This method is much used to join the edges of wrought iron or steel plates up to an inch or more in thickness. Heavy plates conduct away the heat so rapidly that it may not be possible for the burner to raise the seam to welding temperature. Steel castings may also be welded, and cracks or blow holes may be filled with drops of molten iron. Brass castings may be repaired by melting into a crack, cavity or blow hole, drops of brass from a rod of the same composition as the casting.

The difficulty of welding iron is in proportion to the amount of carbon it contains, and, while some operators claim ability to weld or mend cast iron, it is difficult to do so. A cast-iron bar may be fused at the end into drops which will weld to a properly heated cast-iron surface, but the iron will not be homogeneous and it may be too brittle for strength and too hard for machining.

Aluminum may be welded, under blow-pipe heat, by use of a flux which removes the oxide, although this metal cannot be soldered or brazed, at least by usual means.

Steel pipes and flanges may be welded together, and lengths or branches welded in one piece.

A particular application of gas welding is that used in welding the longitudinal seam of a boiler drum made of one sheet of metal. The drum is mounted on a machine especially built to handle it readily and quickly. The machine carries two specially constructed burners which travel along on opposite sides of the lapped plate edges, heating them to welding heat. The burners are followed by rollers which exert great enough pressure to make the weld and to press the seam down to the thickness of the plate. Water gas and air are used in the burners. The strength of such a welded joint is about 90 or 95% that of the solid plate, although the ordinary oxy-acetylene-welded joint is not over about 80 or 85% of the plate strength.