Hydrocarbons such as methane, ethylene, acetylene, propane, and
propylene or hydrogen are used as fuel gases for flame heating. The flame
temperature and intensity depends on the fuel gases used and oxygen mixing
ratio. Acetylene and hydrogen flames can be set to a normal or reducing
atmosphere whereas all other common fuel gases have an oxidizing
flame.
The main heating applications are described below:
Preheating: Certain materials must be preheated before
welding to avoid hardening cracks. After welding, stress relief heat
treatment may be necessary in order to reduce welding stress.
Flame straightening: This technique is employed to change
or restore the shape of parts that have been distorted. When a defined
section is heated beyond its yield limit and the surrounding prevent
expansion, plastic deformation will occur. Once cooled, the material will
shrink in size. A skilled operator can heat isolated spots to straighten the
deformation.
Melting: This involves soldering, brazing and fusing of
sprayed coatings as well as flame welding.
Forming: The part to be formed is heated locally. An
external force is then applied to form the part. For example, necking out
pipe branch stubs.
Shrink fitting: To achieve a change in size used for shrink
fitting to securely fit shafts/bearings. Liquid nitrogen can be used for
cooling (shrinking) and an oxy-acetylene flame for heating (expanding).
Stress relief heat treatment: This may be necessary after
welding.
Metal structure changes: This involves flame hardening.
