Cast iron braze welding can be used for the repair of breaks in iron castings, from very small castings to those weighing several tons. The term brazing includes metal arc processes and oxyacetylene braze welding.
Castings must be thoroughly clean and free of graphite; they must be preheated before brazing is applied. Joints are prepared for brazing in the same way as for welding. See CAST IRON; ARC WELDING, JOINT PREPARATION; and PREHEAT.
When braze welding, a “black heat” is all that is necessary (instead of the dull red heat necessary when fusion-welding with the torch and cast iron rods). There are a number of bronze welding rods on the market which may be used for brazing cast iron.
A flux should be used when braze welding cast iron with the oxyacetylene process. The torch flame should be either neutral, slightly carburizing or slightly oxidizing, depending on the recommendations of the rod manufacturer for the type of rod and the specific application.
It is good practice to position the pieces being braze welded so that the weld is made uphill. A short length of seam, about 5 cm (2 in.), is heated with the torch, the rod is dipped into the flux, and this area tinned. It is easy to recognize when the bronze is tinning well because of the manner in which it flows over the hot casting. If it does not flow readily, tinning is not being properly accomplished; the flux should be placed so that the metal ahead of the molten film of bronze is covered. If tinning becomes difficult, as is sometimes the case with an old casting, it is often possible to file the surface with a coarse file while the surface is hot so that the bronze will adhere.
After a two-inch section of the joint has been welded, another two-inch section is tinned, and subsequently built up until the joint is completed. Any tendency of the bronze to become liquid and run can always be prevented by drawing the torch away immediately. This enables the welder to control the bronze pool and to make an acceptable weld.
When braze welded in this manner, the strength of this joint is such that if the weld is broken, pieces of the bronze will actually pull areas of the cast iron out of the cast iron base metal, indicating that the weld is stronger than the cast iron base.