How to Gas Weld Cast Iron

Written by chris stevenson
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How to Gas Weld Cast Iron
Always use the proper welding safety equipment when working with a gas welder or any welding device. (welding image by glgec from

Welding cast iron can be challenging if you do not know the correct process and the properties of the metal. Cast iron contracts and expands with temperature all the time. Cast iron has brittle properties that allow it to crack easily, and it rusts easily. Several kinds and grades of cast iron exist, making it difficult to know its carbon content. However, cast iron can be successfully welded with a gas welder, called an oxyacetylene system, along with some special welding rods and proven welding techniques.

Skill level:

Things you need

  • Drill motor
  • Conical grinding bits
  • Gloves
  • Safety goggles
  • Wire brush
  • Old barbecue (optional)
  • Welding hood
  • Oxyacetylene welding system
  • Respirator (optional)
  • Hydrochloric acid (optional)
  • Tinning flux
  • Flux brush
  • Braze welding rod (copper alloy filler)
  • Slag hammer

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  1. 1

    Place the cast iron piece on a firm bench, in a vice or on the floor. Make certain that no combustible materials are within range of any sparks produced from grinding. Place a conical, pointed grinding bit in a drill motor. Don gloves and safety goggles. For a crack or break in the stock iron piece, turn the drill on and grind out a V-groove channel inside the crack until you have removed all rust and reached bare, shiny metal.

  2. 2

    Grind the outside edges of the crack area for at least two inches on all sides. There should be nothing but bare, shiny metal showing. Inspect the ground-down area for pits or holes in the iron. You might see small pressure cracks. For iron in this condition, move the piece to a well ventilated area outside. Don a respirator and pour a few ounces of hydrochloric acid on the crack and move away from it. Let it air dry.

  3. 3

    Place the cast iron piece over a metal plate that rests over a solid surface made of cement or bare earth. Or use a vice to secure it, if the area contains no combustible materials. If the stock piece can fit inside an old barbecue, place it there on top of the grille.

  4. 4

    Don a welding helmet. Turn your oxyacetylene welding set valves on and ignite your welding torch. Adjust the flame for a neutral dispersion. Preheat the iron piece with the torch, bringing the surface to a dull red colour. Move the flame away and wire brush the crack area.

  5. 5

    Heat the iron piece up with the torch again until dull red. Use a flux brush to paste flux over the crack area, inside and out. Adjust the oxygen setting on your welder to produce slightly more oxygen flow to the tip flame. Place a braze welding rod into the crack area at one end, while holding the torch on the tip of the rod.

  6. 6

    Let the rod melt and flow into the crack area. Push the bead of melted rod along the crack. Stop after a few inches. Pull the torch tip away and gently tap the bead with a slag hammer. Let the piece cool completely.

  7. 7

    Heat the iron piece again until it glows dull red. Paste more flux on the crack. Insert your welding rod over the crack where you left off. Run another two-inch bead of weld over the crack. Tap with the slag hammer. Let it cool. Continue to weld two-inch sections in this manner until you have reached the end of the crack. Tap the bead weld with the slag hammer and use the wire brush to burnish away any ash. Let the piece cool and inspect the weld.

Tips and warnings

  • To weld two flat pieces of iron together, grind both mating surfaces with a drill motor grinding bit until shiny. Treat with acid, if necessary. Join the pieces with welding clamps. Weld a bead over the front portion of the seam in two-inch sections, like the above example. You can flip the piece over and weld another bead on the underside for added strength, if you wish.
  • Be very careful when using hydrochloric acid. Wear a respirator, gloves and a mask. Acid is used to leach out oils and debris from holes and micro-cracks in the iron, since iron can be very porous. Do not breath acid fumes or stand over a piece that has been treated with acid. Use a hose to flush the area where the acid was used.

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