Once we have completed the analysis, we can remove the existing coating. We select a suitable chemical strip bath according to the coating’s condition and composition.
We document the results of the NDT inspection and mark them on the components. Then we measure the dimensions of the parts with tools ranging from Vernier calipers to the latest optical 3-D scanners, and we document the results.
The next step is an inspection via Non-Destructive Testing (NDT). We apply one of three main techniques:
- In most cases, we perform FPI (Fluorizing dye Penetrant Inspection) to inspect non-magnetic alloys like nickel- and cobalt-base superalloys. During this process, we spray the parts with a liquid fluorizing dye. Due to capillary forces, the liquid is absorbed by cracks and other defects. After soaking, rinsing, and drying the surface, even minute cracks will be visible under UV light.
- Eddy current inspection is an alternative to FPI. This far more elaborate technique is suitable for the detection of closed cracks, and more especially high-cycle fatigue cracks. These may not be open wide enough for FPI inspection, but they are detected with Eddy current inspection.
- X-ray detection is also used to detect cracks, but only for cracks running parallel to the X-ray. Therefore it is not used routinely, but only when necessary.
The repair process essentially involves the controlled removal and addition of material. Heat treatments may be required between these steps. Removing material is one of the most tedious parts of the repair. It is crucial to meticulously remove non-rejuvenable material before adding new material.
Cleanliness of the surface is of the utmost importance for most processes which add material. We degrease and grit blast the components after almost every step in the repair process. There are two main grit blasting techniques:
- through erosive particles which remove surface layers by abrasion
- through metallic particles which smooth the surface and remove ceramic contaminants
In welding, a filler metal is used to add material. Welding creates intermediate layers which are liquid during welding and have a mixed composition of base material and weld filler. This area is inhomogeneous with respect to its material composition. Where the base material itself does not melt, a zone is formed in which the base material microstructure is changed. This is the heat-affected zone.
Tungsten inert gas welding is commonly used for repairs because it is the most versatile welding process for this purpose. For nickel-based superalloys, welding is performed in an argon atmosphere inside a glove box. Laser welding is used for repetitive serial repairs and also for welding with high-alloy weld filler material in DS and SX materials.
The excess weld material must be removed by machining or manual benching. Well trained operators can perform this with outstanding accuracy.
A brazing alloy contains elements which lower the alloy’s melting point for it to be used to join parts.
Ideally the brazing alloy should melt well below the melting point of the base metal. On the other hand, the brazing alloy must be able to withstand all final heat treatments applied to the component. This determines the permissible melting ranges of high-temperature (nickel- and cobalt-base) brazing alloys.
Nickel-base superalloys contain aluminum and titanium for good creep resistance. These elements are very reactive with oxygen. Thus brazing can only be carried out in high-vacuum furnaces at temperatures well above 1,100°C (2,000°F).
Braze mixes are used for overlay repairs. These are mixtures of superalloy powder and braze that form a dense semi-molten mass on repair areas. After it solidifies, the braze mix has a creep strength that is comparable to cast high-strength superalloys.
Since parts are heated and cooled homogeneously, brazing creates little or no distortion.
Typical applications for brazing are:
- Joining: e.g. brazing of honeycomb seals
- Surface rebuilding (even in high-stressed areas) to restore corroded, eroded, or fretted areas
Field experience of these braze repairs is very good. Sulzer has repaired components with this technology for many years. Components can be repaired repeatedly and the braze mixes are usually in a good condition after service.
The elements which lower the melting point produce a limited amount of brittle phases in the solidified material. During operation, these phases become more rounded and the moderate ductility of the brazed repairs increases over time.
After the repair, we open and size cooling holes through Electrical Discharge Machining (EDM) or milling. Then we flow-test the parts.
The repair process is completed by coating and final heat treatment.
After the main repair, reconditioning and coating, Sulzer Turbo Services can perform a number of additional operations:
- Shot peening is used to create compressive stresses in surfaces likely to be fatigue-prone, such as blade roots.
- Blades are usually balanced as a set by moment-weight calculation and sorting. It is highly recommended to stay as close as possible to the original sequence. This is achieved by the special design of the sorting routine.
- Guide vanes must be repaired to known and homogeneous throat openings. This is checked and documented.
- A report on the condition and life expectancy may be necessary. This requires destructive analysis of one or more components and must be started as soon as possible after receipt of the components. Since the life expectancy of the components selected for repair is rarely very low, we start the repair process in parallel with this investigation. Part of the investigation is creep testing that requires many weeks’ processing.