PWHT full form is Post Weld Heat Treatment. It is a process of heating a welded joint to a specific temperature and holding it there for a specified time. This is done to relieve residual stresses, improve ductility, and temper the micro structure of the weld.
Post Weld Heat Treatment is a thermal process carried out on a welded component after the welding is completed. In this method, the material is heated to a specific temperature, held for a controlled period, and then cooled at a regulated rate. Unlike normal heating, PWHT is carefully designed based on the material type, thickness and applicable industry codes.
The main objective of PWHT is not just heating the weld, but to relieve internal stresses, improve toughness, and restore the material’s metallurgical balance.
PWHT Requirement Based on Pipe Thickness
The decision to carry out Post Weld Heat Treatment is not the same for every weld. It depends on several factors such as the material grade, pipe wall thickness, service environment, and design code requirements (like ASME Section VIII or B31.3).
Thickness Guidelines for Different Materials
Carbon Steel (C–Mn steels)
- When the welded section is thicker than 19 mm (3/4 inch), PWHT is generally mandated.
- For thinner joints, heat treatment is optional unless specified by service conditions (e.g., high pressure or sour service).
Low Alloy Steels (Cr-Mo family)
- For steels such as 1.25Cr-0.5Mo or 2.25Cr-1Mo, stress relief is needed once the thickness exceeds about 13 mm.
- These steels are more crack-sensitive, so codes often require PWHT at a lower thickness compared to carbon steels.
Stainless and High Alloy Steels
- Austenitic grades (304, 316, etc.) usually do not require PWHT but may need solution annealing or stabilization depending on service.
- Martensitic or ferritic stainless steels almost always require a tempering or stress relief step to restore ductility and toughness.
PWHT Temperature Ranges and Soaking Times
Carbon Steel
- Heat treatment is usually performed at 595°C – 675°C.
- Standard rule: 1 hour of soaking per 25 mm of thickness, with at least 30 minutes minimum.
Cr-Mo Alloy Steels
- Typical range is 650°C – 760°C.
- Same guideline of 1 hour per 25 mm applies, though longer times may be used for thicker walls.
Special Stainless Grades
- Stabilized grades like 321/347 require treatment around 870–900°C.
- Martensitic stainless steels are tempered in the 600–750°C range.
Example
- For a carbon steel pipe wall measuring 50 mm thick:
- Heat to about 620°C.
- Hold for 2 hours (since 50 mm ÷ 25 mm = 2 hours).
Summary
- PWHT is typically required for carbon steel above 19 mm and low alloy steels above 13 mm.
- The soak time is about 1 hour per 25 mm of wall thickness.
- The exact range depends on the steel type and governing design code
Applications Where PWHT is Required
1. Pressure Vessels and Piping
Pressure vessels and critical piping systems are often exposed to high internal pressures and fluctuating operating conditions. It helps reduce residual stresses from welding, preventing brittle fracture and improving toughness. It ensures these components can safely withstand long-term service under demanding conditions.
2. Boilers and Heat Exchangers
Boilers and heat exchangers face continuous thermal cycling and extreme temperatures. If not properly treated, welded joints can become stress-prone and susceptible to cracking. It restores ductility, improves weld toughness, and enhances the overall life span of these components.
3. Structural Steel Members
Large steel structures like bridges, cranes, or heavy frames often involve thick welded joints. These joints can hold significant residual stresses after fabrication. It relieves these stresses, ensuring dimensional stability, reducing distortion, and improving structural reliability.
4. Pipelines
Oil, gas, and petrochemical pipelines carry fluids at high pressure and varying temperatures. It minimizes the risk of stress corrosion cracking, hydrogen embrittlement, and weld failure. It ensures pipeline integrity and safe long-distance transport of hydrocarbons.
5. Ships and Offshore Structures
Marine vessels, offshore platforms, and subsea structures are exposed to dynamic loads, seawater corrosion, and harsh environments. It enhances weld toughness, reduces the likelihood of brittle fracture, and extends service life in these challenging conditions.
