Method for relieving residual stress of hard alloy and steel brazed joint

Abstract

The invention discloses a method for relieving residual stress of a hard alloy and steel brazed joint, and relates to a method for relieving residual stress of an alloy and steel brazed joint. The invention aims to solve the technical problems of poor mechanical property and short service life of the welding joint caused by residual stress of the existing hard alloy and steel brazing welding joint. According to the method, a brazed joint is subjected to subzero treatment of liquid nitrogen and then naturally heated to the room temperature, so that the two base materials and the brazing filler metal are subjected to cold contraction and thermal expansion, redistribution of residual stress at the joint is achieved, and the macrohardness, the bonding strength and the service reliability of the brazed joint are finally and comprehensively improved under the influence of the mechanism; early failure of the connector is effectively prevented, residual stress of the connector can be fully relieved by using the liquid nitrogen cryogenic hard alloy and the steel connector, the bonding stability of brazing filler metal and base metal is improved, and the brazed connector with excellent performance is obtained.

Description

Technical Field

[0001] This invention relates to a method for relieving residual stress in alloy-steel brazed joints. Background Technology

[0002] Cemented carbide, often referred to as the "teeth of industry," possesses high hardness, high wear resistance, and excellent red hardness, making it the preferred material for manufacturing high-efficiency cutting tools, mining drills, and precision wear-resistant parts. However, its inherent brittleness, low toughness, and expensive raw materials make it difficult to directly form large or complex structural components. To balance performance and economy, the "local strengthening" approach is commonly used in engineering: brazing a cemented carbide working layer onto a high-toughness, low-cost steel substrate to achieve complementary properties between the two materials. The brazing temperature is much lower than cladding or fusion welding, minimizing heat-affected zone deformation and cemented carbide microcracks. This is currently the most mature and widely used process for joining cemented carbide and steel. However, the thermal expansion coefficients of cemented carbide and steel differ by 2-3 times, resulting in residual tensile stresses of up to hundreds of megapascals in the joint area during the post-weld cooling stage, becoming a bottleneck restricting the service life and reliability of the joint. Therefore, developing an efficient and low-damage residual stress control technology is of great theoretical and engineering significance for promoting the lightweighting and long service life of high-end tools / molds.

[0003] However, during brazing, the joint cools from hundreds of degrees Celsius to room temperature, and the shrinkage of the steel is much greater than that of the cemented carbide. This mismatched deformation ultimately creates a complex stress field within the joint: the cemented carbide side, especially near the interface, bears extremely high residual tensile stress; while the steel matrix side mainly bears residual compressive stress. This residual stress caused by thermal mismatch is extremely detrimental to cemented carbide, a brittle material. First, it significantly weakens the joint's bonding strength and fatigue life. Like a pre-set internal load, when superimposed with external working loads, it easily induces the initiation and propagation of microcracks at the interface. Second, during cooling or under slight impact, excessive tensile stress can directly cause the cemented carbide layer to crack or peel off, resulting in sudden workpiece failure. Furthermore, residual stress can also cause dimensional deformation of the workpiece, affecting its accuracy and stability.

[0004] To address this issue, existing technologies primarily focus on two aspects: firstly, optimizing the brazing process, such as using an interlayer to bridge the difference in thermal expansion coefficients. For instance, Xie Siyao et al. (A cemented carbide-steel composite connection method based on a high-entropy alloy interlayer: 202510662167.2[P]. 2025-08-26) reduced the risk of brittle fracture at the joint by welding cemented carbide and steel using a high-entropy interlayer, ultimately achieving a bending strength of 583.7 MPa. However, such methods often significantly increase process complexity and production costs or have limited effectiveness in stress relaxation. Secondly, traditional heat treatments, such as tempering, are used on the brazed joints. For example, Wu Jiachao (A heat treatment method for a cemented carbide welded end mill: 201410644377.0 [P]. 2016-06-08) performed quenching-tempering treatment on the welded cemented carbide welded end mills, which significantly increased the hardness of the cutter body at the weld joint and greatly increased its service life. However, the tempering process faces a dilemma: if the tempering temperature is too high, although it can effectively eliminate stress, it may lead to the deterioration of the steel matrix or the formation of brittle intermetallic compounds at the interface; while if the tempering temperature is too low, the stress elimination effect is not obvious (Yu Jiyang et al. Influence of tempering temperature on the microstructure and properties of electron beam welded joints of 12Cr12Mo steel [J]. Journal of Materials Heat Treatment, 2025, 46(01): 225-234). Therefore, traditional methods are difficult to achieve a balance between maintaining the original properties of the material and effectively eliminating stress.

