Redistribution of substitutional alloying elements between -Fe matrix and cementite after long-term tempering in a low alloying steel

The partitioning behaviour of non-carbide forming elements (Si, Cu and Ni) and carbide forming elements (Mn and Mo) between alpha-Fe matrix and cementite (theta) after long-term tempering a quenched reactor pressure vessel (RPV) model steel has been studied by atom probe tomography (APT). It has demonstrated that a non-carbide forming element enriched layer near the interface of alpha/theta in the alpha-Fe matrix side is a common feature for tempered steels containing non-carbide forming element, which is widely accepted as the mechanism by which substitutional elements retard the coarsening of cementite. In addition, a carbide forming element enriched layer near the interface of alpha/theta in the cementite side, Si, Cu and Ni enriched clusters formed in the cementite and Mo2C carbide formed beside cementite were observed, and all this is taken as valuable mechanisms by which addition of substitutional alloying elements can increase tempering resistance of alloying steels.