Preparations before vacuum brazing
1- Before vacuum brazing, check whether the cemented carbide has cracks, bending or unequal defects. The brazing surface must be smooth, if it is spherical or rectangular, the brazing surface of cemented carbide should also conform to a certain geometric shape to ensure good contact between the alloy and the matrix, so as to ensure the brazing quality.
2- Sandblasting treatment of cemented carbide. Without sandblasting equipment, the cemented carbide can be held by hand, and the oxide layer on the brazing surface and the Black Brand letters can be ground on the rotating green silicon carbide grinding wheel. If the oxide layer on the brazing surface of cemented carbide is not removed, the filler metal is not easy to wet the cemented carbide. Experience has proved that if there is oxide layer or black brand letters on the brazing surface, sandblasting treatment should be carried out. Otherwise, the brazing filler metal is not easy to wet the cemented carbide, and obvious black letters still appear in the brazing seam, which reduces the brazing area and causes the phenomenon of de-welding.
3- When cleaning the brazing surface of cemented carbide, it is better not to use chemical mechanical grinding or electrolytic grinding methods, because they rely on the binder (cobalt) which corrodes the surface layer of cemented carbide to speed up grinding or improve grinding efficiency. When the cobalt on the surface of cemented carbide is corroded, it is difficult for the solder to wet the cemented carbide again and easy to cause debonding phenomenon. The material surface must be cleaned before vacuum brazing.
The method is to apply W3.5 diamond abrasive paste on the cast iron block and grind the brazing part of the cemented carbide until it is bright. The surface treatment of 40Cr steel is also carried out: firstly, the surface dirt is removed on W 150 sand paper, and then polished on W 40 sand paper. Solder should also be polished on W150 sandpaper to make the surface clean.
Finally, 40Cr steel, YG8 cemented carbide and filler metal are washed in acetone to remove oil stains on the surface. After the treatment, the brazing filler metal is placed between the cemented carbide and 40Cr steel (the cemented carbide is on the top), and then placed in the fixture. In special cases, when the cemented carbide brazing surface must be treated by the above method or WEDM, the treated cemented carbide can be sandblasted or ground with silicon carbide grinding wheel to remove the surface layer. The cemented carbide after sandblasting can be cleaned with gasoline and alcohol to remove oil pollution.
4- Before vacuum brazing, the groove shape on the steel base should be carefully checked. Especially for crackable cemented carbide and large brazed surface cemented carbide workpieces, more stringent requirements should be required. The knife groove is also sandblasted and cleaned to remove oil stains. When the amount of cleaning is large, alkaline solution can be used to boil for 10-15 minutes. High frequency or copper dipping brazing multi-edged cutting tools and complex measuring tools, it is best to boil with saturated borax solution for 20-30 minutes, take out and dry before welding.
5- The filler metal is wiped off with alcohol or gasoline before use, and is cut and formed according to the brazing surface. When brazing cemented carbide tools or dies, the thickness of filler metal is about 0.4-0.5mm, which is similar to the size of the brazing surface. When the brazing furnace is heated with continuous nitrogen protection, the brazing filler metal can be increased appropriately.
When brazing cemented carbide multi-edged cutting tools and measuring tools, the area of brazing sheet should be reduced as far as possible. Generally, the brazing sheet can be cut into about 1/2 of the brazing surface. When the brazing technology is skilled, the brazing sheet can be reduced to 1/3 or less of the brazing surface. Reducing the solder filler metal can make the workpiece appearance beautiful and grinding more convenient.