In order to investigate the behavior of creep crack propagation in a boiler superheater tube (Type 321 stainless steel) after long service, the unused and 120000 hours used materials produced at the same lot were made into test specimens and creep tests on the smooth specimens and creep crack propagation tests on the center notched plate specimens were carried out.
The results obtained were summarized as follows:
(1) The crack propagation rate, dl/dt, of the used material was faster than that of the unused material at higher stress levels. However, this difference in dl/dt decreased as the applied stress decreased. These results arose from the difference in creep deformation property of two materials.
(2) Roughly, the crack propagation rate, dl/dt, was correlated well with the creep J integral (i.e., C* parameter or modified J integral), Jc, and the correlation was identical for both materials.
(3) Strictly speaking, the crack propagation rate, dl/dt, was accelerated at lower stress levels, because of decrease in creep fracture strain, εf. Therefore, better correlation was achieved between dl/dt and the fracture strain modified creep J integral, Jc/εf.
