The nitriding of gears has been a controversial subject over the past 20 years or so, but it is now becoming a favored thermal-processing technique. (It is also a growth process technology.)
The thermal treatment of gears (particularly the process of nitriding gears) necessitates a very careful selection of the choice of steel in relation to the gear application. When choosing the steel for a gear to be nitrided, one needs to consider the working environmental conditions that the gear will operate under at the initial design stage of the gear. Some of the design considerations could be:
• Tooth pitch
• Tooth design loading
• Fatigue bending performance
• Case-depth requirement
• Operating temperature
• Gear type (helical, spiral bevel, spur gear, pinion driveshaft)
• Core hardness required
• Core tensile strength
• Allowable surface growth
• Grinding tolerance
• Surface metallurgy (compound layer, type of case to be formed in relation to the steel chemistry)
Many gear types are carburized, quenched and tempered in today’s manufacturing environment, which requires high-temperature processing conditions, particularly for the diffusion of carbon into the steel surface. This must be followed by austenitizing and quenching. The obvious net result is the potential for distortion. The distortion aspect will also be influenced by the machining conditions prior to the carburizing.
The process of nitriding, on the other hand – gas, salt-bath, low-pressure and plasma nitriding – offers the engineer the ability to pre-treat the steel in order to develop the required core hardness and tensile strength. The appropriate core hardness is necessary to support the formed metallurgical case when the gear is nitrided.
We will finish our discussion of this topic next time.