The Silicon Effect in the Tempering of Martensite in Steels

High-silicon steels have long been known to possess exceptional resistance to tempering, but the reasons for this effect were unknown. We report here a study of the third stage of tempering of these steels, in which cementite is formed. In a 0.75 wt% C 1.4 wt% Si alloy steel, tempered for 1 hour at 380°C, it is found that silicon atoms are rejected from the growing cementite into the ferrite matrix, and form a siliconenriched atmosphere around the particles. The concentration of silicon in the segregated zone reaches levels of 12% in a region 1-2 nm wide adjacent to the cementite. The results are consistent with a mechanism in which the silicon-rich region is the kinetic barrier to the further growth of the cementite phase, and the diffusion of silicon away from the interface controls the growth and coarsening processes.