Figure 1. How gears fail - factors influencing gear performance[1]

It seems to me that all the articles I read about the low-pressure, vacuum carburizing process compare it only to conventional atmosphere carburizing. I am curious why I never see a comparison to controlled gas nitriding? I can understand why if we were to look only at plain carbon steels, for example, which cannot be effectively nitrided, but many (if not all) of the materials run in the low-pressure carburizing process are highly alloyed materials that, I assume, would easily nitride after heat treating and finish machining. Can you straighten me out on this?

Figure 2. Gear design considerations – areas of maximum stress[1]

You raise an interesting point. I assume one of the reasons that low-pressure, vacuum carburizing is compared most often to atmosphere carburizing is that they are technologies that more directly compete with one another. In some products (common rail fuel-injection systems come to mind), there is a movement away from vacuum carburizing and back to gas nitriding.

Figure 3. AGMA gear rating system[2]

I've put together some excerpts from several recent gear presentations to show you an example of how nitrided gears compare to carburized gears.

Major failure modes in gears can be traced to: (a) wear, (b) scoring, (c) profile pitting, (d) tooth breakage and (e) spalling.

Figure 4. Sca AGMA allowable contact-stress number (from AGMA 2001-C95)[2]

Figure 5. Typical gear tooth hardness profiles[2]