22.214.171.124.1 Shot Peening
Shot peening is a defined and reproducible machining process with (virtually) no material removal. The surface being machined is plastically deformed when it is struck by the particles, which are usually spherical in shape. The two types of peening are wet peening and dry peening, depending on whether water or air is used to generate the particle stream. Shot peening should not be confused with abrasive blasting techniques such as sandblasting or oxide blasting (blasting media: Al2O3), which are used to clean and remove surface material through erosion.
Shot peening is often the only possibility of achieving a significant increase in dynamic strength without unallowably altering the part. For this reason, it is used especially as an after-treatment for serially-produced parts with dynamic fatigue. The effect of shot peening is usually not part of a strength design. In addition to the positive influence on dynamic strength, shot peening creates a certain immunization of the surface against small damages that do not penetrate the compressive stress zone. In this context, shot peening has been used as a safeguarding mechanism. In the future, however, it can be expected that the shot peening effect will be included in the design of parts. This means that the life span and overall safety of the engine will depend on the shot peening results. Unfortunately, shot peening can only be verified through process controls. This means that undetected deviations take on a new safety-relevant dimension. This will make the testing of shot-peened surfaces and the monitoring of the shot peening process much more demanding.
Shot peening has proven itself in a large number of engine parts. It is frequently used only when dynamic fatigue damages occur during testing or serial operation and other measures are no longer possible with acceptable costs and work. Examples of applications (Fig. "Benefits of compressive residual stresses") include:
- Compressor blades (blade and root) in order to improve the dynamic strength in the blade. Improvements in dynamic strength at the root contact surfaces in case of fretting, especially in titanium alloys. Improving the corrosion protection effect (cathodic protection) of Al-ceramic lacquers (Fig. "Problems of adhesive systems", “L”) on parts made from heat-treated steel (13% Cr steel).
- Compressor disks: at the contact surfaces of the blades, and in zones under special LCF stress, such as around bores.
- Turbine disks: in areas under especially high LCF and HCF stress, such as around the inner bore.
- Turbine blades: to improve dynamic strength. In this case, fine grain can be created through recrystallization in blade zones under high dynamic strength through a combination of shot peening (deformation) and suitable heat treatment (Fig. "Increasing fatigue strength by shot peening"). However, an application of this possibility has not been reported. It is also possible to increase the dynamic strength of fir tree roots.
- Turbine stators: reworking to deform the outlet edges in order to optimize through-flow.
- Toothed gears, especially case-hardened gears: to improve dynamic tooth strength and improve the gliding properties of the tooth surfaces (reservoir effect). In this case, tribological advantages of the calotte structure of the shot-peened surface are used.
- For closing open porosity to seal oil-carrying housings (gearboxes, bearing casings) made of light metal. This process uses large-diameter shot made from the same material type.
- Parts that are already in serial production when they experience fatigue fractures.
In the following, some technical terms from the field of particle blasting are defined:
Beam intensity is the effect the impacting peening media has on the work surface. It is determined by the kinetic energy transferred from the peening media to the surface. The beam intensity is measured using Almen strips, and given as “Almen intensity” (Fig. "Blasting (Almen) intensity"). The measurement is made using the curvature of a standardized steel strip that is blasted on one side. Depending on the thickness of the Almen strip, Almen intensity is categorized as N, A, or C.
Coverage ratio (overlap ratio, surface overlap)
The proportion of the surface that is covered by impact craters from the peening media is referred to as the coverage ratio (Fig. "Optimizing shot peening process"). In practice, a coverage ratio of over 100% refers to a situation in which a correspondingly longer time than the saturation peening time is used.
Saturation (coverage ratio around 100%) is attained when an increase in peening time will no longer result in a noticeable increase in intensity (less than 10% of total intensity). This is usually indicated by a sharp drop in the intensity/time relationship (Fig. "Blasting (Almen) intensity").
Stress peening, strain peening
During stress peening, the parts are mechanically stressed in the manner as they would be by the expected later operating loads (one-directional). This results in an additional increase in the compression residual stresses and dynamic strength. This process is used especially on springs and shafts.
