WEB Investigation of cementite dissolution in 100Cr6 bearing steel under high pressure torsion (HPT)
Cementite is one of the common phases in steels. It has a complex orthorhombic crystal structure and usually hard in nature . Nevertheless, Cementite (and its variant carbides e.g. Fe2.4C) also deforms, decomposes and dissolves under certain circumstances. Examples include the deformation & dissolution of cementite in cold drawn pearlite wires  and carbides decomposition in white etching areas (WEAs) in wind turbine gearbox bearings (WTGBs) [3,4]. Therefore, it is worthwhile to understand the behavior of carbides under similar circumstances.
Here we focused on comprehending the cementite dissolution in standard 100Cr6 bearing steel in the soft-annealed state under severe plastic deformation (SPD) conditions. For SPD, high pressure torsion (HPT) was employed (load = 7.56 GPa, 2 rotations). We used electron channeling contrast imaging (ECCI), electron backscattered diffraction (EBSD), transmission kikuchi diffraction (TKD) and atom probe tomography (APT) techniques for robust characterization down to atom scale.
The undeformed microstructure consists of primary cementite and pearlite (ferrite + secondary cementite) whereas ECCI micrographs showed primary cementite and ferrite in the HPT condition. APT data revealed difference in Cr content in primary (~ 12 at. %) and secondary (lamellar) cementite (~ 1-4 at.%). Also there is no observable change in C content (~ 25 at. %) in primary cementite in both deformed and HPT states, but fragmentation is evident. EBSD and TKD data disclosed grain refinement from ~2 µm to 60 nm upon HPT. The primary cementite is stable because of the larger size (350 nm-0.8 µm) and higher Cr content. For counter reasons, secondary cementite got dissolved. Our conclusions were further confirmed by thermocalc predictions, first principles  and ab-initio calculations .