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Replica Metallography: An NDT Technique for Condition Assessment of Gas Turbine Components

There is a need to evaluate the microstructure of the serviced gas turbine components without destroying or cutting samples from the components. The nondestructive methods and techniques, such as penetrant testing (PT), eddy current testing, and radiographic testing (RT), cannot determine the cracks along the grain boundaries in the gas turbine components. Conventional laboratory metallography calls for sampling, but with the replica metallography technique, the microstructure of the components can be evaluated nondestructively. Replica metallography is an effective nondestructive technique that is used to assess the condition of the high temperature components. Replica evaluation is performed using a light optical microscope (LOM) and a scanning electron microscope (SEM). Replica metallography has gained wide acceptance in the utility industry since it is cost effective, quick, and reliable. Condition assessment and life extension programs for rotating and non-rotating gas turbine components are popular and widely practiced in the industry. Replica metallography is one of the vital techniques used for assessment of gas turbine components. Gas turbine components experience various types of damage, such as creep, fatigue, and high/low temperature corrosion. These damages are intrinsically reflected in the microstructure to some extent. The reliability, availability, and efficiency of the gas turbine operation are very often based on condition assessment and remaining life analysis. As a part of the incoming inspection of the gas turbine components, there is a need to examine the microstructure of the serviced gas turbine components such as turbine wheels, compressor wheels, combustion liners, and transition pieces nondestructively. The prime objective is to determine the condition of the components by evaluating the microstructure for continuation in service. Replica metallography technique was performed on the serviced gas turbine components. The microstructure was then evaluated using a light optical microscope and a scanning electron microscope. The replica technique details and significant test result findings of replica metallography are presented and discussed in this paper.


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