The range of diffusion and pre-mixed flame temperatures in air are not close to the temperatures needed to melt carbon steel. Yes, there is a range of flame temperatures based on fuel chemistry (think adiabatic flame temp calcs) and location of the measurement in the flame sheet but the range is much lower than the melting temperature of carbon steel.

Fuel loading and heat release rate are not significant factors in the determination of flame temperatures of diffusion and premixed flames in air. Gasoline and wood have similar flame temperatures, but very different normalized heat release rates.

The reason is basic chemistry. Nitrogen is in air and air is the oxidizer. Thus, Nitrogen is on the reactant side of the chemical equation. But it is also on the products side of the chemical equation and in the same amount (molar concentration or atoms). Thus, the nitrogen does not react with the hydrocarbon fuel.

Nitrogen has a relatively high heat capacity and can absorb a lot of thermal energy with relatively little increase in temperature. Much like water; a good fire suppressant. The flame temperatures stay reasonably consistent due to the Nitrogen acting as a "thermal ballast" and absorbs thermal energy that might otherwise contribute to the combustion process where it could increase the flame temperature. That is also why hydrocarbon flame temperatures are much higher when the oxidant is pure oxygen as opposed to air.

If you are seeing melted carbon steel near diffusion or pre-mixed flames, there is something else going on then just the flames melting the steel.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com