The important role of martensite laths to fracture toughness for the ductile fracture controlled by the strain in EA4T axle steel

The Hall-Petch relationship was used to investigate the role of martensite lath on fracture toughness (KIC) during ductile fracture in a low-carbon EA4T axle steel. The hierarchical structures of lath martensite was clarified by means of optical microscope (OM), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD). Firstly, in such hierarchical structures, packet size (dp) and block size (db) increase significantly with the size of prior austenite (dr), while the martensite lath width (dl) decreases. Subsequently, KIC was measured and follows the Hall-Petch relationship with dl. It depends on the rotation, bending and direct shear during crack propagation of laths, confirmed by EBSD. Besides, fracture toughness (KIC) is proportional to a parameter εv, the matrix strain, which is related to the plastic deformation of laths. Therefore, the martensite lath in hierarchical structures is the effective control unit of KIC during ductile fracture controlled by the strain.