Microstructural evolution and strength-toughness synergy in 40CrNiMo steel via deep cryogenic treatment following tempering

40CrNiMo medium-carbon alloy steel serves as an essential material for aerospace harmonic reducers, yet conventional approaches focusing on multiphase microstructure development face persistent challenges in dimensional stability under cryogenic cyclic loading. This study applies deep cryogenic treatment - a technique commonly used for high-speed steels and tool steels - to medium-carbon alloy steels, and establishing the "quenching, tempering, deep cryogenic treatment" (QTDCT) heat treatment process. Through comprehensive microstructural characterization and mechanical testing, we reveal that QTDCT effectively refines martensitic substructures while promoting homogeneous dispersion of M 3 C carbides. At the same time, the aspect ratio of martensite decreases from 2.36 to 1.91, showing a transition toward a reduced-aspect-ratio lath morphology. The above microstructure evolution significantly improves the mechanical properties: yield strength increases from 729 MPa to 777 MPa, accompanied by a 15.6 % improvement in −60 °C Charpy impact energy from 59.8 J to 69.1 J. The strength augmentation derives from the combined effect of multiple strengthening mechanisms, while toughness enhancement arises from three coordinated effects: increased high-angle grain boundaries for crack deflection, refined carbides suppressing microcrack nucleation, and equiaxed martensite facilitating multi-slip system activation. These findings demonstrate that QTDCT enables concurrent enhancement of strength and low-temperature toughness. This study provides a reliable way for the dimensional stability and service performance of medium-carbon alloy steel in extreme aerospace environments.