Advanced ultra high strength bainitic steels

The addition of about 2wt.% of silicon to steel enables the production of a distinctive microstructure consisting of a mixture of bainitic ferrite, carbon-enriched retained austenite, and some martensite. With careful design, impressive combinations of strength and toughness have been reported for high-silicon bainitic steels. More recently, it has been demonstrated experimentally that models based on phase transformation theory can be applied successfully to the design of carbide-free bainitic steels. Toughness values of nearly 130 MPa m(1/2) were obtained for strength in the range of 1600-1700 MPa. However, the concepts of bainite transformation theory can be exploited even further to design steels that transform to bainite at temperatures as low as 150 degrees C. Microstructure obtained is so refined that it is possible to achieve a strength in excess of 2.5 GPa in a material which has considerable toughness ( 30MPam(1/2)). Such properties have never been achieved before with bainite. It is intended to provide a description of the characteristics and significance of this remarkable microstructure in the context of the mechanism of transformation.