Originally Posted by
Concentric
I am just going to throw this speculative idea up in the air for consideration. Is it possible there may have been hydrogen embrittlement of the gear material? I noted from AAIB report 2-2011 that the carburised gear raceway is chemically etched before final polishing. I am not a materials specialist but I wonder if a combination of materials susceptibility, environment and stress have combined with the presence of diffused hydrogen trapped underneath the carburised layer to initiate a defect and cyclic stress has done the rest?
Carburized surfaces of the planet gear that get finish ground after heat treat are given a surface temper etch inspection procedure (AMS 2649 or similar) to check for manufacturing damage such as overheating (re-hardening or over-tempering) caused by abusive grinding, or to detect areas of local discontinuous carburization. The surface temper etch inspection is performed after finish grinding but prior to any honing/lapping/polishing operations on the surface. The etchant used is typically a 3-5% nitric acid solution with an anti-smut additive containing hydrochloric acid. Immediately after etching the parts are cleaned with an alkaline solution followed by hot water rinse. After inspection the parts are given hydrogen embrittlement relief baking (AMS 2759/9 or similar). The time between etching and baking should be less than 24 hours.
So the short answer to your question is that it is
possible there may have been some hydrogen embrittlement issues from the temper etch inspection, but only if the procedures were not performed correctly.