Cast iron is a strong but brittle metal, and it is prone to cracks and breaks if it experiences large thermal or physical stresses. Cast-iron pieces can be repaired, however, using brazing techniques.
Methods for Repairing a Cast-Iron Break
Brazing is a fabrication process that can be used to repair cracked or broken cast-iron parts. Brazing is similar to welding, and some of the same tools are used, but the base material is not melted. The brazing material is melted to coat the edges of each side of the crack or break, and it acts like an adhesive to hold the parts together or to fill the crack.
Cast iron can be welded, but it is extremely difficult. For most parts and applications, brazing is a satisfactory method for repairing cracked or broken cast-iron parts. Brazing also results in better tolerances in the cast-iron part than welding because the intense heat of the welding process can warp or crack the cast iron.
Preparing Cast Iron for Repair
A filler material has to be selected for brazing. For most applications, nickel is used for brazing cast iron. Other materials, such as stainless steel or brass filler, however, can be used, depending on the desired appearance and applications.
The first step in preparing the cast iron for repair is to grind the edges of the part where it will be brazed. Grinding removes any oxides, coatings or other impurities from the surface of the cast iron. The grinding process also can introduce some particles from the grinder, however. So after grinding, use a stainless steel brush to remove any dust from the surface thoroughly.
After the surface has been prepared, the cast-iron part has to be heated. By heating the part, large thermal gradients between the brazing location and adjacent metals are avoided. Cast iron must be heated slowly to avoid further cracking. An oven or gas grill can be used to bring the cast-iron part up to temperature.
Brazing a Cast-Iron Break
If the filler rod does not contain flux, apply a paste or powder flux to the surfaces to be brazed. Use a torch to heat the brazing surfaces to a red-hot glow. The cast iron should be hot enough to allow the filler rod to flow like lead solder.
After the crack or break has been brazed, the part must be allowed to cool very slowly. Rapid cooling can cause the cast iron to crack further. The hot cast-iron part should be placed in a container and surrounded by clean, packed sand or lime. The sand acts as an insulator to help the cast-iron part to cool very slowly, over a period of days.