Microstructural and micromechanical study of ODS steel tubes transformed by cold pilgering process

Nowadays nuclear power is a significant part of electricity production in the world. Many countries forecast an important growth of capacities of nuclear power units, keeping in mind the reduction of a final production cost and insurance of an improved safety. Various international programs aim to develop and approve materials for the coming generation (GEN IV) of fission reactors. In this context, ODS ferritic-martensitic stainless steels, due to their specific crystal structure and the presence of strengthening nano-particles of yttrium-titanium oxides, show high performance in nuclear environment. ODS steel manufacturing process includes a hot extrusion of bars made from a mechanically alloyed powder mixture and further cold rolling operations. The study focuses on the 9 wt% Cr ODS stainless steel grade that exhibits a phase transition at high temperature promoting recovery between pilgering steps. For this project the first tubes set, produced by industrial Mannesman pilgering technology, is considered. During this cold forming process, severe plastic deformations (on the order of several hundred percent) are applied to the material. This study present a wide microstructural characterization of the material at various processing step. A peculiar texture memory effect is observed during intermediate heat treatments. Microstructural evolutions are then prospected by synchrotron in-situ heat treatments that reveal the specific phase transition behavior of the 9% Cr ODS steel. Texture evolution during phase transformation is simulated by a variant selection model based on Double Kurdjumov-Sachs (DKS) relation. At the same time the mechanical properties are determined in various metallurgical states showing the interest to reduce the crystallographic texture. To improve rolling sequence engineering, a polycrystalline modeling is proposed to simulate texture evolution during pilgering of ODS steel tubes. This study investigates the paths towards the process optimization, looking in particular to the risk of mechanical properties degradation and the possibility to reduce the total production costs by minimizing the number of rolling passes and the duration of heat treatments. (author)