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

오스템퍼링 시간에 따른 초고강도 나노 베이나이트강의 미세조직과 기계적 특성

  • 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)
  • 이지민 (서울과학기술대학교 신소재공학과) ;
  • 황병철 (서울과학기술대학교 신소재공학과)
  • Received : 2019.12.21
  • Accepted : 2020.01.31
  • Published : 2020.02.27

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

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