Creep property measurement of service-exposed SUS 316 austenitic stainless steel by the small-punch creep-testing techni
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Shin-ichi Komazaki Materials Science and Engineering, Muroran Institute of Technology, 27-1 Mizumotocho, Muroran, Hokkaido 050-8585, Japan
Toru Takahashi, Toshiyuki Hashida, and Tetsuo Shoji Fracture Research Institute, Faculty of Engineering, Tohoku University, Sendai 980-8579, Japan (Received 13 January 2002; accepted 6 May 2002)
The creep properties for SUS 316 HTB austenitic stainless steel were evaluated by using the small-punch creep test at 650 °C for loads of 234, 286, 338, 408, and 478 N and at 700 °C for loads of 199 and 234 N. The creep curves, determined by means of the small-punch creep test, were similar to those obtained from a conventional uniaxial creep test. That is, they exhibited clearly the three creep stages. The width of secondary creep stage and rupture time tr decreased with the increase in testing load level. The creep rupture strength for the service-exposed material was lower than that of the as-received material at high testing loads. However, the creep resistance behavior was opposite at relatively low load levels. This difference in creep resistance was explained on the basis of the difference in the creep deformation and microstructural evolution during tests. It was also found that the ratio between the load of small-punch creep test and the stress of uniaxial creep test was about 1 for having the same value of creep rupture life.
I. INTRODUCTION
The evaluation of creep behavior in materials is one of the most important factors to assess the integrity of elevated temperature structural components. The SUS 316 HTB stainless steel has been used widely in elevated temperature applications, including components in nuclear power plants and chemical plants.1,2 That is, it is used in the creep regime. The creep resistance is usually assessed by carrying out a uniaxial creep test at selected values of stress and temperature.3 The data determined in this way are then analyzed to establish appropriate relationships between testing conditions and selected parameters such as rupture life. This information is subsequently used for design purposes. This kind of test requires the removal of material from specific components locations; however, the procedure for extracting the large standard uniaxial creep specimens needs a subsequent weld repair, which a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 8, Aug 2002
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may cause severe damage in the mechanical properties of materials. Furthermore, the creep rupture strength deteriorates after prolonged exposures at elevated temperatures as a result of microstructural changes, such as the formation of carbides and intermetallic compounds.2 To evaluate directly the degradation of creep properties for materials in-service, it is necessary to use miniature-sized specimens because the volume of removable samples is limited and the extraction of a standard uniaxial test specimen from critical components is not practical. The small-punch (SP)
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