Characteristics of ultrafine/nanostructured bainite formation in low-temperature ausformed medium-carbon steels

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Oulun Puhelin auditorium (L5)

Topic of the dissertation

Characteristics of ultrafine/nanostructured bainite formation in low-temperature ausformed medium-carbon steels

Doctoral candidate

Master of Science (Technology) Pentti Kaikkonen

Faculty and unit

University of Oulu Graduate School, Faculty of Technology, Materials and Mechanical Engineering

Subject of study

Materials Engineering


Professor Peter Hedström, KTH Royal Institute of Technology

Second opponent

Associate Professor Ilchat Sabirov, IMDEA Materials Institute


Professor Mahesh Somani, University of Oulu, Materials and Mechanical Engineering

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Low-Temperature Rolling and Annealing: Essential Elements for Ultra-High Strength Nanostructures in Steel

This dissertation focuses on a novel microstructure in steel called nanobainite, which is achieved through a specific heat treatment process at low temperatures. Nanobainite is particularly intriguing because it not only enables exceptionally high tensile strength but also outstanding ductility and toughness properties.

Typically, the successful formation of nanobainite requires a high carbon content (≥0.6%), which lowers the formation temperature of martensite and allows for bainite formation at low temperatures. However, it has been demonstrated that nanobainite can be produced in medium-carbon steels (0.3–0.5%) through certain rolling and/or heat treatment processes.

The aim of this research is to understand how nanobainite forms in medium-carbon steel and what mechanical properties it imparts. Research findings indicate that low-temperature rolling may slow down the onset of bainite formation but enhance its overall kinetics. Additionally, it reduces the size of bainite plates, resulting in increased hardness and strength in the steel. A portion of the original microstructure remains in the steel, resulting in a unique composite structure composed of bainite, martensite, and retained austenite, which enables exceptional mechanical properties.

This study opens up new possibilities for developing ultra-strong and tough nanobainitic medium-carbon steels through innovative processing methods.
Last updated: 23.1.2024