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(3) cavitation or void formation.
A considerable fraction of the intermediate temperature strength of nickel-based turbine blade alloys results from the fine .. precipitate Ni3(Al， Ti). The .. particles generally coarsen as a function of time to the one-thirdpower growth law， with a corresponding decrease in strength. Grain bound-ary carbide morphology and amount can also change with time. Since thealloy heat treatments， which cause carbide formation， are generally opti-mized for short-term properties， long-term changes in carbide structure areusually detrimental， particularly with respect to such properties as ductility and notch sensitivity. Cavitation represents the initial step in creep failure. It consists of the nucleation and growth of voids on grain boundaries. Withtime， the isolated voids link up to form cracks. It appears that blade material has to be near or in tertiary creep before cavitation can readily be detected by optical microscopy.
It is not known to what extent each of the previous mechanisms contri-butes to turbine blade degradation during service. It is also probable that each alloy will respond differently to a particular temperature.stress combi-nation. Figure 21-12 shows the typical variation in stress.rupture life deter-mined at 1350 0F (3.5 0C) with service time for forged Inconel .-.50 blades.

Figure 21-12. The variation of remaining stress rupture life at 1350 0F (735 0C) .ith service time in forged Inconel alloy X-750 turbine blades. (Courtesy ofWestinghouse ElectricCorp.， Gas Turbine Div.).
Internal service damage due to precipitate coarsening and changes in grain boundary carbides should generally be reversible with conventional heat treatment involving complete solutioning followed by controlled precipita-tion at lower temperatures. For creepvoids， it is not clear if cavitation damage can be removed by conventional heat treatment.
Normal reheat treatment can partially restore blade properties; however，it does not appear to be capable of full property recovery， although the miscrostructures are comparable to new blades. This shortcoming implies that cavitation may be present and was not removed by conventional reheat treatment. Hot isostatic press (HIP) processing is an alternative that ensures void removal. It has demonstrated its ability to remove even gross internal shrinkage porosity in investment castings.. The results of HIP treatment
. The HIP treatment is conducted in a five-zone furnace located in a steel pressure vessel thatcan be pressurized to20，000 psi with argon and capable of temperatures of 2250 0F. The blades placed inside the furnace must not be subjected to any undue loads and thus are stacked inseparate tiers. Betweencycles， the furnace temperature is maintained at 14.0 0F to obtain maximum life of the .anthal elements. The pressurizing argon is provided by two diaphragm compressors in series and can be reclaimed at the end of each cycle.

Figure 21-13. Comparison of stress rupture life at 50 ksi.1350 0F (3.5 M.a.735 0C) inservicee.posed， commerciallyreheat-treated， laboratoryreheat-treated， and HI. reheat-treated used Inconel X-750 turbine blades. (Courtesy of Westing-house ElectricCorp.， Gas Turbine Div.)
Maintenance Techniques 7.3
(Figure 21-13) clearly show that the HIP-processed material is superior to both commercial and laboratory conventional reheat-treated material. Cost estimates indicate that used blades can be rejuvenated at a fraction of the cost of a new set of blades.
.epair and .ehabilitation of Turbomachinery Foundations
In many instances， vibration problems in turbomachinery can be attrib-uted to faulty support. Once the problem areas have been identified， correct-ing defects can be a logical procedure. What is novel is that this result can often be accomplished through the proper selection and application of adhesives.
Most turbomachinery is mounted on structural steel platforms sometimes referred to as base plates or skids. These platforms are then installed on a mass of concrete at the jobsite (either by direct grouting or mounting on sole plates) to become the machinery foundation. Platforms should always be considered as part of the foundation rather than as part of the machinery.
　
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