Overview
Localised Post Weld Heat Treatment (PWHT) using low voltage equipment is a specialized technique employed to relieve welding-induced stresses in a specific area of a component, particularly around the weld and heat-affected zone (HAZ). This method is ideal for field applications where full-furnace treatment is impractical and is commonly used in industries like power generation, oil & gas, petrochemical, and structural fabrication.
Low voltage systems (typically 30–60 volts AC) are electrically powered, safe, portable, and provide precise thermal control through resistance heating pads or ceramic heaters.
Purpose of Localised PWHT
- Relieve residual stresses from welding
- Reduce hardness in the HAZ and restore ductility
- Prevent cracking, brittle failure, or stress corrosion
- Meet code and specification requirements (e.g., ASME B31.3, Section VIII)
Principle of Operation
Low voltage electric current is passed through resistance heating elements (typically flexible ceramic pad heaters) that are in direct contact with the component. The heat generated raises the temperature of the weld region in a controlled and uniform manner.
Equipment Setup
- Low Voltage Power Source (30–60V, 3-phase)
- Flexible Ceramic Pad Heaters (FCPHs) or Resistance Heating Blankets
- Thermocouples (Type K or J) welded or clamped near the weld and control zones
- Temperature Controller Units with multiple channel support
- Temperature Recorders/Loggers (chart recorders or digital systems)
- High-temperature insulation (e.g., ceramic fiber blankets)
PWHT Cycle Parameters
| Stage | Typical Range |
| Heat-Up | ≤100–200 °C/hr (as per material spec) |
| Soak Temp | 590–750 °C (depends on material type) |
| Hold Time | 1 hour per 25 mm thickness (min. 30 minutes) |
| Cool-Down | ≤100 °C/hr until 300 °C, then air cooling |
Note: These values vary based on material (e.g., carbon steel, Cr-Mo alloy) and applicable code.
Advantages
- Highly portable – ideal for field or confined-space operations
- Precise temperature control with automated ramp/soak cycles
- Safe low-voltage operation reduces electrical hazard risks
- Code-compliant for most industry standards
- Allows simultaneous heating of multiple zones
Limitations
- Only suitable for localized areas – not full-body treatment
- Surface preparation is essential for thermocouple attachment
- Requires trained technicians for setup, monitoring, and documentation
- Limited by power availability in remote areas (generators may be needed)
Applications
- Field welds on pressure vessels, piping, and structural joints
- Shutdown and turnaround repairs in plants
- Offshore welding applications
- Components where full furnace treatment is impractical
Compliance and Documentation
- Follow a PWHT Procedure Specification (PWHT-PQR)
- Place thermocouples per code (e.g., minimum 1 in control zone, 1 in monitoring zone)
- Maintain time-temperature charts for quality assurance
- Comply with relevant codes (e.g., ASME, API, AWS, ISO)
Conclusion
Localised PWHT using low voltage equipment is a safe, accurate, and cost-effective method to achieve stress relief on-site. It enables reliable thermal treatment of critical welded joints without the need for full-scale furnaces or excessive downtime, making it essential for modern field and maintenance operations.