Cast iron is a strong, durable, and inexpensive material that is frequently used in a variety of home, commercial, and industrial settings. Despite its strength, cast iron is brittle, and can break easily. Cast iron that is dropped or subjected to other sudden forces can crack or break, and large temperature changes can also cause parts to break. Welding, a repair technique often associated with metals, is difficult to perform on cast iron. However, brazing can be used to create strong repairs in broken cast iron parts. Like welding, brazing uses heat to apply a filler material to connect two parts together. However, brazing uses less heat than welding, and the base material is not allowed to melt. The filler material is like a glue that coats the cast iron parts and bonds them together.
Select a filler rod based on the application. Filler rods for cast iron brazing are generally nickel or brass, and may or may not contain flux. Some fabricators use a separate powder or paste flux.
Prepare the edges of the broken pieces. The edges have to be cleaned before brazing to remove dirt, oils, and oxides that have built up on the surface. The edges should be ground out using a grinder so that the two pieces together have a U-shaped groove between them, allowing the filler material to flow into the groove.
Use a stainless steel brush to thoroughly clean away any dust or particulates left over from grinding.
Use a heat source to heat up the cast iron pieces. Depending on the size of the pieces, an oven or gas grill can be used to heat the parts.
Arrange the cast iron pieces on a work surface, and support the pieces so they are aligned properly. If the pieces are large, use a heater to maintain the temperature of the pieces.
Heat the edges of the broken pieces with the torch until they are bright red in colour. Use the filler rod to fill in the gap; the temperature of the cast iron pieces should be hot enough to melt the filler rod. Progress along the gap until the whole gap is filled. Use the torch to maintain the temperatures of the cast iron at the gap.
After the brazing is complete, put the part into a container full of sand. Make sure the part is completely covered and packed in with sand. This allows the part to cool very slowly and prevents cracking. The part should be allowed to cool for at least one day, or longer for large parts.
When brazing, use protective equipment, such as gloves, coveralls, and eye protection, to protect from the hot temperatures of the parts and torch.