BALANCE OF PLANT | STEAM TURBINE REPAIRS
Above left, figure 2: Shroud erosion removed Above right, figure 3: Shroud welding in progress
V It was also important to check the position of the lacing wire hole and Z lock shroud, which was also done within the gauging fixture. This data would highlight any anomalies upon goods receipt and demonstrate that each blade had been repaired within the technical specification. The following inspections were also performed prior to
the repair: magnetic particle inspection (crack detection); visual inspection (to identify any anomalies); dimensional inspection (chords, max thickness); and hardness check. Based on the initial engineering assessment, it was
agreed that a Stellite bar-nose (basic size 305 mm x 25 mm), machined to the required aerofoil geometry, would be welded to the turbine blade leading edges. It was also agreed that up to 365 mm of erosion below the bar-nose would be repaired using Jethete M190 filler material. The erosion below the bar-nose seating was dressed out, dye penetrant inspected and welded. This weld was then dressed to profile and checked using profile gauges to ensure the correct geometry had been achieved. Due to the level of erosion on the tip shroud it was
necessary to undertake an extensive repair to reclaim the geometry and ensure the blades interlocked during installation. The erosion was removed, and the shroud welded to allow reprofiling. Figure 2, above, shows the shroud erosion removed and Figure 3 the shroud welding in progress.
Once the shroud repair was complete the leading edge
was prepared in readiness for the Stellite bar-nose. Stellite 6B bar-noses were manufactured from 3/8” thick plate and individually fitted to each blade. They were welded into position using Inconel 82 filler material and polished for inspection. A dye penetrant inspection was used to check for surface defects and volumetric examination was performed by radiography. Figure 4, below, shows the Stellite bar-nose inclusion. Once all the blades had passed inspection, they had to
be stress relieved. Critical to the success of the weld repair is the development of the appropriate heat treatment process. As rotating parts suffer from cyclic stressing in a hostile environment it is essential to control the material hardness and metallurgical structure of both the components and weld filler material, with particular attention given to the materials structure within the heat affected zone (HAZ). A bespoke blade furnace (see Figure 5) is used for stress
relieving turbine blades. The design enables only the repair zone to be stress relieved, eliminating the need to apply any heat to the blade root. Numerous trials were conducted to calculate the correct positioning of the blades within the furnace to ensure sufficient heat deposition was maintained throughout the
Above, figure 4: Stellite bar-nose inclusion 28 | September 2022 |
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