For NM grades in thicker gauges and/or with high carbon equivalent preheating is necessary to avoid cold cracking.
The need for preheating is dependent on
- NM grade carbon equivalent
- type of electrodes used
- welding method
- stress condition in the weld joint
Before start of welding the whole length of the seam should have reached the recommended preheating temperature.
A zone of about 100mm ( or at leat 4 times plate thickness) should be reached on each side of
the weld.
The temperature at single or multi pass welding should not fall below the recommended preheating temperature.
In order to reduce residual stresses it is important to select a weld sequence that ensure the individual components to shrink freely as long as possible.
Root welds and tack welds should be thick enough considering the minimum preheating temperature.
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