, you can use a propane torch instead of the more powerful and dangerous oxygen-acetylene torch. With the right preparation and materials, you can solder cast iron in a few minutes.
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Clean both cast iron pieces with an isopropyl solvent and a rag to remove grease and oil. Scrub the pieces with a bronze or steel brush to remove any remaining dirt.
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Paint both pieces with a thin layer of liquid flux to prevent oxidation and improve the flow of the solder.
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Put on heat-resistant gloves and safety glasses, and heat the fluxed cast iron pieces with a propane torch until warm. Scrub the pieces again, and re-apply the flux to each piece.
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Clamp the cast iron pieces together in the desired position. Heat a small amount of solder on the tip of a soldering iron, and paint the solder into the seam between the pieces when it begins to melt.
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Heat the cast iron pieces with the propane torch. Direct the tip of the blue cone in the torch’s flame against the cast iron pieces, moving the torch around the area adjacent to the seam until the solder changes color and flows into the seam. Do not heat the solder directly.
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Allow the cast iron pieces to cool completely before unclamping them.