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.

Development, reliability evaluation and service experiences of gas turbine blade life regeneration technology

“…In recent years, various methods have been utilized for the purpose of reducing gas turbine maintenance costs by extension of the service life of gas turbine hot parts [1,2,3,4,5]. Toshiba started such research and development programs since 1986 to develop service-life assessment and refurbishment technologies to extend the life of turbine components.…”

Evaluation of Fatigue Crack Propagation in The Post-Service Gas Turbine Vane

“…Meanwhile there are two possible metallurgical factors which lead to the reduction of fatigue crack propagation resistance shown in Figs. 6-8: one is a depletion of aluminum from the IN939 alloy resulting from a formation of alumina during the service period, and the other is a reduction of coherency between γ matrix  and γ' precipitates which may reduce a magnitude of internal stress in superalloys (1,2,4,21) . The recovery of the hardness expressed in Fig.…”

“…In general it is often too hard to extract a large size specimen enough to satisfy, e.g., ASTM standard, from the actual components in service (4,10) . This is also the case in this work.…”

“…Therefore, it is of considerable technical and financial benefit if the condition of the blades can be assessed and their remaining life determined. Whilst some efforts have been made to investigate the microstructural evolutions (2,3) or the degraded creep properties of post-service gas turbine superalloys (4) , little is known about their properties of fatigue; the most frequent failure mode (9)(10)(11)(12)(13)(14)(15)(16)(17) . Here, the properties may be often depending on the location in the components.…”

“…This service condition inevitably causes many types of damages and degradations in components during the operation period. Materials used in them, especially Ni-base superalloys, are so expensive items that they can be a substantial part of an operating and a maintenance budget (1)(2)(3)(4)(5)(6)(7)(8)(9) . Therefore, it is of considerable technical and financial benefit if the condition of the blades can be assessed and their remaining life determined.…”