Evolution of Microstructure and Mechanical Properties during Tempering of Continuously Cooled Bainitic Steels

Abstract:

With the increasing demand for high performance engine or suspension components, bainitic steels are receiving interest as potential replacement of their quench and tempered counterparts. Indeed, for a number of mechanical components, ferrite pearlite microstructures are no longer sufficient in terms of mechanical properties. Bainitic steel grades allow production of hot-rolled bars or forged components exhibiting a homogeneous bainitic microstructure and achieving UTS up to 1200 MPa without the need for additional heat-treatments [1]. During tempering, these V-microalloyed bainitic steels exhibit unusual yield strength variations, with a very pronounced increase around 250-300 °C followed by the better known secondary hardening peak for temperatures around 600-650 °C. Indeed, after tempering at 250-300 °C, some of these steels exhibit an increase in yield strength of up to 200 MPa, concurrent with an increase in impact toughness of up to 25%. This, however, goes unnoticed if hardness measurements are used to characterize tempering. In the following, results are presented for three different bainitic steel grades, and the origins of the changes in mechanical properties are discussed.

• Periodical: Materials Science Forum (Volumes 706-709)
• Pages: 2308-2313
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.706-709.2308
• Online since: January 2012
• Authors: Thomas Sourmail, Véronique Smanio, Francisca G. Caballero, J. Cornide, C. Capdevilla, Carlos Garcia-Mateo
• Keywords: Bainite, Retained Austenite Stability, Tempering

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