POST WELD HEAT TREATMENT(PWHT)

 PWHT (Post Weld Heat Treatment) is a heat treatment process that is commonly used in the welding industry to reduce residual stresses and improve the mechanical properties of welded materials.

After welding, the welded components can experience residual stresses due to the heat generated during welding and the subsequent cooling process, which can lead to distortion, cracking, or reduced strength. PWHT involves heating the welded component to a specific temperature and holding it at that temperature for a certain duration, followed by controlled cooling.

 

1.1 Methods of Heat treatments: 

There are several methods commonly used for Post Weld Heat Treatment (PWHT), depending on the type of material, size and shape of the welded component, and the specific requirements of the application.

Some of the commonly used methods of PWHT include: 

1. Furnace Heating: This method involves placing the welded component in a furnace and heating it to the required temperature for a specific duration of time. Furnace heating provides uniform heating and controlled cooling, which allows for precise control of the heat treatment process. It is commonly used for large and heavy components that can be accommodated in a furnace.

 

2. Electrical Resistance Heating: In this method, electrical resistance heaters are attached to the welded component, and electric current is passed through the heaters to generate heat. This method allows for localized and controlled heating of specific areas, making it suitable for smaller components or components with complex geometries.

 

 

3. Induction Heating: Induction heating uses electromagnetic induction to heat the welded component. An alternating magnetic field is generated by passing alternating current through a coil, and the component is placed within this field, causing it to heat up due to induced currents. Induction heating provides fast and localized heating, making it suitable for components with localized heat treatment requirements.

4. Flame Heating: Flame heating involves directing a high-temperature flame onto the surface of the welded component to heat it. This method is typically used for small components or areas that require localized heat treatment. Care must be taken to ensure proper temperature control and even heating to avoid distortion or other issues. 

5. Resistance/Thermal Blanket: In this method, a thermal blanket or resistance material is wrapped around the welded component, and heat is applied using external heaters or electric current passed through the resistance material. The blanket helps to contain the heat and promote uniform heating of the component.

    

     The specific method of PWHT used will depend on various factors, such as the material being welded, the size and shape of the component, the desired heat treatment parameters, and the available equipment and facilities. 

1.2 Criteria for PWHT: 

The criteria for Post-Weld Heat Treatment (PWHT) are typically determined based on several factors, including the material type, thickness, joint configuration, welding process, and service conditions. The specific criteria for PWHT can vary depending on the applicable codes, standards, and project specifications.

However, some common criteria for PWHT may include:

1. Material Type: Certain materials, such as carbon steel, low alloy steel, stainless steel, and nickel-based alloys, may require PWHT based on their welding characteristics, susceptibility to cracking, and service conditions. The specific material type and its properties, including tensile strength, toughness, and weld ability, may be considered in determining the need for PWHT.
2. Thickness: Thick sections of materials may generate higher levels of residual stresses during welding, which can increase the risk of distortion, cracking, and reduced mechanical properties. PWHT may be required for thicker sections to relieve residual stresses and minimize the risk of welding-related issues.
3. Joint Configuration: Joint configurations, such as butt joints, T-joints, and corner joints, can affect the stress distribution and welding conditions, which may impact the need for PWHT. Complex joint configurations or joints with high stress concentrations may require PWHT to mitigate the risk of cracking and improve the integrity of the welded joint.
4. Welding Process: Different welding processes, such as shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW), and submerged arc welding (SAW), have different heat input, cooling rates, and metallurgical effects. PWHT requirements may vary depending on the welding process used and its impact on the material properties and residual stresses.
5. Service Conditions: The service conditions, including temperature, pressure, and environment, to which the welded structure will be exposed may influence the need for PWHT. High-temperature or high-pressure applications, aggressive environments, or cyclic loading conditions may require PWHT to ensure the desired material properties and performance in service.
6. Applicable Codes and Standards: The criteria for PWHT may be specified in applicable codes and standards, such as ASME Boiler and Pressure Vessel Code, ASME B31.3 for process piping, and API standards for oil and gas equipment. These codes and standards provide guidelines on PWHT requirements based on materials, thicknesses, joint configurations, and service conditions. 

As per Standard ASME B31.3, Chapter V, Para. 331 explains about heat treatment’s criteria,

 

1.3 Advantages of PWHT:

1. Stress Relief: PWHT can relieve residual stresses that may have developed during welding, reducing the risk of distortion, cracking, and premature failure of the welded structure. This can result in improved dimensional stability and reduced distortion, leading to better fit-up and alignment of welded components.
2. Improved Mechanical Properties: PWHT can improve the mechanical properties of the welded material, such as strength, toughness, and ductility. It can also refine the microstructure of the welded material, which can lead to improved fracture resistance and reduced susceptibility to brittle fracture.
3. Reduced Risk of Cracking: PWHT can help to reduce the risk of cracking in welded structures, particularly in materials and joint configurations that are more susceptible to cracking during welding, such as high-strength steels, thick sections, and complex joint configurations.
4. Reduced Hydrogen Embrittlement: PWHT can mitigate the risk of hydrogen embrittlement, which is a phenomenon where hydrogen can diffuse into the material during welding and cause embrittlement, leading to reduced mechanical properties and increased risk of cracking. PWHT can help to diffuse and reduce the hydrogen content in the welded material, thereby reducing the risk of hydrogen embrittlement.

1.4 Disadvantages of PWHT:

1. Additional Time and Cost: PWHT can add time and cost to the welding process, as it requires additional heating and cooling time, specialized equipment, and skilled personnel. This can increase the overall fabrication costs and project timeline.
2. Material and Energy Consumption: PWHT requires additional energy for heating and cooling, which can result in increased material and energy consumption, and may have environmental implications.

1.5 Applications of PWHT:

1. Pressure Vessels and Boiler Components: PWHT is often required for pressure vessels, boilers, and other critical equipment used in industries such as oil and gas, petrochemical, power generation, and chemical processing. These structures are subjected to high-pressure, high-temperature, and cyclic loading conditions, and PWHT can help improve their mechanical properties and reduce the risk of cracking and failure.
2. Piping Systems: PWHT may be required for certain piping systems, particularly in high-temperature, high-pressure, and critical service applications where the welded joints are subjected to significant stress and loading. PWHT can help to reduce the risk of cracking and improve the integrity of the welded joints.
3. Structural Steel: PWHT may be applied to welded structural steel components, such as bridges, offshore structures, and heavy machinery, to relieve residual stresses, improve mechanical properties, and reduce the risk of distortion and cracking.
4. High-Strength and Thick Sections: PWHT may be required for materials with high-strength properties and thick sections, as these materials are more susceptible to cracking during welding and may require stress relief and improved mechanical properties through PWHT.

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