A Comparative Study of Negligible Creep Curves for Rational Elevated Temperature Design
Authors: Ando Masanori , Isobe Nobuhiro , Kawasaki Nobuchika , Sukekawa Masayuki , Kasahara Naoto 
Published: July 2010

In Japanese elevated temperature standard, creep considering design is required for all ferrite steels applied over 375°C and all austenitic stainless steels applied over 425°C regardless of the operating time. On the other hands, ASME Sec.III Subsection NH, RCC-MR and R5 provide the additional rules to determine the negligible creep range. In those standards, each material is evaluated as non-creep considering design region, although there are varieties of applicable materials and the rules to settle the negligible creep range in each standard. 316FR and Mod.9Cr-1Mo are candidate materials of Japan sodium-cooled fast reactor (JSFR), and those high creep resistant properties extend the negligible creep damage area over the conventional temperature limits. Extension of non-creep design area widens design windows and simplifies the creep analysis procedure. To reply those requirements, authors already proposed original negligible criterion and discussed about it. In this paper, we recall the backgrounds of the negligible creep criterion which have already been proposed. Then the negligible creep criterion and relating property in each standard were compared. For estimating the evaluation procedure of each criterion, the common material properties used in “Elevated Temperature Structural Design Guide for Commercialized Fast Reactor (FDS)” were applied to each standard’s criteria. All standards have the negligible creep curves/regions for type 18Cr-8Ni steels and type 18Cr-12Ni-2.5Mo steels, although ASME Sec.III Subsection NH defines just the criteria of negligible creep for the rule of inelastic strain limits. On the diagram of temperature-negligible creep time, the negligible creep curves of 316L(N)(1S) in RCC-MR and R5 exist between those of SUS316 and 316FR in FDS. The negligible creep regions defined in all standards are similar for austenitic stainless steels, although those criteria are different. Comparison of the negligible creep curves by each criterion with FDS’s material properties indicated that the criterion in FDS provides the most conservative curve. In case of Mod.9Cr-1Mo steel, FDS and R5 provide relationship between temperatures and time for estimating the negligible creep time. ASME Sec. III Subsection NH provides only procedures and has no practical allowable values, and RCC-MR doesn’t have the negligible creep curve. Comparison of the negligible creep curves in each criterion with FDS’s material properties indicated that FDS’s criterion allows the longest negligible creep. The negligible creep criteria in ASME Sec.III Subsection NH, RCC-MR and R5 are not practicable for Mod.9Cr-1Mo. On the other hands, FDS criterion raises the temperature limits from conventional 375°C to about 425°C even when the components designed lifetime is 60years. Sensitivities to the difference of criteria and material properties were discussed and concluded that negligible creep curve is strongly dependent on the combination of criteria and material properties. Some evaluations proved that the negligible creep curves in FDS are moderately conservative and practicable.