Gas Turbine Parts Lifetime Extension Clinic

Mechanical Dynamics & Analysis (MD&A) can extend the service life of gas turbine components whose OEM service limit has passed or whose configuration is no longer suitable to operate. Significant savings can be realized by opting for lower-cost repaired spares instead of new parts.

Elevated temperatures take a big toll on gas turbine components. Repair enhancements can extend a part’s life by slowing its degradation and extending its usable service life.

Formerly PW Powers Systems’ IGT division, the MD&A San Antonio Texas Service Center team have years of experience evaluating the life-limiting factors for gas turbine vanes and blades. Such factors include cracks, corrosion, wall thickness, and material condition.  Most crack limiting locations are found at the platform or shroud transition. Smaller percentages occur at the blade tip, the blade airfoil, or the root. Vane limiting factors are usually related to the wall thickness and material condition.

Platform degradation can be reduced in many ways: by operating in baseload only ( although dispatch conditions may prevent that from being a practical solution), by pursuing a robust platform weld repair program, by improving the integrity of the platform coating, and by making platform improvements.

Thermal mechanical fatigue cracks will eventually lead to platform failure. Design modifications often are required, incorporating a robust repair procedure. The life cycle enhancement process used by our experts exceeds OEM platform properties.

Our experts at the San Antonio Service Center combine years of experience with state-of-the-art equipment to deliver exactly the right solutions to your specific set of parts!

Several Case Studies of our Life Time Extension approach and methodology are presented as examples:

Case Study 1) – 7FA 2^nd Stage Blade Case Study

Our San Antonio facility evaluated two sets of 7FA 2^nd Stage Blades to determine if they are capable to operate for a third service interval. The blades had previously operated through two service intervals and underwent one repair.

An x-ray inspection was performed on the components in their as-received condition. No areas of concern were detected, and no clogging was observed on the cooling holes.

An FPI inspection was performed to look for the presence of cracks. The area around the shroud radius was also evaluated for cracks.  No major areas of concern were found during the preliminary FPI inspection.

The condition of the internal coating was deemed acceptable and in good condition to operate for one more service interval. The external coating protected the external surfaces and was recommended for removal to allow a good FPI inspection of the hot gas-path surfaces.

Microhardness readings taken at several blade locations showed a significant drop in the base material hardness where parts had been exposed to higher temperatures. These areas also showed significant degradation to the gamma prime structure under inspection by a Scanning Electron Microscope.

Hardness values after heat treatment demonstrated that the GTD 111 responded well to the process and the overall blade hardness values were improved.

For heat treatment, our experts utilize a vacuum heat treat furnace, where we control the heating and cooling of metals to alter their physical and mechanical properties.  The specific heat treatments selected for a set of parts depend on a number of factors.  Several separate heat treatment cycles are conducted throughout the course of repair including  1) pre-weld solution heat treatment, 2) post-weld stress relief heat treatment, 3) coating diffusion, and 4) precipitation hardening (or aging) heat treatment.

Samples were removed from the blades for stress rupture testing and in the as-received condition did not meet the minimum expected strength requirements for GTD 111. The samples tested after heat treatment exceeded the minimum requirements.

After the repair heat treatment, the stress rupture test results improved by roughly 100 test-hours (tested @ 1600F / 50KSI). This demonstrates that the repair heat treatment was able to rejuvenate the parts sufficiently to operate for one more service interval.

In conclusion, our experts evaluated each component in the as-received condition and in the post-repair heat-treated condition. Improvements in the alloy mechanical properties were evaluated to determine if the two sets are candidates for a Life Time Extension (LTE) repair.  Once established as candidates for Life Time Extension, our Experts employed repair practices to further extend the useable life of the components.

Case Study 2) – 9FA 3^rd Stage Blade Extended to 120K FFH

Three 9FA 3^rd stage blades were received by our San Antonio facility where they underwent detailed inspections, including microstructural assessment and high-temperature testing.

