The main products that need to be used for brazing carbide brazed tools are the bimetallic structure of cutting tools, measuring tools, dies and excavation tools. The characteristic of this bimetallic structure is that its cutting parts are cemented carbide, and the matrix is carbon steel or low alloy steel, usually medium carbon steel. A bimetallic structure can cushion the tremendous stress acting on this kind of workpiece, especially under compressive bending, impact or alternating loads. Most of the carbide brazed tools are welded to the medium carbon steel or low alloy steel matrix. The brazing process is closely related to the performance of the cemented carbide, and the brazing performance directly affects the use effect of the cemented carbide.
Cemented carbides contain a high content of carbides and alloying elements. Although they can be welded, they are prone to structure and cracks during brazing. Limited technological measures must be adopted to obtain satisfactory welded joints. Another reason for the need to weld carbide brazed tool is that the carbide brazed tool has the shortcomings of high brittleness, poor toughness, and high price.
Brazing characteristics of carbide brazed tools
The relationship between linear expansion coefficient and brazing crack
The carbide brazed tool in the carbide brazed tool weldments has a small composition and is fixed on a relatively thick steel support material. The linear expansion coefficient of carbide brazed tool is 401-7.0*10, which is quite different from that of common steel. The difference between linear expansion coefficient of carbide brazed tool and steel will cause great stress when the seam is cooled. Both carbides brazed tool and steel expand freely when heated, but the shrinkage of steel is much larger than that of carbide brazed tool when cooled. At this time, the weld is subjected to compressive stress while the surface of carbide brazed tool is subjected to tensile stress. If the residual stress is greater than the requirement of tensile strength or crack resistance of cemented carbide, cracks may occur on the surface of cemented carbide, which is one of the main causes of cracks in carbide brazed tool brazing.
Effect of brazing Stress
Residual stress in welded joint area is a potential hazard. Although cracks may not be found immediately after brazing, they are prone to occur during subsequent grinding, storage or use, resulting in tool scrap. The larger the brazing area of cemented carbide, the greater the brazing stress and the greater the possibility of cracking.
In the brazing of carbide brazed tool tools, measures must be taken to minimize the brazing stress, such as reducing the brazing temperature, preheating and slowly cooling before brazing, choosing plastic filler metal, adding compensating gaskets and improving joint structure. Brazing large area cemented carbide
Special measures should be taken to reduce brazing stress and prevent crack production, regardless of strength.
Oxidation when brazing carbide
When the carbide brazed tool is heated to more than 800 degrees Celsius in air, the surface of the carbide brazed tool begins to oxidize to form a loose oxide layer, accompanied by decarburization phenomenon. When heated to 950 degrees Celsius to 1100 degrees Celsius, the surface layer of the carbide brazed tool will undergo rapid oxidation, which can reduce the mechanical properties of the cemented carbide. Oxidation film. The existence of oxide layer on the surface of carbide brazed tool also reduces the hardness of the weld. Therefore, measures should be taken to minimize the oxidation phenomenon in the brazing parts of cemented carbide, which is an important measure to improve the brazing quality.
Brazing Method of carbide brazed tool and Steel
The main brazing methods of carbide brazed tool and steel are oxygen-acetylene flame brazing, high frequency induction brazing, contact resistance brazing, and brazing in heating furnace.
Oxygen-acetylene flame brazing
This is one of the most commonly used brazing methods. carbide brazed tool brazing can be carried out with general oxygen-acetylene equipment without additional special equipment. High-quality carbide brazed tool can be welded by using reasonable heating method and proper process according to the characteristics of oxygen-acetylene flame. Oxygen-acetylene flame brazing is suitable for small and medium-sized carbide brazed tool cutting tools, dies and measuring tools in batches, and also for repairing damaged carbide brazed tool mining tools in the field.
The core temperature of oxygen-acetylene flame is as high as about 3000 C. When brazing is heated, it should be avoided to spray carbide brazed tool directly from the core, so as to avoid cracking caused by excessive temperature. Before brazing, the filler metal, filler metal and carbide brazed tool are placed in turn, and the reduction flame is used to preheat the base part near the cemented carbide.
Hi-frequency induction brazing
In high frequency induction brazing, high frequency induction heating power supply with frequency of 600 kHz and power between 10 and 100 kW will generate high frequency current. When the high frequency current passes through the inductor, the high frequency alternating magnetic field is generated, and the induced current is also generated in the welded metal of the inductor. The high frequency heating speed is very fast. It can be heated to a very high temperature in a very short time to melt the solder. The inductors used in high frequency induction brazing are mostly made of copper tubes with diameters of 5 to 10 mm. Whether the geometry and size of inductor are suitable or not is one of the important factors that determine the heating speed, temperature uniformity, production efficiency and brazing quality of high frequency induction brazing.
This is carried out on a brazing machine or butt brazing machine specially used for brazing carbide cutters. The secondary coil voltage of the brazing transformer is less than 36V and the current is over 1000A. When brazing, the workpiece is clamped between two copper electrodes. When the strong current from the secondary coil passes through the welded workpiece, the heat generated by the contact resistance between the carbide brazed tool and the steel matrix is used as the brazing heat source to melt the brazing filler metal.
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