It is very difficult to braze cast iron on account of more carbon percentage in metal and the oxide formation during heating. Principle employed to braze cast iron is to heat the work first and to cause alloy to melt by placing it in contact with the hot work. The process is simple provided certain fundamental principles are followed.
The brazing spelter used for cast iron brazing consists of Cu 60% and Zn 40%. The parts to be joined are first cleaned either by filing, grinding, scrapping or sand blasting. A special flux, comprising of boric acid 16 ozs, pulverized potassium chloride 4 ozs and carbonate of irons 3 ozs thoroughly mixed and kept dry is used for brazing.
It is very essential to clean the cast iron parts before brazing and then to secure the parts in position by means of clamps, bolts, wires, or pegs to prevent movement during brazing operation.
To make a joint, the parts are heated to bright yellow colour. It is important not to exceed a temperature of 11000C in this process. After joints have been filled with molten spelter, remove the work from heat source and allow it to cool down slowly. Covering the work with the fine ashes will prevent sudden cooling. Casting may be severely stressed internally and may ultimately fracture if the work is cooled quickly.