M. Rieth (Sp), M. Klimiankou, E. Materna-Morris, Forschungszentrum Karlsruhe, Eggenstein-Leopoldshafen (Germany)
Among many other applications the austenitic 17/12/2–CrNiMo steel 316 L(N) (DIN 1.4909) is used or envisaged for both conventional and nuclear power plant construction as well as for ITER within the International Nuclear Fusion Project. Worldwide a huge number of experimental investigations have already been carried out to determine the material properties (including creep behavior) of this steel type in the conventional stress and temperature range. In the design relevant low-stress range at 550 °C and 600 °C, however, creep data allowing statements to be made about the stress dependence of the minimum creep rate or about the technically relevant creep strain limits are almost unavailable. This is not only due to reasons of time, but to technical reasons, too. In this stress-temperature range, the expected creep or strain rates are so small that they can hardly be measured by conventional creep tests. Therefore, we have started a special long-term creep testing program at 550 °C and 600 °C, respectively, in 1991. After an experimental period of about 10 years the creep tests have been either finished or aborted, and evaluated. Now, this low-stress creep data not only allow for a much better long-term prediction of the reliability of 316 L(N) applications but also enable deformation modeling for a broader stress range. The present work focuses mainly on the set-up of a steady-state creep model for the 316 L(N) steel which explains the accelerated creep velocity observed at the low stress range. Due to the small number of adjustable parameters it may also be easily adapted to other materials. Since austenitic stainless steels are well known for their problematic aging behaviour at elevated temperatures, microstructure and precipitation formation as well as their impact on creep are outlined above all.
