Carbide Precipitation in Austenite of a Titanium-Tungsten-Bearing Low-Carbon Steel

In this study, the carbide precipitation at 925 °C in austenite (γ) of a 0.04C–1.5Mn–0.10Ti–0.39 W (wt%) low-carbon steel was investigated by stress relaxation (SR) high-resolution transmission electron microscopy and atom probe tomography. First-principles calculations were employed to reveal the precipitation mechanism. Results indicate that a high dispersion of W- and Fe-rich MC-type ultrafine carbides (< 10 nm) forms during the very early stage prior to the onset of precipitation determined by SR. These ultrafine carbides possess a B1-crystal structure with a lattice parameter of 3.696 A, which is quite close to that of γ (3.56 A). It can significantly decrease the misfit of carbide/γ interface with a cube-on-cube relationship, thus assisting the carbide nucleation. As the time prolongs, a few spherical or polygonal Ti-rich (Ti, W)C particles (18–60 nm) are formed at the expense of the ultrafine carbides by nucleation and growth on them. These (Ti, W)C particles are identified with a “core–shell” structure (Ti-rich core and Ti, W-rich shell), which leads to a better-coarsening resistance compared with pure TiC in Ti steel. Calculation results show that the composition and structure of carbides at certain stage are closely related to a combined effect of W, Fe, and Ti atoms together with interstitial vacancies.