Corrosion Fatigue Failure of Exhaust Valve of a Diesel Generator

The preliminary defect investigation, as part of the causative analysis, has revealed that the damage to diesel generator is caused by a failed exhaust valve. Damaged exhaust valve head has been analyzed in the present case study. A fast fracture appears to have happened, as typified by the Chevron marks. Presence of Cl on the whole fracture surface and corrosion pits in microstructure close to fracture indicates that the corrosion has also an important role in the failure of exhaust valve. Intergranular cracks have often been found around the edges initiating from the corrosion pits, while microstructure is examined close to the fracture surface. The presence of large amount of Cl along with substantial existence of Ca and V on the whole fracture surface indicates the possibility of hot corrosion. More interestingly, corrosion products have been found in all the three distinct regions of the fatigued fracture surface, viz. initiation, rubbing (propagation), and final fracture. Moreover, striations have been observed in the crack initiation as well as crack propagation regimes. It appears that the failure of the exhaust valve has occurred due to an impact loading with secondary contribution from corrosion fatigue damage. Presence of intergranular weakness due to corrosion fatigue along with existence of impact loading has led to mixed appearance (of inter-plus transgranular) of the whole fracture surface.