Although the deep carburizing of large heavy duty gears improves the bearing capacity and prolongs the service life, it still has the disadvantages of long production cycle, high cost and large deformation. Therefore, the deep nitriding process is being studied abroad to replace the deep carburizing process.
ZHY gear believes that nitriding has the advantages of short time, low cost and small deformation, which creates convenient conditions for finishing after heat treatment. Nitriding treatment can also improve the contact fatigue strength of gears and maintain sufficient strength and toughness within a certain temperature range. However, as mentioned above, the nitriding layer is too shallow to be used in large heavy duty gears with large curvature radius. If the hardness of the gear center is increased to HRC40-50, and the nitrided layer on the surface is deeper, it will be an ideal heat treatment method for large and heavy duty gears.
Some foreign manufacturers use precipitation hardening steel as nitrided. For example, ams6475 steel of Cr Ni Al Mo system in the United States, NT100 and NT200 steel of Cr-Mo-V system in Japan, 39crmov139 steel and 11crmo6 steel in Germany, etc. When the precipitation hardening steel is used as gear, the gear blank is first quenched as a whole (quenching treatment), and the hardness is about HB320; then rough hobbing is carried out, leaving a very small amount of fine hobbing allowance, and nitriding treatment is carried out (570 ℃). In the process of nitriding treatment, the center of the gear is also precipitation hardened. In this way, after nitriding treatment, not only the surface nitriding layer can be obtained, but also the hardness of the substrate can be increased to HRC45-50. Finally, the gear can be precisely rolled (or ground or developed) to achieve the required gear precision and finish. The research work in this field is also being carried out in China, and the industrial test has been carried out with small rolling mill gears. At present, the application of precipitation hardening steel nitriding treatment to gears still needs to solve the problems of cutting performance, steel cost and comprehensive mechanical properties after precipitation, and the nitriding depth must also be improved.