Selection of cemented carbide solder

Brazing fillers for cemented carbide and steel can be divided into three categories according to melting point and brazing temperature: high temperature filler metal, normal temperature filler metal and low temperature filler metal.

Brazing temperature above 1000 °C is called high temperature brazing filler metal, such as copper and 106 brazing filler metal; brazing temperature of medium temperature brazing filler metal is between 850 and 1000 °C, such as H62 and H68 brass filler metal; low temperature brazing filler metal refers to brazing temperature between 650 and 850 °C, such as B-Ag-1 and L-Ag-49 silver-containing filler metal.

The brazing temperature of copper filler metal is high and the strength of the weld is low, so it is mostly used in vacuum brazing. Pure copper brazing filler metal belongs to single phase structure. It is easy to control brazing temperature. It has good wettability for all kinds of cemented carbides, good plasticity and the cheapest price.

The shear stress of brazed seam of copper is about 150 MPa, which can be used below 400 °C. The brazing temperature of H68 brass is much lower than that of copper, but it is not often used because of the low strength of the weld. The melting point and brazing temperature of H62 brass are relatively low, and the welding seam has a certain room temperature strength, so it is a common cemented carbide filler metal.

Generally used for brazing cemented carbide tools under medium and small load conditions. 105 brazing filler metal should be used in the case of small weld or weld surface requiring high temperature strength. L-Ag-49 low-temperature silver solder is widely used abroad because of its low melting point (690-710 °C), good wettability to cemented carbide, easy brazing and low stress. When necessary, copper sheet can also be used as compensation gasket, which can almost completely eliminate brazing stress and avoid brazing cracks.

It can be used to braze some crackable cemented carbides or some large brazing surface cemented carbide tools. As the strength of the weld decreases rapidly with the increase of the service temperature, the working temperature of the workpiece brazed with L-Ag-49 silver filler metal should be limited to less than 200 °C. The fluoride and chloride in the flux used with L-Ag-49 silver filler metal are high, which requires higher cleaning work after welding, otherwise the surface of workpiece will be corroded because of unclean cleaning.

B-Ag-1 solder is a kind of ultra-low temperature silver solder. Its melting point is about 600-610 °C, which can further reduce the residual stress of cemented carbide joints. It can also be used as compensation gasket to braze some fragile workpieces.

Because B-Ag-1 silver solder has ultra-low melting point and good wettability to tungsten carbide, it is also suitable for brazing some diamond tools such as diamond saw blades.

However, B-Ag-1 silver solder has high price and low high temperature strength, and is only suitable for use at temperatures below 150 °C. The content of cadmium in the brazing filler metal is 24%. When the brazing temperature is high, it is easy to evaporate, which is harmful to human health. In addition to controlling the brazing temperature, the exhaust device should be installed at the brazing operation.

After brazing, the workpiece should also be cleaned to avoid corrosion.