Creep-fatigue evaluation on butt welded joints of type 304 SS

In high temperature plant systems such as thermal power generation and petro-chemical complexes, various weldments are used for pressure vessels and piping. Usually in structural design methods for weldment of conventional pressure vessels and piping systems, load controlled stresses are dominant, and to protect early failure in weldments tensile and creep strengths of each welded joint have not to be lower than the standard values of base metal. While under cyclic deformation controlled stress such as thermal loading in LMFBR, creep-fatigue phenomenon caused by reversal loading and residual stress relaxation is dominant. Therefore, a creep-fatigue evaluation method of Type 304 ss butt welded joints has to be investigated under cyclic thermal loading. For a study of this effect, the strain concentration factor Kε; Kε = Max[1+(qwq-1)(1-2 γyσy/E εn), Kε0], which is derived from the generalized elastic follow-up concept as the plastic strain redistribution, was applied, and in the Kε factor the metallurgical discontinuity effects; qw and γy were considered. The elastic follow-up increment factor for weldment; qw is equal to 2 under uni-axial mechanical loading. And under bi-axial thermal loading in shell-type structures, it is clarified that 1.5 as the qw value can be applied. As the yield ration γy, the value of 0.8 is the best for Type 304 ss weldment. As for the creep damage estimation, the simplified procedure based on the time fraction rule is discussed, and it is investigated how the creep damage of weldment can be evaluated by using mechanical properties of base metal (design standard values). The initial value of stress relaxation is determined by the cyclic stress-strain relation of base metal and the above Kε value. Stress relaxation during strain hold can be analyzed by using creep strain behaviour of base metal. Finally the allowable creep-fatigue life can be estimated by using the design curves. (author). 8 refs, 14 figs