Even the most brilliant gear design will fail if the wrong material is used to make the parts. Our understanding of metallurgy and alloying elements has continued to evolve, so what was once an acceptable material is no longer so. Part size and heat treat process also influence your selections.
Do not feel embarrassed if you did not know that AISI 8620 is not the perfect steel for all carburized gears. Many “experts” erroneously think all steels are interchangeable. I once attended a very high-level summit on wind energy where the boffins (British slang term for one engaged in technical or scientific research and development.) in charge got that deer-in- the-headlights look when I asked how they planned to alleviate the shortage of 17NiCrMo6-4 bearing quality steel. (Having the answer to this question is particularly important during the current tariffs-induced steel uncertainty.) Their analysis indicated the total amount of steel needed to make all the turbines planned was only 4% of U.S. steel production. They were startled that the steel in gears was much different than that used in bridges or car bodies.
One needn’t be a metallurgist to design gears, but it helps to know enough about this dark science to avoid the misinformation floating around. Good gears can quickly fail if the material is not up to the task asked of them. This is true regardless of the gear type or heat treat method or quality level. You cannot tell if a gear has the required core hardness unless you have the lab results.
Fifty years ago, it was thought that as long as the surface of a carburized gear was “file-hard” you were good to go. Some 1 NDP gears were even made from AISI 1018 steel — which meant their core hardness was around 160 HB! We now have a much more nuanced understanding of case depths, hardenability, core hardness, and cleanliness. Through-hardened gears have many of the same issues, i.e. — you must match the hardenability of the steel to the tooth size and critical section of the part.
Unfortunately, engineering schools do not have the classroom time to cover this topic adequately, so you have to do it yourself. I added this to the second addition of my book (free download at www.beytagear.com) because it was so hard to find good advice on material selection. Good gears need the right stuff to go along with the proper design. Do not handicap your products with inadequate materials.