Abstract: On-line quenching and tempering heat treatment of bars can not only significantly reduce costs, but also help improve material properties. Laboratory tests, organization performance testing and finite element simulation were performed on typical 45# steel and 40Cr steel. The results show that the depth of the hardened layer of 45# steel and 40Cr steel is about 4.8 and 18 mm, respectively, and the critical cooling rate is about 72, 16 ℃/s. After high temperature tempering and holding at 600 ℃ for 20 minutes, the carbides in the steel are distributed uniformly. Strong plasticity and uniformity along the cross section have been significantly improved. The finite element simulation shows that the thermal stress changes in the quenching: the surface layer is first pulled and then pressed, while the core is the opposite.