[0005] In summary, there is currently a lack of an economical, efficient, and non-degradable method to effectively address the residual stress problem in cemented carbide-steel brazed joints. Therefore, there is an urgent need to develop a novel brazing and post-processing technology to significantly reduce residual stress in cemented carbide joints, thereby improving joint reliability and service life. Summary of the Invention

[0006] The present invention aims to solve the technical problem that residual stress in existing cemented carbide and steel brazed joints leads to poor mechanical properties and short service life of the welded joints, and provides a method to alleviate residual stress in cemented carbide and steel brazed joints.

[0007] The method of the present invention for relieving residual stress in cemented carbide-steel brazed joints is carried out according to the following steps:

[0008] 1. Apply CuMnNi solder paste evenly to the surface of the treated base steel, and then assemble the treated base cemented carbide to form a sandwich structure.

[0009] 2. The sandwich structure prepared in step one is placed in a vacuum brazing furnace for brazing, and finally cooled to room temperature with the furnace to obtain the brazed joint;

[0010] Third, the brazed joint obtained in step two is immersed in liquid nitrogen and brought into direct contact with the liquid nitrogen. After soaking, it is taken out and then naturally heated to room temperature, which relieves the residual stress of the cemented carbide and steel brazed joint.

[0011] The design principle of this invention is as follows: In step three, the brazed joint is subjected to deep cryogenic treatment with liquid nitrogen and then naturally heated to room temperature. This causes the two base materials and the brazing filler metal to undergo "cold contraction and thermal expansion," which redistributes the residual stress at the joint. Under the influence of this mechanism, the macroscopic hardness, bonding strength, and service reliability of the brazed joint are comprehensively improved, effectively preventing early failure of the joint. The use of liquid nitrogen cryogenic hard alloy and steel joint can fully alleviate the residual stress of the joint, improve the bonding stability between the brazing filler metal and the base material, and obtain a brazed joint with excellent performance.

[0012] This invention employs cryogenic treatment technology to alleviate residual stress in cemented carbide-steel brazed joints, thereby improving the interfacial stability between the brazing filler metal and the base material and enhancing the mechanical properties of the joint. The room temperature shear strength of the cemented carbide / steel brazed joint obtained by direct brazing at 1050℃ is 273±33MPa. After further cryogenic treatment with liquid nitrogen (-196℃) for 1 hour, the room temperature shear strength increases to 510±24MPa, an improvement of 87%. Attached Figure Description

[0013] Figure 1 The image shows a backscattered electron microscope image of the joint structure obtained when the welding temperature in step two of the above experiment was 1030℃ and held for 5 minutes.

[0014] Figure 2 The backscattered electron image is a photograph of the joint after the cryogenic treatment in step three, in which the welding temperature in step two was 1030℃ and held for 5 minutes.

[0015] Figure 3 The image shows a comparison of the room temperature shear strength of the joint after only completing step two (without cryogenic treatment) and after completing step three (cryotherapy). Detailed Implementation

[0016] Specific Implementation Method 1: This implementation method is a method for relieving residual stress in cemented carbide and steel brazed joints, specifically carried out according to the following steps:

[0017] 1. Apply CuMnNi solder paste evenly to the surface of the treated base steel, and then assemble the treated base cemented carbide to form a sandwich structure.

[0018] 2. The sandwich structure prepared in step one is placed in a vacuum brazing furnace for brazing, and finally cooled to room temperature with the furnace to obtain the brazed joint;

[0019] Third, the brazed joint obtained in step two is immersed in liquid nitrogen and brought into direct contact with the liquid nitrogen. After soaking, it is taken out and then naturally heated to room temperature, which relieves the residual stress of the cemented carbide and steel brazed joint.

[0020] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the base material steel mentioned in step one is 45# steel. Everything else is the same as in Specific Implementation Method One.

[0021] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method Two in that the method for treating the base steel in step one is to sequentially polish it with 600#, 800#, 1000#, and 2000# sandpaper. Everything else is the same as in Specific Implementation Method Two.

[0022] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the thickness of the CuMnNi solder paste in the sandwich structure described in step one is 200 μm. Everything else is the same as in Specific Implementation Methods One to Three.

