TWI Industrial Member Report Summary 679/1999
R H Leggatt, A Muhammed, A T Smith and M J Cheaitani
Background
Surveys of Member companies conducted by TWI and EWI have shown that there is a strong demand for research aimed at reducing the requirement for postweld heat treatment (PWHT).
Current design codes for the design of pressure vessels, boilers and piping specify that PWHT is required if the thickness of the parts being welded exceeds a specified value. This value depends on the type of material being used, and varies from code to code. The use of a thickness criterion for PWHT provides a simple and direct method for determining whether PWHT is required.
An alternative procedure for deciding whether PWHT is necessary to avoid the risk of failure by fracture is by conducting a fracture mechanics assessment using a recognised procedure such as that described in PD6493: 1991. A criterion for PWHT based on a fracture mechanics assessment is more complicated than a criterion based on thickness alone. It seems unlikely that designers, owners or certifying authorities would, in general, wish to abandon the thickness-based criteria in favour of a more complicated approach.
However, there are many cases in which PWHT is required by the appropriate code, but may be considered to be unnecessary, excessively expensive, or impossible. In these cases, a fracture mechanics assessment may be used, subject to the agreement of the concerned parties, to determine whether PWHT is necessary for the avoidance of failure by fracture.
Objectives
- To illustrate how the fracture assessment procedures of PD6493: 1991 may be used to make a case for exemption from PWHT.
- To provide examples of the technical and economic benefits which may be obtained using these procedures.