Cyclic deformation of superduplex stainless steels at intermediate temperatures
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ITIC stainless steels are often used for applications involving both alternating loads and relatively high temperatures, i.e., up to 600 °C, for example, in power generation and chemical processing.[1] Therefore, cyclic deformation response at elevated temperatures of these alloys has emerged as a relevant structural design topic. Several works dealing with this subject, for the most part regarding temperature effects on fatigue life, may be found in the literature.[2–9] Meanwhile, in the last decades, the replacement of austenitic stainless steels by duplex stainless steels (DSSs) has also become a technological trend because of the superior combination of mechanical properties and corrosion resistance offered by DSSs.[10] However, concerning high-temperature applications, it is well known that when DSSs operate between 250 °C and 500 °C, an interval usually defined as the intermediate range of temperatures, they are susceptible to suffer ‘‘475 °C embrittlement.’’ This phenomenon leads to increments in mechanical strength and hardness, together with a reduction of ductility and toughness, the latter being a serious restriction on the maximum temperature for safe operation. From this perspective, although thermal aging effects on the cyclic response of DSSs at room temperature have been extensively explored,[11–14] studies on their cyclic properties at intermediate temperatures are rather scarce.[15,16,17] Regarding physical mechanisms, monotonic and cyclic mechanical responses exhibited for single-phase stainless steels at intermediate temperatures are usually related with the operation of dynamic strain aging (DSA) phenomena. Most of the information has been provided for austenitic grades,[2,4–6,18,19] although there also exists some literature regarding ferritic and martensitic stainless steels.[20,21] Recently, DSA effects have been reported on DSSs,[15–17,22] even though for these steels experimental data are quite limited. The DSA is an anomalous behavior, which is characterized by the appearance of several effects on the deforA. GIRONE`S, formerly Doctor, with the Department of Materials Science and Metallurgical Engineering, Universitat Polite`cnica de Catalunya, 08028 Barcelona, Spain, is now with SKF, The Netherlands. L. LLANES, M. ANGLADA, and A. MATEO, Professors, are with the Department of Materials Science and Metallurgical Engineering, Universitat Polite`cnica de Catalunya, 08028 Barcelona, Spain. Contact e-mail: antonio.manuel.mateo@ upc.edu Manuscript submitted February 28, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
mation behavior as the temperature is raised: negative strain rate sensitivity (SRS), high rates of work hardening, and yield and ultimate tensile stress maxima together with a ductility minimum.[23,24] Moreover, under certain conditions, the serrated flow or Portevin–Le Chatelier effect is observed. On the other hand, DSA operation also introduces changes in the dislocation substructures, promoting planar slip.[8,25,26] Following the preceding ideas, it is the objective of this stud
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