bt, Just remember the relitive effect of each factor.
Stress, In any assembly that sees temperature cycling or mechanical loading some locations will be at over 75% YS. There is almost always 'enough' stress.
Cl content, Low O and S help offset this some, but data from nuclear power applications show that even with ppb levels of O and S, and <5ppm Cl you can get Cl stress cracking. A clean system helps though. Almost every case that I know of where there was cracking in 'clean' system there were areas that trapped impurities.
Temperature is the big driver. Even small increases in temperature can have a very big impact on CSCC. We all know tales of equipment that ran fine for years, but when the process temp was raised slightly it went to pieces.
If you are in a low Cl, low O, low temp environment it is reasonable to expect low risk of CSCC in a 300 series stainless. If you do experience CSCC don't expect envrionmental changes to move you into a safe region.
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Corrosion never sleeps, but it can be managed.