Effect of Microstructural Evolution on Creep and Rupture Behavior of Inconel 617 Alloy

Abstract

The Inconel 617 alloy has been creep-tested at different combinations of loads (80-350 MPa) and temperatures (650-800 degrees C), considering its use in the advanced ultra-super critical (AUSC) boiler. The values of Monkman-Grant and modified Monkman-Grant constants obtained by analyzing the creep data are considered as related to the observed microstructural instability during creep at similar to 650-800 degrees C. The presence of intragranular secondary gamma' precipitates has led to significantly higher strength inside the grains compared to that of their boundaries, found to have a scanty population of secondary (Cr,Mo)(23)C-6 precipitates. As a result, formation of creep cavities leading to intergranular fracture has been observed in the temperature range of 650-750 degrees C. The creep damage tolerance factor, lambda, found as similar to 2.5 by empirical damage analysis on the basis of test results is in tune with the predominance of creep cavitation in the temperature range of 650-750 degrees C. In contrast, both the obtained value of lambda similar to 10.51 and the mixed mode nature of the fracture surface observed for the sample creep tested at 800 degrees C are considered due to substantial localized plastic deformation or necking. Increased creep ductility with necking is ascribed primarily to the reduced the grain body strength caused by the absence of gamma' precipitates and the grain boundary sliding being restricted by the discrete secondary (Cr,Mo)(23)C-6 precipitates present in abundance along the grain boundary. Based on the plot of applied stress against Larson-Miller parameter, it is possible to predict that the Inconel 617 alloy would withstand the steam pressure of 30 MPa at 750 degrees C for a design life of 10(5) h experienced in a typical AUSC boiler tube.