Everything About Post Weld Heat Treatment (PWHT)
Post weld heat treatment or post welding heat treatment (PWHT) is a heat treatment process which is carried out after welding process itself. Often PWHT is a mandatory procedure due to industry codes such as ASME to ensure a safe design and specific metal characteristics. Although any heat treatment performed after welding can be referred to as post weld heat treatment, in some industries such as the oil and gas and nuclear industries, PWHT has a clearly defined meaning. In steel fabrication, the two most carried procedures of post weld heat treatments are post heating and stress relieving.
When is PWHT needed?
The most common reasons to apply post weld heat treatment are the improvement of the microstructural changes and the residual stress which occurs during the process of welding. Furthermore, PWHT increases the resistance to brittle fracture, as well. Other advantages of PWHT are hardness reduction and the enhancement of material strength. In summary, PWHT improves the mechanical properties of the material after the welding process.
However, the need for post welding heat treatment is often driven by industry codes, such as the ASME and are usually a mandatory requirement to ensure a safe design and optimal material characteristics.
Post Heating
Post heating is a process with the goal to minimize hydrogen induced cracking (HIC). For HIC to occur, the following requirements must be met: the base material must have a sensitive microstructure, a sufficient level and a high level of stress must be present. It is possible to avoid HIC in ferritic steels by diffusing hydrogen from the welded are before it cools to room temperature. Instead of cooling, the workpiece must be immediately heated to the post heat temperature and held there for a specific amount of time. If not dictated by code and service requirements, the common post heating temperature is about 150°C – 300°C. Usually, post heating is not a necessity for most applications. However, the procedure can be a code requirement, for example the ASME Section III. Further, the application can be crucial for critical repairs, as well. When the danger of hydrogen cracking is not present, post heating is often neglected due to higher production costs.
Stress relieving
During the manufacturing process, materials develop residual stress due to high level of restraint and shrinkage forces. These stresses shorten the fatigue life of the workpiece significantly. Therefore, stress relieve heat treatment are applied to reduce the stresses which remain in the material. To relax these residual stresses, the material must be heated to a sufficient but below the transformation temperature range. Then it must be cooled uniformly. Typical temperatures for carbon steels are 600 to 675 °C, while the hold time is determined by the thickness of the steel (1 hour for 25 mm).
The benefits of stress relieving
Stress relieving provides greater dimensional stability in machining. This leads to increased dimensional reliability of the material. Further, stress relieving prevents stress corrosion cracking and improves the metallurgic structure of the workpiece. In addition, stress relieving leads to a lower probability of HIC. However, post heating is the less expensive option for the minimization of HIC.
Considerations before performing PWHT
Before performing post welding heat treatment, the previous heat treatments and the alloying materials of the steels must be considered. If the working temperature exceeds the tampering temperature of the material, PWHT can have negative effects on the characteristics of quenched and tempered alloy steels. Furthermore, stress relive cracking can occur during the process. On the other hand, some materials, such as chrome-molybdenum, almost always require post welding heat treatment Therefore, the steel grade and the application of the material must be determined before PWHT.
Enhance Your Welding Outcomes with Advanced Surface Treatments!
After understanding the significance of Post Weld Heat Treatment (PWHT), it’s crucial to consider advanced surface treatments that can further enhance the durability and performance of your welded components. At BorTec, we offer a range of specialized services:
- Boronizing with BOROCOAT®: Achieve high hardness and exceptional protection against wear. Perfect for industries like valve construction, power plant technology, and more. Discover the benefits of Boronizing.
- Hardening of Stainless Steel using BORINOX®: Elevate the hardness of your stainless steel components, ensuring high wear protection. Suitable for a wide range of stainless steels. Learn about the BORINOX® Process.
- Nickel Plating with NICKELCOAT®: Get distortion-free, hardenable nickel plating that offers superior corrosion resistance, especially in acidic environments. Ideal for components in the oil & gas industry, valves, drive shafts, and more. Explore the advantages of Nickel Plating.
Don’t compromise on the quality and longevity of your welded components. Choose BorTec for state-of-the-art surface treatments that deliver unmatched results!
