Oxygen - In the preheat flame, a fuel gas burns in oxygen, generating heat. The flame consists of an outer and an inner combustion zone. The main purpose of the preheat flame in most flame processes is to achieve rapid, local heating. This imposes requirements on the combustion properties of the gas that is used.
Acetylene is the primary fuel for oxy-fuel welding and is the fuel of choice for repair work and general cutting and welding. Acetylene produces the hottest and most concentrated primary flame of all industrial fuel gases. Its calorific value is fairly low, but the portion emitted by the primary flame is very high, about 30%, making acetylene the fuel gas that produces the most heat in the primary flame. All fuel gases except acetylene produce an oxidising flame. This is not acceptable in gas welding, since the weld metal is thereby oxidised, resulting in reduced strength. Acetylene is therefore the only gas suitable for gas welding.