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

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.