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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 considerably less costly than manufacturing new parts. Besides that, flame straightening is often the only option. In shipbuilding, as much as half of the time required to produce the body-in-white is devoted to removal of distortions. Flame straightening is based on the physical principle that metals expand when heated and contract when cooled. If expansion is limited, compressive stresses build up and result in plastic deformations if the temperatures are high enough. Upon cooling, the plastic deformations remain. In practice, an oxy-fuel flame is used to rapidly heat a well defined section of the work piece. Upon cooling, the metal contracts more than it could expand when heated and any resulting distortions can thus be straightened out. Suitable materials include steel, nickel, copper, brass, aluminum and titanium. 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 thickness of up to 15 mm can be straightened with a standard torch available in most workshops. For straightening of large plates, such as decks and deck houses on ships, adjustable attachments with three or more single-flame nozzles are available, mounted on a small wheel car for easy movement across large surfaces. Working with fuel gases and oxygen can be risky if the person using the equipment does not have sufficient knowledge of the equipment - its installation, handling and care.
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