Welding cast iron: nickel electrodes and cold repair

Cast iron contains two to four per cent carbon — ten times what weldable steel carries. Everything about welding it is an attempt to work around that one number.

Why it cracks

Three reasons, all rooted in the carbon.

The heat-affected zone turns to white iron. Rapid cooling converts the graphite-bearing structure into hard, brittle cementite. That zone is glass-hard, cannot be machined, and cracks under the shrinkage stress of the weld itself.

Cast iron has almost no ductility. Steel accommodates shrinkage by yielding a little. Cast iron does not yield; it breaks.

Old castings are contaminated. Decades of oil, coolant and combustion products soak into the porous surface, and they come out as gas during welding. An engine block that looks clean is not.

Cold welding — the workshop method

Nickel electrodes, low current, short runs, peening, and never letting the part get hot. The nickel deposit stays soft and ductile, so it absorbs the shrinkage instead of transmitting it into the casting.

• Electrode: pure nickel (Ni 99) for machinable repairs, nickel-iron (NiFe) for stronger joints and for ductile iron. NiFe is the more forgiving of the two on unknown material.
• Current: at the very bottom of the range for the diameter — around 25 A per millimetre rather than 35.
• Runs: 25–40 mm at a time, scattered around the joint rather than sequential.
• Peen each run immediately with a round-nosed hammer while it is still hot.
• Let it cool to hand temperature before the next run. If you cannot hold your hand on the casting, wait.

Hot welding

Preheat the whole casting to 400–600 °C, weld it with a cast-iron filler rod, and cool it slowly under insulation or in a closing furnace over several hours. The result is machinable and matches the parent metal — and it requires an oven, so it belongs to a foundry or a specialist repair shop rather than a bench.

The critical part is the cooling, not the welding. Cool a preheated casting in air and you will have produced the exact hardened structure the preheat was meant to prevent.

Preparation

Find the ends of the crack — cast iron cracks run further than they look — and drill a small stop-hole just past each end. Grind the crack out to a V, completely through. Then burn the contamination out: run the arc over the prepared surface without filler, which drives oil and gas out of the pores. It will smoke, and the weld metal will sit far better afterwards.

What to expect

A cold nickel repair to a bracket, a housing or a manifold is a genuine repair that will hold. A cold repair to a part that must be machined afterwards will disappoint, because the heat-affected zone is harder than the cutter. And a crack through a heavily loaded section of an old casting may simply not be worth welding: replacement is often cheaper than the third attempt.

Updated: 14 August 2026