The role of finely divided retained austenite on the mechanical properties of QP and ART processed novel 0.3C ultrahigh strength steels.

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Oulun Puhelin auditorium (L5), Linnanmaa, University of Oulu

Topic of the dissertation

The role of finely divided retained austenite on the mechanical properties of QP and ART processed novel 0.3C ultrahigh strength steels.

Doctoral candidate

Master of Science in Engineering Pekka Kantanen

Faculty and unit

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

Subject of study

Materials Engineering


Professor Esa Vuorinen, Division of Materials Science Department of Engineering Sciences and Mathematics Luleå University of Technology

Second opponent

Associate Professor Ilchat Sabirov, IMDEA Materials Institute, Calle Eric Kandel 2, Getafe, 28906, Madrid


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

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The effect of two different heat treatment processes on the microstructure and mechanical properties of novel ultrahigh strength steels.

Futuristic high strength steels are intended to meet the growing demands of the industrial sector, such as reducing CO2 emissions and manufacturing costs. The use of higher strength steels enables lighter structures, which can be used to increase, inter alia, the payloads of vehicles and structures and increase energy efficiency. This also reduces CO2 emissions. Besides, the use of high strength steels in vehicles improves their crash safety. In addition, steel manufacturing cost can be reduced by using lean, inexpensive alloy compositions and developing energy-efficient steel manufacturing and heat treatment processes. The main applications of the new generation high strength steels will be in demanding structures, construction and mining equipment, and automotive products.

In this thesis, an altogether new generation of high strength steels were studied, and their properties evaluated. The purpose of this scientific investigation was to develop novel high strength steel concepts in order to achieve an excellent combination of strength and toughness than possible with existing steels. Other desirable properties were high wear- and fatigue-resistance as well as improved formability. However, compromises may have to be made to achieve a desired combination of properties, which makes it more difficult to optimize the processing.

In order to achieve good strength-toughness properties, two different steel manufacturing processes were used in this study: interrupted quenching combined with low temperature annealing i.e., quenching and partitioning (QP) and austenite reversion transformation (ART). The aim of the study was to understand how different characteristics of the steel microstructures affect the properties of QP and ART treated steels.

The results showed that the impact toughness of steel can be significantly influenced by the studied heat treatments. Optimized QP and ART treatments imparted good tensile strength/elongation combinations depending on the amount, size, shape, distribution, and phase composition of the respective microstructures. The drawbacks of the studied heat treatments were the associated practical challenges for implementation in industrial production. The difficulty of QP processing is to precisely reach a desired temperature in interrupted quenching, which can have a major impact on the desired properties of the steel. In ART treatment, the challenge lies in identifying the narrow temperature range and time window of the reversion heating to achieve the desired properties. On the other hand, it is noteworthy that innovative industrial processes are also under development alongside these novel materials, which will render manufacturing of these steels possible in near future.
Last updated: 3.3.2023