Peening processes and peening machines
Depending on the transport media for the peening particles, peening processes are divided into wet peening and dry peening. Wet peening usually reaches somewhat lower intensities (damping by a film of liquid) than a comparable dry peening process. It can only be done with corrosion-resistant peening media (e.g. glass or ceramic beads/shot). Advantages include its non-explosiveness (light metal wear products!) and lack of dust. Depending on the principle of peening media acceleration, peening machines can be categorized as air pressure machines, injector machines, and centrifugal machines.
Figure "Effects of shot peening": The many influences related to shot peening (see descriptions in Fig. "Blasting (Almen) intensity") can have very different effects on the finishing process and later operating behavior. As one can see, there is a complex network of effects and influences. Their reciprocal influence is not necessarily positive.
Influences on the finishing process:
Positive: Immunization against stress corrosion cracking (SCC, Volume 1, Ill. 126.96.36.199-4; Fig. "Dangers of residual stresses") and intergranular corrosion (IGC, Fig. "Preventing corrosion cracking by shot peening,", Ref. 188.8.131.52-5) means a considerable risk reduction for the finishing process. It prevents dangerous, material-specific, crack-initiating corrosive attacks that can be initiated by etching baths, coating baths, and corrosive fouling.
Raising and securing the dynamic fatigue strength is an especially outstanding property of shot peening. Deviations within the limits of the specified process parameters during machining processes such as grinding (Fig. "Fjnishing caused grinding cracks"), milling, and turning can be evened out by shot peening. This creates a protective effect for scratches and damages whose influence does not penetrate the compressive stress zone.
Compressive stresses can also be used in a targeted manner, such as to straighten thin-walled parts. One example is suppressing warping in a longitudinally split compressor housing made from a titanium alloy during the first few hours of operation (Volume 2, Ill. 7.1.3-20). The residual stresses created by milling are countered by compressive residual stresses from an optimized shot peening process.
Compressive stresses can also be used to test adhesive strength. In areas with poor adhesion of thin ductile coatings, such as galvanic silver plating, blistering can occur (Fig. "Shot peening as testing method").
Local differences in surface hardness are revealed following shot peening, since the deformations will be different than in the surrounding material (Fig. "Shot peening as testing method").
Negative: Shot peening can plastically close cracks (Fig. "Limiting influences non-destructive testing"), compromising penetrant testing.
If deformation occurs during shot peening in parts with very tight tolerances, it may not be possible to rework the peened surface if this would result in a considerable breakdown of compressive stresses and hardening.
Freshly peened metal surfaces can be highly reactive. If smeared depositing of the metallic shot material (steel) also occured, corrosion may occur even in otherwise corrosion-resistant materials (stainless steel, titanium alloys).
Wear products from blasting particles and embedded peening particle fragments (loading effect, e.g. glass beads, Fig. "Optimizing shot peening process") can compromise coating processes such as the diffusion-coating of hot parts (Fig. "Oxidation protecting Al diffusion coating problems"). Fe wear products are also responsible for welding flaws in titanium alloys (see Fig. "Fouling of Titanium welds"). Naturally, plastic deformation of the surface can cause thin, brittle coatings (e.g. Al diffusion coatings, ceramic coatings) to spall. However, metallic coatings such as galvanic Ni or Cr coatings are not threatened (Fig. "Increasing fatigue strength by shot peening"). Thin-walled parts can be unallowably plastically deformed during shot peening (Fig. "Shot peening undesirable effects i"). This includes the formation of burrs (elephant tail), which can dangerously lower the dynamic fatigue strength (Fig. "Shot peening undesirable effects i" and Chapter 184.108.40.206).
Influences on the operating behavior:
Positive: If flaws do not act as deeply as the compressive stress zone of the shot peening (Fig. "Blasting (Almen) intensity"), they are “defused” and dynamic crack formation is prevented. Some parts, especially rotor components in modern engines, are under extremely high cyclical loads. Therefore, even minor unnoticed notches such as scratches (e.g. from handling during assembly and maintenance), unusual machining grooves (Fig. "Fatigue strength and work or machining direction"), and sparks (Fig. "Dynamic fatigue lowered by electric arcs") can lead to catastrophic failure of the part during operation.
