Mechanical Behavior in Local Post Weld Heat Treatment (Report Ⅰ): Visco-Elastic-Plastic FEM Analysis of Local PWHT

摘要

Local post weld heat treatment (local PWHT) is usually performed when it is impractical to heat treat the whole vessel in a furnace. Many factors have an influence on PWHT procedures, such as size of the pipe, heated widths, insulation conditions, heating rates, soak temperatures and hold times, material composition etc. However up to now the influences these factors have on PWHT are not very clearly understood and different criteria for sizing the parameters can be found in different codes. This study provides a direct method to assess the effectiveness of local PWHT. An axisymmetric model was used based on the thermal-visco-elastic-plastic Finite Element Method with the consideration of creep phenomena. Investigations show that the thermal stresses induced by local PWHT are much affected by creep behavior and the changes of Young's Modulus very much. The stress relief history during whole local PWHT was studied using the present method. The results show that the stresses decrease quickly in the heating stage, they decrease slowly according to creep law in the holding stage and then suddenly increase when the cooling stage starts. The study shows the possibility that through a series of computations the effects of many factors can be assessed and the optimum parameters can be found by using this model.