Abstract: Based on the phase transition points of a new designed 4140 steel determined using a dilatometer, the Gleeble-3800 thermal simulator was used to conduct different spheroidizing annealing simulations, including subcritical spheroidizing annealing, step cooling spheroidizing annealing, and isothermal spheroidizing annealing. The results show that during subcritical spheroidizing annealing, even after a long holding time, the original martensitic region is spheroidized, while the original bainitic region just begins to spheroidize. In the step cooling spheroidizing annealing test, the spheroidization rate is increased to 75%, and as the heating temperature increases, the amount of lamellar cementite also increases. For the isothermal spheroidizing annealing process, heating at 760 ℃ followed by isothermal holding at 720 ℃ results in a spheroidization rate of 82%, with hardness meeting the user's requirement of ≤185 HBW. However, localized carbide aggregation is observed. Therefore, based on Thermo-Calc calculations of the isothermal transformation curve for the 4140 steel, a preheating stage at 700 ℃ is added before the isothermal spheroidizing annealing. With the optimized preheating+isothermal spheroidizing annealing process, the continuous heat treatment furnace in the production line successfully produces 4140 round steel that meets the design specifications, achieving spheroidization rate of ≥90%, Brinell hardness of ≤175 HBW, with uniformly distributed microstructure.

Key words: 4140 steel, spheroidizing annealing, spheroidization rate, Brinell hardness