as Fe and Co in the base metal on both sides will be affected, thereby affecting the metallurgical combination of the brazing structure and the joint. The optimum interlayer thickness is 0.2mm.

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