Blog on Post Weld Heat Treatment (PWHT)
What is PWHT?
Post-Weld Heat Treatment (PWHT) is a controlled heat treatment process applied to welded materials to reduce residual stresses and improve the mechanical properties of the weld and adjoining area. It is a heat treatment process heated to a lower critical transformation temperature, and then held at that temperature for a specified time before being cooled at a controlled temperature.
This heat treatment involves a specific heating cycle, i.e. rate of heating, rate of cooling, holding temperature & time as per applicable code.
PWHT is sometimes referred to as stress relief heat treatment.
Why is PWHT Important?
During welding, metal expands and contracts due to rapid heating and cooling and induces residual stresses, which affect below mechanical properties.
• Hardness and brittleness in the Heat-Affected Zone (HAZ)
• Distortion and cracking
• Reduced toughness and ductility
• Unacceptable microstructures
During the welding, pre-heating is also necessary to avoid the above effects.
PWHT aims to relieve residual stresses induced during welding, which can cause distortion, cracking and reduced strength of material.
PWHT can improve the weld’s toughness, ductility, and resistance to cracking.
When does PWHT require?
PWHT is commonly used for thick-walled metals. generally, PWHT is applicable over 38 mm wall thickness for carbon steels, high-strength materials, PWHT is mandatory.
PWHT helps to:
• Relieve internal stresses caused by welding
• Improve ductility and toughness
• Reduce the risk of brittle fracture
• Meet code and specification requirements (like ASME, API, ISO)
Applicable Standards
PWHT must be done as per specific standards and codes.
• ASME Section VIII Div. 1 (Pressure vessels)
• ASME B31.3 (Process piping)
• API 650 / 653 (Storage tanks)
• NACE, ISO, and client-specific specifications
These codes define when PWHT is required, what temperatures and durations to use, and how to document the process.
Where is PWHT Commonly Used?
PWHT is widely used in industries such as:
• Oil & Gas
• Power & Energy
• Petrochemicals
• Pressure Vessel Fabrication
• Pipelines and Refineries
• Structural Welding (in high-stress applications)
PWHT Methods
• Furnace PWHT: For large batch jobs, using industrial ovens
• Local PWHT: Using electrical resistance pads or induction coils for field welds
• In-situ PWHT: Heat treatment done on-site for large or immovable components
Documentation
A proper PWHT process requires:
• PWHT Procedure Specification (PWHTPS)
• Thermocouple placement and recording
• Calibration of equipment
• PWHT chart or graph record showing time vs. temperature
Final Thoughts
After PWHT, hardness shall be verified within the material specifications. Post-Weld Heat Treatment is not just a technical process—it’s a safety and reliability enabler. Neglecting PWHT where it’s required can lead to weld failures, downtime and even hazardous incidents.