Compressive stresses, hardening, and probably also the (material-specific) callotte structure (topographic) typical of shot peened surfaces prevent extreme reductions in dynamic fatigue strength (Fig. "Fretting damage loweing by shot peening"). This is especially true of titanium alloys being worn by micro-movements (fretting, Volume 2, Ill. 6.1-19).
Compressive stresses are a protection against stress corrosion cracking. They can protect parts with sensitive structural conditions (Volume 1, Ills. 220.127.116.11-4 and 18.104.22.168-16) from failure during operation.
Aluminum-pigmented inorganic lacquer systems used on steels, such as the 13% Cr steels used in compressor blades of older engine types, are made electrically conductive by shot peening. This improves their cathodic protective effect considerably.
Negative: Under especially unfavorable conditions, shot peening can also have a negative effect on the operating behavior of parts. Wear products from the peening shot deposited on corrosion-resistant materials such as titanium alloys and high-alloys steels can cause corrosion (Ill. 22.214.171.124-2). In the case of thick cross-sections under bending loads, the flatter stress gradients cause compressive stresses to lose their protective effectiveness relative to flaws lying directly below the surface (Fig. "Influences at fatigue strength by shot peening"). If the cross-section is under even tension loads, such as rotating ring-shaped racks under tangential stress (Fig. "Problems by compressive residual stresses"), it is even possible that the dynamic strength (LCF) may be lowered.
Figure "Blasting (Almen) intensity": The effect of shot peening is indirectly specified (Fig. "Common Almen intensities") and monitored (Refs. 126.96.36.199-1 and 188.8.131.52-5) by the Almen intensity. The Almen intensity primarily serves as a process control (Fig. "Shot peening parameters"). It is a characteristic value for the entire shot peening effect, and dependent on many factors (Fig. "Shot peening parameters"). In the bottom left diagram, one can see a generally linear relationship between the Almen intensity and the depth of the induced compressive stresses. In contrast, the strength of the induced compressive stresses does not have a simple relationship to the Almen intensity (Fig. "Shot peening parameters"). In order to determine this, a defined metal plate is peened on one side, as shown in the top frame. After the plate is removed from the testing block, the induced compressive stresses will cause the plate to bend, and this flexure can be measured. This principle is used in the determination of optimal process parameters during development, as well as during serial production. The testing procedure is invitingly simple, but certain problems can be expected. Most of all, the Almen measurement can only indicate the peening intensity on the metal plate, not the entire part surface. Examples of parts where this is relevant are blade slots in disks or fir tree roots on turbine blades.
The peening time is usually not sufficient for saturation. This is attained when the intensity does not increase by more than 10% of the entire value, regardless of the duration of peening (top diagram in the right frame). In this range, the intensity curve flattens considerably over the peening time. At saturation, coverage reaches about 100%. Often, in order to ensure the required intensity in every case, coverage of more than 100% is prescribed. This requires a corresponding multiple of the peening time required for saturation. The increase in the intensity, i.e. the bending of the thin metal plate over the peening time, often progresses in a different manner to the increase in the depth and levels of the induced compressive residual stresses (left diagram). The pattern of the compressive residual stresses in the part shows an especially pronounced saturation behavior. Depending on the shot material and base material, considerably different mean compressive residual stresses can be achieved at saturation using the same depth of compressive residual stresses and same Almen intensity.
It is important to note that the peening intensity must not lead to overlapping (PSEF, Fig. "Peened surface extrusion folds"). This can be seen as a characteristic of overpeening, and can compromise the dynamic fatigue strength.
The diameter of the shot particles (grain diameter) given in the specifications is usually a mean value. Depending on quality, a certain percentage of shot particles will be larger and smaller than this. Therefore, the grain size distribution is determined in a sieve analysis. The tightest possible distribution of diameters is desirable. The grain diameter influences the attainable saturation intensity, which generally increases along with the diameter and therefore the particle energy (bottom frame). Interestingly, it is possible that grains with larger diameters will induce lower residual stresses than smaller particles. It appears that the influence of the grain diameter increases in softer materials. The scattering of the grain diameters has an important influence on the increase of the intensity with the peening time (bottom frame). With minor scattering, the increase of intensity over peening time will be virtually linear to saturation. A shot material with widely varying grain diameters, on the other hand, will result in a flat, bow-shaped increase in the saturation curve. Only after a relatively long time will the saturation limit be reached. This means that the peening machine and the shot material are not being used optimally.
