Page 278 - The Jet Engine

Overhaul

these alloys, which have to withstand high stress
loadings in service, are often welded in a bag or
plastic dome that is purged by an inert gas before
welding commences.

34. More advanced materials and constructions
may have to be welded by electron-beam welding.
This method not only enables dissimilar metals to be
welded, but also complete sections of the more
advanced fabricated constructions, e.g. a section of
a fabricated rotor drum, to be replaced at a low
percentage cost of a new drum.

35. Some repair methods, such as welding, may
affect the properties of the materials and, to restore
the materials to a satisfactory condition, it may be
necessary to heat treat the parts to remove the
stresses, reduce the hardness of the weld area or
restore the strength of the material in the heat
Fig. 25-4 Cracks revealed by magnetic affected area, Heat treatment techniques are also
crack detection. used for removing distortions after welding. The parts
are heated to a temperature sufficient to remove the
stresses and, during the heat treatment process,
fixtures are often used to ensure the parts maintain
30. When the requirement for a detailed inspection their correct configuration.
on a component such as a turbine disc is necessary,
etching of the disc surfaces would be followed by 36. Electro-plating methods are also widely used for
binocular inspection of the blade retention areas. The repair purposes and these range from chromium
whole disc would then be subjected to magnetic plating, which can be used to provide a very hard
crack test, followed by re-inspection of the disc surface, to thin coatings of copper or silver plating,
including a further binocular inspection of the blade which can be applied to such areas as bearing
retention areas. locations on a shaft to restore a fitting diameter that
is only slightly worn.
Repair
31. To ensure that costs are maintained at the
lowest possible level, a wide variety of techniques 37. Many repairs are effected by machining
are used to repair engine parts to make them suitable diameters and/or faces to undersize dimensions or
for further service. Welding, the fitting of interference bores to oversize dimensions and then fitting shims,
sleeves or liners, machining and electro-plating are liners or metal spraying coatings of wear resistant
some of the techniques employed during repair. material. The effected surfaces are then restored to
their original dimensions by machining or grinding.
32. The welding techniques detailed in Part 22 are
extensively used and range from welding of cracks 38. The inspection of parts after they have been
by inert gas welding to the renewing of sections of repaired consists mainly of a penetrant or magnetic
flame tubes and jet pipes by electric resistance inspection. However, further inspection may be
welding. required on parts that have been extensively
repaired and this may involve pressure testing or X-
33. On some materials now being used for gas ray inspection of welded areas.
turbine engine parts, different techniques may have
to be employed. An example of this is the high 39. Re-balancing of the main rotating assembly will
strength titanium alloys which suffer from brittle be necessary during overhaul, even though all the
welds if they are allowed to become contaminated by original parts may be refitted, and this is done as
oxygen during the cooling period. Parts made in described in para 40.

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