Welding cast iron with a torch and cast iron welding rods can readily be accomp1ished.Preheating the weld area or the entire casting is required, except when very small castings are involved. See PREHEAT.
The same techniques for cleaning and preparing the casting for welding that are used for other processes are used with gas welding. See CAST IRON, ARC WELDING.
The cast iron weld joint is prepared and preheated. A neutral oxyacetylene flame is used, along with a cast iron welding flux, which is essential to break down the surface oxide and increase the flowing qualities of the metal. Assuming that the weld is to be made on a V-joint, the torch is applied to the edges of the V, and the cast iron welding rod is heated and dipped into the
flux, which adheres to it. The sides of the V are melted down, and the molten rod is added to the puddle.
A short section of the weld is built up in this manner; successive sections are added until the weld is finished. With cast iron, it is often good practice to begin at the center of a weld and back the torch out to the edge of the weld. Then the flame is always pointing toward the completed weld, and as the edge is approached, the metal can be controlled to good advantage. The edge can be built up square, or to the appropriate shape. If the torch is pointing toward the edge, the hot metal is likely to be driven over the edge by the force of the flame and will produce a weld with a ragged, incomplete finish.
Manipulating the torch helps finish the weld. If the torch is withdrawn momentarily from the hot puddle, the weld metal will instantly solidify, enabling the welder to control the metal and make a neat, square edge weld. The opposite side of the weld can be completed using the same procedure.
Porosity in a weld is formed by gas entrapment during solidification. The cavities or holes (blowholes) which appear in the molten puddle during welding are usually the result of overheating the metal, or holding the flame in one place too long, thus driving the gases produced by the flame into the molten cast iron. Porosity can also be caused by gas produced in the metal
during overheating. If gases are allowed to remain in the metal, the weld will be porous and will lack strength.
Porosity in cast iron welds can be avoided in many cases by correct manipulation of the torch. The flame should be directed so that it is not pointed at the molten puddle for more than a moment, then it is quickly moved to a colder part of the seam, or to the rod as it is held in the molten metal. Standards for welding cast iron are contained in ANSYAWS D11.2, Guidefor Welding Iron Castings.