Avoiding PWHT - Can it be justified?

Amin Muhammed and Julian Speck

Dr Amin Muhammed ( amin.muhammed@twi.co.uk) is a principal integrity engineer, who undertakes a range of risk and reliability assessments on safety-critical structures.

Julian Speck ( julian.speck@twi.co.uk) is TWI's structural integrity department manager, responsible for FFS and RBI projects and development activities.

Paper published in Inspectioneering Journal, May/June 2005 Volume 11, Issue 3

Code requirements

Current BSI and ASME codes for the construction of pressure vessels, boilers and piping specify that post-weld heat treatment is required if the thickness of the components being welded exceeds a specified value. This value dependson the type of material being used, and varies from code to code. An alternative procedure is available for deciding whether or not PWHT is necessary to avoid the risk of failure by fracture. This involves conducting a fracturemechanics assessment using procedures such as those in BSI 7910 (or API 579). The use of these procedures is permitted in the British pressure vessel standard BS PD 5500:2003.

Alternative to code rules

Welding thick walled components generates residual stresses that can be the cause of failure mechanisms such as brittle fracture and stress corrosion cracking. A criterion for PWHT based on a fracture mechanics assessment is morecomplicated than the code criterion of thickness alone. It may at first seem unlikely that designers, owners or certifying authorities would abandon the thickness-based criteria in favour of a more complicated approach. However, thereare cases when PWHT is a code requirement but it may be considered unnecessary, excessively expensive, or practically impossible. In these cases, a fracture mechanics assessment may be used, subject to the agreement of the concernedparties. A fracture mechanics approach is based entirely on avoidance of failure by fracture or plastic collapse, Fig.1. Inspection engineers should also give consideration to the influence of heat treatment on avoiding other mechanisms such as fatigue and stress corrosion cracking, before adopting this approach.

The boiler operator argued that a plausible embedded flaw height might be up to 12.5mm (½in). (This would be the height of a root flaw which could have extended as a hydrogen crack in the weld metal and HAZ, mainly below theheader outer surface). Such a flaw may be either longitudinal or transverse with regard to the welding direction. Longitudinal flaws were considered to have a maximum length equal to the weld circumference, and transverse flaws wouldbe limited to the width of the weld.

Assessment results

The results of the fracture mechanics assessment demonstrated that the assumed embedded flaws in the as-welded condition were acceptable, i.e. are non-critical in terms of fracture and plastic collapse. The 12mm (~½in) deepsurface flaws assumed to exist in the header were also acceptable. The critical height of a surface flaw in the stub, with a length equal to the weld toe circumference, was found to be only 5.3mm (0.2in). If the minimum surface flawheight that can be reliably detected using visual or magnetic particle NDE is less than the tolerable height, say 3mm (~1/8in), then larger unacceptable flaws (height >5.3mm) can be detected by NDE and dealt with. Therefore, it maybe concluded that non-detectable surface flaws do not threaten the integrity of the stub repair in the as-welded condition. Based on the above and assuming that no other mechanisms (eg. creep-fatigue) may lead to extension of theoriginal flaws, it was concluded that the weld repair was fit-for-service under operating loading in the as-welded condition.

Financial justification

It was shown that avoiding PWHT was technically justified. The cost of this fracture mechanics analysis was negligible in comparison with the total cost associated with carrying out PWHT on site which was cumbersome and expensive.The main advantage of the codes' thickness criterion is its simplicity. Unfortunately, it is not possible to use fracture mechanics to justify a general relaxation or rationalisation of the thickness criteria in the codes. The chancesof making a successful case for avoidance of PWHT are best with a good knowledge of the fracture mechanics input parameters. Assumptions regarding fracture toughness, flaw sizes and applied stresses can be crucial to the outcome of theanalysis.