Heat Treatments in Welding: Complete Guide
Introduction
Heat treatments are fundamental in welding, particularly for medium/high carbon steels, alloy steels, and high-load structures. The objective is controlling joint mechanical properties and relieving residual stresses.
Why are they necessary?
During welding, the HAZ experiences temperatures between 200–1100 °C. This gradient causes high residual stresses, microstructural changes, cold cracking risk, and reduced toughness.
Types of Heat Treatments
1. Preheating
Raising workpiece temperature before welding. Range: 100–300 °C. IIW formula:
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
2. Post-heating (Dehydrogenation)
Maintaining 200–350 °C for 1–4 hours after welding. Allows hydrogen diffusion.
3. Post Weld Heat Treatment (PWHT)
| Material | PWHT Temperature | Minimum time |
|---|---|---|
| Carbon steel (P1) | 580–620 °C | 1 h per 25 mm |
| Cr-Mo steel (P4/P5) | 680–720 °C | 1 h per 25 mm |
| Austenitic stainless | Generally not required | — |
| Ferritic stainless | 730–790 °C | 1 h per 25 mm |
Objectives: stress relief (80–90%), hardness reduction, improved toughness.
4. Normalising
Heating above Ac3 (870–920 °C) and air cooling.
5. Annealing
Heating followed by slow furnace cooling. Minimum hardness.
When mandatory?
EN 13445, EN 13480, ASME BPVC. Qualified WPS with PWHT. High CE steels. Critical service conditions.
References
- CHIAVERINI, Vicente. *Aços e Ferros Fundidos*. 7th ed. ABM, 2005.
- LANCASTER, J.F. *Metallurgy of Welding*. 6th ed. Woodhead Publishing, 1999.
- AWS. *Welding Handbook, Vol. 1*. 9th ed. AWS, 2001.