PSI - Issue 68

Marwa Ben Bettaieb et al. / Procedia Structural Integrity 68 (2025) 297–302 M. Ben Bettaieb et al. / Structural Integrity Procedia 00 (2025) 000–000

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b

Fig. 6. Magnified SEM observations of longitudinal specimens of the damaged parts of the (a) Nb-free material (SE image); (b) Nb-rich alloy (BSE image) tested at 575 °C/310 MPa.

Very few precipitates are observed in the head compared to the damaged part for the Nb-free alloy. Therefore, one can conclude that the creep accelerates the precipitation in this material. On the other hand, no differences are noticed between the head and the damaged section of the Nb-rich alloy. Due to the small size of the precipitates, this must be confirmed by further TEM observations. 5. Conclusion Comparing mechanical behaviors of two 316L (N) alloys, a Nb-free material and a Nb-rich one, lower tensile yield strength are noticed for the Nb-rich alloy. This observation can be associated to a Hall-Petch effect since the grain size of the Nb-free plate is twice smaller than the one of the Nb-rich alloy. However, this relationship is no longer valid for the creep test carried out at 575 °C/310 MPa where the Nb-rich alloy has a drastically lower minimum creep rate compared to the Nb-free material. The reduction of minimum creep rate of the studied 316L (N) steel is believed to be related to the addition of niobium. The creep fracture strain is also drastically reduced for the Nb-rich material. The Nb-free alloy as well as the Nb-rich specimen tested at 575 °C/310 MPa exhibit damage nucleation at grain boundaries from intergranular precipitates. However, final coalescence occurs through ductile transgranular mechanisms for the Nb-free material associated with substantial plastic deformation, whereas for the Nb-containing steel the coalescence mechanisms are mainly intergranular and are accompanied with very limited plastic deformation. The exact role of Nb, and associated phases, on the damage mechanisms is yet to be established. Acknowledgements Florent Lefebvre and Sébastien Vincent (CEA) are acknowledged for running the creep tests. References Feaugas, X., Haddou, H., 2003. Grain-Size Effects on Tensile Behavior of Nickel and AISI 316L Stainless Steel. METALLURGICAL AND MATERIALS TRANSACTIONS A (34A) 2329–2340. Li, Y., Liu, Y., Liu, C., Li, C., Li, H., 2018. Mechanism for the formation of Z-phase in 25Cr-20Ni-Nb-N austenitic stainless steel. Materials Letters (233) 16–19. Mathew, M.D., Laha, K., Ganesan, V., 2012. Improving creep strength of 316L stainless steel by alloying with nitrogen. Materials Science and Engineering A (535) 76–83. Nassour, A., Bose, W.W., Spinelli, D., 2001. Creep Properties of Austenitic Stainless-Steel Weld Metals. Journal of Materials Engineering and Performance (10) 693–698. Padilha, A.F., Escriba, D.M., Materna-Morris, E., Rieth, M., Klimenkov, M., 2007. Precipitation in AISI 316L(N) during creep tests at 550 and 600 C up to 10 years. Journal of Nuclear Materials (362) 132–138. Sourmail, T., 2001. Precipitation in creep resistant austenitic stainless steels. Materials Science and Technology (17) 1–14. Vodarek, V., 2011. Creep behaviour and microstructural evolution in AISI 316LN +Nb steels at 650 ◦C. Materials Science and Engineering A (528) 4232–4238.