The Effect of Cooling Rate on the Banded Structure of 20CrMnTi Gear Steel Based on C Diffusion and Partitioning Analysis

This study, through experimental analysis and numerical calculations, not only identifies the critical cooling rate for eliminating banded structures in 20CrMnTi gear steel but also analyzes the impact of cooling rate on banded structures, including the mechanisms related to element distribution and interface migration. The results indicate that the fundamental cause of banded structure formation is the presence of chemical segregation bands, and increasing the cooling rate leads to a reduction in the grade of ferrite/pearlite banded structures, with a critical cooling rate of 0.55 degrees C s-1. Bainite and martensite banded structures are observed in samples cooled at rates of 3 and 5.8 degrees C s-1, respectively. As the cooling rate increases from 0.07 to 1 degrees C s-1. The ferrite phase transformation time in the solute-poor zone is shortened from 171 to 12 s within the ferrite phase transformation temperature difference (Delta Ar3$A_{\text{r} 3}$) between the poor and rich solute zones. The concentration difference of C on both sides of the alpha/gamma phase interface increases from 0.350 mass% to 0.431 mass%, and the diffusion distance of C decreases from 34.38 to 7.57 mu m. When the diffusion distance of C is less than half of the width of the solute-rich banding (12.5 mu m), the ferrite banded structure disappears. At low cooling rates, the longer phase transition time allows for a substantial diffusion distance of C toward the solute-rich region, promoting the formation of banded structures. Conversely, at high cooling rates, the short phase transition time results in a limited diffusion distance of C into the solute-rich region, making it less conducive to the formation of banded structures.image (c) 2023 WILEY-VCH GmbH