Auto-tempering-induced nanoprecipitate strengthening of ultrastrong low-alloy high-carbon steel

Ultrahigh-strength steels, which have become vital components in energy-efficient structural systems, can be realized by incorporating expensive alloying elements into them. In this study, increasing the strength of a low-alloy high‑carbon steel, by precipitating its granular and rod-like ε-carbides, formed during its auto-tempering and low-temperature tempering, respectively, was explored. The crystallographic orientation relationships between the ε-carbides in the low-alloy high‑carbon steel and its martensitic matrix were determined. The number density and average size of the granular ε-carbides in the steel were 4.8 × 10²³ m⁻³ and 2.2 ± 0.5 nm, respectively. The volume fraction of the rod-like ε-carbides in the steel was 4%. The diameters of the rod-like ε-carbides in the steel were between 10 and 20 nm, and their lengths were between 50 and 250 nm. The granular and rod-like ε-carbides in the steel contributed 949 and 70 MPa, respectively, to its yield strength. Thus, the granular ε-carbides were primarily responsible for the ultrahigh yield strength (2250 MPa) of the steel. In addition, the semi-coherent interfaces between the granular ε-carbides and the martensitic matrix in the steel may facilitate dislocation motions without subjecting the steel to severe local stress concentrations, thereby contributing to its total elongation of 11.4%. This study employed inexpensive carbides to produce high-performance steels, leading to a sustainable, lightweight design.