Overview of Post Weld Heat Treatment (PWHT)
by Admin
Posted on 12-09-2023 05:14 PM
Some of the factors that cause pipe damage that often occur in pipeline system are corrosion, erosion, welding defects, material defects, vibration stress and mechanical damage. Damage in the weld area, often related to the quality of the weld in the haz (heat affected zone) area and its residual stress.
One of the ways to improve the quality of welds is by doing post weld heat treatment or we call it pwht. Pwht is part of the heat treatment process which aims to remove the residual stress that is formed after the welding process is complete. The pwht process can be done in two ways, namely inserting the object into the furnace ( full pwht for complete equipment e.
What is post weld heat treatment (pwht)? pwht is a controlled process in which a welded material is reheated to a temperature below the lower critical transformation temperature and held at that temperature for a period of time. The need for pwht is mainly due to the residual stresses and microstructural changes that occur after welding. At delta heat services, we specialize in heat treatments and are happy to explain the entire process.
Post welding heat treatment (pwht), also referred to as stress relieving, is an application of uniform heat applied after welding to a structure, or portion thereof, at a sufficient temperature – below the transformation temperature range and with the intention of retaining the material’s strength/quality and ductility. The primary benefits of post welding heat treatment are recognized as relaxation of residual stress, tempering, and hydrogen removal. If performed incorrectly, or neglected altogether, it can lead to reduced tensile strength, creep strength, notch toughness of the component, as well as the increased susceptibility to corrosion. An important aspect of post welding heat treatment is to prevent brittle trans-granular cracking that can potentially result in catastrophic failures, such as the collapse of a pressure vessel or major piping component.
Recommend Revisions to Post Weld Heat Treatment (PWHT)
The process performed to relieve stresses that developed in welding and to improver microstructure of weld and heat-affected zone is post weld heat treatment (pwht) or stress reliving (sr)
post weld heat treatment (pwht) is the heating process carried out after welding is complete. In some cases, an intermediate bake-out / de-gas will be performed if not going directly into the stress relieving phase. This involves a soak at around 300°c to remove any hydrogen from the weldment and heat-affected zone (haz). Generally, pwht is associated with the stress-relieving process. This is where the metal is heated in a controlled manner to a temperature below its lower critical limit. It is held for a specified duration and then controlled cooling is required. The requirement for pwht is mainly due to the residual stresses and microstructural changes that occur after the welding process.