PWHT (Post Weld Heat Treatment) is the process of heating a welded component to a specified temperature and cooling it in a controlled manner. Purposes: relieve welding residual stresses, stabilise microstructure and diffuse hydrogen to eliminate cold cracking risk.
Why Perform PWHT?
- Stress relief: Residual tensile stresses can be 50–80% of yield strength. PWHT reduces them by up to 80%.
- Hydrogen diffusion: Diffusible hydrogen that causes delayed cold cracking escapes during heat treatment.
- HAZ toughness improvement: The hardened HAZ is tempered; Charpy values improve.
- Dimensional stability: Internal stresses reduced to prevent in-service deformation.
- Corrosion resistance: Sensitisation in stainless steels is prevented or resolved.
When is PWHT Mandatory?
| Standard | PWHT Requirement |
|---|---|
| EN 13445 (pressure vessel) | Carbon steel t > 35 mm; low-alloy steel t > 19 mm |
| EN 13480 (process piping) | P-No 1: 19 mm; P-No 4–6: 13 mm |
| ASME VIII Div 1 | Carbon steel t > 38 mm |
| API 650 (storage tank) | Generally above 38 mm |
PWHT Temperature and Duration
| Steel Class | Temperature (°C) | Duration Rule |
|---|---|---|
| P-No 1 (S235, S355) | 580–620 | 1 h / 25 mm, min 30 min |
| P-No 3 (1.25Cr-0.5Mo) | 620–680 | 1 h / 25 mm |
| P-No 4 (2.25Cr-1Mo) | 680–720 | 1 h / 25 mm |
| P-No 6 (12–13Cr martensitic) | 720–780 | 2 h minimum |
| P-No 8 (304L/316L) | 1010–1100 (solution anneal) | By thickness |
S690QL and S960QL: PWHT is generally PROHIBITED for these QT steels. Exceeding the tempering temperature irreversibly reduces yield strength.
Heating and Cooling Rates
- Heating rate (above 300°C): Max 220°C/h × (25 / t), min 50°C/h
- Soak temperature tolerance: ±20°C (general)
- Cooling rate (above 300°C): Max 275°C/h × (25 / t)
- Below 300°C: Free cooling in still air
Heating Methods
- Furnace: Most uniform, ideal for small-to-medium parts
- Local induction: Common for circumferential pipe welds
- Resistance blanket: Economic for local welds
- Gas torch: Site repairs, temperature control is difficult
Thermocouple Placement
- Minimum 2 thermocouples (inlet/outlet control)
- Wall thickness > 50 mm: separate inner and outer surface thermocouples
- Local heating: at the weld line + control points at 2× wall thickness distance
- All temperature-time records stored (10 years)
Local Heating Band Width
- Heated band: 5 × Ri × t
- Soaked band: Inner half of heated band
- Gradient control band: Insulated area extending 2.5 × Ri × t beyond heated band
Common Mistakes
- Insufficient soak time
- Cooling too rapidly
- Missing thermocouples or incorrect placement
- Insufficient band width for local heating
- Applying PWHT to S690QL / S960QL (PROHIBITED)
Post-PWHT Verification
- Hardness test (HV10): within WPS limits?
- NDT: PT/MT for new cracks?
- Dimensional check: has distortion occurred?
ArmaWeld scope: Furnace PWHT for parts up to 8 m × 3 m × 3 m. EN 13445 and ASME VIII thermal treatment process charts and temperature records supplied.
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