Welding Cast Iron
Author: Harold Sharon
Whenever this subject comes up no one wants to talk
about anything else. Welding is a complex, and controversial topic, and welding cast iron is especially complex.
To use the proper technical terms, cast iron can be soldered or brazed, as well as welded. Because a lower temperature is reached, these methods are less risky than welding. Cast iron has so much carbon in the alloy that there are tiny flakes of graphite on the surface that is prepared for repair. An oxidizing flame is all that is needed to vaporize this graphite, then with a bit of appropriate flux, soldering or brazing can be done quite conventionally. De-oxidized copper bronze, or brass rod can be used. (bronze is much stronger than brass.) The ductility of any of these materials will allow the repair to stretch, rather than crack, as the opposite sides of the iron contract during cooling
Now let's talk about welding. Stainless steel and cast iron rod are both popular. The most serious problem with stainless is its great hot strength. As the weld metal is solidifying it has enormous strength at, say, 1800 degrees. Although cast iron has a higher melting temperature than stainless steel it has no appreciable strength at "red" heat so as the stainless weld shrinks in cooling, the cast iron tears the adjacent weld. This area is called the HAZ.
HAZ in welding literature means the Heat Affected Zone. High temperature weakness is one of many problems of metals in the HAZ. . While welding you are, in reality, heat treating, and altering the grain structure of the metal, usually for the worse.
Cast iron rod shares the hot weakness of the piece being welded so usually the base metal in the HAZ won't tear; the weld itself will crack along its center. If the part is small and can all be heated red hot (about 1200 degrees) excellent welds are possible. The repairs that car people usually need to make are on engine blocks or gear housings and heating these big parts is a problem! If the part has been finish machined welding is likely to cause distortion that will make re-machining necessary. If stainless or cast iron welds can be hammered while red hot, to squash the weld and widen it while the parent metal is cooling and pulling away, excellent results can be had. Picture the delicate logistics: one man welding, one hammering a half inch away from the arc (or flame)
Steel rod is useless for cast iron welding because it would instantaneously acquire, from the cast iron, enough carbon to make it more brittle than glass. Solder, braze, and and stainless alloys also acquire carbon but are not carbon hardenable. It would be nice if all the stress cracks incurred while welding cast iron could be avoided by "stress - relieving". It is possible but it must be done before the weld cools.
When the cracks occur the game's over. The best advice for someone who needs a block repaired: go to a welder who has a record of success with this difficult alloy.
For more of Harold's Great technical knowledge, check out His Book