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

TL;DR

This study reveals how nanoscale austenite reversion, driven by carbon partitioning and segregation, enhances the strength and ductility of a Fe-Cr-C steel through controlled tempering processes.

Contribution

It demonstrates the mechanisms of austenite reversion at the nanoscale and how tempering parameters can be tailored to optimize steel's mechanical properties.

Findings

Achieved 2 GPa ultimate tensile strength with 14% elongation.

Controlled tempering modifies strength and ductility profiles.

Characterized microstructural changes using advanced microscopy techniques.

Abstract

Austenite reversion during tempering of a Fe-13.6Cr-0.44C (wt.%) martensite results in an ultrahigh 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{\deg}C for 1 min. produces a 2 GPa ultimate tensile strength (UTS) and 14% elongation while 30 min. at 400{\deg}C results in a UTS of ~ 1.75 GPa with an elongation of 23%). The austenite reversion process, carbide precipitation, and carbon segregation have been characterized by XRD, EBSD, TEM, and atom probe tomography (APT) in order to develop the structure-property relationships that control the material's strength and ductility.