Breathing Support and Safety Systems
The support bearing of a hydrofoil vessel failed after only 220 h of operation. The bearing consisted of an outer ring made of chromium-plated AISI type 416 stainless steel and an inner ring with a spherical outer surface made of AISI type 440C stainless steel, with a plastic material, bonded to the outer ring, between the two. The inner ring was found to have failed in four places. The two metallic rings were allowed to come in contact with each other by the disappearance of the plastic material. It was revealed by examination of the fracture surfaces of the inner ring that the failure was caused by fatigue initiated in corrosion pits (caused by seawater). The fracture was found to be transgranular. It was recommended that the inner and outer rings should both be made from the more corrosion resistant 17-4 PH (AISI type 630) stainless steel.