Elemental redistribution and precipitation reactions of 9Cr1.5Mo1CoB (FB2) steel during tempering

Yongqiang Zhang, Jianfeng Gu *, Lizhan Han *

Institute of Materials Modification and Modelling, Shanghai Jiao Tong University, Shanghai 200240, China

ABSTRACT

The effect of tempering temperature on the precipitation reactions and elemental redistribution in
9Cr1.5Mo1CoB(FB2) steel has been studied using scanning electron microscope(SEM), transmission electron microscope(TEM), secondary ion mass spectrometry(Tof-SIMS) and atom probe technique (APT). The results show that water-cooling from normalization at 1100 °C produced lathy martensite. During tempering at 500 °C, only C and B atoms were segregated, and formed needle-like Fe3(C, B); After tempering at 600 °C, the segregation of Cr, Mo and Mn showed noticeable difference, forming Cr + Mn + Mo + C + B and Mo + Cr + C + B clusters. Consequently, (Cr,Mo)₂(C,B), Cr-rich M₇(C,B)₃ and Cr-rich M₂₃(C,B)₆ were observed in sample tempered at 600 °C. And both (Cr,Mo)₂(C,B) and M7(C,B)3 particles were entirely replaced by Cr-rich M₂₃(C,B)₆ by enhancing the tempering temperature to 700 °C. Therefore, the evolution of carbides in FB2 steel during tempering at
500 °C–700 °C can be summarised as follows: Fe₃(C,B) → (Cr,Mo)₂(C,B) + Cr-rich M₇(C,B)₃ → Cr-rich M₂₃(C,B)₆. Besides, the Mo element in (V,Nb,Mo)(N,C) particles was gradually replaced by V and Nb with the increase of temperature from 600 °C to 700 °C. While Si and Co were rejected to the interfaces of carbides/matrix during tempering, forming a flux of Si and Co. Then, this flux may suppress carbides coarsening by retarding the diffusion of Cr, Mo and Mn from the matrix into carbides.