Abstract:
Fractured gear teeth assembly samples have been received from Belka Power Station Saharanpur for its failure investigation from metallurgical point of view.
Service report has been studied for its failure investigation. Samples have been taken by cutting across a damaged tooth of gear to study the morphology of structure of fractured gear and for Vickers Hardness and tensile Testing. Chemical composition of gear steel was obtained by spectroscopic analysis. Tensile testing has been done on core material of gear. Fracture surface of tooth of gear was tested under Scanning Electron Microscopy (SEM) for revealing fracture mode.
Service report shows fractured gear teeth have been repaired previously by welding process after some damage had been noticed on the gear teeth. The service report also reveals abnormal sound from the whole assembly right from the beginning when the unit was installed and made operational.
Spectroscopic analysis shows the steel used in making the gear is satisfactory. Microstructures of gear sample reveals coarsening of grains at a point which is nearer to HAZ (heat affected zone). Hardness testing also show decrease in harness value at a point nearer to HAZ.
Tensile testing of gear material shows variation in tensile properties which may be due to variation in microstructure due to the heating caused by repair welding. Scanning electron micrograph reveals fatigue fracture. Repair welding on damaged teeth of gears is not recommended as this process results in coarsening of microstructure and decrease in value of hardness at region which is nearer to HAZ. Initial damage in the gear assembly appears due to excessive vibrations in the unit and these vibrations are due to improper alignment of gear assembly while installing the unit.
