Thermomechanical investigation of the production process of a creep resistant martensitic steel

During the production process of creep resistant martensitic steels, the microstructure of the material undergoes a number of transformations due to thermomechanical loading. The final microstructural features have direct influence on the mechanical properties of the alloy such as creep, fatigue and corrosion resistance, as well as toughness. In order to study the effect of each production step, the thermomechanical history of the material is reproduced in a controlled manner at lab scale for detailed examination of the flow curves during hot deformation. In addition, microstructural investigations are applied to samples after each step of the simulated production process. This procedure provides an overview of the influence of processing parameters on the material's microstructure and allows the improvement of the processing steps. The objective of this work is to reproduce parts of the production and manufacturing process of forged parts and tubes in a controlled way and to study the microstructural evolution with respect to phenomena such as recrystallization and strengthening. For this purpose hot-rolled experimental 11%Cr steel is investigated using the thermomechanical simulator Gleeble^®3800. The deformed samples are investigated via LOM, SEM and EBSD. For comparison, as-received samples are included in the investigations. The interpretation of the microstructural investigation and of the obtained flow curves during the hot compression tests allow conclusions on dynamic recrystallization and recovery. Results indicate dynamic recovery as main softening process for both tested temperatures, whereas the higher temperature leads to a significant formation of delta ferrite. These results allow for improved precipitation kinetic simulations, and for further optimizing the thermomechanical treatment with respect to improved microstructure.