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

In this study,the carbide precipitation at 925 ℃ 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 M C-type ultrafi ne carbides(<10 nm)forms during the very early stage prior to the onset of precipitation determined by SR.These ultrafi ne carbides possess a B1-crystal structure with a lattice parameter of 3.696Å,which is quite close to that ofγ(3.56Å).It can signifi cantly decrease the misfi t of carbide/γinterface 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 ultrafi ne carbides by nucleation and growth on them.These(Ti,W)C particles are identifi ed 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 eff ect of W,Fe,and Ti atoms together with interstitial vacancies.