ASME PWHT Requirements

Power and process industries face issues related to variations between codes in terms of PWHT requirements for boilers. While some of these differences can be justified, others seem less important from a technical viewpoint.

Current fabrication codes typically exempt certain structures from PWHT due to their Charpy fracture toughness at their minimum service temperature; however, these criteria are often subjective and vaguely defined.

What is PWHT?

Post Weld Heat Treatment (PWHT) of steel components is an essential step in the welding process, involving heating them at high temperatures for an extended period to correct thermal distortion, weakening strength and toughness of metal, which may render pressure vessel equipment more prone to failure. PWHT’s primary goal is restoring these properties so as to restore strength to both tensile and yield strengths of material.

Current fabrication standards for pressure vessels and piping, such as ASME Section VIII and ASME B31.3, mandate PWHT for weldments that exceed a specified thickness threshold. This limit thickness requirement typically takes into account Charpy test properties of material as well as intended service conditions when setting its thickness requirement – however this approach has been known to be too conservative, leading to anomalous values across various codes for PWHT requirements.

As Table 1 indicates, various exemptions from PWHT are provided by these codes, making rationalization opportunities available within each code more achievable. It would be difficult, however, reconcile petrochemical industry specifications with power generation sector ones; so it is vitally important to recognize their differences and conduct weld procedure qualifications on every weldment according to specifications and ensure their conformance.

Why is PWHT required?

PWHT is essential due to conventional welding processes generating significant residual stresses that could reach or surpass yield strength of either parent metal or weld material. Postweld heat treatment serves to mitigate these residual stresses while tempering weld metal microstructure, decreasing risk of environmental assisted cracking.

However, heating and cooling large steel structures is an expensive endeavor, especially if multiple cycles are required over the lifecycle of pressure equipment. Furthermore, this process can cause distortion to equipment that compromises its dimensional accuracy and structural integrity.

Users seeking to reduce costs will naturally want to minimise the time their equipment is exposed to PWHT processes, making composite materials a viable alternative option. As a result, pressure equipment repair specialists must understand these potential solutions for PWHT treatment.

Review of PWHT exemption requirements in various codes, such as those for pressure vessels and piping (Table 1) as well as general structural standards like PD 5500 [22] and EEMUA 158 [23], has shown considerable variance among them. The differences stem mainly from differences in design stress criteria, inherent Charpy toughness requirements and allowable defect sizes that vary among them.

PWHT Temperature Ranges

Petrochemical and power industries each have unique requirements for post weld heat treatment, partly due to varying codes used; but more importantly due to how each code treats its respective material involved.

PWHT (Post Weld Heat Treatment) involves heating a weld to temperatures below its lower critical transformation temperature and holding it there for a specific amount of time, in order to relieve metal stress and increase toughness. PWHT temperatures differ between petrochemical and power industry codes; those within the former typically have higher limits for treatment temperatures.

As such, there can be significant variance in the PWHT requirements across various code sections, leading to inconsistency in PWHT requirements and creating safety risks when heating to PWHT temperatures that result in distortion of components which subsequently struggle to support their own weight when heated to their required PWHT temperatures. To prevent such potential complications from arising during PWHT operations, supports tailored to each component must be strategically positioned throughout a structure before PWHT begins in order to mitigate such potential hazards.

EPRI testing (Ref. 1) and research by Lundin and Khan (Ref. 6) suggests that the lower critical temperature used by B31.1 Code for Power Piping of 1100degF may be too low; instead, these researchers found that using 1200degF as the PWHT would result in faster changes to properties, with increased toughness and decreased hardness of weld HAZs.

PWHT Exemptions

Multiple Code sections exempt certain thicknesses of P-4 and P-5A chromium-molybdenum steel weldments from mandatory postweld heat treatment (PWHT), though rules vary from Code to Code and depend on thickness; ASME BP&V Section VIII allows thicker than 5/8 in weldments to avoid PWHT; other codes use other criteria, including weldability or the ability of weldments to resist hydrogen uptake as indicators for exemption.

Weldability is often used as the sole determinant for whether a material requires PWHT treatment. In particular, its impact in terms of welding components without incurring defects during or immediately post welding (weldability) is crucial. Furthermore, this aspect also plays a vital role when designing nuclear-service welds and weldments.

Weldability refers to the ability of welded structures or weldments to resist stress concentrations caused by residual stresses from welding process residual stresses and/or geometry of their weldment. To minimize hydrogen delayed cracking risks, an appropriate postweld bake may include electrical resistance heating or driving off any trapped hydrogen with inert gas such as nitrogen. Considering these considerations it would seem prudent to relax current Code requirements so as to significantly decrease PWHTs conducted for nuclear pipe materials.