The impact of the gas turbine blade heating temperature in the presence of aviation kerosene on coating and alloy microstructure

Under operating conditions, gas turbine blades may experience overheating. The degree of unfavourable modifications of the condition of both the protective insulating coating and the alloy (microstructure degradation) depends, among other factors, on the temperature and its exposure time. In this study, under laboratory conditions, in the presence of aviation kerosene exhaust gases, the influence of temperature (mainly outside the range of nominal operating temperatures) on the condition of uncooled polycrystalline rotor blades of aircraft turbine jet engines was examined. The object of the research were new gas turbine blades of the SO-3 aircraft engine made of the EI-867 WD alloy, which were exposed to high temperatures for a period of two hours in a laboratory furnace in the temperature range T = 1123 - 1523K, every 100K. The article determines the nature of changes (modifications) both in the state of the coatings and in the core material (alloy). A multifactor analysis was taken into account, including in the case of coatings modifications: morphological microstructure of the coating, chemical composition of oxides and roughness parameters, and in the case of the alloy mainly grain growth, and modification of the strengthening γ' phase. As a result of exposure to high temperatures in the surroundings of exhaust gases, the roughness of the surface changes and various types of oxides are formed, and its thickness increases. An increasing number of carbides appeared in the EI-867 WD alloy and grain growth was found as a function of the heating temperature. In particular, the blade alloy structure experienced the growth of the reinforcing γ’ phase, which is adverse in terms of heat resistance, and the percentage-wise depletion of this phase in the alloy structure. Due to the aforementioned changes, heated blades experiences significant reduction in heat resistance and high-temperature creep resistance. The article also indicates the possibility of using the characteristics of microstructural changes to determine the technical condition of the tested turbine element in a non-destructive way.