Just became a member. Sorry to be so late in responding to your question.
I’ve worked with girth gear problems in many plants, on various rotary kilns & ball mills (up to 5000 hp). The previous responders are spot on regarding the cause of pitting.
There are some additional methods for correcting pitting on the girth gear regarding carburized pinions and machine in place technology. Had good luck with that procedure on a 4000 hp ball mill over 4 years ago, which saved a $600,000 girth gear reverse, and also saved the back side of the girth gear for the future reverse. Still running well, with low vibration, etc. The service companies involved have done over 50 girth gears over the last 15 years.
Regarding your tooth root cracks: Sounds like you have axial mis-alignment, which will cause alternating side root cracking. We developed a method to shim large girth gears (60 ton - 26’ diameter) to correct axial mis-alignment from 0.120” to within 0.010” total axial run-out. Another service company has an excellent, inexpensive method to look for root cracks in big gears.
Welding gear teeth is possible in many applications. When designing the repair the base material should be thoroughly investigated to better plan the welding. Some older cast gears respond better to mechanical / welded connections using high-strength (100,000 psi) silver solder. Other materials are readily weld-able.
If you need more information, call me at 641-423-9363 with more details about your application. Good Luck, Paul Juhnke, Maintenance Engineering Services