Hardening of the gear surfaces has a profound effect on their performance. Reduction in friction, enhancing rolling contact fatigue resistance, and other tribological properties of the surface can be utilized by application of various surface techniques available to the manufacturing industry. For surfaces hardened by a diffusion process, what really matters is the depth of the hardened layer and its hardness and resulting tribological performance. Two leading methods used to produce hard layers in gears made of low alloy carbon steels are carburizing and nitriding. Both methods produce surfaces of significant hardness and may be applied to finished products if their control is at a sufficiently high level. Nitriding is often used as an alternative process to carburizing. Therefore, comparing the wear resistance of nitrided and carburized cases formed on the same steel could be important to designers to determine which method to use. A good example for this purpose is the SAE 5120 (European equivalent 18HGT) steel, which can be either carburized or nitrided. A typical carburized case is a few times thicker than the nitrided case to allow for grinding stock removal, and its surface hardness is slightly lower (see Table 1).
As shown in the graphs, despite the lower case depth, tribological performance of the nitrided case is similar to the performance of the much deeper carburized case. Remember that the nitriding process represents a low-temperature (350-650°C) thermochemical treatment technology that is widely applied in the industry to various products mainly made of steel, both to single units as well as bulk or serial production, and its popularity is permanently growing. It is a cost-effective alternative to widespread high-temperature (800-950°C) thermochemical treatment methods, such as carburizing or carbonitriding. Low temperatures of the nitriding process make it possible to reduce deformation and undesired changes of workpiece dimensions, as compared to carburizing or carbonitriding. Therefore, final machining costs of nitrided gears might be reduced. Also, service temperature of the nitrided layer can be higher than that of the carburized case.
References
- J. Senatorski, J. Tacikowski, and E. Rolinski, Nitrided and Nitrocarburized Layers and Their Tribological Characteristics, ASM Metals Handbook, Chapter in Vol. 18, in preparation.