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 ℃ or higher. The specimens isothermally heat-treated at 250 ℃ 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 ℃ and 300 ℃.

Abstract
 1. 서 론
 2. 실험 방법
 3. 결과 및 고찰
  3.1 등온 열처리에 따른 미세조직 변화
  3.2 미세조직 변화에 따른 경도 및 충격 특성
 4. 결 론
 References

• 이지민(서울과학기술대학교 신소재공학과) | Ji-Min Lee (Department of Materials Science and Engineering Seoul National University of Science and Technology)
• 이상인(서울과학기술대학교 신소재공학과) | Sang-In Lee (Department of Materials Science and Engineering Seoul National University of Science and Technology)
• 임현석(서울과학기술대학교 신소재공학과) | Hyeon-Seok Lim (Department of Materials Science and Engineering Seoul National University of Science and Technology)
• 황병철(서울과학기술대학교 신소재공학과) | Byoungchul Hwang (Department of Materials Science and Engineering Seoul National University of Science and Technology) Corresponding author