The effect of soaking time after ultrafast heating on the microstructure and mechanical behavior of a low carbon steel

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2019-09, Vol.765, p.138276, Article 138276
• Main Authors: Valdes-Tabernero, M.A., Petrov, R.H., Monclus, M.A., Molina-Aldareguia, J.M., Sabirov, I.
• Format: Article
• Language: English
• Subjects: Dislocation density; Ferrites; Ferritic stainless steels; Grain growth; Grain size; Heat treatment; Heating rate; Low carbon steel; Low carbon steels; Martensite; Mechanical analysis; Mechanical properties; Microstructure; Nanohardness; Nanoindentation; Nucleation; Pop-in; Recrystallization; Soaking; Steel; Tensile tests; Ultrafast heating
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2019.138276

The main objective of this study is to understand the effect of the soaking time during the ultrafast heat treatment of a low carbon steel on its complex multi-phase microstructure, tensile mechanical behavior and properties of individual microconstituents. Tensile tests were performed to determine the macro-mechanical properties. Nanoindentation testing was carried out on individual microconstituents (martensite, recrystallized ferrite and non-recrystallized ferrite) identified a priori via EBSD analysis to measure their properties. It is shown that ultrafast heating combined with short soaking times results in improved macro-mechanical properties due to finer grain size and higher fraction of non-recrystallized ferrite, that has a higher nanohardness than recrystallized ferrite. Prolonged soaking times eliminate the advantages of the ultrafast heat treatment. This occurs because, even though a long soaking time promotes a higher volume fraction of martensite than a short one, it also induces substantial grain growth and complete recrystallization of the ferritic matrix. On the micro-scale, the ferritic grains show two different types of mechanical response. The recrystallized ferritic grains are prone to show pop-in events on the nanoindentation curves that are associated to dislocation nucleation events as a consequence of their low dislocation density, while non-recrystallized ferritic grains demonstrate a continuous response. The relationship between microstructure and mechanical properties on the macro- and micro-scales is discussed with respect to the microstructure, which in turn strongly depends on the applied heating rate and soaking time. A general recipe for microstructural design to improve the tensile mechanical behavior of low carbon steels implementing controlled heating and soaking conditions is outlined.