gurig-
Using simple factors (like a Kt temp factor) to design your gears when you have operating conditions far beyond the norm is a bit risky. As AGMA 2001-D04 notes, for bulk oil temps above 250degF reduction of gear material strength can occur due to loss of temper. Carburized gear steels like 9310, or through hardened bearing steels like E52100, are commonly tempered for operation at around 300degF max. If your bulk oil temp is 350degF, and your gear contact conditions produce another 100degF in flash temp rise, the 450degF local temp at your gear flank will result in surface de-tempering and significant loss of strength/hardness.
If you are going to design for bulk oil temps of >350degF, it would be prudent to perform a detailed analysis using one of the better software tools available for this task. The fluid mechanics and heat transfer conditions occurring in the hydrodynamic oil film of a gear mesh contact is quite complex. But if you choose to employ the simplified design approach of using adjustment factors (like Kt for temp) then you must also accept that your design will be very conservative.
The only thing I can suggest doing with a simplified approach is to assume your gear flanks will be subjected to temperatures equal to the bulk oil temp plus any flash temp rise, which will result in a reduced case hardness. You can then use this reduced case hardness to adjust other factors like Ch accordingly.
Good luck.
Terry
