Abstract: Charpy-V impact tests at serial low temperature (–99～20 ℃) are carried out for 16MnR steel of three different microstructures (fine grain + fine carbide- particle microstructure, fine grain + coarse carbide particle microstructure and coarse grain + coarse carbide particle microstructure) obtained through three different heat treatment systems. The effects of grain size and carbide size on the impact toughness of 16MnR steel are studied through impact toughness comparison, fracture morphology observation and fracture micro-parameter measurement. The results reveal that the impact toughness of materials with different microstructures decreases with the decrease of test temperature, the ductile-brittle transition temperature (DBTT) of fine grain fine carbide microstructure is lower than those of coarse grain coarse carbide microstructure and fine grain coarse carbide microstructure, so the impact toughness of fine grain fine carbide microstructure is higher than those of the other microstructures at same temperature. The effects of grain size on DBTT and fracture toughness value are much more obvious than those of carbide size. Through fracture micro-parameter measurement, it is revealed that the fracture energy of DBTT zone is mainly consumed in blunting of crack tip and propagation of plastic crack, so in the low temperature zone of ductile to brittle transition, much energy is absorbed in the blunting process of crack tip, thus the toughness becomes very high.

Key words: Cleavage fracture, Impact toughness, Low temperature impact toughness, Microstructure parameter