Low temperature kinetics of bainite formation in high carbon steels

This work is concerned with quantifying and discussing the influence of alloying elements on the kinetics of bainite formation at low temperatures (220-250 °C) in high carbon, high silicon steels (100Cr6 and similar grades). In a first step, it is shown that the austenite carbon content is strongly influenced not only by the austenitizing temperature, but also by its duration. A method is thus proposed and validated to estimate this content and ensure that later comparisons are meaningful. In a second step, the influence of Cr, Mn, Mo and Si are evaluated. The relative effects of C, Mn, Cr and Mo are shown to be quantitatively in reasonable agreement with calculated driving forces, with C being by far the strongest retardant of bainite formation. The influences of Mn and Cr are found to be of similar order of magnitude, though with a stronger influence of Mn. Si is shown to continuously slow down bainite kinetics with increasing content, with no threshold content identified and an influence that is stronger than that of Mn or Cr. The role of Si is discussed and the current accepted mechanism is shown to be inconsistent with present and published observations. A new possibility is discussed for how Si influences kinetics in the investigated conditions (carbon content, temperature).