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Effect of Tempering Temperature on Microstructure Evolution and Hardness of 9Cr1. 5Mo1CoB(FB2) Steel
Yongqiang Zhang, Chuanwei Li, Gang Shen, Lizhan Han,* and Jianfeng Gu*
ABSTRACT
The microstructure evolution and hardness variation of FB2 steel influenced by tempering are investigated. The results show that water-cooling from 1100 °C produces lathy martensite microstructure. After tempering at 500 C, the steel exhibits a martensite structure with 4.2% needle-like Fe3C particles. The (Cr, Mo)2C and Cr-rich M7C3 particles are detected in the sample tempered at 570 °C. As the tempering temperature enhances from 620 to 700 °C the (Cr,Mo)2C and Cr-rich M7C3 in the matrix are replaced by Cr-rich M23C6. Besides, the (V, Nb)C particles are identified in samples
tempered above 620 °C. Thus, the evolution of carbides in FB2 steel during tempering can be summarized as: Fe3C→(Cr, Mo)2C + Cr-rich M7C3→(Cr, Mo)2C + Cr-rich M7C3 + Cr-rich M23C6→Cr-rich M23C6. Furthermore, the results suggest that M3C, M2C + M7C3, M23C6, and MX have high reaction rates at 500, 570, 620, and 700 °C respectively. In addition, the dislocation density reduces from 6.8×1014 m-2 to 2.1 × 1014 m-2, and the volume fraction of carbides increases from 4.2% to 12.6% with increasing temperatures from 500 to 700 C leading to hardness decrease from 485 to 284 Hv. Finally, the quantitative relationship between the hardness and microstructure evolution during tempering is discussed.
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