Why PWHT is Required?
Welding is essentially a process of melting and rejoining metals. This rapid heating and cooling creates problems in the welded zone. PWHT is required to overcome these issues:
1. Relief of Residual Stresses
- During welding, localized heating and fast cooling cause uneven expansion and contraction.
- This leaves behind locked-in residual stresses that can lead to cracking or premature failure.
- PWHT gradually reduces these stresses, ensuring a more stable weld.
2. Improvement in Ductility and Toughness
- Fresh welds and heat-affected zones often become hard and brittle.
- PWHT tempers these zones, softening the microstructure slightly and improving toughness.
- This makes the weld capable of handling shocks, vibrations, and temperature cycles.
3. Hydrogen Diffusion and Crack Prevention
- Hydrogen from electrodes, shielding gas, or moisture can get trapped inside the weld.
- At high pressure, this hydrogen can cause hydrogen-induced cracking.
- PWHT allows hydrogen to diffuse out of the weld, preventing cracks.
4. Refinement of Microstructure
- Welding can form undesirable structures like martensite in carbon and alloy steels.
- These phases are hard but brittle.
- PWHT transforms them into softer, tougher structures, improving long-term reliability.
5. Compliance with Industry Standards
- Codes such as ASME Section VIII, ASME B31.3, and API standards require PWHT for certain materials, thicknesses, and operating conditions.
- This ensures safety and reliability in high-pressure or high-temperature service.
Benefits of PWHT
1. Reduction of Residual Stresses
During welding, rapid heating and cooling create uneven expansion and contraction within the material. This leaves behind residual stresses, which may cause distortion, cracking, or premature failure under service conditions. PWHT relieves these stresses by allowing the metal to relax at elevated temperatures, improving dimensional stability and extending service life.
2. Improved Toughness and Ductility
PWHT refines the microstructure of the weld and heat-affected zone (HAZ). By tempering brittle phases formed during welding, the material becomes less prone to sudden fracture and gains better ductility. This ensures the welded joint can withstand shocks, vibrations, and varying loads without failure.
3. Enhanced Corrosion Resistance
Unrelieved stresses and localized hard zones often make welded materials more vulnerable to stress corrosion cracking. PWHT reduces hardness variations and stress concentrations, significantly improving the resistance of equipment exposed to corrosive environments such as offshore platforms, boilers, and chemical plants.
4. Increased Fatigue Life
Cyclic loading conditions in pipelines, pressure vessels, and structural components can lead to fatigue cracks over time. By minimizing internal stresses and improving structural uniformity, PWHT enhances the fatigue resistance of welded joints, ensuring long-term reliability.
5. Compliance with Industry Codes and Standards
Many standards such as ASME, API, and AWS mandate PWHT for certain materials, thicknesses, and applications. Performing PWHT ensures compliance with these codes, guarantees weld integrity, and provides confidence during inspections and certifications.
6. Improved Safety in Service
Since PWHT stabilizes the weld structure, reduces the likelihood of brittle fracture, and minimizes the risk of sudden equipment failure, it directly contributes to safer operation of critical assets like pressure vessels, pipelines, and offshore structures.
Limitations of PWHT
1. High Cost and Time Requirement
It involves heating welded components to a specific temperature and holding them for a controlled duration. This process demands specialized equipment, skilled labor, and significant energy, making it both time-intensive and costly.
2. Challenges in Temperature and Time Control
Accurate control of temperature and holding time is critical during PWHT. Even small deviations can affect the outcome. Poor control may result in incomplete stress relief or, worse, damage to the metallurgical structure of the weld and base material.
3. Potential Reduction in Mechanical Properties
While PWHT is generally beneficial, in certain materials—especially high-strength or temper-sensitive steels—the process can reduce hardness, toughness, or other desirable properties. This may weaken the weld instead of improving it if not properly managed.