Study Of Micronstructure-Property Relationships Of Austenitic Stainless Steel Based On Electron Backscatter Diffraction And Nano-indentation Techniques

Generally, material microstructure is closely related with its property. "In-situ" observing the material microstructure during high temperature, and measuring micrometer-scale residual stress of materials etc., are important to analyze the failure mechanism of materials. For a long time, however, these studies are research hotspot, but also are research difficulties. In the recent years, with the development and improvement of material characterization techniques and theories, it is possible to overcome the above difficulties. In the present work, the microstructural transformation during 1200℃super-high temperature and the micrometer-scale residual stress at the weld interface of austenitic stainless steels and welded joints widely used in the engineering are examined systematically using electron backscatter diffraction (EBSD) technique, nano-indentation technique, and scanning electron microscopy (SEM) etc.. It is respected to provide scientifical basis to the application of austenitic stainless steels serviced under complicated conditions and its failure analysis.This dissertation consists of nine chapters. Chapter one is an introduction, in which the origin and support of the project, significance and the major author's work are introduced briefly. The research status and development of both EBSD and nano-indentation techniques in materials science are reviewed.In order to better understand this dissertation, and reflecting the system of dissertation, in chapter two the fundamental knowledge, the system structure, the principle of data acquisition, and data dependability of EBSD and nano-indentation techniques are introduced briefly.Chapter three illustrates the experimental materials and methods used in this dissertation, on the emphasis of the metallographic samples preparation of austenitic stainless steels and welded joints. In addition, the characterization techniques for morphologies and microstructures, measurements of properties are also demonstrated.In chapter four, a novel "in-situ-tracking" approach for evaluating microstructural variations using SEM, EDS and EBSD is introduced. Comparing with the regular methods and the strict "in-situ" techniques, the present "in-situ-tracking" approach provides a possibility to achieve the examination in series on the variations of microstructures, compositions and properties at a fixed place during treatments simply and economically. It is an effective approach for studying more materials microstructure-property.In chapter five, the "in-situ-tracking" approach using SEM and EBSD is proposed to examine the "recovery and recrystallization" process of austenitic stainless steel, especially the grain orientation evolution and grain boundary variation, during 1200℃super-high temperature. It is found that comparing to the regular high temperature service (below 900℃), the present "recovery and recrystallization" process is accelerated due to dislocation fastened movement and intensive interaction. The grain growth mechanism still meets the well-accepted dislocation model of subgrain combination.In chapter six, the crystallographic relationship of the "liquid-solid" interface of the weld pool of austenitic stainless steel is investigated. It has experimentally demonstrated the reasonability of the competitive growth resulting from the solidification of weld pool. That is to say, the<100> texture has been formed in the weld metal. Additionally, the microstructural transformation and grain orientation evolution at the weld interface during high temperature are studied using the EBSD "in-situ-tracking" method.In chapter seven, the formation and failure mechanism of oxidation scale on the stainless steel substrate surface served at 1200℃with corrosion environment are analyzed. The results indicate that the scale on the stainless steel substrate surface consisted of poly-film layers, depending on the oxidation environment, and the elements distribution and diffusion at the scale-substrate interface. The formation of cracks in the scale and the fall off of the non-compact scale are the main problem at the failure behavior of stainless steel oxidation scale. In chapter eight, the nano-indentation technology with higher resolution and measuring precise is firstly adopt to measure micrometer-scale residual stress around the fusion bound. The results suggest that in the as-welded joint, the residual stresses around the fusion bound are entirely compressive, and the maximum compressive stresses occurrs in the fusion bound; after post-weld heat treated (PWHT), the compressive stresses around fusion bound are reduced by thermal stress relief, especially the heat-affected zone (HAZ).Chapter nine is the conclusions of all the research works mentioned in this dissertation. The last part of the dissertation lists author's published papers finished during the doctorate study period.