Effects of Tempering Temperature and Mo/Ni on Microstructures and Properties of Lath Martensitic Wear-Resistant Steels

The tempering behavior was experimentally studied in lath martensitic wear-resistant steels with various Mo/Ni contents after tempering at different temperatures from 200 to 600 degrees C. It is shown that a good combination of hardness (HV) (420-450) and -20 degrees C impact toughness (38-70 J) can be obtained after quenching and tempering at 200-250 degrees C. The microstructure at this temperature is lath structure with rod-like and/or flake-like epsilon-carbide with about 10 nm in width and 100 nm in length in the matrix, and the fracture mechanism is quasi-cleavage fracture combining with ductile fracture. Tempering at temperature from 300 to 400 degrees C results in the primary quasi-cleavage fracture due to the carbide transformation from resolved retained austenite and impurity segregation between laths or blocks. However, when the tempering temperature is higher than 500 degrees C, the hardness (HV) is lower than 330 and the fracture mechanism changes to ductile fracture due to the spheroidization and coarsening of cementite. Additions of Mo and Ni have no significant effects on the carbides morphologies at low tempering temperatures, but improve the resistance to softening and embrittling for steels when tempered at above 350 degrees C.