Microstructure and Property Assessment of Conventionally Cast and Directionally Solidified Buckets Refurbished After Long-Term Service

Abstract

Conventionally cast GTD-111EA first stage buckets from GE Frame 5001P-NT engine after 42,000 hours of service, directionally solidified GTD-111DS Frame 5002C buckets after 49,000 hours of service, and conventionally cast IN-738 Frame 5002B buckets after 81,000 hours of service were evaluated before and after refurbishment. These buckets were coated with GT-29, GT-29 plus and RT-22 coatings respectively. Coating condition, microstructural degradation, tensile properties, and creep properties were evaluated in the service aged condition. Microstructure was assessed after the hot isostatic pressing (HIP) and solution heat treatment and again after the full refurbishment and recoating of the buckets. It was found that the GT-29 coating was breached 100% by oxidation in the 5001P engine after service. The GT-29 plus coating on the 5002C engine was breached very slightly and the majority of the coating was still in good condition. The RT-22 coating on the IN-738 bucket was stripped before it was received for evaluation at SwRI. Gamma prime growth, coalescence and agglomeration were found in all of the buckets. Continuous carbide network near the airfoil leading edge was found in the GTD-111EA bucket. Significant improvement in the microstructural condition was observed after the refurbishment for the GTD-111 buckets. However, the gamma prime morphology was not normal for the IN-738 bucket. Both tensile and creep properties of the GTD-111 buckets showed significant improvement after refurbishment. However, for the IN-738 bucket little improvement on the mechanical properties was observed. The reasons for this and the correlation between the microstructure and properties are discussed in this paper.