Full Rejuvenation Heat Treatment of MGA 1400DS

Abstract

Abstract Directionally Solidified (DS) nickel-base superalloys have been extensively used for power generation turbine blades. These blades are exposed to high stresses and temperatures for extended periods of time and aging the microstructural and reducing the mechanical properties. After an operational cycle a repair is typically performed to restore dimensional and coating integrity as well as restore metallurgical and mechanical properties of the base material to extend the life of components manufactured from DS alloys. These repair methods can include heat treatments for stress relieving weldment, application of braze, coatings and rejuvenation of the aged base material. Industry standard heat treatments (ISHT) for these DS alloys do not always provide a (full) rejuvenation. Therefore TEServices, Sulzer and EPRI improved ISHT for DS alloys, such as MGA 1400DS and GTD 111DS, for obtaining a full rejuvenation these base materials and obtaining metallurgical and mechanical properties that, in some cases, exceeds the new material properties [1]. The study in this paper focuses on modifying MGA 1400DS ISHT to obtain a full rejuvenation (solutioning the gamma prime during solution heat treatment in the dendritic and inter-dendritic regions) without negatively impacting the metallurgical structure (such as incipient melting) and mechanical properties. This study was performed using optical and scanning electron microscopy evaluating the microstructure of MGA 1400DS in as new, artificially aged and heat treated condition to establish the impact of full rejuvenation heat treatment (FRHT). Mechanical testing was performed on new, aged and FRHT MGA 1400DS to establish the impact of aging and FRHT compared to new material. For this, creep and stress rupture, fatigue, and combination of stress rupture and fatigue testing was performed. The findings in this study show that FRHT provides superior metallurgical and mechanical properties of MGA 1400DS when compared to new and aged material.