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.
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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.