Characterization of Inelastic Strain in Type 316H Austenitic Stainless Steel using Electron Backscatter Diffraction

Unnikrishnan, Rahul (2019). Characterization of Inelastic Strain in Type 316H Austenitic Stainless Steel using Electron Backscatter Diffraction. PhD thesis The Open University.



Type 316H austenitic stainless steel is widely used in the UK’s advanced gas-cooled reactors, for example in boiler components, due to its good corrosion, creep and fatigue properties at temperatures around 550°C. The influence of service conditions such as the type of loading, strain rate, load and temperature histories and the environment have to be considered in assessing the remaining safe lifetime of plant operating at high temperature, so that fatigue and creep-fatigue damage can be quantified to ensure that the material will still be able to withstand the required future loads in the creep-fatigue-damaged condition.

Electron backscatter diffraction (EBSD), which can measure crystallographic orientations in polycrystalline materials, has been used to quantify and map inelastic strains in materials in terms of metrics derived from the local orientation changes or by calculating the geometrically necessary dislocation densities. This thesis explores the potential of EBSD for characterizing localised inelastic strain from lattice orientation measurements during uniaxial stress relaxation and cyclic loading.

The study was conducted on ex-service Type 316H austenitic stainless steels, through a series of monotonic tests under tensile primary load, secondary stress relaxation and under low cycle fatigue/creep-fatigue conditions. The EBSD metrics termed kernel average misorientation (KAM), grain orientation spread (GOS), low-angle boundary fraction (LABF) and deformed grain fraction (DGF) all showed a linear variation with inelastic strain accumulation in both tensile and cyclic tests. In contrast the proportion of twin boundaries reduced with increasing inelastic strains. This study demonstrated that the development of misorientation depends on the temperature and precipitate distribution. The misorientation developed in solution annealed material was much lower than that in ex-service material. Near grain boundaries, the local misorientations were found to increase with increasing stress relaxation and cavities were identified from secondary electron images. A misorientation-based strain assessment method considering the deformed grain fraction was found to be more consistent than using mean misorientation values as this reduced the scatter associated with inhomogeneities in grain size. After tensile deformation at different strains and cyclic deformation at different strain ranges, a good correlation was found between the behaviour of different EBSD metrics and the measured mean microhardness. However, hardness measurements did not clearly detect the inelastic strain developed during stress relaxation and cyclic strain accumulation (i.e. the strain accumulated over different numbers of cycles). Scanning transmission electron microscopy showed the dislocations in tensile samples after stress relaxation to form homogeneous diffuse cell structures, and clear cell structures were also evident after cyclic deformation. The cell structures elongated to veins with further cyclic strain accumulation.

Local misorientation maps were used to study the inelastic deformation around a reheat crack in an ex-service component. The EBSD misorientation maps showed good correlation with microhardness maps. Local misorientation maps showed strain accumulation along grain boundaries. Comparison with Small Angle Neutron Scattering (SANS) cavitation measurements showed that the areas with more cavities had higher local misorientations. Although stress concentration at grain boundaries is well documented, such a relationship between high local misorientation and cavitation has never been seen before in austenitic materials subject to stress relaxation.

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