Post Weld Heat Treatment (PWHT) Requirements As Per ASME Section VIII, Division 1

ASME Section VIII, Division 1 is an important engineering standard that provides clear guidelines for creating, inspecting, and certifying pressure vessels. This contributes to standard engineering practices while improving safety and efficiencies across different industries.

Post weld heat treatment (PWHT) is a controlled process designed to eliminate residual stresses in welded materials and is defined in clause 331.11 along with tables 331.1.1.1 and 331.1.3. It must comply with requirements set out by clause 331.11.1.2 as set out below for piping applications.

Minimum Temperature

Post Weld Heat Treatment (PWHT) involves heating weld metal to temperatures below its lower critical transformation temperature and holding for an extended period. This helps reduce residual stresses and microstructural changes caused by welding; however, PWHT is often performed uncontrolled with temperature gradients potentially harming weld metal, leading to stress cracking or increasing its susceptibility to brittle fracture.

Current codes requiring PWHT for power piping stipulate a minimum PWHT temperature of 1100degF, which is too low and can lead to premature corrosion damage. An EPRI sponsored study conducted by Souder and Lundin and Khan advocate for raising it to 1200degF; doing so would result in significant changes to weld metal properties like increased toughness and decreased hardness.

Reducing the minimum PWHT temperature could significantly decrease its ductile-to-brittle transition temperature of weld metal, potentially leading to unacceptable reductions in its tensile strength and ductility, necessitating an increase in design pressure of code stamped vessel as well as being very costly for both pipe makers and pressure vessel manufacturers. It is thus imperative that manufacturers find a balance between these requirements when setting minimum PWHT temperatures.

الحد الأدنى من وقت الانتظار

PWHT (post weld heat treatment) is a post weld heat treatment process in which steel is heated to its lower critical transformation temperature and held there for a specific duration to reduce residual stresses and microstructural changes in weld metal. PWHT is often required when fabricating pressure vessels and pipes; however, as it can be costly and time consuming. In order to save costs it may be worthwhile trying to secure exemption from PWHT where possible.

Pressure vessels, piping and storage tanks must meet PWHT requirements determined by design codes used during their construction. Requirements vary widely across these codes – for instance BS 2633’s requirements differ significantly from ASME VIII Div 1 or B31.1/B31.3 which set maximum thickness limits at 32mm; there may also be significant variance between general structural codes such as BS 5400/5750 as well as those from industries seeking exemption from PWHT requirements like petrochemical and power generation industries who have sought wider exemption.

Current design codes rely on Charpy impact test results as their limiting thickness criterion; however, such requirements may not always reflect real toughness and residual stress data available for steels used for fabrication.

درجة الحرارة الدنيا للتسخين المسبق

Preheat temperature is essential to the success of post weld heat treatment (PWHT). PWHT serves to redistribute residual stresses created by welding, making the material more ductile while attenuating residual stresses created during welding process. Furthermore, post weld heat treatment also serves to remove microstructural changes caused by high-temperature gradients between weld metal and parent material during welding processes.

PWHT testing is currently mandated for most pipe and pressure vessel materials; its requirements typically follow Charpy energy absorption limits; thus there can be considerable variation in the minimum thickness at which exemption from PWHT can be granted due to different design codes used by petrochemical industry which have developed their specifications over time through experience and engineering practice.

Some requirements have become marginal from a technical viewpoint and, specifically, the minimum PWHT temperature has come under scrutiny. An EPRI sponsored report (Ref 1) suggested lowering this requirement to 1100degF which would bring it close to meeting minimum Charpy energy absorption limits used in both NBEP and B31.1 codes for P No 4 material used as weld metal; however this lower preheat temperature has been rejected by Subcommittee on Pressure Vessels of US due to concerns of reheat cracking occurring due to concerns of cracking of weld metal during welding operations (Ref 1).

Minimum Preheat Time

As part of ASME Code Section VIII Div 1, to qualify as exempt from postweld heat treatment (PWHT), weldments must meet two criteria. They must feature both a minimum preheat temperature and holding time before qualifying as exempt. PWHT treatments reduce brittle fracture by lowering welding residual stresses while tempering hard, potentially brittle microstructures, helping avoid potential failure scenarios.

Different fabrication codes impose various exemption requirements for steels. Some require PWHT even in thinner weldments than others, depending on its carbon content; for instance, BS 1113 [23] and 2633 [24] do not mandate it when welding weldments with carbon content exceeding 0.25 percent.

Even though PWHT is generally understood, its precise requirements remain contentious. Particularly within power generation applications, its necessity has come under debate. Variations between fabrication codes’ exemption requirements could be attributable to different professional bodies drawing them up according to their individual experiences; yet such variances exist and it seems unlikely that one unified requirement for exemption from PWHT can ever be reached; this article explores these similarities and variations while proposing possible rationalisation strategies.