The goal of this work is to improve the high temperature oxidation resistance of boiler steels. These steels are categorized as low (9 wt.%), medium (12 wt.%) and high (18 wt.%) Cr steels. The isothermal oxidation studies are done with the help of thermogravimetry in lab air in the temperature range of 700-750°C. The oxide scales grown on the steels are analysed and oxidation mechanisms were understood. Cr- content and the diffusion of Cr towards the oxidation front in the alloy has a major ascendency on the protection behaviour of the steels. Main emphasis is kept on the effect of shot-peening on the oxidation behaviour of medium and high Cr steels. It is found that shot-peening has a considerable influence on the diffusion of Cr and protection behaviour of medium Cr steels. This effect is more pronounced in high Cr steel, where shot-peening has largely reduced the oxidation rate. A protective chromia layer is found to be formed on the surface of the shot-peened high Cr steel. ‘Dislocation engineering’ is applied on the shot-peened steels to improve the oxidation resistance of the steels. This is done by pre-annealing the shot-peened steels under high vacuum for different times at the same temperature as the oxidation temperature. The pre-annealed steels are subjected to oxidation at 750°C. With the help of Focussed Ion Beam (FIB) and Transmission Electron Microscope (TEM), the subsurfaces of these steels are analysed. It is found that the pre-annealing for shorter times has resulted in forming a stable network of dislocations. The dislocation engineering method has proved to be very effective in protecting the medium Cr steels against the oxidation. The diffusion of Cr has been enhanced through this network and a protective chromia layer is developed at the oxidation front of these steels.
Based on the oxidation mechanisms observed from the experiments, a model for the simultaneous oxide scale growth (internal and external oxide formation) is developed. This model is incorporated in the existing simulation tool Incorr. This tool simulates the internal corrosion phenomenon only. It works basically by solving Fick’s second law using the finite difference method (diffusion kinetics) in combination with thermodynamic equilibrium calculations. The major contribution of this work is the addition of external scale growth in the simulation. The effect of shot-peening on the microstructure of the alloying system is also considered and implemented in the model. By modifying the Incorr the simultaneous oxide layer growth on the studied steels is simulated and the results are found to be in reasonable agreement with the experimental results.
The modification of the mesh from a square shaped grain structure to the honeycomb shaped structure was implemented by using a more advanced Finite Element Method (FEM). Simulation of the internal oxide scale growth is performed using FEM and the results are presented in the work.