Failure analysis of a T-joint of AISI type 316 L stainless steel

The flow of sodium through the various circuits of a PFBR is controlled by centrifugal pumps. T-joints, made of type 316 LN stainless steel, will be used as pipe fittings to connect sodium pumps in the secondary side of the PFBR. However, two such T-joints received from the manufacturer were found to have developed fine cracks on the surface, which were seen after the pickling and passivation operation was carried out at the user's site. Visual, metallographic and fractographic examinations suggested that the failure occurred due to fatigue, which initiated because of surface roughening caused by the fabrication process. Initial large grain size of the material caused surface roughening to occur during fabrication. The rough surface led to initiation of fatigue cracks during fabrication, which was not carried out in one stretch as recommended by the user. Absence of corrosion product on the surface suggested no role of corrosion in the failure. The high hardness of the material indicated non-compliance with the user's specifications by the manufacturer vis-à-vis post-fabrication annealing. To avoid future failures, it was recommended that proper selection of material with respect to starting grain size should be made; qualified fabrication procedures should be adopted and followed so as to avoid cyclic loading on the component during fabrication; and appropriate solution annealing at 1323 K should be carried out to relieve cold work.