Microstructural evolution of hydroformed Inconel 625 bellows

Fatigue cycle tests of Inconel 625 superalloy bellows expansion joints were conducted using a Fatigue testing machine at both room and elevated (650 °C) temperatures. Optical Microscope, Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were employed to investigate the microstructure evolution of grains and its boundaries. The intermetallic phases like γ″ were found and carbide precipitates were observed on the grain boundaries at elevated temperature. The recrystallization of the grains and its growth at the elevated temperature is characterized.