Formation of Bainite Studied by In-situ High-energy X-ray Diffraction

Sammanfattning:       Bainitic steels have attracted great attentions in recent years due to their excellent combination of properties to accommodate a wide range of applications. A deep and comprehensive understanding of how bainite forms is required to better design the production process and optimize the properties of bainitic steels. Extensive experimental investigations, mostly using the post-mortem techniques, have been conducted to shed light on the bainitic transformation. Unfortunately, the nature of bainitic transformation is still a subject of debate, which hinders further development.      The bainitic transformation involves multiple events that occur concurrently, such as formation of bainitic ferrite and cementite, dislocation generation and annihilation, and carbon diffusion, etc. These events may also affect each other and, in turn, affect the overall bainitic transformation kinetics. It is difficult to quantify the evolution of these events in a large sample volume and reveal their interrelationship during bainitic transformation using conventional experimental methods. Moreover, some bainitic steel grades, such as high-strength low-alloy (HSLA) steels used in the automotive industry, possess a very rapid phase transformation kinetics and a complex final microstructure. So, it is challenging to understand the transformation progress and correlate it with the final microstructure by the sole assistance of post-mortem techniques. In this circumstance, in-situ techniques, such as high-energy X-ray diffraction (HEXRD), appear to be a better solution to these challenges.      Synchrotron sources provide extremely brilliant X-rays. It enables the detection of minor phases, such as carbides, and facilitates the rapid data acquisition to resolve the rapid transformation progress. Therefore, this thesis is dedicated to utilizing of HEXRD with state-of-the-art instrumentation to study bainite formation. One objective is to explore the feasibility of HEXRD for industrially relevant questions, e.g., rapid bainitic transformation in HSLA steels. At the same time, Si and Mo are two important elements and their content is often tuned in HSLA steels. So, another objective is to systematically study the influence of Si, Mo, and temperature on the bainitic transformation and other events that occur concurrently. The thesis thus brings industrial applications, fundamental transformation mechanisms, and HEXRD methodology development together.      Two commercial HSLA steels with different hardenabilities were austenitized and fast cooled to different isothermal temperatures. Austenite decomposition occurred during cooling with high transformation rates. Several transformation products, i.e. polygonal ferrite, bainitic ferrite, degenerate pearlite, and martensite, were separated by combining HEXRD and electron backscatter diffraction analyzes. The steel with higher hardenability was found to have a smaller fraction of polygonal ferrite and a higher amount of bainite, which was speculated to be caused by the larger addition of Mo. On the other hand, the low-hardenability steel with a higher addition of Si show a higher carbon content in the retained austenite, probably because of suppressed carbide formation.      Following the study of HSLA steels, the effects of Si and Mo were investigated using a series of Fe-C-Mn-Si and Fe-C-Mn-Si-Mo alloys with various of heat treatment conditions. These investigations aimed to reveal the influences of the alloying elements with a focus on the correlation between the formation of bainitic ferrite, carbon diffusion, carbide formation, and dislocation density evolution during the bainitic transformation. In general, bainite formation is retarded by increasing the Si content and the isothermal temperature, and the carbon contents in retained austenite at transformation stasis were close to the Widmanstätten/bainitic ferrite start temperatures (WBs). A minor addition of Mo had a negligible effect, but a larger addition introduced a bay area in the time-temperature-transformation that can be a result of solute drag effect. Transition carbides were only found in Si-added alloys, whereas cementite was found in both Si-added and Mo-added alloys. The carbide formation had similar kinetics as bainitic ferrite but no correlation to the dislocation density evolution was found for Si-added alloys.      Furthermore, an ongoing work is introduced. A HEXRD study using the state-of-the-art PILATUS area detector was performed to shed more light on the transformation mechanism of bainite in comparison with martensite. The result shows that at the same range of diffraction angle, for bainitic ferrite, a single symmetric diffraction peak was found in Fe-3.0Mn-0.4 and 0.6C (in weight percent) alloys; for martensite, the diffraction peak of the Fe-3.0Mn-0.4C alloy was similar to that of bainitic ferrite, whereas two peaks adjacent to each other were found for martensite in the Fe-3Mn-0.6C alloy.      This thesis demonstrates the versatility of HEXRD in phase transformation studies. Crystallographic, chemical, volume fraction, and stress/strain information extracted from the data is of particular interest for scientific and industrial studies.

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