Enhancing technological prospect of nanostructured bainitic steels by the control of thermal stability of austenite

Highlights: • Enhancing thermal stability allow for the industrialization of bainitic steels. • Thermal stability may be controlled by chemical composition and heat treatment. • Retained austenite determines reversion of the structure to thermodynamic equilibrium. • Thermal stability particularly depends on morphology and carbon content of austenite. • Solution and precipitation strengthening, secondary hardening are apt prospects. Excellent mechanical properties of nanostructured bainitic steels are primarily related to the ultra fine scale of their structure constituents. Typically, such structure is exhibited by suitably treated steels with high carbon and high silicon content. Regardless, the common application of this steel grade in the industry is limited by some in-used properties that have not yet been explained. This mainly concerns the possibility of joining, long-term operationing at elevated temperatures and ensuring corrosion resistance. The feature promoting the provision of these properties is the sufficiently high thermal stability of the retained austenite. Enhancing thermal stability would allow for the application of these steels in the automotive, energy and marine industries. In contrast to the well-known control of the mechanical stability of the retained austenite, thermal stability requires a different approach in the context of chemical composition design and heat treatment parameters. This work presents the concept of increasing the thermal stability of the retained austenite and its influence on the improvement of the in-used properties of nanostructured bainitic steels. Mechanisms ensuring higher stability of the microstructure at elevated temperatures were reviewed, such as controlling the morphology and carbon content of retained austenite, solution strengthening, precipitation of intermetallic phases, and the prospect of secondary hardening.