Creep mechanisms in a vanadium alloyed austenitic stainless steel

In certain alloys it is possible to stop any movement of the original or strain introduced dislocations by a strong, heterogeneous nucleation of alloying elements. Diffusion will be the only active process left to contribute to the straining during a high temperature creep test. The dislocation structure generated before or during the nucleation process resists the deformation for a very long time and therefore forbids a true secondary structure to develop. A creep resistant steel, UHB SS 724 LN of the type AISI 316, has been intensively studied for the internal structure by TEM through all the three classical stages. This steel appears to have a critical stress above which dislocations at grain boundaries are generated in the first few seconds of the testing. The secondary creep stage is significant by showing an extremely low minimum creep rate at stresses below the critical one contrary to the creep rate predicted by the Coble model. The steel has a very developed tertiary creep stage, and observations of the dislocations generated by the initial straining indicate that the dislocations during this stage climb away from the precipitated particles. The contribution from void growth to the tertiary stage will probably still exist, and this stage therefore has at least two contributions.