Braze Repair of Industrial Gas TurbinesKevin McCurdy - Bodycoat Inc.
Traditionally large industrial gas turbines have been repaired by welding. Welding generally leads to high distortion. With the use of newer (higher temperature and higher efficiency) materials, many turbines are simply no longer weldable. OEMs are using Aerospace materials and most weld repair shops simply cannot work with these.
Repairing a turbine is generally much much less expensive that replacement, but needs to be accomplished quickly as the down time costs for a turbine are very high.
What are the sort of defects that need to be repaired?
Thermal Fatigue
Craze Cracks
Oxidation
Corrosion
Erosion
FOD (Flying Object Damage)
There is also a need to worry about wear when determining if a component can if fact be repaired at all.
The repairable components of the stationary turbine are nozzles (non-rotating) and blades (rotating). Frequently these have coatings (thermal barrier or wear resistant) that also must be repaired.
The typical materials used in the stationary turbines are:
FSX-414 (Cobalt based) and older material which can be repair welded
GTD-111 (Nickel Based) material which is welded with great difficulty
Rene N4 (Single Crystal) material which cannot be welded and is difficult to braze
The process of a Braze Repair:
Inspection - determine if the item is repairable
(most turbines can be repaired a couple times a 30,000 hr intervals).
Remove Coatings - Blast and chemical processes
Dimensional Corrections - Straighten, machine, HIP, etc.
Cleaning - (the MOST CRITICAL step for the braze)
Vacuum clean, H(sub)2(/sub) Clean, Vacuum H(sub)2(/sub) clean, Fluoride Ion Clean, Water Jet (experimental).
Apply Braze Materials
Vacuum braze with adequate diffusion time.
Dimension checks
Inspection
Repairable turbines have lots of cracks. Fluoride ion cleaning is quite but very costly and difficult to find set-ups to handle large turbines. It is also not environmentally friendly. Water-jet cleaning holds some promise. It exposes and widens cracks, but first round testing found it to not clean the full depth of cracks, leaving unbrazable areas.
The application of braze alloy can be by slurry, tape, rope, preforms, and spread inserts.
Braze diffusion cycles are typically long (up to 30 hours for some turbines) with the temperature determined by the material. The long diffusion is important to get the melt inhibitors of the alloys to be diffused.
After brazing there must be dimensional restoration (usually regeneration of holes). This is followed by extensive non-destructive testing (Ultrasonic for wall thicknesses, and visual for braze).
Parts passing inspection then must have the appropriate coating re-applied. These can be aluminized coatings. Thermal Spray, air of low pressure Plasma, HVOF Spray are all method that have been employed.
Final inspection of the turbine consists of air-flow testing and pressure tests.
Bodycoat is involved with all aspects of stationary turbine repair except the Fluoride-ion cleaning process which they must out-source.
There are some materials being used in turbines, that are so new, that none have ever been repaired yet. It will be a challenge to develop the right techniques as these require repair.