Here are links to the subject of carburizing v. nitriding
What is the cost difference between carburizing and nitriding? | Eurotherm Products and Solutions Blog
Case hardening - Wikipedia, the free encyclopedia
I don't understand much of it, but if all that was changed in the EC225 shaft design was the case hardening process, then what is to prevent EC from producing new shafts using the old process, which gave no trouble for 4.5 million hours?