2025 FEMS Master Thesis Award – Finalists

Oskar Wikner

SFMT, Sweden – Linköping University

Mechanical properties and microstructures of steels with different manganese contents

This thesis study comprises a cooperation between Linköping University and Alleima AB, one of the globally prominent high alloy steel manufacturers. In the thesis study, a series of novel steel alloys with different manganese contents were characterized by examining their mechanical properties and microstructures through different stages of processing. The study had two main objectives. Firstly, to investigate the effects of manganese substitution in a commercial Fe-Cr-Ni alloy, and secondly, to examine two steels belonging to the TWinning Induced Plasticity (TWIP) steel category. Five different materials, including a reference alloy, were manufactured by ingot casting. The materials underwent hot and cold rolling, followed by heat treatment. Samples for mechanical testing were taken from different stages of processing. Charpy impact toughness, as well as tensile properties including tensile strength, yield strength, and elongation to fracture, were measured in hot and cold rolled states. Microstructures were examined in light optical microscopy (LOM) and scanning electron microscopy (SEM) in as-cast, hot-rolled, cold-rolled, and heat-treated states. An electron back scatter diffraction (EBSD) study was conducted to evaluate the potential for generating deformation twins (TWIP-effect) in the materials. The results indicate that all examined materials can be processed by flat rolling, with forming limits beyond those investigated in this work. Manganese substitution in the commercial Fe-Cr-Ni alloy resulted in generally deteriorated tensile properties and impact toughness. The two TWIP steel alloys showed comparable or, in some cases, improved tensile properties relative to the commercial Fe-Cr-Ni alloy; however, their impact properties were clearly inferior. The microscopic investigation revealed that the final heat treatment was unsuitable for the Mn-substituted variants, as it led to partially recrystallized microstructures. The EBSD study showed that all examined materials exhibit TWIP characteristics, displaying evidence of deformation twins in cold-worked states. In conclusion, one of the two examined TWIP steel alloys shows good potential for further development based on the evaluated properties. However, all materials require more extensive testing. Mechanical testing at cryogenic temperatures and investigation of corrosion properties are suggested as future research areas. Additionally, it is recommended that new ingots be prepared using improved protective atmospheres, such as vacuum or shielding gases, to reduce the occurrence of inclusions and provide a fairer representation of the materials’ potential.