Figure "Shot peening parameters": During shot peening, over a specified unit of time, a certain amount of (usually) spherical particles are blasted onto the surface being treated. This is done with the aid of a shot blasting machine. The shot is conveyed mechanically with a blasting wheel or with compressed air (Ref. 184.108.40.206-1). The particle beam strikes the part surface at an angle at a specified distance from the nozzle. The surface is plastically deformed in the impact zone of the peening particle. This creates the following desired effects (Fig. "Effects of shot peening"):
- Hardening of a surface zone to a depth of several tenths of a millimeter.
- Creation of residual stress conditions with compressive stresses in the surface zone. These are many times greater than the tension and shear stresses in the layers below.
- Prevention of dangerous tensile stress peaks in notches (e.g. in blade slots and fir tree roots).
- Evening of the surface conditions. This is true of both the mechanical properties as well as the roughness and its shape.
- Creation of a special, usually calotte-like surface topography (Fig. "Fretting damage loweing by shot peening").
The hardening and induced compressive residual stresses (Fig. "Increasing fatigue strength by shot peening") usually increase the dynamic fatigue resistance considerably. Depending on the materials, the proportion of hardening and compressive stresses can vary considerably (Ref. 220.127.116.11-3). Compressive residual stresses increase dynamic fatigue resistance by overlaying with dynamic operating loads, reducing the mean stress (Ills. 18.104.22.168-5 and 22.214.171.124-4 ).
The degree to which the dynamic strength can be increased through shot peening also depends on the stress gradients near the part surface under dynamic loads (Fig. "Influences at fatigue strength by shot peening").
Interestingly, in most materials, the highest dynamic fatigue resistance is attained with a moderate blasting intensity. This intensity must be sufficiently below the values of overpeening, which is indicated by overlapping (PSEF, Fig. "Peened surface extrusion folds" ).
Experiences indicate that, in some cases, the calotte-like surface texture apparently has a positive effect on the dynamic fatigue resistance during simultaneous fretting (Fig. "Fretting damage loweing by shot peening").
In addition to raising the dynamic fatigue resistance, compressive residual stresses can also be used in other ways:
- Protection against stress corrosion cracking (Fig. "Preventing corrosion cracking by shot peening,").· Protection against intergranular corrosion (Fig. "Preventing corrosion cracking by shot peening,") .
- Improvement of the corrosion resistance by closing pores in cast parts.
- Increase in electric conductivity of Al ceramic lacquers, which improves their cathodic protective effect.
- Prevents crack growth from cracked surface layers (e.g. Cr coatings) into the base material.
Shot peening is a defined machining process, and therefore requires intensive optimization and monitoring of the process parameters. The most important parameter is the blasting intensity (Almen intensity). This is the determining factor for the part-specific optimization of the process. The constancy of the calculated process parameters must be monitored during the peening process. This is usually done using a simple method of measuring the intensity with the aid of an Almen plate (Fig. "Blasting (Almen) intensity").
The diagrams in the bottom frame provide an idea of the complexity of the relationship between Almen intensity and the compressive stress development (Ref. 126.96.36.199-4). The sequence of the saturation intensities corresponds to the depth of the induced compressive stresses, but does not
correspond to the stress levels. This difference should be even more pronounced in some parts, depending on the materials used (titanium alloys, Ni alloys, light metals, and steels).
The blasting intensity (Almen intensity) is dependent on a large number of influencing parameters:
- Shot blasting machine: Type of blasting process (injection, blasting disk, dry, wet).
- Process parameters: Working pressure, number of nozzles.
- Nozzle diameter (throughput), blasting angle (Fig. "Problems by angular shot peening"), distance between nozzle and part, blasting time in which the part is in the middle of the blasting jet.
- Speed of the part`s movement in a straight line or turning.
- Coverage (Fig. "Optimizing shot peening process")
- Blasting media: Material properties (hardness, toughness/brittleness), grain size, and grain shape.
- Quality assurance: Position of the Almen strips relative to the part, intensity, coverage, saturation.Common Almen intensities (Ref. 188.8.131.52-1) are shown in Fig. "Common Almen intensities". In the following, more specific information is given regarding important influencing factors.
