ASME Section VIII Div 1 PWHT Requirements
Complete Guide to UW-40 Procedures & UCS-56 Tables
Post Weld Heat Treatment (PWHT) under ASME Section VIII Division 1 is one of the most critical requirements in pressure vessel fabrication. Unlike structural codes, ASME strictly mandates PWHT based on material type, thickness, and service conditions.
Quick Navigation
- UW-40 Procedures
- Soak Band Concept
- Nominal Thickness (t)
- UCS-56 Requirements
- Heating & Cooling Rates
- Repair Welding Rules
UW-40: Procedures for Post Weld Heat Treatment
Clause UW-40 defines how PWHT must be performed. There are eight permitted procedures depending on vessel geometry and practicality.
1. Full Furnace Heating
Preferred method ensuring uniform temperature.2. Multi-Heat Furnace
Requires 5 ft overlap and thermal shielding.3. Sectional Heating
Used before final assembly.4. Internal Heating
Controlled with insulation and pressure limits.5. Circumferential Band Heating
Used for nozzle areas.6. Pipe Joint Heating
Full circumference heating required.7. Local Nozzle Heating
Used for spherical heads.8. Special Configurations
Requires engineering justification.Soak Band Concept (Critical for Compliance)
- Includes weld + HAZ + base metal
- Width = weld width + (1t or 50 mm max)
- Must be uniform temperature
Nominal Thickness Definition (UW-40 f)
| Joint Type | Definition of Thickness (t) |
|---|---|
| Butt Weld | Depth of weld |
| Fillet Weld | Throat thickness |
| Groove Weld | Depth of groove |
| Unequal Thickness | Thinner part |
| Repair Weld | Depth of repair |
UCS-56: PWHT Requirements by Material
UCS-56 defines when PWHT is mandatory. Requirements depend on P-Number classification.
| P-Number | Material | Temp (°F) | PWHT Requirement |
|---|---|---|---|
| P-No.1 | Carbon Steel | 1100°F | >38mm thickness mandatory |
| P-No.3 | Low Alloy | 1100°F | Mandatory >16mm |
| P-No.4 | Cr-Mo Steel | 1200°F | Generally mandatory |
| P-No.5A | 2.25Cr-1Mo | 1250°F | Mandatory almost always |
| P-No.15E | Grade 91 | 1300°F | Strict control required |
Heating & Cooling Rate Control
Heating Rate Rule
Max Heating Rate = 400°F/hr ÷ Thickness (inches)
Cooling Rate Rule
Max Cooling Rate = 500°F/hr ÷ Thickness (inches)
- Heating: max 250°F variation
- Holding: max 150°F variation
Repair Welding After PWHT
ASME allows repairs after PWHT under strict conditions:
- Prior approval required
- Depth limits: 38mm (P1), 16mm (P3)
- Mandatory NDT (MT/PT)
- Preheat required
Temper Bead Technique
For alloy steels, half-bead technique is used to control HAZ hardness without PWHT.
Quick Quiz (Test Your Knowledge)
Q1: What is minimum PWHT temperature for P-No.1?
Answer: 1100°F (595°C)
Q2: What defines nominal thickness?
Answer: Thickness of welded joint
Q3: What is soak band width?
Answer: Weld width + 1t or 50mm
Key Takeaways
- PWHT is mandatory based on thickness & material
- UW-40 defines procedures
- UCS-56 defines requirements
- Strict heating/cooling rates prevent damage
- Repairs require strict compliance
Frequently Asked Questions
Is PWHT always mandatory?
No, depends on thickness and material classification.
What is soak band?
The heated zone including weld, HAZ and adjacent base metal.
Can PWHT be done locally?
Yes, but must ensure uniform heating and controlled gradients.
What happens if PWHT is skipped?
Risk of cracking, brittle fracture and code non-compliance.
PWHT Thickness Requirements (UCS-56 Summary)
| Material (P-No.) | PWHT Mandatory Thickness | Condition |
|---|---|---|
| P-No.1 (Carbon Steel) | > 38 mm (1.5 in) | Mandatory |
| P-No.1 (With Preheat ≥ 95°C / 200°F) | > 32 mm (1.25 in) | Mandatory |
| P-No.3 (Low Alloy Steel) | > 16 mm (5/8 in) | Mandatory |
| P-No.4 & P-No.5 | All thicknesses | Generally mandatory |
PWHT Holding Time Requirements
| Thickness | Holding Time Rule | Minimum Time |
|---|---|---|
| Up to 50 mm (2 in) | 1 hour per inch thickness | 15 minutes minimum |
| Above 50 mm (2 in) | 2 hours + 15 min per additional inch | — |
Heating and Cooling Rate Limits
| Condition | Formula | Maximum Limit |
|---|---|---|
| Heating Rate (Above 425°C / 800°F) | 400°F/hr ÷ Thickness (inches) | Max 400°F/hr |
| Cooling Rate (Above 425°C / 800°F) | 500°F/hr ÷ Thickness (inches) | Max 500°F/hr |
Temperature Uniformity Requirements
| Stage | Maximum Temperature Variation |
|---|---|
| Heating Stage | 250°F (140°C) |
| Holding Stage | 150°F (83°C) |
Repair Welding After PWHT (UCS-56)
| Material | Maximum Repair Depth | Special Requirements |
|---|---|---|
| P-No.1 | 38 mm (1.5 in) | Preheat ≥ 95°C (200°F), NDT required |
| P-No.3 | 16 mm (5/8 in) | Temper bead technique mandatory |
Soak Band Requirements
| Parameter | Requirement |
|---|---|
| Coverage | Weld + HAZ + adjacent base metal |
| Minimum Width | Weld width + (1t or 50 mm, whichever is less) |
| Temperature | Must reach minimum PWHT temperature |
PWHT Temperature Limits Based on Ni + Mn Content
| Condition | Maximum Temperature |
|---|---|
| Ni + Mn ≤ 1.0% | 1455°F (790°C) |
| Ni + Mn > 1.0% to 1.2% | 1435°F (780°C) |
| Minimum Temperature (General) | 1300°F (705°C) |