Post-Weld Heat Treatment
, , hydrogen release annealing — why the weld is post-treated
AI generated After , the job is often not done. reduces residual stresses, neutralizes the hardened , and hydrogen release annealing drives out the crack-triggering hydrogen. These post-treatments determine whether a weld holds — especially for thick, high-strength, or safety-critical parts.
The weld bead is laid, the slag removed — and still the part isn't finished. For many materials, a heat treatment follows that's what really makes the joint hold. It has three goals:
1. Relieve residual stresses — . As it cools, the weld shrinks and pulls on the cold . This creates that in the worst case can reach the of the material. (550–650 °C) lets these stresses relax in a controlled way.
2. Neutralize the hardened — . In the , can form during — hard and brittle. at 600–700 °C transforms it into a tougher, tempered structure and lowers the to acceptable values.
3. Drive out hydrogen — hydrogen release annealing. The most dangerous trigger for cracking is from damp electrodes, , or the part itself. It migrates to the weld and triggers there with a delay — sometimes only hours after . Immediate post-heating (hydrogen release annealing, 200–300 °C, several hours) drives the hydrogen out before it can do damage.
When is which post-treatment necessary? That depends on four factors:
• Material: High-strength, high-carbon, and quenched and tempered steels almost always need post-treatment. Plain structural steels with low CEV often don't.
• Thickness: Thick plates create more residual stresses and cool faster → more and hydrogen.
• Load: Load-bearing, pressure-bearing, and dynamically loaded parts are post-treated because stresses and brittleness are critical there.
• Standard/specification: For pressure vessels and steel structures, post-treatment is often mandatory and specified in the WPS.
The sequence matters: hydrogen release annealing right after (before the hydrogen can do damage), then /. And watch out for : some steels become brittle at 250–450 °C — you have to skip that range when .