Blasting particles (shot)
Blasting particles can consist of different materials, and are produced in various processes.
- Rounded steel wire sections, usually shorter than 1 mm with a hardness of about 50 HRC.
- Hard cast steel shot
- Glass and ceramic beads with a diameter of several tenths of a millimeter.The shot particles should all have similar sizes and no sharp edges, in order to prevent fatigue resistance-lowering notches (Fig. "Optimizing shot peening process").
These requirements demand high quality in shot production. For example, cast spheres with hollow cavities are highly prone to fractures with sharp edges. For this reason, even during the blasting process, constant monitoring and sorting of the shot is necessary. One kilogram of shot with a mean grain diameter of 0.3 mm contains about 107 particles and creates about 100,000 impacts during a normal blasting process.
Blasting angle (striking angle) see Fig. "Problems by angular shot peening".
The blasting distance (distance between the blasting nozzle and the part surface) is usually less than 0.5 meters, and is closer to 10 cm. The blasting distance influences the blasting intensity because the shot comes out of the nozzle in a conical stream, which means that the number of striking particles per surface area and time decreases as the blasting distance increases.
Often, not only a single nozzle, but several nozzles are used simultaneously in order to make the best possible use of the shot blasting machine. The nozzle diameter affects the shot throughput. For every grain size and shot material, there is an optimal setting for the throughput amount. When this is exceeded, the nozzle begins to stutter, i.e. the shot is discharged unevenly due to jamming particles.
Shot particle speed
Depending on the blasting process or impact conditions, the speed of the shot will be between 30 and 200 m/sec. It determines the kinetic particle energy and has a quadratic influence on the blasting effect. The shot particle speed is generally not measured directly during the blasting process. Therefore, it is not a practical parameter of the process.
Shot particles see Fig. "Blasting (Almen) intensity".
Blasting time see Fig. "Blasting (Almen) intensity"
Blasting jet movement relative to the part surface see Fig. "Optimizing shot peening process".
Preparation of the parts
The parts being shot blasted should be clean and grease-free ahead of the blasting process. Coatings such as lacquers, galvanic coatings, or brinneling must be removed beforehand, unless they are intentionally being peened, as is usually the case with Cr and Ni coatings (Fig. "Influence of Cr and Ni coatings at fatigue strength"). The lowest possible surface roughness (Ra = 1.6 mm) before blasting is desirable in order to attain an optimal peening effect. This is also true of cases in which blasting increases roughness. Bores (danger of contamination) and sensitive areas of thin-walled parts such as labyrinth tips or fine threads should be covered with suitable covers or auxiliary material. Lead sheets should not be used due to the risk of shot contamination and the possibility of lead being transferred to other parts.
After-treatment of parts
Shot peening should always be the last work step on the surface being hardened. This requirement is based on the realization that any subsequent material-removing work will have a negative effect on the relatively thin compressive stress zone (Ills. 184.108.40.206-2 and 220.127.116.11-3). If this thin zone is partially or completely removed, the desired induced compressive stresses will rapidly break down in accordance with the stress patterns in the cross-section (Fig. "Increasing fatigue strength by shot peening"). In addition, after-treatment is accompanied by the risk of heat development (e.g. during mechanical polishing or grinding) resulting in relaxation. Surface damages such as file scratches or scribing scratches must absolutely be avoided, despite a certain protective effect of the shot peening. Following shot peening, there must not be any heat treatments that exceed the maximum operating temperatures of the peened parts. Otherwise, the shot peening effect will break down excessively through relaxation (Fig. "Annealing time effect on residual stress level"). The maximum allowable/practical operating temperature (Fig. "Relaxion limits application of shot peening") depends on the high-temperature strength, i.e. creep resistance, of the base material. In steels, this is around 250 °C, while in Al and Mg alloys, this is around 130 °C. Superalloys can be heated up to several hundred 0C (some up to a maximum of 750 °C). This is the reason why shot peening is also useful for the fir tree roots of cooled high-pressure turbine rotor blades.
