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Conjunction gears, which are also known as helical gears with clutch gears, are used mainly in the transmission and shift design of automobile gearboxes. In this paper, focusing on the hot forging process and microstructure changes after forging, hot forging is proposed for manufacturing a conjunction gear according to the characteristics of a combination gear. Combined with finite element numerical simulation, by comparing changes in temperature, equivalent stress, equivalent strain, recrystallization fraction, and average grain size, the effect of the optimization of the forging process of a combined tooth billet is achieved using the response surface methodology. Finally, optimal forging process parameters are obtained through experimental research, and the quenched structure after hot forging is observed.

42CrMo steel belongs to low-alloy medium carbon steel and also belongs to ultra-high-strength steel. In this study, thermal compression tests (800°C - 1200°C, 0.01s-1 - 10s-1) and subsequent calculations proved that many material constants of 42CrMo alloy steel have a similar functional relationship with Z parameter: Material constants = a*Zb. The thermal processing maps under strains of 0.2, 0.4, 0.6 and 0.8 were established, and the practical application of the processing maps was verified in combination with metallographic diagram analysis. The optimum thermal processing parameters range of the material was obtained: the temperature range of 1000°C - 1125°C and the strain rate range of 3.16 s-1 - 10s-1.The dynamic recrystallization volume fraction model and grain size model of the material were established, and the typical sigmodal curve was obtained. The hot workability and recrystallization behavior of 42CrMo were further analyzed.