Abstract: There are two main purposes should be considered at the same time during postweld heat treatment (PWHT). One is to relieve the residual stresses, and the other is to improve the properties of the welding joint. Local PWHT is usually performed when it is impractical to heat treat the whole vessel in a furnace. However up to now the assessing criteria in local PWHT are still very different from one code to another. In this study, the numerical method such as thermal-visco-elastic-plastic FEM is used and compared with the experiments. The pipe with original welding residual stresses is analyzed under different conditions of PWHT especially for determination of the heated width. Based on above investigations two criteria are proposed for assessing the effectiveness of local PWHT. One is the residual stress criterion which is related with the relaxation of residual stresses, the other is the criterion of temperature different in soak band which is more related to the improvement of the properties. Both of criteria should be satisfied during local PWHT.

Key words: Assessing criterion, Finite element method, Postweld heat treatment, Residual stress