The duplex failure is micro-cracks via fatigue enhance crevice corrosion in the cracks which lessen the load bearing which enhances cracking which enhances propensity for crevice corrosion, ad nauseum.
Stainless 300 series (90000 psi UTS) should NEVER be designed for more than 30000 psi (30ksi) UTS (the generally accepted 'plain vanilla' fatigue endurance limit) in 'cyclical stress' applications AND should be mirror polished to remove all the micro/macroscopic 'anomalies' of surface roughness where 'moisture' enters the micro-cracks.