A special type of after-treatment that is often used is the two-stage process. This refers to two subsequent steel treatments. Blasting is done first with steel shot, and then with glass beads. This process is done especially when there is a concern that remnants of the shot material could cause corrosion or other reactions on the part surface. These reactions include element formation in Al alloys or Cr/Ni steels, liquid metal embrittlement in Ni alloys, and welding problems in titanium alloys.
All coverings need to be removed immediately after peening, and the part must be completely cleaned of shot material. Special attention must be given to hollow spaces for the cooling air flow in hot parts and bores for oil flow. After wet blasting, the parts must be dried immediately (e.g. blow-dried with compressed air). In corrosion-sensitive materials, the chemically highly active freshly peened surface must be coated with a suitable preservative.
Influence of the part material see Fig. "Relaxion limits application of shot peening"
Figure "Optimizing shot peening process": In order to attain an even blasting effect over a large part surface, it is necessary to move the particle jet over the part surface. In this way, the surface is moved under the shot jet one or several times. Often, both the blasting jet and part are moved. For example, cylindrical surfacs are peened using an axial movement of the nozzle and a rotating motion of the part. In this case, especially at high nozzle movement speeds, there is a danger that periodically peened and unpeened zones will occur. Rapid oscillating motions of the nozzle relative to slow part movments tend to result in a tile pattern. If the movement of the nozzle is slow relative to that of the part rotation, striping may occur (top diagrams). These anomalies in the spray pattern must be prevented through optimal tuning of the movements. A good spray pattern will have even, complete (100%) coverage across the entire surface. This can be verified on the part using an planimeter or, if the area is poorly accessible, it can be done using a microscope on a surface impression (bottom diagrams, Refs. 18.104.22.168-7, 22.214.171.124-3). A fully automated analysis with the aid of a computer program is possible.
Figure "Increasing fatigue strength by shot peening": The most commonly used effect of shot peening is the increase and/or assurance of dynamic fatigue strength. This effect is based on induced compressive residual stresses (top right and middle diagrams) near the surface, which reduce tensile stresses during operation. This lowers the mean stress and increases the tolerable stress amplitude (Volume 3, Ill. 12.6.1-2). An additional dynamic fatigue strength-increasing influence is a hardening of the material.
The benefits of shot peening are especially pronounced and stable over the long term under the following operating conditions:
- High numbers of fatigue load cycles, i.e. the life span is as far as possible in the HCF range, and the dynamic loads are around the endurance limit.
- Low operating temperatures in the peened engine parts
- Less hardened surface (Fig. "Relaxion limits application of shot peening").
The increase in dynamic fatigue strength is used in many parts:
- In galvanically coated parts (Fig. "Aftertreatment cracks on Cr-coating"). Shot peening before coating evidently prevents crack growth into the base material (Fig. "Aftertreatment cracks on Cr-coating"). Shot peening can be done before or after Cr or Ni coatings are applied. Contrary to expectations, shot peening is actually more effective following the application of the brittly acting coatings.
- In welds, shot peening can reduce dynamic fatigue strength-reducing tensile residual stresses (Fig. "Residual stresses of welds").· Shafts: Improves wear resistance (fretting) and the fatigue resistance of spline toothing (Volume 2, Ill. 6.2-18). The reservoir effect of the unique topography of shot-peened surfaces can be used for permanent lubricants.
Shot peening has proven especially effective at predetermined breaking points. In areas of relatively low static strength (geometrically weakened zones), high dynamic strength is required (Volume 1, Ill. 4.5-12).
- Dimensional notches in parts such as disks (accepting slots for blades) and turbine blades (fir tree roots) can be defused with the aid of shot peening. It is necessary to have sufficient accessibility and covering on the base of the slot.· Parts with high tensile residual stresses in the surface zone, such as compressor rotor blade root contact surfaces, that are simultaneously subjected to dynamic loads and fretting (Fig. "Fretting damage loweing by shot peening").
- Defusing finishing-related weak points (e.g. scratches, Fig. "Effects of shot peening") and tensile residual stresses (e.g. from grinding, Fig. "Aftertreatment cracks on Cr-coating"; bottom right diagram).
Inside cast turbine blades, the high creep resistance of coarse grain (frame, bottom diagram) is desirable.