[0023] Specific Implementation Method Five: This implementation method differs from Specific Implementation Method Four in that the base material cemented carbide mentioned in step one is YG8 cemented carbide. Everything else is the same as in Specific Implementation Method Four.

[0024] Specific Implementation Method Six: This implementation method differs from Specific Implementation Method Five in that the method for treating the base material cemented carbide in step one is as follows: the cemented carbide is treated sequentially with polishing pastes of W3.5, W2.5, and W1. Everything else is the same as in Specific Implementation Method Five.

[0025] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Method Six in that the specific brazing process described in step two is as follows: First, the temperature is raised to 300℃ and held for 30 minutes to allow the organic matter in the brazing paste to fully volatilize. Then, the temperature is raised to 960℃, close to the melting point of the brazing filler metal. Next, the temperature is raised to 1010℃~1070℃ and held for 5~60 minutes, and then lowered to 300℃. Everything else is the same as in Specific Implementation Method Six.

[0026] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Method Seven in that the specific brazing process described in step two is as follows: First, the temperature is raised to 300℃ at a rate of 10℃ / min and held for 30 minutes to allow the organic matter in the brazing paste to fully volatilize. Then, the temperature is raised to 960℃ at a rate of 10℃ / min, close to the melting point of the brazing filler metal. Next, the temperature is raised to 1010℃~1070℃ at a rate of 5℃ / min and held for 5~60 minutes. Finally, the temperature is lowered to 300℃ at a rate of 5℃ / min. Everything else is the same as in Specific Implementation Method Seven.

[0027] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Method Eight in that the temperature of the liquid nitrogen mentioned in step three is -196℃. Everything else is the same as in Specific Implementation Method Eight.

[0028] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Method Nine in that the sample is removed after soaking for 1 hour in step three. Everything else is the same as in Specific Implementation Method Nine.

[0029] The invention was verified using the following experiments:

[0030] This experiment presents a method for alleviating residual stress in cemented carbide-steel brazed joints, specifically conducted according to the following steps:

[0031] 1. Apply CuMnNi solder paste evenly to the treated base steel surface with a thickness of 200μm; then assemble the treated base cemented carbide to form a sandwich structure.

[0032] The base steel is 45# steel, and the method for treating the base steel is to grind it sequentially with 600#, 800#, 1000# and 2000# sandpaper;

[0033] The base material cemented carbide is YG8 cemented carbide, and the method for treating the base material cemented carbide is as follows: the cemented carbide is treated sequentially with polishing pastes of W3.5, W2.5, and W1.

[0034] 2. Place the sandwich structure prepared in step one into a vacuum brazing furnace. First, heat it to 300℃ at 10℃ / min and hold it for 30min to allow the organic matter in the brazing paste to fully volatilize. Then, heat it to 960℃ at 10℃ / min, close to the melting point of the brazing paste. Next, heat it to 1010℃~1070℃ at 5℃ / min and hold it for 5min (see Table 1 for specific temperatures). Then, cool it down to 300℃ at 5℃ / min. Finally, cool it to room temperature with the furnace to obtain the brazed joint. The room temperature shear strength is shown in Table 1.

[0035] 3. The brazed joint obtained in step 2 is immersed in liquid nitrogen (-196℃) and brought into direct contact with the liquid nitrogen. After soaking for 1 hour, it is taken out and then naturally heated to room temperature. This achieves the relief of residual stress in the brazed joint between cemented carbide and steel. The room temperature shear strength of the joint is shown in Table 1.

[0036] Table 1

[0037]

[0038] Figure 1The backscattered electron micrograph of the joint microstructure obtained during the welding process in step two (1030℃ for 5 min) in the above experiment (only step two was performed, without the cryogenic treatment with liquid nitrogen in step three) shows that the joint is dense and defect-free. The brazing filler metal on the left side bonds well with the cemented carbide and exhibits a thin, light gray reaction layer. On the right side, the brazing filler metal and 45# steel show good dissolution, resulting in areas of different contrast. Based on energy dispersive spectroscopy analysis... Figure 1 The composition and phase composition at points 1-4 are shown in Table 2. From left to right, the phase composition is cemented carbide / (Fe, Co)-based solid solution + Cu-based solid solution + (Fe, Co)-based solid solution / 45# steel. The energy dispersive spectroscopy (EDS) analysis before cryogenic treatment in Table 2 shows that the cemented carbide and 45# steel achieved good metallurgical bonding through Cu-based brazing filler metal. The weld center is mainly composed of Cu-based solid solution, while near the interface of the base materials on both sides, a transition layer of (Fe, Co)-based solid solution with different compositions is formed due to element diffusion and reaction.

