Aluminum washer stress-corrosion cracking failure

Customer bought aluminum in T651 (stress-relieved) condition, machined washers, reheat-treated them, and did not re-relieve the residual stresses. Resulting 8 ksi residual stress exceeded the 5 ksi SCC threshold for the alloy; washers cracked. Resolved by going to overage temper with higher SCC tolerance.

That washer I showed you — they bought it in the T651 condition. It had been stress relieved, and then they cut a piece off — washer shaped like this — and they reheat-treated it. When they reheat-treated it they reintroduced the residual stresses, and they never did anything to relieve them. They said, oh well we bought it stress relieved. Yeah, but then you reintroduced residual stresses, and when they actually shipped it, it had 8 ksi residual stresses. Stress corrosion cracking for that alloy is 5 ksi, so they got cracking of the washers. They changed the heat treatment to go to an overage condition, which now you have 20 or 25 ksi, and so you can tolerate 8 ksi residual stress.