The heat applied to the base metal or substrate to attain and maintain preheat temperature is called pre heating.
The right preheat and interpass temperature is important in order to avoid hydrogen cracking.
Preheating may be performed by the use of gas burners, oxy-gas flames, electric blankets, induction heating, or by heating in a furnace. For good results, it is essential for the heating to be uniform around the joint area.
When preheating is specified, the entire weld joint should be heated evenly through the material thickness to the desired minimum temperature.
To obtain a uniform temperature through the material thickness, it is desirable to apply the heating sources to one side of the material surface and to measure the material temperature on the opposite side.