PWHT Requirements & Processes: Getting it Right is Mission-Critical

In critical sectors like oil & gas, power generation, and petrochemicals, Post Weld Heat Treatment (PWHT) isn't just a procedural step—it's a mission-critical engineering process. Properly executed PWHT directly impacts equipment integrity, personnel safety, and long-term service reliability.

🔍 What is PWHT and Why Is It Done?

PWHT is a controlled heating and cooling cycle applied to welds after welding is completed. It aims to:

• Relieve residual stresses induced by rapid heating/cooling

• Refine grain structure and microhardness in the Heat-Affected Zone (HAZ)

• Mitigate cracking risks such as:

  • Hydrogen-Induced Cracking (HIC)

  • Stress Corrosion Cracking (SCC)

  • Brittle fracture in low-toughness zones

This process is especially critical for sour service environments with H₂S, and materials operating under creep-prone high temperatures.

✅ Benefits of PWHT

• 🔹 Hydrogen diffusion: Removes trapped hydrogen to prevent delayed cracking

• 🔹 Stress relaxation: Reduces tensile stresses in and around the weld

• 🔹 Improved toughness & ductility: Especially in low-alloy steels

• 🔹 Restores mechanical properties: After welding distortion

• 🔹 Creep resistance: In Cr-Mo alloys and pressure-retaining components

🛠️ When is PWHT Required?

PWHT is mandated by engineering codes, project specs, and risk-based assessments:

• Material:

  • Carbon steels

  • Low-alloy steels (e.g., 1¼Cr-½Mo, 2¼Cr-1Mo)

  • Martensitic stainless steels

• Weld thickness:

  • Per ASME B31.3, PWHT is required for carbon steel welds over 19 mm (¾")

• Service conditions:

  • Sour service (NACE MR0175/ISO 15156)

  • High pressure/high temperature

  • Thermal cycling or fatigue-sensitive applications

• Codes & Standards:

  • ASME B31.3 (Process Piping)

  • ASME Section VIII Div. 1 & 2 (Pressure Vessels)

  • API 582, API 934-A

  • NACE MR0103, MR0175

🔄 The PWHT Process: Step-by-Step

1. Planning & WPS/PQR

   • Define PWHT parameters (temp, time, rate)

   • Thermocouple placement and monitoring setup

2. Preheating (if applicable)

   • Prevents thermal shock and cold cracking (especially for Cr-Mo steels)

3. Controlled Heating

   • Typically ≤55°C/hr to reduce temperature gradients

   • Prevents distortion and cracking in thick-wall weldments

4. Soaking Phase

   • Maintain target temp (620–740°C)

   • Hold time: 1 hour per inch of thickness (minimum)

   • Use multiple thermocouples to confirm uniformity

5. Controlled Cooling

   • Cool down ≤55°C/hr to ~300°C

   • Air-cooling allowed beyond 300°C for most steels

6. Inspection & Documentation

   • Review temperature charts and time/temperature profiles

   • Include records in MDR (Manufacturing Data Record) or TOP (Turnover Package)

   • Final inspection by QA/QC and client

📌 Conclusion

Incorrect or skipped PWHT can lead to catastrophic failures—from brittle fracture to hydrogen damage—especially in high-risk environments. By following code requirements, client specs, and best practices, you ensure that your weldments deliver maximum safety, performance, and longevity.

Always treat PWHT as an engineered, quality-controlled step—not just a heat-up and cool-down task.