Carbide precipitation and coarsening kinetics in low carbon and low alloy steel during quenching and subsequently tempering

Abstract

As-quenched microstructure in a low carbon and low alloy steel has been examined by transmission electron microscopy and atom probe tomography. The coarsening kinetics of the cementite was investigated for the tempered steel. The results show that a number of nanosized auto-tempered transition‑iron-ε' carbide and cementite precipitate in the coarse lath martensite during quenching. The carbon concentrations of auto-tempered transition‑iron-carbide are typically around 10–15 at.%, lower than that for stoichiometric Fe2.4C or Fe2C transition ε carbide. Partitioning of Mn, Cr and Mo into the cementite occurs during tempering. The coarsening rate of the cementite is governed by volume diffusion of Mn, Cr and Mo, and Cr retards cementite coarsening more effectively than Mn and Mo. More stable M7C3 type carbide transforms from the cementite partially, and retains smaller size than the cementite during tempering with prolonged time.