Welding Cast Iron

Cast iron cracks if you look at it wrong during welding. The 2-4% carbon content creates a heat-affected zone that forms hard, brittle martensite when it cools too fast. Successful cast iron repair is 80% heat management and 20% welding.

Preheat is the single most important step. Heat the entire casting to 500-700F (or as high as 1200F for large, heavily-restrained pieces) before striking an arc. Use a rosebud torch and a temperature crayon or infrared thermometer to verify. Skip the preheat and the repair will crack, sometimes before you even finish welding.

Nickel-based electrodes are the standard for cast iron repair. ENi-CI (99% nickel) produces a soft, machinable deposit and works best on thin sections and finish surfaces. ENiFe-CI (55% nickel, 45% iron) is cheaper, has higher strength, and handles thicker sections better. Both run on DCEP at low amperage with short stringer beads, 1 inch maximum, followed by peening each bead while still red to relieve shrinkage stress.

Short beads and skip welding prevent concentrated heat buildup. Weld a 1-inch bead, peen it, let it cool to a temperature you can still touch with a gloved hand, then weld the next bead in a different area. This distributes thermal stress across the casting instead of concentrating it.

After welding, slow cooling is critical. Cover the casting in dry sand, vermiculite, or a welding blanket and let it cool over 12-24 hours. Forced cooling (air blast, water quench) guarantees cracks. Patience is literally what separates a successful cast iron repair from a broken one.