Recent progress on the substructure of Fe-C martensite and pearlites: A review

Martensitic phase transformation plays a crucial role in the functional properties of shape memory metals and alloys, profoundly affecting the mechanical behavior, including transformation-induced plasticity effect in metallic materials, especially steels. Considering the wide-ranging applications and the absolute usage volume, iron (or steels) is undoubtedly the metal among functional and structural materials that is used the most in the world. Quenching has long been employed as a straightforward and historically recognized method for hardening carbon steels, yet its underlying scientific mechanisms remain incompletely understood. Recent transmission electron microscopy investigations have revealed that quenched Fe–C martensite consists of ultrafine ferrite and carbide grains, typically measuring about ∼1-2 nm in size. Rapid quenching can prevent the coarsening of the ultrafine grains. These ultrafine two-phase grains ― ferrite (α-Fe) and carbide (ω-Fe 3 C) ― are responsible for quenching induced hardening and forming twinned martensite with the ω-Fe 3 C located at the α-Fe twinning-boundaries. Tempering (auto/post) results in coarsening or recrystallization of fine crystals, leading to the formation of various microstructure, such as pearlite, bainite and tempered martensite. The present study seeks to elucidate the atomic scale mechanisms of quenching and hardening in carbon steels, with particular emphasis on the behavior of carbon atoms during the formation of diverse microstructures. Carbon atoms cannot be super-saturated in the α-Fe lattice, instead, carbides can only precipitate from austenite. The ultrafine grains resulting from martensitic transformation are the origin of quenching hardening. These ultrafine grains consist of both α-Fe and carbides (including θ-Fe 3 C cementite and its precursors).