[Chaplain]

In my opinion there's no way that part should have failed even with the most severe duty. I believe that if you were to check the hardness of the part you will find that it was either heat treated improperly or that during use lubrication wasn't able to get to the part and it work hardened and failed. If this was a part that was supposed to be "lifetime lubricated" I think a lack of or insufficient lubrication from the very beginning is your culprit. If you have access or live close to a metallurgical (SP) lab I would have them perform an analysis to find out for certain what the cause was.