Formation mechanisms of the mixed-grain structures in 20CrNiMo steel during warm forging and subsequent heat treatment

Gears are essential components in mechanical systems, which are critical for motion transmission in industrial equipment. However, localized mixed-grain structures form in 20CrNiMo steel during warm forging at 800–900 °C, reducing gear service life. This study integrated finite element simulations and isothermal compression experiments to explore the formation mechanism of mixed-grain structures in steel during warm forging and subsequent heat treatment. Results showed that mixed-grain structure formation was collectively governed by strain-rate-dominated deformation-induced thermal effects, dynamic recrystallization mechanisms, and stored energy heterogeneity. Discontinuous dynamic recrystallization ‌dominated‌ at low strain rates due to relatively weak deformation-induced thermal effects. The recrystallization volume fraction remained low, with stored strain energy distributed uniformly. During subsequent heat treatment, nucleation preferentially initiated in unrecrystallized regions, promoting uniform grain growth. In contrast, at high strain rates, thermomechanical coupling induced dominant continuous dynamic recrystallization. Significant local kernel average misorientation differences generated stored energy gradients, driving grain boundary migration during heat treatment. Without precipitate phases to suppress this, abnormal grains grew via boundary migration, leading to mixed-grain structures. These findings establish theoretical foundations for suppressing mixed-grain defects in 20CrNiMo steel during warm forging and subsequent heat treatment.