Brazing of cast iron

1. Brazing material

(1) Brazing filler metal cast iron brazing mainly adopts copper zinc brazing filler metal and silver copper brazing filler metal. The commonly used copper zinc brazing filler metal brands are b-cu62znnimusir, b-cu60zusnr and b-cu58znfer. The tensile strength of the brazed cast iron joint generally reaches 120 ~ 150MPa. On the basis of copper zinc brazing filler metal, Mn, Ni, Sn, AI and other elements are added to make the brazed joint have the same strength with the base metal.

The melting temperature of silver copper brazing filler metal is low. Harmful structure can be avoided when brazing cast iron. The brazing joint has good performance, especially the brazing filler metal containing Ni, such as b-ag50cuzncdni and b-ag40cuznsnni, which enhance the binding force between the brazing filler metal and cast iron. It is especially suitable for the brazing of nodular cast iron, which can make the joint have the same strength with the base metal.

(2) When copper and zinc are used for brazing cast iron, fb301 and fb302 are mainly used, that is, borax or a mixture of borax and boric acid. In addition, the flux composed of h3bo340%, li2co316%, na2co324%, naf7.4% and nac112.6% is better.

When brazing cast iron with silver copper filler metal, fluxes such as fb101 and fb102 can be selected, i.e. the mixture of borax, boric acid, potassium fluoride and potassium fluoroborate.

2. Brazing technology

Before brazing cast iron, graphite, oxide, sand, oil stain and other sundries on the casting surface shall be carefully removed. Organic solvent scrubbing can be used to remove oil stains, while mechanical methods such as sand blasting or shot blasting, or electrochemical methods can be used to remove graphite and oxides. In addition, graphite can be removed by burning it with an oxidizing flame.

Brazing cast iron can be heated by flame, furnace or induction. Since SiO2 is easy to form on the surface of cast iron, the brazing effect in protective atmosphere is not good. Generally, brazing flux is used for brazing. When brazing large workpieces with copper zinc brazing filler metal, a layer of brazing flux shall be sprayed on the cleaned surface first, and then the workpieces shall be put into the furnace for heating or heated with a welding torch. When the workpiece is heated to about 800 ℃, add supplementary flux, heat it to the brazing temperature, and then scrape the needle material at the edge of the joint to melt the solder and fill the gap. In order to improve the strength of brazed joint, annealing treatment shall be carried out at 700 ~ 750 ℃ for 20min after brazing, and then slow cooling shall be carried out.

After brazing, excess flux and residue can be removed by washing with warm water. If it is difficult to remove, it can be cleaned with 10% sulfuric acid aqueous solution or 5% ~ 10% phosphoric acid aqueous solution, and then cleaned with clean water.