Preheating and intermediate pass temperature

Preheating can be done for the following reasons:

• to reduce humidity before welding i.e. welding on site 
• to remove irregularities at arc start 
• to provide heat adjustment when welding different thicknesses 
• to reduce the effect of cooling when welding thick plates 

The time of preheating should be as short as possible to prevent unfavourable effects. A too high preheat temperature may infl uence the strength of the welded joint in a negative way. By using Argon-Helium mixtures or pure Helium as shielding gas the preheat temperature may be reduced or completely omitted.

The inter-pass temperature should be controlled for the following reasons: 

• to prevent a reduction of the physical properties from overheating 
• to reduce the size of the soft zone in the heat affected zone 
• to reduce precipitation in the heat affected zone i.e. by superannuation 

It is recommended that the temperature at the beginning of each successive weld does not exceed the values as shown by the table above.