Hot straightening

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Flame straightening on a double-deck wagon, Görlitz , 1952

Hot straightening is a production technique from metal processing .

In the case of steel structures , partial heating, for example by welding , and the subsequent internal tension can result in geometric deviations in the structural geometry from the target shape, which can be corrected with this method.

background

If a sheet metal that has been (slightly) deformed by welding processes is partially heated again locally, so-called heat wedges are created. These heated areas of the sheet tend to expand . However, they are partially hindered in their expansion by the surrounding cold areas. In connection with the drop in the yield strength of the steel under the influence of temperature, local plastic compressions occur in the area of ​​the heat wedges. As a result, the sheet metal shortens in the area of ​​the heat wedges when it cools down again and thus counteracts the prevailing deformation.

Disadvantages of hot straightening

The edges of these welding wedges are often visible in the steel through the tempering colors . In addition, changes in the structure of the steel can occur in the area of ​​the welding wedges.

Hot straightening can introduce considerable additional local stresses in the steel structure.

These areas then often represent a weakening and thus a danger for the static and dynamic load-bearing capacity of the construction. They can even cause local pre-damage and, in extreme cases, cracks in the component as soon as it cools down.

Measures to avoid cracks and distortions

In order to reduce these undesirable stresses, these constructions are usually subsequently heat-treated by means of stress relief annealing in order to reduce the internal stresses present in the steel.

With a suitable arrangement of the weld seams in the welded construction and an optimal sequence in the application of the weld seams, as well as with a welding technology tailored to the component, the most important causes of the geometric deviations (distortions) can already be reduced during the production planning.

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