Effect of Tempering Temperature on the Microstructure and Properties of 40CrNiMoA Steel Used for the Flexspline of Harmonic Drive

Abstract This study investigates the effect of tempering temperatures ranging from 400°C to 600 ° C on the microstructure and mechanical properties of 40CrNiMoA steel for harmonic reducer flexsplines. The results demonstrate that increasing the tempering temperature induces a microstructural transition from tempered troostite to tempered sorbite. Concurrently, ferrite undergoes recovery and recrystallization, transforming from fine lamellar to rod-like morphologies, while cementite evolves from fine to coarse granular particles. As the tempering temperature rises, the yield strength, tensile strength, and hardness of the steel progressively decrease, whereas both room-temperature and -40°C low-temperature impact energies increase. Elongation and reduction of area exhibit slight improvements but no significant variations. Considering the operational demands of harmonic reducer flexsplines, tempering within the 400–450°C range is recommended for downstream users, with the 450°C-tempered condition achieving the optimal balance of strength, ductility, and toughness.