Improvement of AISI 4340 steel properties by intermediate quenching – microstructure, mechanical properties, and fractography

AISI 4340 is one of the most widely used steels in high-risk industries such as military, nuclear and aerospace. The strength of this steel is capable of increasing to 1 825 MPa with quench and temper heat treatment, but it results in low toughness, low impact properties and brittle-fracture especially at low temperatures. In this study, the intermediate quenching treatment was used to induce ferritic–martensitic dual-phase (*50/50 ferrite/martensite) microstructure that led to an impact energy of 93.6 kJ m–2, which was 241 % higher than that of quench and temper treatment. Moreover, mechanical tests revealed tensile strength and hardness of 911.5 MPa and 43 HRC, respectively. Also, fractographic analysis confirmed the occurrence of the desirable ductile fracture mechanism. # Carl Hanser Verlag GmbH & Co. KG