Turbine Technology: Brazing Repair

CF6-80 series engines are high bypass turbo-fan engines with two shafts, one stage of fan, three stages of LP compressor and five stages of LP turbine installed at LP shaft, 14 stages of HP compressor and two stages of HP turbine connected by HP shaft. The first stage of HP turbine is located behind the combustion chamber. HP turbine shroud support is used for supporting and maintaining high-pressure turbine shroud in place in hot gas path and operates in a high temperature condition. Front surface of HPT shroud support contacts turbine nozzle support to sustain load from turbine nozzle. Location of HPT shroud support is shown in Figure 1. HPT shroud support is made from Rene 41 or Rene 77 Nickel superalloy.

HPT shroud support stage 1 exhibits erosion, scratches, nicks, wear, dents, and cracks on cast area of parts after removal from gas turbine engine during engine overhaul. See Figure 2. It is normal for hot section component in high time operation. Engine repair manual has a welding repair for cracks, erosion, and other defects on surface of HPT shroud support. However, superalloy Rene 41 or Rene 77 is sensitive to cracking during post welding heat treatment, resulting in excessive welding cycles and thus causing significant dimensional distortion to the part.

Repair proposal and procedures
In this repair design brazing process is implemented to avoid cracks resulting from excessive welding and heat treatment cycles and thus ensures that defected part is salvaged efficiently without causing distortion or other discrepancies. The following are procedures which are used in this repair.

• Grit blast surface of HPT shroud support using aluminum oxide grit and pressurized air.
• Remove air baffle from shroud support in accordance with engine repair manual.
• Remove plasma coating or hard coat from forward face lip in accordance with engine repair manual.
• Polish or blend to clean repair surface and remove abrasive contaminants from repaired surface. Blend to remove cracks for brazing preparation.
• Fluorescent penetration inspection for machined surface for cracks.
• Cap weld seal slot break out with Inconel 718 welding filler.
• Set up and put shroud support into furnace for vacuum cleaning. Evacuate furnace to vacuum and run furnace per setting temperature profile.
• Prepare braze filler and braze alloy at certain ratio and mechanically mix powder to ensure alloy powder is mixed thoroughly. Add binder into mixture and completely mix blended powder with binder.
• Put several beads of alloy mixture over the worn or cracked area to get a buildup. Apply braze stop-off to the area adjacent to the repair alloy, seal slot, and holes to prevent hole plugging of non-reworked areas during alloy flow.
• Set up and put shroud support into furnace. Evacuate furnace to vacuum and run furnace per setting temperature profile to braze and diffuse shroud support.
• Gas fan cool and remove shroud from furnace to perform metallographic inspection. Put test sample in mount and metallographic prepare it. Section and examine samples for porosity, void, delineation, and cracks using magnifier. Ensure there are 100 percent bonding between braze alloy and substrate material.
• Machine repaired area per dimensions specified in engine repair manual.
• Blend excessive braze alloy flush with parental material.
• EDM machine seal slot forward end per engine manual.
• FPI inspection for HPT shroud support for crack after EDM machining.
• Install air baffle and do age heat treatment per engine repair manual.
• Apply thermal plasma coating on front surface of lip.

Table 1