Heat Treatment

Preheating involves raising the temperature of the parent material to a value above ambient on either side of a joint to be welded.

The need for preheat is normally determined by the relevant fabrication code and verified by the weld procedure.

Preheat is normally required as an aid to welding, to drive out moisture, and for any of the reasons given below.

1. To slow the rate of cooling in the heat affected zone, reducing the risk of hardness and cracking.
2. To control the diffusion rate of hydrogen in the welded joint.
3. To reduce thermal stresses.
4. To compensate for heat loss.

PWHT is used when required by the fabrication Specification Code and verified by the WPS. (In some cases it is mandatory.) It involves raising the temperature of the weld and the heat affected zone uniformly and in a controlled manner.

The purpose of PWHT is primary to reduce residues and the internal stresses caused by welding, quench hardening or cold working.

It can also be beneficial in improving resistance to stress corrosion, cracking and brittle fracture. It improves the elasticity, ductility and strength of the material, and enhances its machineability.