The standard conditions for stress relief annealing (SR-treatment) of welded structures such as pressure vessels are indicated by JIS or ASME code. However, application of these conditions to high quality thick plates seems to be unreasonable, since they require for such plates to simply keep higher temperature for longer time than actually needed. This is partially due to the lack of sufficient knowledge on the effect of SR-treatment over the reduction of welding residual stresses, especially for thick plates. On the other hand, the theory of thermal elastic-plastic creep analysis based on the finite element method has been well developed. Although the SR-treatments of welded joints of very thick plates have been analyzed by this method, its application. is not easy but requires a great deal of computation. Accordingly, it is necessary to develop some simple calculation method to accurately estimate residual stresses remained in a very thick plate after SR-treatment.
In this fundamental study, using the various relations between stresses and strains during SR-treatment, estimating equations are formulated in order to simply calculate residual stresses after SR-treatment as in a uniaxial stress state. As a demonstration, approximate equations are developed for the relaxation test both at a constant and changing temperatures because of its similar stress relaxation phenomenon to that of SR-treatment. The results are compared with the highly accurate analytical result based on the finite element method. The both results show such a good coincidence that the appropriateness of the adopted method is confirmed.
In the next report, this method will be developed and applied to SR-treatment of welded structures with more general stress state and boundary condition in order to obtain the change of residual stresses in a welded joint of a very thick plate.
