Examples of heating include flame-straightening, flame-hardening, hot-forming and heating in conjunction with welding. The flame is used for melting purposes in flame brazing and for fusing purposes with flame sprayed coatings. With oxyfuel cutting and gouging, the flame heats to ignition temperature in order to start and maintain the cutting process.
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 processes
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.
