Amélioration de la stabilité mécanique de l’austénite dans un acier inoxydable 13Cr-4Ni lors de déformations monotones et oligo-cycliques

Abstract

The fundamental objective of this project is to optimize the microstructural factors of the reformed austenite of 13Cr-4Ni martensitic stainless steel, namely its size and morphology, to improve its mechanical stability. After austenitization and intercritical tempering, the microstructure of these steels consists of a tempered martensitic matrix, carbides and a quantity of reformed austenite, the quantity and morphology of which varies according to the chosen parameters. The transformation induced plasticity (TRIP) of austenite is a good candidate to slow down the propagation of cracks generated by fatigue because it is accompanied by an increase in volume, capable of generating compressive stresses, which can close the crack tip. For this, the austenite must be sufficiently mechanically stable (resistant to complete transformation during small applied deformations). Heat treatments were carried out in order to produce the same quantity of austenite at different temperatures. A volume of 10% was produced for the intercritical tempering conditions of 620°C, 1h; 600°C, 4h and 580°C, 10h. A new austenite morphology has been discovered for 580°C with a contrast in composition such that the austenite seems to surround a region having the same composition as the martensitic phase that constitutes the matrix. Moreover, the size of these particles is much smaller than those produced at other temperatures. Mechanical tests on samples treated at 620°C and 580°C with 14% austenite showed notable differences in terms of mechanical stability of austenite as both, tensile and low cycle fatigue tests, showed a considerable improvement in the mechanical stability of the austenite formed at 580°C. The objective of the study is thus well and truly achieved and it is recommended to target low inter-critical tempering temperatures by extending the holding time to produce a more mechanically stable austenite.

Date	7 Mar 2024
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Philippe Bocher (Supervisor) & Denis Thibault (Co-supervisor)