Abstract: 18CrNiMo7-6 carburizing steel is treated with conventional isothermal normalizing,which is prone to produce granular bainite microstructure and difficult to change the secondary strip effectively.Microstructure and performance of the 18CrNiMo7-6 carburizing steel and isothermal normalizing new process were studied.The specific process consisted of,after austenitizing and holding,for the 40 mm specimen,directly immersed in the normalizing liquid and cooled to 700-750 ℃.The specimen was then quickly transferred to the isothermal furnace,at 650 ℃ for 5-7 h.The results show that the equilibrium microstructure of ferrite and lamellar pearlite is obtained by using the new isothermal normalizing process,the granular bainite is eliminated,the grain size is uniform,and there is no obvious crystal mixing,which is beneficial to reduce the heat treatment distortion.The hardness can be guaranteed at 170-185 HBW,which is conducive to machining.

Key words: isothermal normalizing, granular bainite, normalizing liquid, equilibrium microstructure, heat treatment distortion