Korean Journal of Materials Research (한국재료학회지)

Materials Research Society of Korea (한국재료학회)
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Microstructural Evolution of Grade 91 Steel upon Heating at 760~1000℃

  • He, Yinsheng (School of Nano & Advanced Materials Engineering, Changwon National University) ;
  • Chang, Jungchel (Technology Policy and Planning Department, Korea Electric Power Corporation) ;
  • Lee, Je-Hyun (School of Nano & Advanced Materials Engineering, Changwon National University) ;
  • Shin, Keesam (School of Nano & Advanced Materials Engineering, Changwon National University)
  • Received : 2015.07.27
  • Accepted : 2015.10.01
  • Published : 2015.11.27
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Abstract

The microstructural evolution of Grade 91 tempered martensite ferritic steels heat treated at $760{\sim}1000^{\circ}C$ for two hours was investigated using scanning electron microscopy(SEM), energy disperse spectroscopy(EDS), electron backscattered diffraction (EBSD), and transmission electron microscopy(TEM); a microhardness tester was also employed, with a focus on the grain and precipitate evolution process as well as on the main hardening element. It was found that an evolution of tempered martensite to ferrite($760{\sim}850^{\circ}C$), and to fresh martensite($900{\sim}1000^{\circ}C$), occurred with the increase of temperature. Simultaneously, the parabolic evolution characteristics of the low angle grain boundary(LAGB) increased with the increase of the heating temperature(highest fraction of LAGB at $925^{\circ}C$), indicating grain recovery upon intercritical heating. The main precipitate, $M_{23}C_6$, was found to be coarsened slightly at $760{\sim}850^{\circ}C$; it then dissolved at $850{\sim}1000^{\circ}C$. Besides this, $M_3C$ cementite was formed at $900{\sim}1000^{\circ}C$. Finally, the experimental results show that the hardness of the steel depended largely on the matrix structure, rather than on the precipitates, with the fresh martensite showing the highest hardness value.

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References

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