First-stage nozzle repair for 7F engines combines brazing, welding

Gas-turbine repair techniques continue to evolve and proliferate against the backdrop of ever-present cost pressure, machines ageing in service, and competition among repair service firms. Ultimately, the choice will depend on many technical factors, but especially the owner/operator’s approach and budget for managing risk and the life stage the unit/components are at.

Latest entrant for the repair of first-stage nozzles in 7F.03 and later machines is a technique which combines welding and brazing and, according to supplier ProEnergy Services, Sedalia, Mo, can reduce repair cost by 40% or more.

Brazing is defined as a process which bonds materials. The base material is heated to the “brazing temperature” in the presence of a filler metal having a liquidus (temperature above which material is completely liquid) above 450C and below the solidus (the locus of temperatures below which a material is completely solid) of the base or parent metal.

The important distinction from welding is that the parent metal does not melt. The filler metal is distributed among the closely fitted mating surfaces of the joint through capillary action. A braze is a fine-grain structure (Fig 1).

Pro En 1

Repair experts always point out that one risk with brazing techniques is that they are proprietary. Confidence, or even faith, in the service company’s expertise is paramount, as is field experience at similar sites with similar components. In addition, no qualification standards exist for a braze repair, although industry level efforts are being initiated to change that, according to ProEnergy’s Warren Miglietti, who presented to users attending CTOTF’s™ GE F-class Roundtable at the organization’s 2016 spring meeting, last April.

ProEnergy’s approach is to combine the best of both worlds. Cracks above a proprietary width are weld-repaired and cracks below are braze-repaired. The company claims to have repaired 7F first-stage nozzles three times and doubled their life for customers, even though GER3260 states that these components should only be repaired once.

According to Miglietti:

      • If done properly, diffusion brazing results in mechanical properties equivalent to the parent metal—that is, if done at the right temperatures for the correct intervals and joint gaps (Fig 2).

      • Does not result in distortion because the brazing is done in a vacuum furnace.

      • Repairing as a “batch” process reduces the schedule.

      • A section of the brazed-repair nozzle is removed to guarantee that a sound repair, not a cosmetic repair, was conducted.

Pro En 2

The dramatic cost and schedule savings comes from using a vacuum furnace instead of individually welding each crack. “A first-stage nozzle can have hundreds of cracks,” Miglietti stresses, “and it takes an individual welder numerous hours to weld them up one crack at a time. We take all 24 nozzles, an entire nozzle set, into the vacuum furnace and braze all the cracks on all the nozzles at one time.” This takes 24 hours; to do that with weld repair would take dozens of welders. “I don’t know of many repair shops with that many welders,” Miglietti added.

Miglietti stresses that the notion peddled by some that braze repair will make future weld repair in that area more difficult (because of embrittlement in the brazed region) is out of date. Welding technologies and brazing wire material have evolved and welds can be done successfully over or near brazed areas without cracking.

It’s no secret that owner/operators are starved of metallurgical engineering expertise. To offer assurance to end users, ProEnergy provides tensile, stress rupture, and low-cycle fatigue properties of the braze repair through testing of a section of a repaired nozzle.

Successful braze repair depends on at least two dimensions: the brazing material composition and the thermal cycle. Compared to most competitors, Miglietti said, ProEnergy uses less melt-point depressant elements in its mix which avoids a brittle repair area, as well as depressants which make the repair area more ductile. Powders of -325 mesh are used to achieve a fine-grain microstructure.

Even when the material composition is the same, the thermal cycle plays a critical role in the final microstructure achieved (Fig 3). ProEnergy’s technique employs an order-of-magnitude longer heating cycle—640 minutes compared to less than an hour—a than some techniques. Optimizing various heating and cooling steps results in smaller intermetallic phases, a less brittle joint, and better mechanical properties.

Pro En 3,4

Finally, much experience is required to know which types of cracks can successfully be repaired through brazing. As the joint gap widens, the stress rupture properties of a brazed repair worsen (Fig 4). “Just because you can physically braze a ¼-in crack doesn’t mean you should,” stresses Miglietti.

Brazing repair techniques can be applied to all stationary hot-gas-path components, as well as combustion liners and transition pieces.

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