Effect of quenching temperature on martensite multi-level microstructures and properties of strength and toughness in 20CrNi2Mo steel

The martensite multi-level microstructures of 20CrNi2Mo steel, which were quenched at the different temperatures of 900-1200 °C and tempered at 200 °C, were investigated by optical microscope (OM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM), and the relationship between the microstructures and properties of strength and toughness was discussed by the classic formula of Hall-Petch. The results show that the size of prior austenite grain (dr), martensite packet (dp) and block (db) increase with increasing of the quenching temperature, while the martensite lath (dl) size is opposite. On another hand, the confusion degree of the martensite packets changes from disorder to order. The boundaries of prior austenite grain, packet, block and the martensite lath are high angle boundaries (HBs) and low angle boundaries (LBs), respectively, and the ratio of the low angle boundaries increase with the quenching temperature by calculating to the multi-level microstructure size with the mathematical model established by myself. In addition, the relationship between the packet/block and strength follows the classical formula of Hall-Petch, and the size of db is far lower than the size of dp, db is the effective control unit of the strength. Meanwhile, dl is the effective control unit of toughness because it strongly impacts the crack initiation and propagation and follows also the Hall-Petch with toughness in 20CrNi2Mo steel.