Microstructural Evolution and the Resultant Mechanical Behavior of Duplex Stainless Steels

Abstract

As the current generation of commercial light water nuclear reactors approach initial design life specifications (40-50 years), the plausibility of extending the operational life of duplex stainless steel piping to 80 years has become an important research focus. Successful evaluation of this potential requires an improved understanding of microstructural evolution and corresponding changes in mechanical behavior that occur during continuous operation at temperatures up to 320 °C, which notably results in aging embrittlement in these systems. This investigation characterizes the effects of thermal aging on the mechanical properties of cast CF–3 and CF–8 stainless steels at operational (280 °C, 320 °C) and accelerated temperatures (360 °C, 400 °C) by a variety of test methods.

Bulk mechanical tests have been performed to measure changes in properties such as tensile strength, impact energy, and ductility during aging embrittlement. The results show an increase in strength and decrease in ductility and impact energy after aging to 17,200 h. The phase structure is investigated by electron microscopy and correlated to the mechanical properties and aging conditions in order to form a comprehensive understanding of the progression of embrittlement and elucidate trends. Smaller length scale tests, such as instrumented nanoindentation, reveal the effects of aging on local properties of the constituent ferrite and austenite phases. The resulting data are utilized to evaluate the influence of local microstructural changes, such as spinodal decomposition, on thermal aging embrittlement of the steels. Finite element method (FEM) models have been developed based on the real microstructure and local properties of the steels in order to analyze the micromechanical relationships between phases at different stages in the aging process. This research combines mechanical, microstructural, and computational characterization methods to build a comprehensive evaluation of the effects of thermal aging on structure-property relationships of these important structural stainless steels.