Phase transformation during tensile and low cycle fatigue deformation of AISI 304LN stainless steel

Abstract

Austenitic stainless steels, such as AISI 304LN, are candidate materials for structural applications in many critical components, including for heat transport piping of nuclear power plants. The design of such components requires a thorough understanding of the deformation behaviour, fracture mechanisms and phase transformation characteristics of the construction material. Metastable austenitic stainless steels, like AISI 304LN stainless steel undergo deformation induced martensitic transformation which enhances the work hardening of these alloys and affects their ductility. This thesis concentrates on the formation, quantication and understanding the nucleation micro{mechanisms of deformation induced martensitic transformation during monotonic and cyclic deformation of AISI 304LN stainless steel. The location, nature and orientation of deformation induced martensite formed in the alloy under study during tensile and strain controlled cyclic plastic deformation has been characterised. The sequences of evolution of deformation induced martensite have been documented. An excellent correlation between the mechanical properties and the dimple geometry on tensile fractured surfaces at various strain rates has been found for this alloy. A strong connection between the evolved dislocation cell substructures and the deformation induced martensite during room temperature low cycle fatigue deformation has been established. An attempt has been made to prove that the martensitic transformation evolving with deformation can be explained through thermodynamic effect of the applied stress. It is well known that deformation induced martensitic transformation is controlled by the chemistry of the material, stress, strain, stress triaxility, temperature of deformation, strain rate, grain size, initial microtexture etc. In this research work, the influence of each parameter on martensitic transformation has been isolated through empirical analysis. The relative role of applied stress and resulting plastic strain on variant selection of martensitic transformation in the alloy under consideration has been investigated during monotonic and cyclic deformation at low temperature. In all studies, deformation induced martensitic transformation and the resultant deformation and fracture behaviour have been understood through the extensive use of advanced experimental and analytical tools such as advanced mechanical testing, optical microscopy, image processing, scanning and transmission electron microscopy, X-Ray diraction, electron back scattered diraction, orientation imaging micrography, thermodynamic analysis and modelling on articial neural network and programming platforms. A holistic overview of the manifestations in such austenitic stainless steels, subjected to deformation has been provided through the research carried out and documented in this thesis.