The carburizing layer of large low-speed and heavy-duty gears is deep, and the surface carbon concentration is low. The carburizing transition zone is required to be gentle. In the process of Carburizing for a long time (usually 100-200h), a certain carbon potential in the furnace should be controlled stably. In addition, the furnace volume of large carburizing furnace is large, the surface area of large gear is relatively small, the supply of atmosphere is insufficient, the stable pressure in the furnace can not be maintained, and too much atmosphere causes a large number of carbon black. All these bring difficulties to the deep carburizing of large heavy duty gear, especially the carbon potential control is more difficult with the drop injection process of furnace cracking.

The herringbone gear shafts of large rolling mills in Japan are generally spheroidized after carburizing, then quenched at low temperature (800 ℃), quenched by strong circulating oil injection, and finally cooled at 180 ℃.

Figure 1 shows the carburizing, quenching and tempering process of herringbone gear shaft of large rolling mill in Japan.

A number of large gears and gear shafts have been carburized successfully by using deep carburizing method. The effective depth of hardened layer is 8 mm, the surface carbon concentration is 0.83%, the morphology of carbide is spherical and granular, and the average diameter is 0.5 μ M. Fig. 2 shows the discharging condition of M16 gear shaft with diameter of 650x1800mm after deep carburizing.