What is the different between backing and backing gas in QW-402.5 and QW-408.9?
QG-109.2 states (ASME IX 2017 edition)
Backing: a material placed at the root of a weld joint for the purpose of supporting molten weld metal so as to facilitate complete joint penetration. The material may or may not fuse into the joint.
Backing gas: a gas, such as argon, helium, nitrogen, or reactive gas, which is employed to excluded oxygen from the root side (opposite from the welding side) of the weld joints.