Observations indicated galling in root serrations and seal pin surfaces, as well as the presence of two distinct casting houses to produce the row of buckets.

Scope of work included a visual inspection, dimensional measurements, and an x-ray inspection. Also included were a microstructural assessment, high-temperature mechanical properties testing, assessment of material condition at multiple locations, heat treatment of specimens to demonstrate the effects of repairs, and high-temperature mechanical property testing performed post-heat treatment.

Inspections showed that the manufacturer’s heat treatment had not achieved full solutioning. All blades also displayed a two-phase gamma prime structure within each grain, indicating that the previous repair heat treatments were not optimized to achieve a good structure. In addition, all three blades had surface oxidation and depletion. Even so, the components all were deemed repairable.

Surface Oxidation on Lead Edge

Additional repair work included grit blasting and polishing the gas path surfaces to remove an oxide layer, a complete X-ray inspection, and HVOF coating and diffusion to reduce surface degradation.

MD&A’s Repair heat treatment was applied to several samples from the test blades, then evaluated to determine the material’s response to these heat treatments. The material responded well to the heat treatments on all three blades. The gamma prime transformed to a more cuboidal morphology, which provides for improved component creep life.

Microstructure

These third-stage blades were evaluated and subsequently determined to be candidates for a Lifetime Extension (LTE) repair. The 9FA 3rd stage blades’ lives were extended to 120K FFH.

Case Study 3) – V94.3A4 2nd Stage Blade Extended to 75K EOH

A V94.3A4 2nd Stage Turbine Blade set had 50K EOH and operated through two service intervals, along with one repair performed by the OEM at 25K EOH.

Our San Antonio service center experts performed testing and evaluation on the blade set to determine if the hardware is deemed repairable and serviceable for another interval, as well as, evaluation of the previous repairs.

One 2nd Stage Blade tested in the as-received condition had an internal and external Coating Evaluation, along with a mechanical properties & metallurgical properties inspection.

The other 2nd Stage Blade was evaluated in the post-heat-treat condition with mechanical properties & metallurgical properties inspection, and then evaluation of metallurgical results. The heat-treated part was then evaluated for reparability.

The external TBC appeared in good condition. However, the blade tip has significant oxidation. This is most likely due to the weld wire selected during the previous repair.  All other inspections were normal.

Our experts performed a pre-weld heat treatment, post-weld heat treatment, and a diffusion/aging combined cycle heat treat solution utilizing our vacuum heat treat furnace in our shop.

One heat-treated blade’s metallurgical properties were then evaluated and compared to the other blade in the as-received condition. The evaluation confirmed that the heat treatment process will heal and improve the gamma prime structure.

Based on all the evaluations performed on the two 2nd Stage turbine blades, our experts considered that the full set is a viable candidate for life extension to 75K EOH.

The measurements of the blade tip cap indicated that there was sufficient material to perform a successful weld build-up repair, so they removed most of the previous weld repair. Because of heavy oxidation on the blade tip, an oxidation-resistant alloy was used. Our weld repair procedure has demonstrated to provide excellent oxidation resistance during operation.

A coating system consisting of a NiCoCrAlY applied by HVOF and a TBC topcoat was applied with our propriety coating. The V94.3A4 2nd Stage Blades’ lives were extended to operate to a total of 75K EOH.

Our San Antonio Service Center, or Gas Turbine Parts Service Facility, exemplifies best in service because of the deep experience in industrial gas turbine engineering and technology, repair techniques and equipment, and proven expertise on multiple frame gas components.  Take a virtual Tour here.

MD&A’s available Gas Turbine services range from full turnkey major inspections to rotor unstacking, gas turbine alignment, and control services. MD&A is a global full-service OEM alternative for services, parts, and repairs.

Call our MD&A San Antonio Service Center, or our Gas Turbine Parts Service Facility today at +1 (210) 256-5000.

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