Also commonly referred to as stress relieving, PWHT is the application of controlled heat applied after the welding is complete. During the welding of metals, the high temperatures and rapid cool down creates stresses in the molecular structure of the parts being welded. When the weld has a PWHT carried out the metallurgical structures and molecules of the weld and parent metal blend together during the heat up, soak period, and cool down to create a finished product that meets the intended design criteria of the parts being heated.
This is achieved by the application of thermocouples, electrical resistance heating elements or induction coils, and insulation being placed around the welded area to achieve a controlled heating temperature, soak period and cooling period. The heating elements and thermocouples are connected to cabling (triple cable sets) that are connected to and controlled by a control console, most control console requires a primary power supply of 480 volts 90 amps 3 phase.
The control consoles on the secondary side have outputs/zones that reduce the voltage output from the primary side through a series of coils/lumps down to 80 volt 45 amps 3.6 Kilowatt per heater being utilized. Making the post weld heat treat operation lower voltage and a safer application for field heat treatment. Each control point/zone can utilize 1,2 or 3 heating elements dependent on the configuration of the parts being post weld heat treated or the code stipulations and control thermocouples that have to be adhered to.
Some of the most prevalent codes that drive post weld heat treatment requirements are:
Some key elements that drive the need for PWHT or Stress relieve are the process the parts being treated will be used in, the metal thickness, and the pressure retaining welds and corrosion resistance in the heat affected zone. All are reasons to carry out post weld heat treatment on a new weld as well as weld repairs on components that have previously been heat treated.
Industry codes and standards for treating are becoming more stringent, Axiom’s knowledge and ability to meet or exceed these requirements are the reason our customers rely on us for a quality driven approach that delivers the maximum return on investment for their heat treatment spend.
Welding is a widely used method for joining metals and alloys, but it can also introduce stress and distortion into the material. To address these issues, post-weld heat treatment (PWHT) is often used to relieve the residual stresses and improve the mechanical properties of the welded joint.
In this article, we will explore the importance of PWHT for stress relief, the potential issues that can arise without proper PWHT, and the common techniques used in PWHT. We will also provide an overview of best practices for PWHT to optimize stress relief and improve the overall quality of the welded joint.
Post-weld heat treatment (PWHT) is a crucial step in the welding process as it helps to reduce residual stresses that can build up in the welded joint and reduce brittle fractures. Residual stress can cause distortion and cracking in the material, potentially leading to failure of the welded structure. PWHT can help to reduce the stress by heating the material to a specific elevated temperature for a certain period of time, allowing the metal to relax and release the stress.
Additionally, PWHT can also improve the mechanical properties of the welded joint, such as increasing its toughness and ductility. This makes the joint more resistant to cracking and failure, especially under dynamic loading conditions. PWHT can also help to reduce the hardness of the welded material, which can make it easier to machine or form.
Overall, PWHT is a crucial step in the welding process that helps to improve the quality and reliability of the joint.
Post-weld heat treatment (PWHT) plays a crucial role in preventing stress cracking in welded joints by reducing the stress in the material. When a metal is welded, the heat generated during the welding process causes the metal to expand. PWHT addresses this issue by heating the entire welded steel structures to a lower critical transformation temperature for a certain period of time. This localized heating causes the metal to expand again, allowing it to relax and release the stress.
Once the welding is complete and the metal cools down, it contracts back to its original size, but this contraction is not uniform throughout the entire welded joint. The result is residual stress in the welded joint, which can cause distortion and hydrogen cracking. In the heat affected zone of a precipitation hardened metal, the precipitates are likely to have changed and become less effective as a result of welding.
As the metal cools down again, the contraction is more uniform throughout the entire joint, reducing the overall stress levels. This reduction of residual stress helps to prevent stress cracking in the joint by reducing the likelihood of the material cracking under dynamic loading conditions.
Post-weld heat treatment (PWHT) is an essential step in the welding process that offers several benefits for stress relief. The primary benefit is the reduction of residual stress in the welded joint. PWHT addresses stress by heating the entire welded joint to a specific temperature for a certain period of time. This heating causes the metal to expand again, allowing it to relax and release the residual stress.
Another benefit of PWHT is that it can improve the mechanical properties of the welded joint, such as increasing its toughness and ductility. This makes the joint more resistant to cracking and failure, especially under dynamic loading conditions. PWHT can also help to reduce the hardness of the welded material, which can make it easier to machine or form.