On other hand, fine grain with high dynamic fatigue resistance is desirable at the surface area (frame, top diagram). It is thinkable that this grain distribution could be achieved through recrystallization of the surface (top left diagram). This could be done through “critical” plastic deformation of the shot-peened surface in combination with a sufficiently high annealing temperature. However, the potentially attainable increase in dynamic fatigue resitance is limited considerably (Fig. "Tolerating cracks in Ni-cast parts") by material-specific weak points (porosity, Fig. "HIP of cast parts"). Therefore, prerequisites would at least include successfully HIP treatments and the breaking down of any porous surface zones. In any case, trials and sufficient testing would be required before this method could be introduced.
Figure "Preventing corrosion cracking by shot peening,": Stress corrosion cracking (SCC) cannot occur without sufficiently high tensile stress. Therefore, the compressive stresses induced by shot peening offer protection from SCC. Typical cases in the finishing process are shown in the above diagrams (Fig. "Aftertreatment cracks on Cr-coating"). In titanium alloys, SCC occurs at temperatures above 450°C under the influence of chlorine-emitting medias such as sweat or thin reaction coatings with degreasers (“Tri”, “Per”)(Ills. 126.96.36.199-8 and 188.8.131.52-15). Toothed gears made from case-hardened steel can suffer SCC in burnishing baths (Fig. "Cracking in steels processed in burnishing baths").
The lower frame depicts the case of a non-stabilized CrNi steel in which the heat-influenced area around a weld seam has been sensitized. This case involves depletion of the protective Cr content as a result of carbide formation on the grain boundaries. This makes the grain boundaries sensitive to intergranular corrosion (IGC) under the influence of acids (right detail).
If shot peening is done before welding, the grain boundaries near the surface are disrupted by the strain hardening (shearing strain, twinning), and seed crystals for carbide formation are created in the grain. This means that there are no throughgoing grain boundaries at the surface, where they could cause Cr depletion (left detail, Ref. 184.108.40.206-5).
The available literature does not indicate the degree to which this protective effect occurs in welded areas that have been subsequently shot peened, meaning that the effect would be based solely on the interruption of grain boundaries that have already been sensitized.
Figure "Influences at fatigue strength by shot peening": The positive influence of shot peening on dynamic fatigue strength is considerably more pronounced in thin cross-sections under flexural loads than it is in thicker cross-sections or those under tensile stress.
The smaller the stress gradient (flat progression), the greater the stresses in the volume, relative to the surface. Therefore, in thick cross-sections, there are relatively higher tensile stresses below the surface than in thin cross-sections. In this case, there is a danger of fatigue cracks below the surface. For this reason, thick-walled parts and/or those under even tensile stress made from materials with volumnal flaws (casting) will not have their dynamic fatigue strength increased significantly by shot peening.
Figure "Fretting damage loweing by shot peening": Micro-relative movements lead to fretting and therefore also material-specific damaging of the surface, which results in a decrease in dynamic fatigue strength. This effect is especially pronounced in high-strength titanium alloys (top diagram). A decrease in dynamic fatigue strength to about 30% of the unaffected base material can be expected. In contrast, shot-peened contact surfaces under fretting stress have a dynamic fatigue strength of roughly 80% of the base material. For this reason, blade roots in turbines and compressors, especially those of titanium compressor blades (Volume 2, Ill. 6.1-19) are shot peened. These shot treatments must be repeated at regular intervals, dependent on the part temperature and material, in order to regenerate the decreasing protective effect (creep, Fig. "Relaxion limits application of shot peening"; wear).
Beneficial influencing of the dynamic fatigue resistance of surfaces under fretting stress through shot peening is largely due to the induced compressive residual stresses and hardening. Depending on the material, the proportion of both of these influences can be very different. However, not all observed effects on fretting-stressed contact surfaces can be satisfactorily explained by these factors.
Evidently, the topography of the contact surfaces is also important for the fretting behavior. On the one hand, the calotte-like depressions can hold solid film lubricants well, while on the other hand they can also hold friction-increasing wear products (oxides) that are carried out of the contact areas.
