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Pre-Heating / Flame Straightening
Flame heating involves igniting fuel gases mixed with air or oxygen. Examples 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 oxy-fuel cutting and gouging the flame heats to ignition-temperature in order to initiate and maintain the cutting process.
The process
Hydrocarbons such as methane, ethane, acetylene, propane, and propene or hydrogen are used as fuel gases for flame heating.
The flame temperature and intensity depends on both the fuel gases used and the oxygen mixing ratio.
Acetylene and hydrogen flames can be set to a normal or reducing atmosphere whereas all other common fuel gases have an oxidising flame.
Application and use
The following lists the main flame heating techniques where gases are used to obtain the correct temperature:
- Pre-heating. 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 which have become 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 the 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. This is used for shrink fitting in order 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.
Flame pre-heating
The use of high strength steels is becoming increasingly common as they can offer increased component performance at reduced weight and cost. However these materials need special treatment during welding and cutting operations to maintain their material integrity.
Preheating is recommended and specified by many standards and codes of practice, to avoid defects such as cracking. This is usually done immediately prior to the cutting or welding operation. Following welding it is also sometimes necessary to continue heating, before allowing the workpiece to gradually cool.
Other materials also need preheating especially when working with thicker sections. In cutting for instance, preheating provides additional energy allowing thicker sections to be cut with better quality.
Successful preheating is dependant upon heating the material up to the right temperature throughout its thickness. The preheat temperature depends on the type of material, thickness and subsequent cutting or welding process, and its correct measurement is essential.
However with the wrong gas, equipment or setup, you could be wasting valuable time and money.
Flame straightening
Welding (and other manufacturing processes where heat is introduced) will leave stresses in the metal during the subsequent cooling, causing distortion or warping.
Flame straightening is an efficient and long-established method of correcting the distorted parts.
Flame straightening is based on the principle that metals expand when heated and contract when cooled.
If expansion is constrained, compressive stresses build up and result in plastic deformation when the temperature is high enough. Upon cooling, the deformation remains.
In practice, an oxy-fuel flame can be used to rapidly heat a well-defined section of the workpiece. The metal contracts more on cooling than it could expand when heated and any resulting distortions can thus be straightened out.
Materials
Suitable materials include steel, nickel, copper, brass, aluminium and titanium.
Fuel gases
Although various fuel gases can be used, the highest flame temperatures and intensities for rapid heating are achieved with acetylene and oxygen.
The choice of appropriate equipment depends on the type and thickness of material.
In principle, thin sheet and plate in thicknesses of up to 15 mm can be straightened with a standard torch available in most workshops.
For straightening large plates, such as decks and deckhouses on ships, adjustable attachments with three or more single-flame nozzles are available. These are mounted on a small wheel car for easy movement across large surfaces.
For thicker material use our LINDOFLAMM® special torches.
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