The reason not all gears can be made from the same material grade has to do with hardenability. We have long understood that high core hardness was needed for the bending strength of through-hardened gear teeth, but the appreciation of it in surface-hardened parts only became apparent in the last 30 or 40 years. As late as the mid-1970s, people thought the strength of a carburized gear came entirely from the case depth, and even plain carbon steels were used for huge parts. If you are asked to make replacement parts, do not fall into the trap of ignoring the current data on alloy selection. Make sure you understand what is going on inside the deepest reaches of your products.
It is disappointing that more papers have not been published on this topic. It had been one of my goals to use water jet technology to cut up a variety of parts so that actual results could be available to guide future standards development. My book [An Introduction to Gear Design; free download at www.beytagear.com] has some graphs and the outline of a procedure to assist designers, but without cut up parts I am reluctant to publish a more complete study.
Fortunately, much of the information you need to start the process is on the typical materials certification. Even if there is no Jominy hardenability report included, the chemical analysis will allow you to calculate the expected hardness at various depths below the surface. The accepted calculation method is so well thought of that many metallurgists prefer it to the physical test.
If you graph the results for a given steel, you see that the “profile” of the curve is different from other grades. A “low” limit sample might run three or four Rockwell C points below the “high” limit sample, so you have to review the actual chemistry on critical parts.
The next step to predicting “core hardness” is determining the “critical section” of the part. A pinion with an 8-inch root diameter will not respond as well as one with a 6-inch root diameter. A gear that is “slab-sided” will not respond as well as one that is webbed out. The largest circle that can be transcribed in the part’s cross-section is the “critical section” for analysis purposes. Ever wonder why some big pinions have a hole bored through them? It is to reduce the critical section size and allow the use of a less expensive alloy. More about that next time.