Straightening structural steel with an oxyacetylene flame process is based on three combined facts of physics: the expansion of steel as its temperature rises, the lowering of the yield point of steel as the temperature is increased, and the ability of steel to flow plastically when the stress imposed exceeds the yield point. The reverse of the first two facts is also utilized in flame straightening under certain conditions.
As heat is applied to a small area of a given section, there remains enough cold metal to confine, or limit, the expansion in certain directions. The lowered yield point of the heated area is exceeded by the stress caused by expansion, therefore the heat area flows plastically in the confined directions. The flame straightening procedure must be controlled by a person who thoroughly understands structures and has a working knowledge of the behavior of steel under stress at elevated temperatures.
Flame straightening can also be applied to straighten bent plates, angle frames, pipes, and fabrications.