The 1st stage nozzle segments experience damage such as creep, fatigue, high temperature corrosion and foreign object damage in service. Amongst the other gas turbine components the 1st stage nozzle segments needs to be refurbished or repaired after stipulated number of running hours. The decision on the extent of repairs for the 1st stage nozzle segments are based on incoming inspection observations. There are various stages of incoming inspection which includes visual, dimensional and non-destructive testing methods. Metallographic evaluation is one of the most important incoming inspection method based on which the extent or class of repairs can be decided.
The 1st stage nozzle segment material is FSX 414, which is a cobalt based alloy. It is critical to estimate the microstructural condition of the nozzle segments in the as received condition. As a part of incoming inspection sections at various locations are taken on the nozzle segments for metallographic evaluation. The microstructures are examined in the as received condition and after solution annealing heat treatment. Microstructural transformations and damages in terms of overheating, oxidation and hot corrosion can be identified at ease. The microstructural condition of the nozzle segments can thus be assessed after stipulated running hours. It is concluded that metallographic evaluation is effective in assessing the microstructural damage and proves to be vital in deciding the category of repairs. Typical microstructural evaluation case studies of 1st stage nozzle segments from Frame 5, 6001B and 7001EA gas turbine units are presented and discussed in this paper.
