Micromechanical modelling and testing of a 9Cr tempered martensitic steel

The work in this thesis focuses on the martensitic steel known as P91. In this work the mechanical response of the material was examined through nite element material modelling and the results validated through experimental methods. The primary goal of this research is (i) to accurately represent the material response of P91 at a range of temperatures, and (ii) to validate the material model through the analysis of crystallographic orientations on the micro scale. Uniaxial tensile tests were conducted using parent metal, weld metal and cross weld specimens that had been extracted from a functioning power plant with 20,000 hours of service. The results of the parent P91 were consistent with other literature results in the case of yield strength and elastic modulus. However the ductility of the steel was reduced after experiencing service. The weld results showed a con- siderable increase in yield strength when directly compared to P91 parent steel. The uniaxial high temperature data was used to calibrate a crystal plasticity material model based on a ow rule. The ow rule is a slip- system based crystal plasticity model used to represent the material deformation at the micro scale. Specimen level modelling also took place, where the material model was represented by the uniaxial data provided by the experimental testing in an isotropic elastic plastic model formulation. Microscopy was undertaken to investigate the microstructure of P91. Scanning electron microscopy (SEM) was utilised as well as elec- tron backscatter di raction (EBSD) to characterise the complex grain structure associated with martensitic steels. This crystallographic data was utilised in the modelling of the micro scale analysis of the steel. Crystal deformation o ered a means of evaluating how well the ma- terial model predicts the change in orientations. It was found that in general, the material model gave a good prediction of the change in crystallographic orientations for both the room temperature and high temperature cases.