Additionally, PWHT can also help to prevent stress corrosion cracking in the welded joint. Stress corrosion cracking can occur when a material is subjected to both stress and a corrosive environment. PWHT can help to prevent this by reducing the stress in the material and improving its resistance to corrosion. This is particularly important for industries such as petrochemical, aerospace and marine, where corrosion resistance is critical.
There are several potential issues that can arise without proper post weld heat treatment (PWHT) for stress relief. One of the main issues is an increase in stress in the welded joint, which can cause distortion and cracking in the material. This can lead to a reduction in the tensile strength and ductility of the welded joint, making it more susceptible to failure under dynamic loading conditions.
Another potential issue is an increased risk of stress corrosion cracking. This type of cracking occurs when a material is subjected to both stress and a corrosive environment. Without proper PWHT, the residual stress in the welded joint can increase the susceptibility of the material to stress corrosion cracking, potentially leading to failure of the welded structure. PWHT can help to prevent these issues by reducing the stress in the material and improving its resistance to corrosion.
There are several common techniques used in post-weld heat treatment (PWHT) for stress relief. One of the most commonly used techniques is called annealing. Annealing is a heat treatment process that involves heating the welded joint to a specific temperature for a certain period of time and then slowly cooling it. This process helps to relieve stress in the material by allowing it to expand and contract more uniformly.
Another common technique used in PWHT is normalizing. This process involves heating the welded joint to a higher temperature than annealing and then rapidly cooling it. Normalizing helps to improve the mechanical properties of the material by making it more homogenous and reducing the hardness of the joint.
Lastly, "quenching and tempering" is another technique that uses high-temperature heating followed by rapid cooling, generally by immersing in a liquid medium, this process improves the mechanical properties of the material by creating a hardened surface with a tougher core. This technique is mainly used for high strength materials such as steels.
Axiom HT also has a patented Axi-Therm blended heating and cooling process which can save hours off the typical time to perform on-site pwht services and back online quicker.
Post weld heat treatment (PWHT) can be optimized for maximum stress relief in welding by following best practices and considering the specific properties of the material being welded. The first step to optimize PWHT for stress relief is to select the appropriate PWHT method. Different welding processes and materials require different PWHT methods.
For example, annealing is typically used for low-carbon steels, while normalizing is often used for high-carbon steels. It is important to select the appropriate PWHT method based on the material being welded and the desired outcome.
Proper heating and cooling is also crucial, the material should be heated to the appropriate temperature range for the required period of time, and then cooled at the appropriate rate. Proper equipment and facilities, holding temperature, and quality control are also important aspects to consider when optimizing PWHT for stress relief.
Another important aspect to consider when optimizing PWHT for stress relief is the environment in which the welding and heat treatment are being performed. Certain environments such as high humidity or exposure to chemicals can have a significant impact on the stress relief process.
It is important to take these environmental factors into account and make necessary adjustments, such as using appropriate protective coatings or selecting different welding techniques, to ensure that the PWHT is effective and reliable.
Additionally, it is also important to consider the post-weld usage of the structure, if it's a critical component, PWHT should be done accordingly to guarantee its safety, if it's an aesthetic component, PWHT should be done in a way that won't alter the appearance.
By following these best practices, PWHT can be optimized for maximum stress relief in welding, resulting in improved mechanical properties and increased reliability of the welded joint.
Post-weld heat treatment (PWHT) is an essential step in the welding process that helps to relieve stress and improve the mechanical properties of the welded joint. Without proper PWHT, the welded joint can be susceptible to issues such as distortion, cracking, and stress corrosion cracking.
By following best practices, dependent on the materials being welded and strength of the material , PWHT can be optimized for maximum stress relief.
This includes selecting the appropriate post weld heat treat method, proper heating and cooling, using proper equipment and facilities, monitoring temperature, holding temperature and implementing quality control measures.
Additionally, it is important to consider the environmental factors and the intended usage of the structure after the welding process. By following these guidelines, PWHT can help to ensure the reliability and safety of the welded joint.
If you have any questions about pressure vessel and piping, treatment of welded steel, pressure vessel design, pressure vessel design and other weld metal information, contact the experts at Axiom HT today.
