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Is preheating necessary when cutting low carbon steels using thermal cutting processes?


Frequently Asked Questions

Thermal cutting produces a heat affected zone just as welding does. Whilst preheating prior to cutting is not essential, it can be beneficial:

  • Preheating can increase the efficiency of the cutting operation by permitting higher travel speeds.
  • It can reduce the temperature gradient in the steel during cutting. This in turn will reduce thermally induced stresses and prevent the formation of quenching or cooling cracks. It will reduce the hardening effect at the cut surface and the amount of distortion that occurs.

Fabrication codes, such as BSI PD 5500 + A2: 2012 'Specification for unfired fusion welded pressure vessels' and BS2633: 1987 'Class I arc welding of ferritic steel pipework for carrying fluids', specify heat treatments for low carbon steels prior to cutting. However, the contractor is often permitted to avoid preheating when cutting if other procedures have been demonstrated by the contractor to be acceptable. In most circumstances, the cut surface is subsequently welded and the metallurgical structure undergoes further transformation, so there is less need for preheating when cutting.

In practice, this means that a formal preheating procedure is not usually considered economical, or necessary. Local preheating is widely carried out though. The cutting torch preheating flames are often passed slowly over the line of the cut several times to lessen the initial chill.

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