Failure of an Aircraft Controller Diaphragm

A preflight inspection found a broken diaphragm from a side controller fabricated from 17-7 PH stainless steel in the RH 950 heat treatment condition. Failure occurred by cracking of the base of the flange-like diaphragm. The crack traveled 360 deg around the diaphragm. Scanning electron microscopy (SEM) revealed that the failure occurred by a brittle intergranular mechanism and stress-corrosion cracking (SCC), and indicated a failure mode of selective grain-boundary separation. The diaphragms were heat treated in batches of 25. An improper heat treatment could have resulted in the formation of grain boundary precipitates, including chromium carbides. It was concluded that failure of the diaphragm was due to a combination of sensitization caused by improper heat treatment and subsequent SCC. It was recommended that the remaining 24 sensor diaphragms from the affected batch be removed from service. In addition, a sample from each heat treat batch should be submitted to the Strauss test (ASTM A262, practice E) to determine susceptibility to intergranular corrosion. Also, it was recommended that a stress analysis be performed on the system to determine whether a different heat treatment (which would offer lower strength but higher toughness) could be used for this part.