Secondary cementite, characterisation and modelling in hypereutectoid steel

Abweichender Titel nach Übersetzung der Verfasserin/des VerfassersSteel is the most often used construction material. The properties of steel can be varied in a wide range by either varying the composition or production process. These properties are influenced in a wide range by the existing microstructure. Computer-aided simulation is used to forecast these properties and to shorten the material development process. Therefore models are developed which represent reality and try to treat this issue. Providing an accurate prediction of the microstructure of an alloy is of great economic interest. In this work, we dealt with the formation of secondary grain boundary cementite in hypereutectoid steel. Secondary grain boundary cementite is a key phase in hypereutectoid steel and has a huge influence on its properties. We wanted to give insight into the formation process of secondary grain boundary cementite and evaluate the influence of substitutional alloying elements such as Silicon, Manganese or Chromium. With the gained data from the experiments, we verified an already existing thermokinetic simulation model. In the experimental part of this research, we carried out a systematic heat-treatment procedure. We analysed the gathered microstructure in the metallographic laboratory. We classified the microstructure and evaluated the growth kinetics of the secondary cementite. In the theoretical part, we validated the existing model with the gained data from the experiments. We classified the model on its accuracy in predicting reality. We were looking for explanations for deviations between model and reality. In the physical experiments, we found out that cementite formation follows an exponential growth till it reaches saturation. Alloying elements, such as Silicon, Chromium or Manganese, influence the continuity of the secondary grain boundary network or retard the nucleation and growth of cementite to later times and therefore strongly influence the growth kinetics. Our research showed that cementite growth stops before it reaches the equilibrium fraction and that further growth stagnates. The validation of the simulation results with the experiments showed that the simulation is accurate for less complex steel. We also saw that substitutional alloying elements lead to a retardation of the cementite formation. The simulation results exceeded the values of the experiments by a factor of two. Further research on this topic can be done by executing additional heat treatments with different alloy compositions or accomplishing a material analysis of the samples with energy dispersive X-ray analysis (EDX) or electron energy loss spectroscopy(EELS). One should aim to increase the understanding of the influence of the alloying elements and the trajectories of the atoms of the substitutional alloying elements during the transformation process.10