Failure Analysis of Air Tube Inlets in a High-Speed Power Turbine

The present study was conducted to investigate the root cause failure analysis of 316L stainless steel air tube inlets at the weld joint in a High-Speed Power turbine (HSPT). Fractography analysis revealed ratchet marks and striations on the fracture surfaces, indictive of vibration-induced fatigue in the heat affected zone of the tube-to-flange weld joint, resulting from the rotation of the HSPT at high speed, identified as the primary cause of failure. Additionally, radiography and visual inspection unveiled a sharp interface between the weld metal and parent metal within the inner wall, acting as a stress riser. X-ray diffraction analysis demonstrated that the parent metal predominantly consisted of austenite ( γ ), while the weld metal comprised a mix of austenite ( γ , ≈ 90%) and delta ferrite ( δ , ≈ %10). Hardness readings in the weld zone ranged from 260–302 HV, compared to ≈ 140 HV in the parent metal. Microstructural examination showed inclusions and impurities rich in manganese, and silicon, likely introduced during manufacturing and/or welding processes. Conclusively, the study attributes failure to the use of low-quality materials and consumables, poor welding practices, and insufficient vibration condition monitoring.