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

Highlights: • The precipitates sequence of FB2 steel during tempering is proposed. • The effect of tempering temperature on the segregation behavior of interstitial atom and substitional is suggested. • The flux of Si and Co at the carbides/matrix interfaces may inlfuence the coarsening of carbides. 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.