Aeroengine Safety

Figure "HIP process limits" (Lit. 21.2.8-3 and Lit. 21.2.8-4): Hot isostatic pressing (HIP) is a process at which parts are charged in an autoclave with gas under high pressure and material specific temperatures (Fig. "Hot isostatic press device" and volume 3, Ill. 12.5-18). Thereby closed faults to the outside (gastight) are plastically compressed. Are the surfaces of the separation/flaw sufficient clean, they fuse/sinter together. This effect can be used for the repair of parts with inner deteriorations/faults (creep).

The creep of hot parts usually develops with inner creep voids (volume 3, chapter12.5) and micro cracking. As long as no, open to the surface crack formation occures, the microscopic faults are free from oxides. They can completely disappear during the HIP process by diffusion welding/sintering. Unfortunately by far not all materials of hot parts show this type of deterioration under typocal operation conditions. Just the casting alloys of turbine blades and vanes show a significant less pronounced formation of creep voids than wrought alloys. Those can be found only at turbine blades of elder aeroengine types.

The high HIP temperature simultaneously acts as a heat treating. This can improve a material structre which has changed during operation (e.g., orientation and coarsening of the age hardening phase).

A HIP regeneration can be quite succeccessful and is applied in single cases (Fig. "HIP regeneration of turbine blades"/-3.2). For turbine blades from a forging alloy the usable lifetime can be markedly extended with an acceptaple scatter (Lit. 21.2.8-1). However it was observed, that it must be reckoned with renewed accelerated initially creep (Fig. "Regeneration proces of hot parts" and volume 3, Ill. 12.5-3), comparable with new parts.

But a HIP regeneration also retrieves potential risks:

• Faults which are open to the surface can not be closed. This must be especially considered, if hollow parts are concerned. To those belong cooled turbine blades (Fig. "HIP regeneration of turbine blades"). Here it must be reckoned with a crack formation in the cooling channels/bores by thermal fatigue (volume 3, Ill.12.6.2-9). Often this micro cracking can not be sufficient safe identified with NDT tests, suitable for series application.
• Faults which are coated with reaction products, mostly thin oxide layers, allow no diffusion welding. This means, the flaw will be compressed but its detrimental effect remains.Naturally the application of the HIP process for regeneration must be approved by the OEM and the responsible authorities after sufficient proofs, respectively testing.
  

A further application of the HIP treatment during the repair process can follow the high temperature brazing of hot parts (chapter 21.2.2) or the repair welding. This can serve the closing of sufficient oxidation free pores, cracks and separations which are sealed from the joining to the surface.

Figure "Hot isostatic press device" (Lit. 21.2.8-3 and Lit. 21.2.8-4): This sketch shows the construction of a frequent HIP press type. There are such facilities with an inner diameter in the range of 10-cm up to a meter. The adjustable inner pressures can be markedly above 1000 bar and the temperatures fare above 1000°C. Naturally they must be adjusted at the particular application. With a suitable configuration, also a following accelerated coolig can be realised. The costly facility, together with the inert gas (argon) and the energy consumption, makes the process not cheap. Therefore for every case the rentability must be checked..

The evaporation of graphite from the heating can influence accessible surfaces of the parts. If there are concerns (carbonisation), a suitable cover (e.g., gas permeable container) is necessary.

Figure "HIP regeneration of turbine blades" (Lit. 21.2.8-1 and Lit. 21.2.8-2): Concerned is a, since years successful series application of the HIP proscess for the regeneration of turbine rotorblades (sketch middle). In this case the operation lifetime is governed by a creep deterioration with the formation of voids (pores). With this forged blades are especially suited for the regeneration process. However the requirement is, that not yet unacceptable microcracks had formed in the region of the cooling air channels (detail below left). This is checked before the regeneration at six blades, equal distributed at the circumference. They will be destructive tested by metallography. Extensive experiences and investigations allow to use the creep void formation as a criterion for the regeneration ability (Fig. "Regeneration proces of hot parts" and volume 3, Ill. 12.5-7).

Figure "Regeneration proces of hot parts" (Lit. 21.2.8-1): The upper diagram shows the typical creep void formation for the case in Ill. 21.2-3.1. The dependence from the operation time can be seen. Thereby the scatter is typically large. Therefore the ability for a regeneration needs a sufficient safety margin.

The diagram below shows test results at used blades without and with HIP regeneration. Typical for the regenerated blades is a pronounced primary creep and tertiary creep. At 75% of exhausted lifetime an extension of about 50% could be achieved. This would be sufficient for a further overhaul inverval. A drop of the fatigue strength (HCF) or a markedly embrittlement did not occur, although the age hardening phase differed from the new parts condition.