What is Post Weld Heat Treatment (PWHT) and Why is it Necessary in Welding?

Post Weld Heat Treatment (PWHT) is a controlled process in which materials that have been welded are heated back up below their critical transformation temperature and held there for a predetermined period. PWHT is an essential step in welding processes and many codes mandate post weld heat treatments as mandatory steps.

Definition

Welding generates large residual stresses in materials, which combine with load stresses to exceed design limits of materials. As a result, welded structures are more prone to hydrogen induced cracking and stress corrosion cracking; to mitigate this risk post weld heat treatment (PWHT) is often employed.

PWHT (Process for Weld Heat Heat-Treatment) involves heating a weldment to an optimal temperature and then cooling it, with time spent at each temperature varying based on local laws and material specifications. PWHT not only reduces residual stresses but can also help increase toughness and ductility of weldments.

Weldments that require high PWHT temperatures require proper support during heat treatment. Trestles shaped specifically to the component can provide this support and be placed regularly along the structure to manage its thermal gradient between sections experiencing PWHT and those still cold.

PWHT requirements usually depend on the material’s chemical makeup and thickness; ASME Section VIII mandates it for materials over a certain thickness, while others depend on what kind of service the structure will undergo, for instance nuclear power plants must comply with stringent safety and reliability standards when welding components together.

Meaning

Post Weld Heat Treatment or PWHT is a process which alters the microstructure of weld areas to reduce residual stresses and enhance strength, as well as to decrease hydrogen induced cracking risks and stress corrosion cracking risks in weldments. PWHT should be seen as an essential step in welding processes in order to guarantee safe operations of your equipment.

PWHT needs depend on several factors, including material type and thickness as well as any potential for hydrogen-induced cracking and stress corrosion cracking. PWHT may be required by codes or standards based on its chemical makeup rather than just thickness alone.

At some point during welding processes, a temperature gradient between weld metal and parent material occurs, leading to stress in the weld metal that could eventually exceed your design limits. PWHT offers you the opportunity to reduce this internal tension by heating the weld metal for an extended period at a specified temperature.

Though PWHT can reduce residual stresses effectively, its use may prove dangerous if not supported properly during heating process. If any component cannot support its own weight at PWHT temperatures, then appropriate support must be provided with trestles that fit to fit its shape and space appropriately; excessive distortion may occur as result of thermal expansion differential between different parts of a structure.

Scope

Post Weld Heat Treatment (PWHT) is a controlled process where material that has been welded is heated above its lower critical transformation temperature and held there for an extended period. The primary goal of PWHT is to relax residual stresses and realign microstructure within the weld area to decrease susceptibility to cracking, fatigue, and corrosion.

PWHT testing is often necessary when designing pressure vessels and piping systems, with industry codes like ASME Pressure Vessel and Piping Codes mandating its performance on certain materials to ensure safe designs with optimal mechanical and metallurgical properties.

PWHT may be necessary, depending on the chemical make-up and thickness of material as well as service conditions in which the component will operate. PWHT is effective at relaxing residual stresses created during welding as well as reorienting weld metal to improve ductility, toughness, and strength within its weld area.

As stress levels increase, so too do requirements for PWHT. Relaxation of welding stresses depends upon both time and temperature during PWHT treatment cycles; to minimize distortion, temper embrittlement, and oversoftening issues while realizing its benefits without detriment to tested mechanical properties, heating/cooling rates/tolerance/soaking times must all be properly managed; in order to reap all its advantages.

Terminology

Post Weld Heat Treatment (PWHT), also known as postweld heat conditioning is a technique designed to relieve and rebalance residual stresses caused by welding processes as well as microstructural changes introduced into material by welding itself. PWHT may also be used to control hardness or improve mechanical strength – an integral component of most welding procedures and often mandated by industry codes such as ASME Pressure Vessel and Piping Codes.

PWHT helps prevent hydrogen-induced cracking (HIC), by diffusing ambient hydrogen into the weld and increasing overall joint strength.

PWHT (Pressure Wash Heat Treating) is another process used to reduce stress corrosion cracking potential on welds, by gradually heating and slowly cooling them to temperatures lower than critical transformation temperature, gradually decreasing stress levels in the weld. PWHT forms an integral component of all weld procedure qualification specifications – any change to its temperature or soak time requirements requires recertification; similarly it plays an essential role when welding vessel seams that feature combination groove and fillet welds which often produce higher residual stresses than single groove or fillet welds due to volumetrical weld metal usage compared with single groove/fillet welds alone.