The microstructural evolution of Grade 91 tempered martensite ferritic steels heat treated at 760~1000 oC 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(760~850 oC), and to fresh martensite(900~1000 oC), 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 oC), indicating grain recovery upon intercritical heating. The main precipitate, M23C6, was found to be coarsened slightly at 760~850 oC; it then dissolved at 850~1000 oC. Besides this, M3C cementite was formed at 900~1000 oC. 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.

1. Introduction
2. Experimental Procedures
3. Results and Discussion
4. Conclusion
Acknowledgement
References

• Yinsheng He(School of Nano & Advanced Materials Engineering, Changwon National University, Changwon 51140, Korea)
• Jungchel Chang(Technology Policy and Planning Department, Korea Electric Power Corporation, Naju 58217, Korea)
• Je-Hyun Lee(School of Nano & Advanced Materials Engineering, Changwon National University, Changwon 51140, Korea)
• Keesam Shin(School of Nano & Advanced Materials Engineering, Changwon National University, Changwon 51140, Korea) Corresponding Author