Aging response of a Fe–C–Co–Ni secondary hardening steel: The significance of matrix ordering

Abstract

The evolution of precipitates and dislocations during aging dictates the mechanical properties of secondary hardening steels. Such microstructural response of a Fe–C–Co–Ni secondary hardening steel has been investigated via high-resolution transmission electron microscopy in this study. The primary source of strengthening is linked to coherent Molybdenum carbides, which form in a sequence of cementite, cluster and M2C. It is shown that the ordering of the martensitic matrix occurs rapidly upon aging and results in the formation of dispersed B2–FeCo precipitates. The co-location of ordered domains with the cluster and M2C carbides indicates that the ordering greatly affects the nucleation and growth of M2C and thus leads to a more homogeneous spatial distribution of M2C. This yields excellent mechanical properties including an ultimate tensile strength of 2150 MPa, yield strength of 1829 MPa, and total elongation of 12 %. It should be noted that the ordering not only contributes to the mechanical properties by promoting the dispersion of M2C, it also causes strengthening directly as demonstrated by an additive model of yield strength. However, the model fails to predict the yield strength of the as-quenched sample since it contains extra mobile dislocations which considerably modify the mechanical property.