Mechanical Behavior in Local Postweld Heat Treatment (Report II) : Determination of Critical Heated Band during Local PWHT(Mechanics, Strength & Structure Design)

概要

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Local post weld heat treatment (local PWHT) is usually performed for tempering and relaxation of residual stresses. The heated band width is the most important parameter controlling the effectiveness of local PWHT. However the determination rules of this parameter are very different in the different codes. In this study, a direct criteria for heated width was used according to the creep FEM analysis of stress relief during PWHT. This method clearly shows the whole history of stress relief during PWHT and a critical heated width can be obtained. The pipe with original welding residual stresses is analyzed under different conditions of PWHT. Numerical analysis shows that the maximum residual stress after PWHT decreases when the heated width increases. When the heated width becomes large enough, the residual stress after PWHT change very slowly and a critical heated width can be found which gives a residual stress close to the value obtained from the uniform PWHT. A series of different PWHT conditions are studied to find the critical heated widths by using this method. Compared with the heated widths based upon some applicable codes, it is found that a heated area of 2.5√<Rt> on either side of a branch connection seems more reasonable. The through thickness temperature gradient H_i criteria is also discussed in this study.
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