Brazing of hard alloy and steel-1

       The brazing methods of cemented carbide and steel mainly include oxygen-acetylene flame brazing, high frequency induction brazing, contact resistance brazing, copper immersion brazing, and brazing in a heating furnace.

(1) Commonly used brazing methods

① Oxygen-acetylene flame brazing is one of the most commonly used brazing methods. Carbide brazing can use general oxygen-acetylene equipment, no other special equipment is needed. According to the characteristics of the oxygen-acetylene flame, using reasonable heating methods and selecting the correct technology, high-quality carbide tools can be welded. Oxygen-acetylene flame brazing is suitable for small batches of small and medium-sized cemented carbide cutting tools, molds and measuring tools, and also suitable for repairing damaged cemented carbide mining tools in the field.

The temperature of the flame core of the oxygen-acetylene flame is as high as 3000℃. When brazing and heating, it should be avoided to spray the cemented carbide directly with the flame core to avoid cracks caused by excessive temperature. Before brazing, put the flux, the solder and the cemented carbide in sequence, and use the reducing flame to preheat the base part near the cemented carbide. When the preheating temperature reaches 700～800℃, the flux begins to melt, then heat the cemented carbide sheet and the surrounding welds from above until the flux melts into a bright liquid state and penetrates to the surface along the side welds. At this time, the flame should be raised so that the flame tail continues to heat around the weld to maintain the brazing temperature. At the same time, use the metal rod to move the knife sign reciprocatingly along the knife slot 2 to 3 times, adjust and compress the blade, and discharge the excess solder and slag. After the slag is discharged, stop heating and pressurize the center part of the top surface of the cemented carbide with a pressure rod, stay for 2 to 3 seconds, after the brazing filler metal is solidified, it can be sent to an incubator or a heat preservation medium for 2 to 3 hours to keep It cools slowly. The slow-cooled tool can receive better results if it is subjected to stress-relieving tempering treatment. The tempering temperature is about 300℃, after 6h, it is cooled to room temperature with the furnace.

② High-frequency induction brazing High-frequency induction brazing uses a high-frequency induction heating power supply with a frequency of 600kHz and a power of 10-100kW to generate high-frequency current. When a high-frequency current passes through the inductor, a high-frequency alternating magnetic field is generated, and an induced current is generated in the welded metal in the inductor. The high frequency heating speed is very fast and can be heated to a very high temperature in a short time to melt the solder. Most of the inductors used in high-frequency induction brazing are made of red copper tubes with a diameter of 5-10mm. Whether the geometry and size of the inductor is appropriate is one of the important factors that determine the heating speed, temperature uniformity, production efficiency and brazing quality of high-frequency induction brazing.

Before welding, the output power of the high-frequency equipment should be adjusted according to the size of the welding tool, so that the workpiece heating speed is moderate and the temperature is uniform. If the power is too high, it will cause partial overheating of the workpiece and incomplete solder melting, which will easily cause cracks in the cemented carbide; if the power is too low, the heating time will be too long, which will easily cause oxidation of the cutter body and affect production efficiency. Generally, the heating rate of welding is 30-60℃/s, and the heating rate of tungsten-titanium-cobalt alloy should be 10-40℃/s.

High-frequency induction brazing has fast heating speed, high efficiency, simple operation and better working conditions. It is suitable for large-volume automatic or semi-automatic brazing. However, equipment investment is large and power consumption is high.

③ Contact brazing is carried out on a brazing machine or butt welding machine specially used for brazing hard alloy tools. The voltage of the secondary coil of the welding transformer is less than 36V and the current is above 1000A. During brazing, the workpiece is clamped between two copper electrodes. When the strong current output by the secondary coil passes through the welding workpiece, the heat generated by the contact resistance between the cemented carbide and the steel substrate is used as the welding heat source to melt the solder.

Contact brazing is often used for brazing tools such as turning tools and planers. The welding efficiency of contact brazing is high, and it only takes 4 to 5 minutes to weld a large cross-section cemented carbide turning tool and planer. During the heating process, the power is cut off for 1 to 2 times until the bright liquid solder fills the entire weld. Due to the short heating time, the oxidation and thermal deformation are small, and the operation is convenient. However, the electrode is easy to burn the surface of the workpiece during the heating process, and sometimes the electrode or cemented carbide surface is not cleaned, or the resistance of the contact surface is too large to conduct heating.

④ Copper immersion brazing is to immerse the brazing part of the workpiece into the molten liquid solder, and use capillary action to make the liquid solder penetrate along the weld seam of the workpiece to achieve the purpose of brazing. Dip brazing usually uses a salt bath furnace, a coke furnace or an oil furnace as the heat source, and the brazing filler metal and flux are placed in a graphite or heat-resistant stainless steel crucible and heated to a liquid state. This brazing method is suitable for mass production of various moraine alloy tools and cemented carbide drill bits for drilling. The brazing of multi-blade carbide tools can be completed by heating at one time, and the brazing efficiency is higher.

The hard alloy multi-blade tool using copper immersion brazing requires that the blade groove has a 0.3-0.4mm deep craft wall for holding the blade, and the blade and the blade groove should match well. Before assembling, carefully clean the blade and knife groove with carbon tetrachloride, riveting the blade with a sharp piece, and then apply a protective coating layer with a thickness of 1 to 3 mm on the non-welded surface of the tool 2mm away from the weld. After being dried in the shade, put it in an oven at 250～300℃ and bake for 30min before welding.