Clustering and precipitation processes in a ferritic titanium-molybdenum microalloyed steel

Atom Probe Microscopy has been performed on a titanium-molybdenum microalloyed steel after simulation of thermomechanical processing. The results reveal the evolution of clustering and precipitation reactions at the austenite/ferrite interphase interface during isothermal heat treatments of durations ranging between 300 s and 100 h. Shorter isothermal ageing times led to extensive solute clustering at the austenite/ferrite interface, which remained free of precipitates. For ageing times beyond 3600 s, high number densities of interphase nano-precipitates were observed along with extensive solute clustering. Beyond 100 h, there was little evidence of solute clustering at these austenite/ferrite microstructural interfaces, which were instead dominated by a dispersion of coarser nano-precipitates. Both the clusters and nano-precipitates formed during isothermal ageing possessed a disc shape morphology and the growth has taken place in the through thickness direction of the disc without much change in the aspect ratio. The size of clusters and nano-precipitates increased from ∼2 nm after 300 s to ∼15 nm after 100 h of isothermal ageing at 650 °C. The solute clusters contain mainly C, Ti and Mo atoms, while the stoichiometry of the nano-precipitates approached that of MC carbide as their size increased beyond ∼4 nm. It is proposed that Mo controls the process of precipitate growth, resulting in a fine and uniform dispersion of nano-precipitates and a high level of tensile strength after just 300 s ageing at 650 °C. Interestingly, the peak strength was achieved during the very early stages of the precipitation process.