Reduction gearboxes made of machine steel gear have a maximum limit to prevent changing their physical properties (softening). They change their properties as a result of the temperature increase which is subject of many variables: friction between gear teeth, ambient temperature, quality of bearings are among the main dependencies. The most typical engineering solutions are such as making gear of different stronger materials which withstands higher temperatures, oil bath or liquid cooling.
I want to know what are possible ways to increase the durability of the same gear in a given gearbox. I can heat the gear pieces to a high temperature using torch that makes it "tempered" or "annealed" making it stronger. Which one of those known processes intended specifically for this purpose and how do I find the procedure (heating, followed by cooling or not...) and temperatures needed to alter the structure of the steel?
Also are there other ways to increase the rotational speed limit beside gear tempering and gearbox cooling? Assume for this topic that bearings do not limit the speed.