Effect Of Tempering Temperature On Microstructure Evolution And Hardness Of 9cr1. 5mo1cob(Fb2) Steel

The microstructure evolution and hardness variation of FB2 steel influenced by tempering are investigated. The results show that water-cooling from 1100 degrees C produces lathy martensite microstructure. After tempering at 500 degrees C, the steel exhibits a martensite structure with 4.2% needle-like Fe3C particles. The (Cr, Mo)(2)C and Cr-rich M7C3 particles are detected in the sample tempered at 570 degrees CAs the tempering temperature enhances from 620 to 700 degrees C the (Cr, Mo)(2)C 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 degrees C. Thus, the evolution of carbides in FB2 steel during tempering can be summarized as: Fe3C ->(Cr, Mo)(2)C + Cr-rich M7C3 ->(Cr, Mo)(2)C + 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 degrees C respectively. In addition, the dislocation density reduces from 6.8 x 10(14) m(-2) to 2.1 x 10(14) m(-2), and the volume fraction of carbides increases from 4.2% to 12.6% with increasing temperatures from 500 to 700 degrees C leading to hardness decrease from 485 to 284 Hv. Finally, the quantitative relationship between the hardness and microstructure evolution during tempering is discussed.