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Carbide Vacuum Brazing Process

Common process methods of vacuum brazing

The main technological parameters of the vacuum brazing process are brazing temperature and holding time. The brazing temperature is usually selected to be 25-60°C higher than the liquidus temperature of the solder to ensure that the solder can fill the gap.

The brazing holding time depends on the size of the workpiece and the intensity of the interaction between the brazing filler metal and the base metal. The holding time of large pieces should be longer to ensure uniform heating. If the solder has a strong interaction with the base metal, the holding time should be short. Generally speaking, a certain holding time is necessary to promote the mutual diffusion of the solder and the base metal to form a firm bond. But too long holding time will lead to defects such as melting and corrosion. It is very important to clean the surface before vacuum brazing to improve the wettability of the solder on the cemented carbide. If necessary, measures such as copper or nickel plating on the surface can also be taken.

Joints in cemented carbide brazing are prone to cracks. This is because its linear expansion coefficient is only half that of low-carbon steel. When cemented carbide is brazed with the substrate of this type of steel, a large thermal stress will be generated in the joint, resulting in cracking of the joint. Therefore, when cemented carbide is vacuum brazed with different materials, measures should be taken to prevent cracking. When vacuum brazing the cemented carbide blade and the steel cutter bar, it is advisable to adopt the method of increasing the gap between the brazing seam and applying a plastic compensation gasket in the brazing seam, and slowly cooling after welding to reduce the brazing stress and prevent Cracks are generated, prolonging the service life of carbide tool components.

Carbide vacuum brazing steps

1) Preparation before welding

① Before welding, check whether the cemented carbide has cracks, bends or uneven defects. The brazing surface must be flat. If it is a spherical or rectangular cemented carbide brazing surface, it should also conform to a certain geometric shape to ensure good contact between the alloy and the substrate to ensure the quality of vacuum brazing.

②Sandblasting the cemented carbide. If there is no sandblasting equipment, the cemented carbide can be held by hand, and the oxide layer and black brand letters on the brazing surface can be ground on the rotating green silicon carbide grinding wheel. If the oxide layer on the brazing surface of the cemented carbide is not removed, the solder is not easy to wet the cemented carbide. Experience has proved that if there is an oxide layer or black grade letters on the brazing surface, it should be sandblasted, otherwise the brazing filler metal is not easy to wet the cemented carbide, and obvious black letters will still appear in the brazing seam, which will reduce the brazing area and cause Desoldering phenomenon.

③ When cleaning the cemented carbide brazing surface, it is best not to use chemical mechanical grinding or electrolytic grinding, because they rely on the binder (cobalt) that corrodes the surface layer of the cemented carbide to speed up grinding or improve grinding efficiency Yes, but after the cobalt on the surface of the cemented carbide is corroded, it is difficult for the solder to re-wet the cemented carbide, which is likely to cause desoldering. In special cases, when the cemented carbide brazing surface must be treated by the above method or wire electric discharge cutting, the treated cemented carbide can be sandblasted or the surface layer can be ground off with a silicon carbide grinding wheel. The cemented carbide after sandblasting can be cleaned with gasoline and alcohol to remove oil stains.

④Before vacuum brazing, carefully check whether the groove shape on the steel substrate is reasonable, especially for hard alloy workpieces of easy-to-crack grades and hard alloy workpieces with large brazing surfaces, which should be strictly required. The sipes are also sandblasted and cleaned to remove oil. When the cleaning volume is large, an alkaline solution can be used to boil for 10-15 minutes. For multi-edged knives and complex measuring tools brazed by high-frequency or immersion copper, it is best to boil them with saturated borax aqueous solution for 20-30 minutes, take them out and dry them before welding.

⑤Wipe the solder with alcohol or gasoline before use, and cut it according to the brazing surface. When brazing ordinary hard alloy tools or molds, the thickness of the brazing filler metal is about 0.4-0.5mm, which is more suitable, and the size is similar to the brazing surface. When heating with a coke oven, the solder can be appropriately increased. When brazing hard alloy multi-edged tools, measuring tools and other workpieces, the area of the brazing sheet should be reduced as much as possible. Generally, the brazing sheet can be cut into about 1/2 of the brazing surface. The brazing sheet is reduced to 1/3 or less of the brazing surface. Reducing brazing material can make the appearance of workpiece beautiful after welding, and sharpening is more convenient.

(2) Vacuum brazing process

Whether the vacuum brazing process of cemented carbide tools is correct plays a vital role in the welding quality. The heating rate has a significant effect on the welding quality. Rapid heating will cause cracks and temperature unevenness in the cemented carbide sheet; but if the heating is too slow, it will cause surface oxidation and reduce the strength of the joint.

(3) Cleaning after welding

The welded carbide workpiece should be cleaned after welding to remove the residual flux around the weld, otherwise the excess flux will block the grinding wheel when the tool is sharpened, making grinding difficult. The residual flux after welding will also corrode the weld and the substrate. The commonly used removal method is to put the cooled workpiece after welding into boiling water for about 1 to 2 hours, and then perform sandblasting treatment to remove the residual flux and oxides adhered around the weld; or put the workpiece into the Pickling is carried out in the pickling tank (the concentration of hydrochloric acid is 1:1), the pickling time is about 1 to 4 minutes, and then put into the cold water tank and hot water tank for repeated cleaning.

(4) Brazing quality inspection

Mainly check the quality of the cemented carbide and steel brazing joints and whether there are cracks in the cemented carbide. The normal welding seam should be uniform without black spots, the welding seam not filled with solder should not exceed 10% of the total length of the welding seam, and the width of the welding seam should be less than 0.15mm. The brazing of the blade is skewed, and those that do not meet the requirements of the drawing should be re-welded. The crack tendency of cemented carbide inserts can be checked by the following methods.

① After the tool is sandblasted, clean it with kerosene first, and then observe it with the naked eye or a magnifying glass. When there is a crack on the blade, there will be a noticeable black line on the surface.

② Make a solution with 65% kerosene, 30% transformer oil and 5% turpentine, add a small amount of Sudan Red, soak the pre-inspected tools in the solution for about 10-15 minutes, take it out and wash it with clean water, and apply it A layer of kaolin, check the surface after drying, if there are cracks on the tool, the color of the solution will be displayed on the white clay, which can be clearly seen with the naked eye.