Abstract: The 20MnCr5H steel for precision formed gears was subjected to subcritical spheroidizing annealing (710 ℃×8.5 h) and spheroidizing annealing in dual-phase region (750 ℃×1.5 h+680 ℃×7 h, 750 ℃×4 h+710 ℃×4 h, 770 ℃×4 h+720 ℃×4 h), and the spheroidizing effects under different processes were investigated. The results show that under the spheroidizing annealing in dual-phase region, higher austenite zone temperature will make it difficult for lamellar pearlite to spheroidize in a short time, while shorter austenite zone holding time will help to improve the spheroidization efficiency. Good spheroidization effect is achieved under 750 ℃×1.5 h + 680 ℃×7 h process. Compared with the spheroidizing annealing in dual-phase region, the spheroidization effect of the 20MnCr5H steel is the best under 710 ℃×8.5 h subcritical spheroidizing annealing, and the spheroidization rate reaches 98%. This is due to the fact that after the process treatment, more dislocations and small-angle grain boundaries in the microstructure can promote the dissolution and spheroidization of flaky cementite more quickly. It is a reasonable spheroidizing annealing process for precision forming gears.

Key words: 20MnCr5H steel, subcritical spheroidizing annealing, spheroidizing annealing in dual-phase region, spheroidizing rate