Korean Journal of Materials Research (한국재료학회지)

Materials Research Society of Korea (한국재료학회)
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Influence of Mo and Cr Contents on Hardenability of Low-Carbon Boron Steels

저탄소 보론강의 경화능에 미치는 Mo 및 Cr 함량의 영향

  • Hwang, Byoungchul (Department of Materials Science and Engineering, Seoul National University of Science and Technology) ;
  • Suh, Dong-Woo (Graduate Institute of Ferrous Technology, Pohang University of Science and Technology)
  • 황병철 (서울과학기술대학교 신소재공학과) ;
  • 서동우 (포항공과대학교 철강대학원)
  • Received : 2013.08.19
  • Accepted : 2013.09.12
  • Published : 2013.10.27
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Abstract

The hardenability of low-carbon boron steels with different molybdenum and chromium contents was investigated using dilatometry, microstructural observations and secondary ion mass spectroscopy (SIMS), and then discussed in terms of the segregation and precipitation behaviors of boron. The hardenability was quantitatively evaluated by a critical cooling rate obtained from the hardness distribution plotted as a function of cooling rate. It was found that the molybdenum addition was more effective than the chromium addition to increase the hardenability of boron steels, in contrast to boron-free steels. The addition of 0.2 wt.% molybdenum completely suppressed the formation of eutectoid ferrite, even at the slow cooling rate of $0.2^{\circ}C/s$, while the addition of 0.5 wt.% chromium did this at cooling rates above $3^{\circ}C/s$. The SIMS analysis results to observe the boron distribution at the austenite grain boundaries confirmed that the addition of 0.2 wt.% molybdenum effectively increased the hardenability of boron steels, as the boron atoms were significantly segregated to the austenite grain boundaries without the precipitation of borocarbide, thus retarding the austenite-to-ferrite transformation compared to the addition of 0.5 wt.% chromium. On the other hand, the synergistic effect of molybdenum and boron on the hardenability of boron steels could be explained from thermodynamic and kinetic perspectives.

Keywords

References

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