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Thermal Stability and Mechanical Properties of Bainitic Steel

Time: Fri 2022-12-09 14.00

Location: F3, Lindstedtsvägen 26 & 28, Stockholm

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Language: English

Subject area: Metallurgical process science Materials Science and Engineering

Doctoral student: Adam Ståhlkrantz , Strukturer

Opponent: Dr. Joacim Hagström, Swerim AB, Kista

Supervisor: Professor Annika Borgenstam, Strukturer; Professor Peter Hedström, Strukturer, Egenskaper; Dr Niklas Sarius, Husqvarna AB

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For material design it is important to understand and establish the relations between the processing of the steel, the resulting microstructure and how that correlate to the properties of the steel. The main focus of this thesis has been to increase this knowledge for low alloy bainitic steels with medium to high carbon content. This can be used to optimize the material properties in order to obtain a better performance of the final product. It may also be used to achieve a better and more efficient process and thus it also contributes to sustainability. Further, it is also important knowledge when substantial changes in an existing production is required or driven due to changes in regulations and the increasing demand for more sustainable processes. This may require further changes in one or several of the processes in order for the material to maintain its properties and performance.

The present thesis has in depth investigated the effects of austempering temperatures and carbon content, where the microstructure of the bainitic steels was characterized and the different contributions to the strength were investigated. The contributions investigated were grain size, dislocation density, precipitations and dissolved alloying elements. The contributions were characterized through extensive experimental work coupled with complementary thermodynamic calculations. Further, the effects of tempering were examined and compared to each of the strengthening contributions. For the steels investigated, the effects from a varying carbon content were in focus. This since carbon plays a very important role in steels, especially when the other alloying contents are kept low. It was shown that dislocation density and coarseness of the bainitic microstructure is affected by the austempering temperature. However, it was only the dislocation density which were affected by the austempering time. It was also concluded that the decrease in dislocation density was the main reason for the hardness decrease during tempering up to 375 °C. Further it was shown that an increase in carbon content increased the hardness of the bainite, and slightly improved the tempering resistance at shorter times. However, the increased carbon content also resulted in longer austempering times to reach a fully bainitic structure, especially at the lower austempering temperatures. 

Finally, to better understand the bainitic microstructure and the transformation related to austempering temperature, the crystallographic relations in bainite were investigated by detailed electron backscatter diffraction and analysis of the variant pairing. The investigations were performed over the austempering temperature range of 275-450 °C for the bainitic transformation, in low alloyed steels with a varying carbon content. The variant pairing was also correlated to the corresponding bainitic microstructure. It could be concluded that variant pairing had two distinct changes when the austempering temperature was varied in the steel with a medium carbon content. This differs from the literature were two types of bainite is commonly referred to, lower and upper bainite. Further it was shown that a continuous increase of variant pairing within the same Bain group could be observed with increasing austempering temperature. It was also observed that the deviation from the theoretical Kurdjumov–Sachs orientation relationship increased linearly with an increasing austempering temperature for bainite.