Korean Journal of Materials Research (한국재료학회지)

Materials Research Society of Korea (한국재료학회)
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Effect of Isothermal Heat Treatment on the Microstructure and Mechanical Properties of Medium-Carbon Bainitic Steels

등온 열처리에 따른 중탄소 베이나이트강의 미세조직과 기계적 특성

  • Lee, Ji-Min (Department of Materials Science and Engineering Seoul National University of Science and Technology) ;
  • Lee, Sang-In (Department of Materials Science and Engineering Seoul National University of Science and Technology) ;
  • Lim, Hyeon-Seok (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) ;
  • 이상인 (서울과학기술대학교 신소재공학과) ;
  • 임현석 (서울과학기술대학교 신소재공학과) ;
  • 황병철 (서울과학기술대학교 신소재공학과)
  • Received : 2018.08.04
  • Accepted : 2018.09.05
  • Published : 2018.09.27
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Abstract

This study investigates the effects of isothermal holding temperature and time on the microstructure, hardness and Charpy impact properties of medium-carbon bainitic steel specimens. Medium-carbon steel specimens with different bainitic microstructures are fabricated by varying the isothermal conditions and their microstructures are characterized using OM, SEM and EBSD analysis. Hardness and Charpy impact tests are also performed to examine the correlation of microstructure and mechanical properties. The microstructural analysis results reveal that granular bainite, bainitic ferrite, lath martensite and retained austenite form differently in the specimens. The volume fraction of granular bainite and bainitic ferrite increases as the isothermal holding temperature increases, which decreases the hardness of specimens isothermally heat-treated at $300^{\circ}C$ or higher. The specimens isothermally heat-treated at $250^{\circ}C$ exhibit the highest hardness due to the formation of lath martensite, irrespective of isothermal holding time. The Charpy impact test results indicate that increasing isothermal holding time improves the impact toughness because of the increase in volume fraction of granular bainite and bainitic ferrite, which have a relatively soft microstructure compared to lath martensite for specimens isothermally heat-treated at $250^{\circ}C$ and $300^{\circ}C$.

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