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AC/OX CUTTING/WELDING/BRAZING

Heating Techniques

The Acetylene/Oxygen flame is frequently used for heating workpieces onboard, whether it is for preheating/postheating in combination with welding processes or for heating steel for bending or forming. A normal welding flame or a normal flame from special multiflame heating attachments may be used. No special techniques are required except the need to be careful not to overheat and weaken parts. There is, however, one area of heating that needs special know-how, the straightening of steel constructions.

Beside purely mechanical methods, straightening is largely performed with a welding flame. This is a convenient method, normally demanding no other equipment than ordinary gas welding equipment. However, you need a good knowledge of how the work reacts to heating and cooling, and how the shrinkage forces should be exploited in straightening.

The steadily growing use of welding calls for an economical method of dealing with the deformations, which often arise in welded steel constructions.

Also, heat should not be applied to a spot that has been heated previously, as this can cause deterioration.

In flame straightening, the temperature should not exceed 550– 600 °C. The work should be allowed to cool slowly, especially in the case of high tensile steel.

When heat is applied as shown in the sketch, the heated part has a tendency to expand. As longitudinal expansion is prevented by the surrounding cold metal, a convexity is created in the heated part. On cooling, the heated metal contracts, with the result that the end of the

section moves upwards.

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Section bars are normally straightenened with the help of heating wedges, as

shown above. On the next page we will give some guidelines on how to apply these heating wedges.