Nanoscale austenite reversion through partitioning, segregation and kinetic freezing: Example of a ductile 2 GPa Fe-Cr-C steel

Austenite reversion during tempering of a Fe-13.6 Cr-0.44 C (wt.%) martensite results in an ultra-high-strength ferritic stainless steel with excellent ductility. The austenite reversion mechanism is coupled to the kinetic freezing of carbon during low-temperature partitioning at the interfaces between martensite and retained austenite and to carbon segregation at martensite martensite grain boundaries. An advantage of austenite reversion is its scalability, i.e. changing tempering time and temperature tailors the desired strength-ductility profiles (e.g. tempering at 400 degrees C for 1 min produces a 2 GPa ultimate tensile strength (UTS) and 14% elongation while 30 mm at 400 degrees C results in a UTS of similar to 1.75 GPa with an elongation of 23%). The austenite reversion process, carbide precipitation and carbon segregation have been characterized by X-ray diffraction, electron back-scatter diffraction, transmission electron microscopy and atom probe tomography in order to develop the structure-property relationships that control the material's strength and ductility. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.