Advanced repair techniques prolong hot-section component life

By Lloyd Cooke, PE, Liburdi Turbine Services Inc

The repair of gas-turbine (GT) hot-gas-path (HGP) components— turbine blades, vanes, and combustor parts—has developed into a billion-dollar business. The reason plant owner/operators are opting for repair, rather than replacement of these critical components is simple: cost savings. Regardless of how extensive the repair procedures, and whether the potential replacement is provided by the original equipment manufacturer (OEM) or an independent shop, the cost of a repair typically will not exceed onethird of the cost of a replacement.

As you might expect, all hot-section component repairs are not created equal. Hence, operators need to understand what they’re buying when they specify a “standard” versus an “advanced full-life” repair.

Standard component repairs

Hot-section repairs are required because of various types of damage mechanisms that the components are exposed to in an operating GT. For example, components may suffer mechanical distress—such as cracking, or metal loss caused by wear and impact damage. They also may suffer metallurgical or coating damage caused by high-temperature oxidation of the surfaces, or by hot-corrosion attack stemming from contaminants in the fuel or air.

Much of the damage from these mechanisms can be addressed in a “standard” repair workscope. In a standard repair, the surface characteristics of the component are restored, allowing the component to be returned to the end user in a visually acceptable condition. Surface restoration may involve welding of surface cracks and replacement of missing material; dimensional restoration by straightening or welding and machining; replacement of the original coatings; or relatively lowtemperature heat treatments, which improve weldability and coating diffusion but only partially restore the internal microstructure, if at all.

If this level of repair only addresses the surface appearance and not the underlying conditions, then it should be considered cosmetic. Still, such a repair can be the right choice in certain circumstances. Example: when the component only needs to last a little while before reaching its planned retirement—usually at the next HGP service interval.

Quality-assurance problems that can arise with a standard repair include welding with weak weld fillers or inadequate weld penetration, welding with weak materials in high stress areas of turbine-blade airfoils, and brazing over cracks. Another problem is that portions of the components that cannot be visually inspected may be left with a depleted alloy or coating; hence, they will be unprotected during subsequent gasturbine operation.

Advanced, full-life repairs

By contrast, advanced repairs are designed to extend the life of the component through multiple service intervals. These repairs entail one or more sophisticated processes to fully restore the component from all service degradation.

For example, if the components have internal as well as external coatings, both must be fully stripped and new coatings applied. This requires the use of an advanced process to chemically strip the internal surfaces so they can be fully recoated to the as-new condition. With all coatings removed, the part can then be processed through HIP (hot isostatic press) and high-temperature vacuum heat treatments to fully restore the microstructure, properties, and strength of the alloy to meet new-part specifications. Compare this to the lower-temperature heat treatments used in the standard repair, which at best may only partially restore the alloy.

Typically, the service experience of the original coating is evaluated— based on metallurgical analysis of the damaged component—before the new coating is applied. In some cases, a coating other than the original may be selected because it offers superior resistance to oxidation or corrosion, or because it offers higher ductility and is less prone to cracking in service.

Similarly, a different weld alloy may be selected for the advanced repair, when components have suffered metal loss caused by oxida-tion. A superior weld alloy not only restores the original dimension of the part, but also provides enhanced oxidation resistance during future operating intervals.

When repairing vanes or nozzles that have experienced extensive cracking damage, a high-strength repair process may be needed. Whereas a standard repair will use a lower-strength, more ductile weld alloy for crack repairs, an advanced repair will employ higher-strength weld or powder-metallurgy repair processes. The high-strength alternative typically results in minimal cracking during the next operating interval and enables the component to provide extended life though multiple service intervals (Fig 1).

Vanes that have lost wall thickness because of oxidation or corrosion might have been retired in the past. Or they might have been repaired by removing and welding in new castings on the trailing edges—in effect, creating welded inserts. With today’s advanced repairs, however, these vanes can be reconstructed to their original thicknesses using powder metallurgy. This process employs high-strength alloys, and can successfully restore leading edges, airfoil mid-sections, and trailing edges to meet the original new-part standards.

Such reconstruction of the original airfoil thickness is preferred over welding inserts, because it facilitates the re-establishment of critical throat dimensions to produce a successful harmonic analysis even after extensive repairs.

Comparing the two

On the plus side, standard repairs typically are lower cost, use conventional repair processes, and can be completed faster. However, standard repairs may only address the visual or cosmetic aspects of the parts; they are not intended to fully restore the internal cooling passages and alloy strength.

Advanced, full-life repairs rely on processes that are more precise, and more timeconsuming to complete. Main benefit of an advanced repair is that the components likely can be refurbished again after the next service interval, and possibly for multiple service intervals, extending the ultimate life and postponing the day when expensive new replacement parts must be bought.

A recent program involving 7EA buckets quantifies the value of full-life repairs over standard repairs. The latter would have replaced only the external coatings, and would have subjected the blades to lower-temperature heat treatments at 24,000 hours, with the intention of retiring them at 48,000 hours. Both internal and external coatings were replaced with the fulllife repair (Fig 2), and alloy strength was fully restored with high-temperature heat treatments at each 24,000-hr interval. Advanced repair processes have successfully extended the ultimate service life of sets of 7EA buckets to 100,000 to 120,000 hours, while at the same time restoring them to the new-part standard after every 24,000 hours of service.

Another example involves the use of advanced weld materials to extendthe service life of a set of Rolls-Royce RB211 high-pressure turbine (HPT) blades. These parts operate at very high temperatures in the engine and suffer from shroud metal loss caused by oxidation. Standard repairs of these components, conducted in the past, employed conventional weld alloy to restore the shroud dimensions. But the parts again suffered severe metal loss after just 12,000-15,000 service hours, and many sets had to be retired at 35,000 to 40,000 hours.

Several years ago, a full-life repair method using advanced oxidationresistant weld alloy was introduced. Since then, these engines have operated for the full 24,000-hr interval, and the industry-leading sets of HPT blades are now reaching 72,000- 100,000 hours of service (Fig 3). This same advanced weld alloy also has been used for blade-tip repairs of Fclass frames as well—including 7FA, W501F, and V84.3A1 models.

The gas-turbine industry has compiled many other examples of full-life repairs, enabling components to be not just repaired at the present service interval, but repairable again at the next service interval, and possibly at several additional intervals. Because full-life repairs prolong a component’s ultimate service life, the savings compared to the use of standard repairs can be significant. Using full-life repairs on its 7EA buckets, one user with a four-engine fleet saved $8-million in avoided replacement costs over a 10-yr period.

There’s more

For more on gas-turbine hot-section repairs, visit www.psimedia.info/ ccjarchives.htm and access the following articles:

  • 2Q/2006, p 91, Western Turbine Users report, section on “Maintenance cost optimization”
  • 4Q/2005, p 36, CTOTF Fall Turbine Forum report, section on “Meeting the challenge of refurbishing 11N2 HGP parts”
  • 2Q/2005, p 30, “Repair technologies borrowed from aeros produce high yields on F-class blades, vanes”
  • 3Q/2004, Outage Handbook supplement, p OH-30, “Taking the mystery out of GT hot-section coatings” ccj oh