[0039] Table 2

[0040]

[0041] Figure 2 Backscattered electron microscopy (SEM) images of the joint from the above experiment, where the welding temperature in step two was 1030℃ and held for 5 minutes, followed by the cryogenic treatment in step three, show that the cryogenic treatment had no significant impact on the bonding between the solder and the base metal; the joint remained dense and defect-free. Energy dispersive spectroscopy (EDS) analysis revealed... Figure 2 The elemental composition at positions 1 to 4 has undergone slight changes, and the specific values ​​are shown in Table 3. The phase composition is still cemented carbide / (Fe, Co) based solid solution + Cu based solid solution + (Fe, Co) based solid solution / 45# steel.

[0042] Table 3

[0043]

[0044] Figure 3 The figure shows a comparison of the room temperature shear strength of the brazed joints after only completing step two (As-brazed) and step three (DCT) cryogenic treatment. The figure shows that the room temperature shear strength of the brazed joints was effectively improved after cryogenic treatment under various processes with a brazing temperature of 1010℃ to 1070℃ and a holding time of 5 minutes. This indicates that cryogenic treatment has a significant effect on relieving residual stress in the brazed joints of cemented carbide and steel.

Claims

1. A method for relieving residual stress in a cemented carbide-steel brazed joint, characterized in that... The method is performed according to the following steps:

1. Apply CuMnNi solder paste evenly to the surface of the treated base steel, and then assemble the treated base cemented carbide to form a sandwich structure.

2. The sandwich structure prepared in step one is placed in a vacuum brazing furnace for brazing, and finally cooled to room temperature with the furnace to obtain the brazed joint; Third, the brazed joint obtained in step two is immersed in liquid nitrogen and brought into direct contact with the liquid nitrogen. After soaking, it is taken out and then naturally heated to room temperature, which relieves the residual stress of the cemented carbide and steel brazed joint.

2. The method for relieving residual stress in a cemented carbide-steel brazed joint according to claim 1, characterized in that... The base steel mentioned in step one is 45# steel.

3. The method for relieving residual stress in a cemented carbide-steel brazed joint according to claim 2, characterized in that... The method for treating the base steel in step one is to grind it in sequence with 600#, 800#, 1000# and 2000# sandpaper.

4. The method for relieving residual stress in a cemented carbide-steel brazed joint according to claim 1, characterized in that... The thickness of the CuMnNi solder paste in the sandwich structure described in step one is 200 μm.

5. The method for relieving residual stress in a cemented carbide-steel brazed joint according to claim 1, characterized in that... The base material cemented carbide mentioned in step one is YG8 cemented carbide.

6. The method for relieving residual stress in a cemented carbide-steel brazed joint according to claim 5, characterized in that... The method for treating the base material cemented carbide in step one is as follows: treat the cemented carbide with polishing pastes of W3.5, W2.5, and W1 in sequence.

7. The method for relieving residual stress in a cemented carbide-steel brazed joint according to claim 1, characterized in that... The specific brazing process described in step two is as follows: first, heat the temperature to 300℃ and hold it for 30 minutes to allow the organic matter in the brazing paste to fully volatilize; then heat the temperature to 960℃, then heat it to 1010℃~1070℃ and hold it for 5~60 minutes, and then cool it down to 300℃.

8. A method for relieving residual stress in a cemented carbide-steel brazed joint according to claim 7, characterized in that... The specific brazing process described in step two is as follows: first, heat the solder paste to 300°C at 10°C / min and hold it for 30 minutes to allow the organic matter in the solder paste to fully volatilize; then, heat the solder paste to 960°C at 10°C / min, then heat it to 1010°C~1070°C at 5°C / min and hold it for 5~60 minutes; and finally, cool it down to 300°C at 5°C / min.

9. A method for relieving residual stress in a cemented carbide-steel brazed joint according to claim 1, characterized in that... The temperature of the liquid nitrogen mentioned in step three is -196℃.

10. A method for relieving residual stress in a cemented carbide-steel brazed joint according to claim 9, characterized in that... In step three, the product is removed after soaking for 1 hour.

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