Importance of Preheating in Welding – Equipment Options
Preheating reduces the risk of cracking in weld metal and heat-affected zone (HAZ) by:
- Lowering the cooling rate of the weld – prevents formation of brittle weld metal/HAZ, allows more time for hydrogen to escape the molten weld.
- Removing moisture (hydrogen source) from the part.
- Reducing shrinkage by lowering the temperature differential.
Preheat or not?
The requirement and level of preheating for steel is determined by the relevant welding code and is dependent on the weld heat input, chemical composition of steel (carbon equivalent), thicknesses, diffusible hydrogen etc. Non-ferrous materials generally require preheat because of their high thermal conductivity.
Preheating can be carried out using the following methods:
- Applying direct flame – surface of the part heated and then heat conducted through the part, open flame.
- Placing the part in a furnace – uniform heating of complete part if sufficient time provided in furnace, limitations of use due to the size of the furnace.
- Electric blankets – surface of the part heated and then heat conducted through the part.
- Induction heating using blankets – heat generated by electromagnetic currents within the material.
Out of the above methods, induction heating delivers the most consistent and fastest results and allows for the precise temperature control. It is also the safest and most energy efficient method.
The temperature is measured on the workpiece using thermocouples, temperature crayons or infrared pyrometers. The measuring positions are specified by the relevant welding code and are typically dependent on the thickness and geometry of the joint.
Red-D-Arc’s range of induction heating equipment includes air cooled and water cooled induction heating systems with heat applied via blankets and rolling inductors (for moving parts). These systems feature the ability to accurately monitor, control and record temperature. The required accessories including blankets, cables, thermocouples and recorders are also available for rental.
Visit our induction heating equipment page to find out more.