Effect of cryogenic treatment on the mechanical properties of 4340 steel

This experimental study investigated the effects of cryogenic treatment on the mechanical properties and microstructures of AISI 4340 steel. Mechanical tests, including rotating fatigue, impact and hardness were carried out, after various heat treating conditions and the results were compared. Fracture features of specimens were also compared. It was shown that in general, hardness and fatigue strength of the cryogenically treated specimens were a little higher whereas the toughness of the cryogenically treated specimens was lower when compared to that of the conventionally treated steel. Neutron diffraction showed that the transformation of retained austenite to martensite occurred which, along with possible carbide formation during tempering, is a key factor in improving hardness and fatigue resistance of the cryogenically treated specimens.