A welding torch is a mechanical device that produces a hot flame by mixing gases for the welding or cutting of metals. This device is used to join metal surfaces by raising the temperature high enough so as to melt the joining ends and then fuse them with or without a filter metal. After the heat is removed the joint solidifies and fuse permanently. This torch usually produces a flame temperature of 2750°C to 3300°C by mixing acetylene and pure commercial oxygen which is sufficient to melt the metal locally. The torch thoroughly mixes the two gases and has facilities to adjust and regulate the flame.
Welding torches are of two types: low pressure torch and high pressure torch. On a low pressure or injector torch, acetylene enters a mixing chamber where it meets a jet of high pressure oxygen. The amount of acetylene drawn into flame is controlled by the velocity of this oxygen jet. In a high pressure torch both the gases are delivered under pressure. The heat generated at the work is controlled principally by the size of the nozzle or the tip fitted onto the torch. The larger the tip, the greater the gas pressure. Small flames are used with thin-gauge metals. Larger flames are necessary for thick metal parts.
A welding torch mixes the fuel and gas internally and well ahead of the flame. For cutting, the torch delivers an additional jet of pure oxygen to the centre of the flame. The oxyacetylene flame produced by mixed gases raises the metal to its ignition temperature. The central oxygen jet oxidizes the metal, the oxide being blown away by the velocity of the gas jet to leave a narrow slit or kerf. The temperature for the cutting action, once initiated, is maintained by the oxidation of the metal. Nowadays automatic torches have been developed for precision, cutting and welding.