ASME Section VIII PWHT Requirements

ASME Section VIII contains an array of regulations and guidelines designed to protect pressure vessels, from their design through construction, inspection, and testing. It covers developing, constructing, inspecting and testing processes as well.

McEnerney recommended raising the minimum temperature for PWHT testing of SS4 materials from 800degF to 1200degF, in line with Lundin and Khan (Ref. 6).

PWHT Requirements

ASME Section VIII Div 1 table UCS-56 [1] shows there is significant variance among PWHT requirements of different codes for pressure vessels and piping, due to engineering applications (e.g. design stress criteria) or different inherent Charpy test requirements as well as differences in allowable defect sizes through inspection codes.

Post weld heat treatment is a controlled process in which reheated weld metal is brought up to its lower critical transformation temperature for an extended period of time and held there. This helps remove residual stresses and microstructural changes caused by welding’s high-temperature gradient.

PWHT requirements vary among various codes in terms of thickness limiting requirements and minimum preheat temperature and duration allowances, dependent upon steel’s composition, toughness and application type.

PWHT requirements tend to decrease with an increasing alloy content and, consequently, require higher preheat temperatures and/or reduce maximum carbon levels allowed. As such, rationalisation of PWHT requirements becomes difficult; particularly as different user groups such as petrochemical and power generation users often have differing interests in gaining exemption from PWHT and may struggle to agree upon a single limit for exemption from its requirements.

Requirements for Preheat

Preheat is the application of heat to the base metal of an item prior to welding, which aims to prevent cracking in the weld zone and increase strength of that component. Preheat can also help avoid hydrogen induced cold cracking (HICC) while improving final mechanical properties of welds and HAZs. Preheat requirements can be determined by welding codes or welding procedure specification (WPS) documents specific for any given project.

A WPS will detail the minimum preheat temperature and duration required for any project. In general, minimum preheat temperatures should be maintained for at least 30 minutes to achieve optimal results. Parts can be verified to meet this temperature using either thermocouple measurements or crayon tests that mark surfaces when it has reached certain temperatures.

Consideration should also be given to material thickness and restraint level when deciding upon a preheat temperature. When applying the preheat, its temperature should exceed both weld zone and base metal temperatures – keeping in mind it needs to remain at this level until welding has concluded.

When welding codes don’t specify when and how preheat should be used, it is the responsibility of a welding engineer to determine whether and what temperature preheat is needed for their base metal and section thickness. AWS D1.1-96 offers two methods for this determination – HAZ hardness control method and hydrogen control method.

Requirements for Post Weld Heat Treatment

Post Weld Heat Treatment (PWHT) can reduce welding residual stresses, temper hard, potentially brittle microstructural regions and enhance tensile strength and toughness of welded materials. PWHT involves heating welds to a specified temperature for an extended period; sometimes preheating must also be used concurrently in order to achieve maximum results.

Most current fabrication codes mandate welds exceed certain threshold levels to be subjected to PWHT treatment, varying between codes depending on fracture toughness requirements at minimum service temperature and Charpy energy absorption values of various material grades.

PWHT involves heating the welded materials to very high temperatures for an extended period of time, then holding them at that temperature without distortion or other forms of damage occurring to them. PWHT can be costly; thus it makes sense to try to minimize its usage whenever possible.

Fabrication codes often allow for exemptions to the PWHT requirement; these may require negotiation based on detailed fracture mechanics analysis. It may even be possible to omit it altogether if multi-pass welding and preheating are used in combination with PWHT; similarly, certain low carbon steel grades like 1-1/4 Cr Mo may allow for a higher temperature range PWHT process.

Exemptions

ASME Section VIII contains comprehensive requirements for pressure vessel design, fabrication, assembly, erection, examination inspection testing and certification. Careful implementation of this Code section enables manufacturers, users, constructors and designers to comply with regulations within their jurisdiction while reaping cost, safety and operational benefits from numerous industry best practices outlined within.

Although Section VIII’s PWHT requirements are among the strictest in the industry, certain welding codes do offer flexibility when welding materials such as pipe. For instance, ASME B31.3 allows exemption from this requirement for pipes measuring 4 inch NPS or less with nominal wall thickness below 0.625 inch; it should be noted however that there can be significant variations across Codes and industries for PWHT requirements which requires careful consideration to ensure consistency and effectiveness.

At present, newer as-welded and PWHT curves based on advanced fracture toughness technology have shown to help more closely align PWHT requirements with actual weld residual stress levels. Furthermore, pilot operated relief valves (POPRV) can sometimes eliminate the need for PWHT in some applications, thanks to their non-flowing design that ensures consistent overpressure relief well below their PRV set point; this helps significantly lower risks of overpressure damage to equipment as well as costs of ownership while providing operators with increased peace of mind.