Failure analysis of duplex stainless steel weld used in flexible pipes in off shore oil production

This paper presents the results of failure investigation of an auxiliary steam turbine in a power plant. Fractures were occurred at the lacing wires in the L1 blade cascade. The failure was occurred in repaired stages of blades after 47 days of an overhaul operation period. Visual inspection showed some regular fractures in the improper brazed joints and dimensional analysis showed that the lacing wire holes in the blades of the L1 stage are smaller than the originals. Fractographic investigation of fractured surface showed that the lacing wires had been exposed to a fatigue stress phenomenon. Finite element analysis showed that there is a high stress critical point near the brazing regions in comparison with original elements. Vibration analysis was performed experimentally and computationally to find the probable intersection points between the excitation harmonics and natural frequencies of blade cascade. Experimental test results verified the FEM analyses with good agreements. Obtained results from harmonic response analysis showed an approximate resonant condition of L1 blades during the operation of boiler feed pump turbine.