21.2.5 Regenerative shot peening
The shot peenimng process (Lit. 21.2.5-1 up to Lit. 21.2.5-4) is in detail discussed in volume 4, chapter 18.104.22.168. Therefore this chapter is confined to repair specific aspectes. The effect which increases the fatigue strength (strain hardening, internal compression stresses ) of shot peened surfaces will relieve during operation by creep. Therefore on periodic time intervals during oberhaul once more must be shot peened (Regeneration, Fig. "Shot peening during overhaul and repair"). Thereby at first the question arises, which operation caused deteriorations prevent a successful regeneration? Such damages like fretting, must be expected especially at contact surfaces of the rotorblades (diskslots and blade roots, volume 2, Ill. 6.2-6). Limits of deterioration like micro cracking or wear are specified for the parts in the overhaul manuals under consideration of the operation conditions.
Also repairs by welding or brazing as well as heat treatments can require an additional shot peening. Thereby creep effects and possibly introduced harmful internal tensile stresses should be eased. Concerned are consequences of process caused heating/cooling or shrinking during the solidifying of a melt. This is especially true also for repairs at fan blades (Fig. "FOD welding repair of blades").
Is the shot peening process not carried out as specified, it can come instead of an improvement, to a deterioration with a drop in fatigue strength (Fig. "Process parameters of shot peening").
Figure "Shot peening during overhaul and repair" (volume 4, chapter 22.214.171.124): Just the versatile and frequent application of shot peening of new parts is a reason for this process during overhaul and repair. Reason is the relaxation by creep of the shot peened zone. This leads to the reduction of the desired shot peening effects as strain hardening and internal compression stresses. The operation temperatures, at which this effect prevents the use of shot peening are depending from the hot strength respectively the creep strength of the peened material (volume 3, Ill. 12.5-1). The overhaul time interval of an aeroengine respectively a module is attuned with the time period after which the shot peened surfaces need a regeneration. This is especially applied to the contact surfaces at the roots of compressor blades and the disk slots (sketch middle). But also the roots of turbine blades as well as the related disks need a periodic „refresh“ with shot peening Kugelstrahlen (sketch below).
Even at components with sufficient low operation temperatures, where no relaxation occurs, again shot peening can be necessary. Must shotpeened surfaces be removed by machining, for example because of wear (sketch above right), a new shot peening is necessary. This can count also for etched surfaces. Thereby the peening can be applied on the base material or if necessary on a galvanic repair plating (!), depending from the specification (volume 4, Ill. 126.96.36.199.3-2). The same is also valid for parts with peened surfaces, which have been etched during overhaul.
Do casings from light metal casting leak because of the removing of coatings (e.g., stripping, Fig. "Problems by thr removal of coatings"), in some cases a sealing by peening is possible (sketch above left). For this, very large balls (e.g., about 5 mm) of the same alloy type are used.
Fig. "Process parameters of shot peening" (Lit. 21.2.5-6 and Lit. 21.2.5-7): The shot peening during repair respectively regeneration (Fig. "Problems by thr removal of coatings") rather differs at the first glance not from that of the new parts manufacuring. Anyway the series process during new parts production, optimized for a certain part, with devices and machines may have advantages, compared with the peening process in the repair shop. It can not be supposed, that that during repair always enough parts are present, to guarantee a continuity of parameters and personnel comparable with the new parts production. Peening for a strain hardening of reworked damages may rather be case specific and can demand appropriate changes in the adjustment of the machines. Are in the repair shop successive different parts and part zones peened, the parameters and devices must be changed, which may increase the risk potential.
To achieve the desired shot peening effect, that means a fatigue strength as high as possible optimal peening conditions must be met (volume 4, chapter 188.8.131.52). Does this not happen, the fatigue strength can drop dangerously. This show the two displayed examples markedly (frame right = volume 4, Ill. 184.108.40.206-14, frame left = Ill. 220.127.116.11-15). A special problem, are unsuitable peening angles. There are several negative effects of angular peening (frame belowe), which can not only influence the fatigue strength negative:
- Too low strain hardening prevents the desired increase of the strength and the depth effect.
- Induced internal compression stresses are too low and act not deep enough.- During peening form notches and folds (peened surface extrusion folds = PSEFs).
- Remove of material of the part.
- Increases smearing of the ball/shot material on the parts survace (danger of corrosion).
Besides the angular peening there are many further possibilities for failures of a peening process. They are discussed in volume 4, chapter 18.104.22.168.
21.2.5-1 P.Adam, „Fertigungsverfahren von Turboflugtriebwerken”, Birkhäuser Verlag, 1998, ISBN 3-7643-5971-4, page 105-112.
21.2.5-2 ASM „Metals Handbook“, „Volume 5 - Surface engineering”, ISBN 0-87170-377-7, 1999, „Shot Peening“ Seite 126-135, „Abrasive Cleaning” page 55-66, 781.
21.2.5-3 J.Horowitz, „Das `Shot-peening'-Verfahren“, Zeitschrift „Metalloberfläche” 32 (1978) 7, page 285-292.
21.2.5-4 Veröffentlichung der Fa. Metals Improvement Company, „Shot Peening“, page 3-47
21.2.5-17 Australian Transportation Safety Board, Aviation Safety Investigation Report 200205780, „In-flight uncontained engine failure and air turn-back, Boeing 767-219ER, ZK-NBC” page 1-43.
21.2.5-18 J.Hall, „ Safety Recommendation“, National Transportation Safety Board, page 1-8.