Linear Defect Analysis of 1st Turbine Blades

A linear fluorescence defect was found on the surface of one of the 1st turbine blades when a certain type of engine was decomposed for inspection after the factory test run. The fracture morphology, microstructure and composition of the failed blade were observed and analyzed by means of metallography and scanning electron microscopy, etc. Its surface oxidation morphology was compared with the oxidized morphology of the same material’s metallographic smooth surface and impact section. The nature of the linear defect and its cause were determined. The results show that the linear defect of the blade is a cracking caused by a membrane-like defect rich in Ti and Mo carbides. The reason is that the oxide film was formed when the surface of the alloy liquid rotated or splattered on the wall during the casting process, and then the membrane-like carbides were precipitated along the oxide film in the subsequent aging process. The membrane-like defects cracks and undergoes a long period of high temperature oxidation, which forms a two-layer composite oxide film with an outer oxide layer mainly composed of the oxides of Ni and Co and an inner oxide layer mainly composed of the oxides of Cr, Al and Ti. We can use a filter, increase the vacuum and maintain a smooth pouring to avoid such defects in the production process.