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.
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.
Comments and Discussion
Add a Comment
Please log in or register to participate in comments and discussions.