Preheating involves rising the temperature of the parent material locally, on both sides of the joint to a value above ambient.

The need for preheat is usually determined by the pertinent fabrication code and verified by the weld procedure qualification test. Preheat may be required as an aid to welding for one of the four basic reasons:

 To control the rate of cooling,

 To control the diffusion rate of hydrogen in a welded joint.

 To reduce thermal stresses.

 Compensation for heat loss.