Each grade of steel has a distinct “hardenability profile.” Some provide great hardness at shallow depths and then decline rather rapidly to a much softer potential. If you are making small parts this may not be a problem. Others lack that high potential at the surface but are capable of relatively high hardness much deeper into the part.Modern standards predict tooth strength based upon the hardness at the centerline of the tooth at the theoretical root diameter. ISO and AGMA methods disagree on the exact methodology used to locate the “sweet spot,” but they share a goal of having a hardness there of at least 28 HRC. Or, perhaps, 30 HRC — or even more. As I said, they are not in total agreement on the details.When you look up allowable bending stresses in a standard you are offered a range of values. What you use in your calculations will be somewhere in that range and depend upon your achievement of process conditions outlined in supporting documents. An assumption is made that you will have that minimum core hardness, however.While there is no “bonus” for extra core hardness, wise designers know to select an alloy that is capable of delivering a core hardness on a cut up part. Despite my promise not to provide shortcuts to understanding, I think it is worthwhile to post my own “rules of thumb” for this in the next blog because so little published data is available.
The question of hardenability is not confined to surface-hardened gears; the same dynamic is at work in through-hardened parts. In extremely large teeth or cross-sections it may even be necessary to rough-machine the teeth prior to finish cutting so the needed core hardness is present on the resulting part. We do not typically run heat treat coupons with through-hardened parts so this hardness variability is usually discovered during failure analysis. You can learn a great deal from failure analysis; unfortunately, one common lesson is the need to find a new job. Learn from the mistakes of others and think about core hardness up front.