Welding cast iron – repairing cracked castings
Cast iron has a reputation for being unweldable: you repair the crack, and once the part has cooled a new crack has appeared alongside it. The cause is almost always heat that changes too fast and too unevenly. Gas welding is therefore one of the safest methods for cast iron — the whole process is built on controlled preheating and slow cooling.
Why does cast iron crack?
Cast iron is hard but brittle, and won't tolerate the stresses created by rapid temperature differences. An arc welder puts a lot of heat into a very small area. A gas flame lets you preheat the whole region evenly instead, weld at the right temperature, and then let the part cool slowly.
How to do it
- Clean and V-out the crack. Grind the crack into a V and drill a small stop hole at each end so it can't travel.
- Preheat the whole part evenly to a dull red — a larger nozzle or a heating nozzle is a great help here.
- Apply cast iron flux to dissolve oxides and give a clean, fluid pool.
- Weld with cast iron rod in grey iron — the same material as the part, giving a repair with matching properties and machinability.
- Let it cool slowly — bury the part in sand or vermiculite. Slow cooling is the whole secret.
What you'll need
- Dillon Standard Kit — the torch with nozzles for both preheating and welding.
- Cast iron rod (4 or 6 mm depending on thickness) and cast iron flux.
- Alternative method: brass rod with bronze/copper flux for brazing at lower temperature — the gentlest option for brittle parts.