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http://dx.doi.org/10.3740/MRSK.2020.30.2.87

Effect of Austempering Time on the Microstructure and Mechanical Properties of Ultra-High Strength Nanostructured Bainitic Steels  

Lee, Ji-Min (Department of Materials Science and Engineering, Seoul National University of Science and Technology)
Hwang, Byoungchul (Department of Materials Science and Engineering, Seoul National University of Science and Technology)
Publication Information
Korean Journal of Materials Research / v.30, no.2, 2020 , pp. 87-92 More about this Journal
Abstract
This study deals with the effects of austempering time on the microstructure and mechanical properties of ultra-high strength nanostructured bainitic steels with high carbon and silicon contents. The steels are composed of bainite, martensite and retained austenite by austempering and quenching. As the duration of austempering increases, the thickness of bainitic ferrite increases, but the thickness of retained austenite decreases. Some retained austenites with lower stability are more easily transformed to martensite during tensile testing, which has a detrimental effect on the elongation due to the brittleness of transformed martensite. With increasing austempering time, the hardness decreased and then remained stable because the transformation to nanostructured bainite compensates for the decrease in the volume fraction of martensite. Charpy impact test results indicated that increasing austempering time improved the impact toughness because the formation of brittle martensite was prevented by the decreased fraction and increased stability of retained austenite.
Keywords
nanostructured bainite; austempering; retained austenite; mechanical property; ultra-high strength;
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