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 degrees C) for 30 minutes: lower bainite cementite with a carbon content of similar to 25 at. pct, e-carbides with a carbon content close to 30 at. pct, and carbon clusters, small features with a carbon content of similar to 14 at. pct indicative of a stage of tempering prior to precipitation of e-carbide. After tempering at 773 K (500 degrees C) for 30 minutes, the e-carbide-to-cementite transition was observed. Solute concentration profiles across carbide/ferrite interfaces showed the distribution of substitutional elements in e-carbide and cementite for all the tempering conditions.