Forming Gas Mixture

Backing gas is a shielding gas for the root pass, usually a nitrogen-hydrogen mixture. The hydrogen binds the oxygen that would otherwise diffuse into the hot root, preventing porosity and scale. You have to make sure the gas actually reaches the root and not just covers the surface. The exact mixture and pressure depend on the material and process, so you need to follow the gas supplier's or the standard's specifications. If you use too much hydrogen or the temperature isn't right, the material can carburize or embrittle.