Why Do You Heat Cast Iron Before Welding It?

Cast iron can be very difficult to weld, especially if done improperly. It has a very high carbon content, up to ten times that of regular steel, which contributes to its brittleness.

Cooling

Rapid cooling is the downfall for cast iron. If it is allowed to get hotter than about 1450 degrees and then rapidly cool, cracking is sure to occur.

Preheat Entire Cast

Welding will bring cast iron well above the 1450-degree mark, so it is necessary to preheat. When possible, preheat the entire casing instead of just the portion to be welded.

Heat Before and After

Preheating the cast iron will slow the cooling rate after it is welded. Heating the iron up both before and after welding is a good idea to reduce the risk of cracking.

Heat Evenly

Preheat the cast iron slowly and as evenly as possible. The goal is to gradually bring the cast iron up to about 700 to 1200 degrees.

Low Amperage

Weld the iron with as low an amperage as possible and in short bursts. The recommendation is 1 to 2 inches at a time, allowing the casting to cool a bit between arcs.

References

• "Welder's Handbook, RevisedHP1513: A Guide to Plasma Cutting, Oxyacetylene, ARC, MIG and TIG Welding;" Richard Finch; 2007