Isothermal Phase Transformations in a Low Carbon Steel During Single and Two-Step Partitioning
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NTRODUCTION
ISOTHERMAL single and two-step heat treatments below the MS temperature have been recently applied to carbon steel in order to achieve better mechanic behavior.[1] Among these steel grades are the so-called Quenched and Partitioned (Q&P) steels, which belong to the third generation advanced high-strength steels. The objective of the Q&P treatment is to produce microstructures that consist of mixtures of martensite and retained austenite, which could theoretically fill the gap of mechanical properties between first and second generation advanced high-strength steel. Isothermal single and two-step thermal cycles, however, have been extensively applied in the heat treatment of TRIP and Bainitic steels and nodular austempered cast iron. Although the technological challenges of the application
F.M. CASTRO CERDA is with the Department of Metallurgy, University of Santiago de Chile, Alameda 3363, 9170022 Estacio´n Central, Santiago, Chile and the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands. Contact e-mail: [email protected] E.I. HERNA´NDEZ is with the Department of Metallurgy, University of Santiago de Chile and the Department of Electromechanical Systems and Materials, Research group Materials Science and Technology, Ghent University, Tech Lane Science Park Campus A 46, Gent, Belgium. T. ROS-YANEZ is with ArcelorMittal Global R&D, 3001 E. Columbus Dr., East Chicago, IN 46312. R.H. PETROV is with the Department of Electromechanical Systems and Materials, Research group Materials Science and Technology, Ghent University and the Department of Materials Science and Engineering, Delft University of Technology. Manuscript submitted July 10, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
of single and two-step heat treatments in a rolling mill are of a different nature compared to the casting industry, the potential improvement in the properties of iron alloys has been well acknowledged by both scientific and engineering communities. The concept of Quenching and Partitioning of low alloyed, low carbon steel has been proposed to produce enhanced grades with a microstructure of mainly martensite and austenite.[2,3] Q&P cycles consist of full annealing in the austenitic (or intercritical) temperature range, followed by the quenching stage to a temperature below MS. The partitioning stage can be carried out either at the same quenching temperature or at a higher one, for a pre-defined time, known as partitioning time. At this stage the carbon ‘partitions’ from martensite to austenite, the carbon content of the untransformed austenite increases and after final quenching this carbon-enriched austenite remains untransformed, i.e., retained at room temperature. Although the aim of Q&P is to produce large amount of retained austenite, the stabilization of austenite is only one of the processes observed. Several studies have reported the observation of bainite[4–11] and tempered martensite[7,12–15] during the partitioning stage. The formation o
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