Metallurgical Evaluation and Condition Assessment of FSX 414 Nozzle Segments in Gas Turbines by Metallographic Methods

Abstract

Gas turbine components such as nozzle segments, buckets, transition pieces, and combustion liners experience damages such as creep, fatigue, high temperature oxidation, and corrosion. The reliability, availability, and efficiency of high temperature gas turbine parts are based on condition assessment and remaining life analysis. These gas turbine components are normally repaired and refurbished after stipulated operating hours. The decision on the extent of repairs is based on various inspection stages. Among various methodologies of condition assessment, metallography followed by microscopic evaluation has gained wide acceptance since it is cost effective, quick, and reliable. Extensive in-house efforts have been put forth in this field in the development of improved techniques of metallography for accurate determination of material degradation and condition assessment. Experimental studies on frame 6, first stage nozzle segment (FSX 414—cobalt based alloy) were conducted to assess the condition of the nozzle segment by using a laboratory electropolishing technique for metallographic preparation. Sections taken from the nozzle segment were electropolished and examined in light optical microscope (LOM) and scanning electron microscope (SEM). It is concluded that the improved electropolishing technique is effective in assessing creep-fatigue, thermal fatigue, and hot corrosion damage. Based on this, the condition of the nozzle segment is assessed. Typical results of frame 6, first stage nozzle segment are presented and discussed.