Ok, if these stresses come from the welding process, can they be baked out, or do they need to change the process? (I relaize the answer to that may not be available from our readership. I am not as familiar with welding aviation steel alloys as I am simpler structures, and so am asking for informational purposes).
Is hydrogen embrittlement one of the issues here?
Lonewolf_50-
The EB weld is performed after carburize/temper of the bevel gear teeth, so it would be difficult to thermally stress relieve the weld HAZ without affecting the tooth case properties. However, it may be possible to perform some mechanical type of stress relief post welding using vibratory stress relief or controlled shot peening. If they are not already doing so, EC should use controlled preheating & postheating in their EB welding process. This will help minimize any residual stress in the weld HAZ created by asymmetrical heating/cooling around the weld circumference as the weld is being produced.
Like you, I was also puzzled by the article's comment about "unavoidable corrosion". I don't think hydrogen embrittlement is a concern since the EB welding is performed in a vacuum chamber. In modern aerospace manufacturing, there is no such thing as "unavoidable corrosion" when it comes to something like a flight critical component inside a rotorcraft transmission. First of all, there should not have been any corrosion present on the part after manufacturing since this would be cause for rejection. Second, if there were the potential for surface corrosion to occur during the component's service life it should theoretically not have produced a fracture failure. Flight critical components like this bevel gear should be included in the propulsion system fracture control plan, which would include a fracture analysis that takes into account the effect of surface flaws from corrosion. Either the author of the article misspoke, or maybe EC needs to take another look at their fracture control plan for this particular aircraft.