Formation Mechanism of Freckles in Large-Sized UGTC47 Directionally-Solidified Columnar Crystal Blades for Heavy-Duty Gas Turbines and Influence on Stress Rupture Property

Abstract:

The formation mechanism of freckle defects in large-sized UGTC47 directionally-solidified columnar crystal blades for heavy-duty gas turbines and their effect on the stress rupture property were investigated using optical microscope, scanning electron microscope, and endurance performance test. The results indicate that freckle defects form in both the middle and root regions of the blade tenon, whose formation mechanism is the density inversion caused by liquid phase segregation, resulting in fluid convection under the action of gravity, and leading to the fracture of dendrite arms and the formation of freckles. At 900 ℃/380 MPa, the proportion of freckle area has a significant impact on the endurance performance of the UGTC47 alloy. With the increase in freckle content, the stress rupture life of the alloy is decreased from 131.83 h without freckle defects to 33.66 h with freckle content of 62%.