How to Extend the Lifetime of Gas Turbine Parts

Elevated temperatures take a big toll on gas turbine components risking cracks, corrosion, wall thickness, and material condition. Repair enhancements can extend a part’s life by slowing its degradation and pushing out its usable service life. Operators can see significant savings by opting for lower-cost repaired spares instead of new parts as demonstrated in the following case studies.

7FA 2nd Stage Blade Case Study

MD&A’s Gas Turbine Parts Service Facility in San Antonio Texas , evaluated two sets of 7FA 2nd Stage Blades to determine if they were capable to operate for a third service interval. The blades had operated through two service intervals and underwent one repair.

The team performed an x-ray inspection on the components in their as-received condition which did not reveal any areas of concern, nor clogging on the cooling holes.

An FPI inspection was performed to look for significant cracking, paying close attention to the platform side face due to the substantial number of starts reported. The area around the shroud radius was also evaluated for cracks. The team found no major areas of concern during the preliminary FPI inspection but still performed a more thorough inspection once the metallic coating was chemically stripped.

The MD&A San Antonio facility deemed the condition of the internal coating as 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, MD&A experts use a vacuum heat treat furnace where we control the heating and cooling of metals to alter their physical and mechanical properties. There is a pre-weld heat treatment, post-weld heat treatment, and a diffusion or aging heat treatment solution.

 

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 time was improved by roughly 100 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, MD&A 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 candidate for a Lifetime Extension (LTE) repair. Once established as candidates for Lifetime Extension, the team employed repair practices to further extend the useable life of the components.

9FA 3rd Stage Blade Extended to 120,000 FFH

The MD&A San Antonio Service Center received three 9FA 3rd stage blades 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.

The 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 a post-heat treatment.

Inspections showed that the manufacturer’s heat treatment did not achieve 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, all of the components were deemed repairable.

Surface Oxidation on Lead Edge


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

The MD&A team applied repair heat treatment to several samples from the test blades, then evaluated to determine the material’s response to the heat treatment, which resulted to be positive on all three blades. The primary gamma 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 120,000 FFH.


V94.3A4 2nd Stage Blade Extended to 75,000 EOH

V94.3A4 2nd Stage Turbine Blade set had 50,000 EOH and operated through two service intervals, along with one repair performed by the OEM at 25,000 EOH. The MD&A San Antonio Service Center experts performed testing and evaluation on the blade set to determine if the hardware could be deemed repairable and serviceable for another interval, as well as an evaluation of the previous repairs.

One 2nd Stage Blade tested in the as-received condition had an internal and external Coating Evaluation, along with mechanical property and metallurgical properties inspection. The other 2nd Stage Blade was evaluated in the post-heat-treat condition with mechanical properties and metallurgical properties inspection, and then an evaluation of metallurgical results. The heat-treated part was then evaluated for reparability.

MD&A experts performed a pre-weld heat treatment, post-weld heat treatment, and a diffusion or aging heat treat solution using their vacuum heat treat furnace in their shop. The external TBC appeared in good condition, however, the blade tip had significant oxidation. This is most likely due to the weld wire selected during the previous repair. All other inspections were normal.

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

Based on all the evaluation performed on the two 2nd Stage turbine blades, MD&A experts determined that the full set was a viable candidate for life extension to 75,000 EOH.

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

A coating system consisting of a NiCoCrAlY applied by HVOF and a TBC top coat was applied with MD&A’s propriety coating. The V94.3A4 2nd Stage Blades’ lives were extended to operate to a total of 75,000 EOH.

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


Call MD&A Gas Turbine Services’ team today at (518) 399-3616 or use this contact form.




 






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