High-Strength Bainitic Steels for Forged Products

Closed die forging parts of bainitic steels have been used increasingly in the automotive industry, since they obtain remarkable properties under cyclic loading. Bainitic morphologies require a consecutive thermal heat treatment after forging in order to achieve their maximum performance. These microstructures can be formed either after isothermal phase transformation or continuous cooling. These two process routes lead to different results with regard to the mechanical properties. The current research focuses on the kinetic influences of different transformation temperatures in the bainitic field and different cooling rates. The isothermal transformation behaviour was dilatometrically measured using a Gleeble 3800 testing machine. The results provide an overview of the formation of a fine microstructure and the expected mechanical properties. At transformation temperatures just above the Ms-temperature, strength values of almost 1000 MPa can be achieved, i.e. significant improved fatigue endurance is expected. Due to the fine microstructure, the Charpy-V-toughness is also excellent.