The Bainite reaction in Fe-Si-C Alloys: The primary stage

The morphology, crystallography, and substructure of bainitic ferrite formed in silicon alloyed high-carbon steels in the temperature range 290 to 380 °C have been studied. The bainite exhibits a plate shaped morphology, an irrational habit plane, and an irrational orientation relationship. The bainitic ferrite is heavily dislocated, while the surrounding austenite contains thin twins, the density of which is highest in the austenite between closely spaced ferrite plates. The bainite plates can cross these twins in such a manner that the twinned region remains in a crystallographic orientation, which is quite different from that of the other regions of the bainitic ferrite plates. Epsilon carbide subsequently precipitates on the austenite twin/bainitic ferrite boundaries. The bainitic ferrite shape strain direction and magnitude are estimated from displacements of austenite twins inherited in the ferrite. All results, including measurement of the austenite carbon content, are consistent with a shear mode of transformation. B.P.J. SANDVIK, formerly with Laboratory of Physical Metallurgy at Helsinki University of Technology, Finland and Ovako OyAb, Imatra, Finland