Atom Probe Tomography Analysis of Precipitation during Tempering of a Nanostructured Bainitic Steel

Carbon distribution during tempering of a nanostructured bainitic steel was analyzed by atom probe tomography (APT). Three different types of particles are detected on samples tempered at 673 K (400 °C) for 30 minutes: lower bainite cementite with a carbon content of ~25 at. pct, ε-carbides with a carbon content close to 30 at. pct, and carbon clusters, small features with a carbon content of ~14 at. pct indicative of a stage of tempering prior to precipitation of ε-carbide. After tempering at 773 K (500 °C) for 30 minutes, the ε-carbide-to-cementite transition was observed. Solute concentration profiles across carbide/ferrite interfaces showed the distribution of substitutional elements in ε-carbide and cementite for all the tempering conditions.