Abstract: The study on the microstructure and mechanical properties of 40CrMnSi-MoVA steel isothermally quenched in the temperature region from 190℃ to 320℃ is dealt with.It is found from experiments that the impact-toughness an of this steel is increased monotonically with the increasing of isothermal temperatures, but the fracture toughness KIC is varied in a saddle-backed curve with the increasing of isothermal temperature.The explanation of the inconsistency between ak and KIC values has been put forward according to the microstructural characteristics of this steel isothermally quenched at different temperatures.

Key words: 40CrMaSiMoVA steel, isothermal quenching, mixed structure