Post Weld Heat Treatment

Post weld heat treatment (PWHT) is a widely practiced process in field fabrication and repair work, used to reduce residual stress to soften weld material hardness and increase strength.

PWHT requirements depend on both pipe material and ASME code requirements; Table 331.1.1 provides the PWHT holding temperature ranges applicable to various materials in Clause 331.1.1.

Strength

Post Weld Heat Treatment (PWHT) is a controlled heating and cooling process applied to welded components to improve their mechanical properties and relieve residual stresses. PWHT increases the strength and hardness of welded materials, reduces weld defects, and protects against environmental assisted cracking. PWHT also promotes dislocation movement within welded materials’ crystal structures, helping to relieve residual stresses and lower stress concentrations. Piping subjected to environmental stresses typically requires PWHT; for a detailed discussion on requirements specific to various pipe materials please see clause 331.1.1 along with tables 331.1.1, 331.1.2, and 331.1.3 in ASME B31.3.

This SEM photograph depicts the differences in fracture surfaces between samples that have undergone heat treatment and those that do not. Heat treated samples display smoother fracture surfaces, while untreated ones reveal brittle fracture mode with river patterns.

Toughness

Post Weld Heat Treatment (PWHT) is a reheating and cooling process used to increase the toughness of welded materials, alleviate residual stresses, and decrease risk of weld defects such as cracking. PWHT is especially important when pipe is subjected to environmental stress or strain, such as in oil/gas pipelines or nuclear power plants; specific standards like ASME Section VIII frequently stipulate post weld heat treatment requirements for these kinds of piping applications.

PWHT processes typically utilize a controlled ramp-soak profile at temperatures that are lower than their material’s initial transformation temperature, to minimize potential negative side effects such as reheat cracking, distortion and loss in toughness. It is critical that ramp/soak rates and holding temperature be managed effectively in order to achieve successful processing results.

SEM images below compare welded NiTi wire samples without and with PWHT treatments, the latter producing smoother fracture surfaces without river patterns on their fracture surfaces. PWHT treatments have been found to significantly decrease environmental assisted cracking sensitivity of materials while increasing service lives.

Corrosion Resistance

Under conditions of corrosion, welded joints may become brittle and fail. PWHT helps mitigate this risk by lowering stress levels in the weld region; additionally it improves resistance against hydrogen damage and other forms of corrosion.

PWHT is an established technique for increasing the durability of welded structures in chemical plants, petroleum refineries and other industrial facilities. This process helps minimize weld failure risk while simultaneously increasing material strength through both tensile strength and yield strength increases. Furthermore, PWHT helps prevent stress corrosion cracking as well as other forms of corrosion within its materials.

Multiple welding processes generate residual stresses in the weld zone that exceed material design limits, leading to fatigue crack growth. PWHT can significantly lower these residual stresses and alter crack propagation paths/rates.

Many piping weldments require plasma weld heat treatment (PWHT). PWHT can reduce susceptibility of welds to metallurgical degradation, increase strength and ductility, and increase corrosion resistance against aggressive corrosion conditions. Unfortunately, PWHT can have negative repercussions for weldability and service performance; hence it is crucial that its benefits and costs be assessed thoroughly before undertaking this treatment. In this report we investigate its origin and basis as well as current requirements and exemptions of PWHT in nuclear service environments.

Durability

Post weld heat treatment (also referred to as stress relief) is a thermal process designed to lower the risk of brittle failure caused by residual stresses created during welding processes. Residual stresses in weld areas can combine with load stresses beyond material design limits and result in cracking, fatigue and ultimately brittle failure if left unchecked; PWHT promotes dislocation movement within crystal structures to release these tensions and is necessary whenever there is the potential for environmentally assisted cracking in weld joints.

PWHT requirements are set forth by various governing codes, with conditions varying depending on the material of a pipe and its Group number. ASME B31.3 contains mandatory PWHT regulations depending on piping material composition; however, exemptions exist from mandatory testing if welds made with thicker pipes constructed of specific welding materials are constructed.

PWHT requirements in power plant piping become less essential as pipe diameter increases while maintaining constant wall thickness, due to decreasing values for tangent of angle between longitudinal stress vector and weld direction (theta “th”). As such, residual stresses created during welding become an even greater cause for concern with thick walled pipes than with thin walled ones.