Modern Nitriding Techniques For Gear Applications

Improving surface properties of gears can be extremely beneficial since many failures start at the surface in the contact area between the teeth. Rolling contact weary properties of the gear surface are improved when a hard diffusion layer - with the compressive residual stresses built into it by the thermochemical surface engineering methods are applied. Such a layer restrains or delays the formation of premature surface and subsurface cracks , core crashing , or subsurface fatigue. Well-known methods of the thermochemical treatments applied for gears include carburizing , carbonitriding , ferritic nitrocarburizing , and nitriding. Although carburizing produces the thickest years, it is carried out at a high temperature, therefore , it must be followed by quenching and tempering operations. That may cause some undesirable distortion and a need for more aggressive final machining. Nitriding is typically performed at a temperature of 390-570°C , depending on the type of steel and therefore, it is considered as a distortion-free process. 

A substantial increase in a gear’s performance can be realized by using modern nitriding techniques. The process of nitriding is one of the most encouraging of the verity of many thermochemical methods known to the gear industry. For a long time, gas nitriding was used for gear applications in combination with a final grinding operation. This machining process was required because the layer that was formed during gas nitriding had a brittle and hard layer at the surface, called a white layer, which was built of iron nitrides that were frequently too thick and porous.

Ion nitriding is carried out in a vacuum with a very low partial pressure of nitrogen reaching no more than 5 mbar. The process is carried out in the DC-pulse glow discharge/plasma and can be seen through the port window and it’s characterized by the sputtering effect of the surface atoms by high-energy ions of nitrogen. In many situations, no ultimate machining is required since post-nitriding surface roughness changes are minimal, and the dimensional changes are almost nonexisting because the process is carried out at a temperature well below the transition of ferrite to austenite. Also, the process of ion/plasma nitriding is useful in many applications as it offers a simple, local masking or protecting from hardening if required.

The progress achieved in the industry since the introduction of plasma nitriding has stimulated more research and development in the gas nitriding field, resulting in the implementation of superior process control methods and   atmosphere measuring devices such as hydrogen and oxygen probes.