Impact of Thermomechanical Aging on Alloy 625 High Temperature Mechanical Properties

Abstract

Alloy 625 is widely used for petrochemical, marine and aerospace applications owing to its outstanding corrosion and mechanical properties at high temperatures. However, this alloy is prone to complex microstructure evolutions above 500°C that may impact its mechanical properties. In this study, the impact of its micro structure evolutions occurring upon long-term thermal aging on its static and cyclic mechanical properties has been studied. Thermal exposures of up to ~ 2000 hours in the 550°C-900°C temperature range have been investigated. TTT diagrams of the 5 and y" phases were established based on high resolution scanning electron microscopy observations. The evolutions of secondary carbides distributions were also followed. It has been observed a steep increase of the room temperature microhardness after overagings performed at 650°C and 700°C due to the precipitation of the y" phase. Moreover, it is also clearly evidenced a strengthening effect of the 5 phase after long-term thermal exposures at temperatures in excess of 700°C. Finally, the impact of a thermal aging in the y" precipitation domain on the high temperature tensile and low cycle fatigue properties was evaluated. It is shown a beneficial effect of the y' ' precipitation to the fatigue life and to the tensile properties up to 750°C. It is also shown that the loss of high temperature ductility is not correlated to the precipitation of grain boundary secondary carbides. y'' precipitation and the y'' ^5 transformation are also shown to be faster under cyclic loading compared to pure thermal aging.