Inspectioneering
Inspectioneering Journal

Avoiding PWHT - Can It Be Justified?

By Amin Muhammed at TWI, and Julian Speck at TWI Ltd.. This article appears in the May/June 2005 issue of Inspectioneering Journal

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 depends on 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 fracture mechanics 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 more complicated 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, there are 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 concerned parties. 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.

Figure 1. Fracture mechanics assesses the combined effects of flaw size, material properties and stresses acting on the flaw.Figure 1. Fracture mechanics assesses the combined effects of flaw size, material properties and stresses acting on the flaw.

 

Another Option: ‘Patch’ PWHT

When a component is too large to be furnace heat treated, local heat treatment of a circumferential band is allowed. Both API 570 and AWS D10.10 allow for the possibility of local heat treatment band on components, subject to various precautions. (British codes specify the size of local ‘heated band’ and American codes specify the size of the ‘soak band’). The ASME B&PV Code will allow the width of the patch to vary provided the resulting temperature gradients are not harmful.

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Comments and Discussion

Posted by Kalimuthu Karuppan on November 11, 2013
The article is very good. The pictures are... Log in or register to read the rest of this comment.

(Inspectioneering) Posted by Nick Schmoyer on December 2, 2013
Dear Kalimuthu Karuppan, Thank you for pointing... Log in or register to read the rest of this comment.

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