1. Brazing Materials
(1) Filler metals
Copper-zinc filler metals and silver-copper filler metals are mainly used for brazing cast iron. Common grades of copper-zinc filler metal include B-Cu62ZnNiMuSiR, B-Cu60ZuSnR and B-Cu58ZnFeR, etc.
The tensile strength of cast iron welded joints is generally between 120-150 MPa. By adding elements such as Mn, Ni, Sn and Al to copper-zinc filler metals, the strength of welded joints can be equivalent to that of the base material.
Silver-copper filler metals have low melting temperatures and can prevent the formation of harmful structures during brazing of cast iron. The performance of welded joints is good, especially with the addition of Ni, such as B-Ag50CuZnCdNi and B-Ag40CuZnSnNi.
These strengthen the bond between the filler metal and the cast iron, making them particularly suitable for brazing ductile iron, achieving joint strength equivalent to that of the base material.
(2) Flow
When brazing cast iron with copper-zinc filler metals, fluxes FB301 and FB302 are mainly used, which are mixtures of borax or a mixture of borax and boric acid. Furthermore, fluxes composed of 40% H3BO3, 16% Li2CO3, 24% Na2CO3, 7.4% NaF and 12.6% NaCl show better effects.
When brazing cast iron with silver-copper filler metals, fluxes FB101 and FB102 can be selected, which are mixtures of borax, boric acid, potassium fluoride and potassium fluoroborate.
2. Brazing Techniques
Before brazing cast iron, it is necessary to carefully remove impurities such as graphite, oxides, sand and oil from the surface of the casting.
Organic solvents can be used to remove oil, while mechanical methods such as sandblasting or shot blasting can be used to remove graphite and oxides. Electrochemical methods can also be used. Additionally, graphite can be removed by burning it with an oxidizing flame.
Brazing cast iron can be done using flame, furnace, or induction heating methods. Due to the formation of SiO 2 on the surface of cast iron, the brazing effect in a protective atmosphere is not good, so flux is generally used for brazing.
When brazing large parts with copper-zinc filler metals, a layer of flux should be spread on the cleaned surface and then the part should be heated in a furnace or with a torch.
When the workpiece is heated to about 800°C, additional flux must be added and heated to brazing temperature. The filler metal must be melted and filled into the gap by scraping with a needle along the edge of the joint. To improve the strength of the welded joint, annealing treatment should be carried out at 700-750°C for 20 minutes, followed by slow cooling.
Excess flux and residue after brazing can be removed by rinsing with warm water. If it is difficult to remove, it can be washed with 10% sulfuric acid solution or 5-10% phosphoric acid solution, followed by rinsing with clean water.
