Post-Weld Heat Treatment (PWHT) As Per ASME B31 3

Current design codes for pressure vessels and piping specify a thickness threshold at which postweld heat treatment (PWHT) becomes mandatory, typically tied to Charpy test properties of steel material.

PWHT of large carbon steel assemblies is an expensive process due to the lengthy hold times and slow heating/cooling rates required, and it would therefore be advantageous if exemptions could be found from PWHT requirements wherever possible.

Limiting Thickness

Current design rules in the piping and pressure vessel industries dictate that PWHT must be performed whenever the thickness of welded components exceeds a certain value, usually determined by material’s Charpy test parameters and minimum service temperature requirements. While this approach provides an easy, straightforward means of assessing whether PWHT is necessary, its interpretation can sometimes be too conservative; discrepancies among various codes exist as to their limiting thickness requirements.

Inconsistent requirements and exemptions between codes often stem from industry sector-specific practices rather than specific metallurgical or structural considerations. The research conducted for this project investigated whether it would be possible to achieve greater consistency through fracture mechanics methodologies.

Fracture mechanics theory allows us to justify lowering the PWHT limit without jeopardizing weldment fabrication integrity. Studies have demonstrated this effect; for example, socket welds in girth butt-welded pipe materials with low carbon content may qualify for reduced thickness requirements as long as minimum soak periods and special repair welding technique requirements are fulfilled.

Charpy Test

The Charpy test is an indispensable way of measuring impact resistance and toughness of various materials, providing engineers and welding professionals with invaluable insight into how materials will perform under stressful conditions, helping them make informed decisions and take appropriate actions.

The standard test specimen consists of a 10 mm square-section bar measuring 10mm x 10mm x 55 mm in dimensions with one face featuring a V-shaped notching machined out at an angle of 45 degrees, typically with 0.25mm tip radius and typically at 45 degrees from one face. Impacting with a pendulum bar and measuring the energy absorbed Cv, tests are conducted.

Current codes for pressure vessels and piping stipulate that PWHT testing must occur when weldment thickness exceeds a predefined value derived from Charpy test results, yet variations among codes exist considerably, some due to conservatism or differences in engineering practice. This article investigates why there may be differences in requirements among codes as well as providing methods that might allow more consistent approaches to exempting PWHT.

Exemptions

Minor differences exist among various codes regarding requirements and exemptions; these differences can be attributed to different engineering practices or interpretations of technical data. As a result, they could cause unnecessary PMHT which leads to unnecessary maintenance costs and outage durations.

Current pipe code regulations mandate that socket welds using P-4 and P-5A materials do not need to undergo mandatory PWHT if their throat thickness does not fall below 1/2 in. This exemption was developed based on successful practices from the petrochemical industry that date back to the 1920s and 1930s.

An exemption should also be amended to account for weld throat thickness as well as B31.3’s governing thickness requirement. Relaxing this requirement could yield significant cost savings to the power generation industry by decreasing unnecessary PWHTs and shortening outage durations. Furthermore, more uniform treatment of these steels across codes should reduce fatigue stresses and extend equipment life; current exemptions should be modified to account for both weld throat thickness as well as B31.3 thickness requirements.

Exigences

As welding joints can be affected by any number of variables that impede their strength, it’s impossible to provide an exact date and time when PWHT will become necessary. However, thickness limitations based on service needs and material composition may be met through sufficient preheat temperatures, soak times, and controlled cooling systems.

Preheat temperature and soak time requirements vary according to welding technique; as a general guideline, one hour of peak value preheat should be maintained per 25mm weld thickness.

The requirements governing preheat applications vary across various regulating codes. For instance, ASME B31.3 lists its requirements based on pipe materials while BS EN 13445 [25] suggests taking a more conservative approach. To meet PWHT requirements successfully regardless of which standard or code is being adhered to, satisfactory weld procedure qualification must be demonstrated that meets minimum crack growth resistance criteria set by ASME Section IX and meets any other applicable criteria such as these must also be demonstrated.