Determination and modeling of the creep propagation on high chromium steel

Modified 9Cr-1Mo steel (T91) is a candidate material for pressure vessels and for some internal structures of GCR (Gas Cooled Reactors). In order to validate this choice, it is necessary, to check if it is covered by the existing design codes, concerning its procurement, fabrication, welding, examination methods and mechanical design rules. A large R&D program on mod 9Cr-1Mo steel has been undertaken at CEA in order to characterize the behavior of this material and of its welded junctions. In this program, the role of the Laboratory for structural Integrity and Standards (LISN) is to develop high temperature defect assessment procedures under fatigue, creep and creep-fatigue loadings, to validate the existing methods (developed on austenitic stainless steels as 316L(N) for the fast reactors) and to get new experimental data on Mod 9Cr-1Mo steel. This paper presents the experimental program undertaken to develop defect assessment under creep loading and describes the main results obtained. Then a creep propagation law is proposed for the Mod 9Cr-1Mo steel at 550 degrees C. To validate the experimental interpretation, a numerical analysis with a 3D finite element model is proposed and allows to model the propagation of the crack. Finally, a comparison of the experimental and the numerical data and in particular of the C* value is investigated.