Microstructural characterization and heat treatment of A-286 turbine buckets
A-286 is an iron-based superalloy used extensively in land-based gas turbines for 2nd stage (low pressure) buckets and 1 st and 2nd stage wheels. Although A-286 may appear to some as just another austenitic stainless steel, its elevated temperature properties are attributed to γ’ precipitate structures, effective distributions of alloy carbides in the forms of M23C6, M6C and MC, and solid solution strengthening. Unfortunately over the course of these components service lives microstructural degradation develops in the form of η-phase formation. The presence of η-phase, especially in large quantities, can have a negative effect on stress rupture properties. Resistance to creep rupture and dimensional stability are desirable elevated temperature properties necessary for turbine operational capacity. Since a significant portion of the elevated temperature strength of A-286 is accomplished by the formation of ordered FCC γ’ precipitates, utilizing alloy additions of aluminum and titanium, the deleterious η-phase formation depletes the alloy of γ’ particles. This investigation presents the use of in-situ metallography, optical microscopy and scanning electron microscopy to quantify the severity of η-phase formation in its microstructural forms and distribution. Recognizing that gas turbine operators extend the service lives of these components beyond warranty coverage, this research should aid in engineering decisions involving the re-use of these A-286 components. This research also has explored the potential for rehabilitation of A-286 turbine buckets, which have demonstrated the formation of η-phase. Through a solutionizing and aging heat treatment, η-phase can be restored back into a “near new” microstructural state, extending the service life of these various components.^
Engineering, Metallurgy|Engineering, Materials Science
Bradley, Christopher Michael, "Microstructural characterization and heat treatment of A-286 turbine buckets" (2009). ETD Collection for University of Texas, El Paso. AAI1468288.