An additional effect may be related to the separation of local contact surfaces at the callote edges of those under high mechanical stress (bottom diagrams). Therefore, fretting damage does not occur at micro-zones and the highest stress (detail). In addition to the elasticity of the thin callote edges, a more even distribution and transfer of the friction forces can be expected. Depending on the influence of the topography, it can be assumed that the behavior of contact surfaces under fretting stress will also depend on optimization of the shot diameter and blasting angle, and not only on the Almen intensity. However, utilization of this behavior would require basic groundwork and perhaps even operation-relevant tests.
Figure "Relaxion limits application of shot peening": Shot peening can be used on all metallic materials in compressors and turbines. However, the effectiveness of the process is dependent on the material strength and the hardening behavior. This is true of the attainable hardening, the level of the compressive stresses, and their depth. They are especially relevant to increases in dynamic fatigue strength, and their utilization. Naturally, these properties must also remain in sufficient levels at higher operating temperatures. If operating temperatures are high enough to cause notable stress-reducing creep, then permanent improvement of the dynamic fatigue strength through shot peening cannot be expected. The threshold temperatures for the most common material families are shown in the top graph. Creep leads to relaxation (Fig. "Annealing time effect on residual stress level") of the compressive residual stresses, causing the shot blasting effect to break down (compare this with the section on after-treatment in the explanation of Fig. "Shot peening parameters"). For this reason, the shot blasting process must be repeated in part-specific regular intervals within the framework of overhauls (regeneration). The top graph shows the operating temperatures above which this type of creep effect can be expected to occur (Ref. 220.127.116.11-5).
In Ti alloys, especially, considerable relaxation has been observed to occur over several days at ambient temperature following shot blasting (bottom right diagram). Therefore, this occurred well below the potential operating temperatures of about 450 °C.
At 400 °C, a considerable decrease in deflection, i.e. residual stresses, can be observed. The diagram does not show an end of this relaxation during the short testing time. Even at room temperature, a reduction in the deflection of metal platelets made from the Ti alloy Ti6A14V can be confirmed. However, this decrease stabilized at a high level after roughly one day, so that sufficient compressive stresses remained.
A similar effect can also be observed in Ni-based forged alloys (bottom left diagram). It is not clear how great the influence of this effect is on the dynamic fatigue strength.
The maximum attainable compressive residual stresses and corresponding dynamic fatigue strength increases in steels have a very pronounced dependence on their strength/hardness. The potential for improvement increases with the hardness of the base material. The lower toughness of harder materials evidently does not have a negative effect in this case. For example, especially large improvements in dynamic fatigue strength can be achieved in steel springs or case-hardened parts (toothed gears).
Figure "Shot peening as testing method": Shot peening can also be used for quality testing. This is possible as a side benefit of a shot treatment that was planned anyway. This includes the non-destructive testing of parts to detect local differences in hardness. The top left diagram shows a toothed gear with case-hardening on the teeth, but not the gear membrane. In the depicted case, the coating locally penetrated the masking and caused hardening. After the shot-peening that was done to improve hardness and offer protection from burnishing cracks (Fig. "Preventing corrosion cracking by shot peening,"), the hardened areas could clearly be seen as lighter, shiny patches (top diagram).
The bond strength of relatively soft and tough silver coatings can be tested using glass bead peening (diagram). In zones with poor bond strength, induced compressive stresses lead to blister-like delamination (bottom diagram). This process is especially interesting, because there are really no serially-implementable processes for reliably detecting “kissing bonds”.
Figure "Shot peening undesirable effects i": Shot peening is a process designed to prevent damage and remove flaws. However, without proper application and execution, there are a large number of typical flaws and damages that parts can suffer due to shot peening.
- Intensity too low: Typical causes are incorrect blasting parameters, malfunctioning machines, and shot materials that alter over the peening period.When conducting tests to determine the optimal blasting parameters, poor plate positioning can be responsible for deviations in the blasting intensity relative to that used on the part. There is an additional risk that intensity deviations in serial production are not detected (in time) and affected parts are installed.
- Shot with sharp edges: The cause of sharp-edged shot may originate with the supplier. This includes sharp glass beads or hollow spheres of cast steel that tend to fracture. During the peening process, fractures can result in sharp edges on the shot, which can in turn create notches that later act as starting points for dynamic cracks during operation.
- Shot peening of unsuitable materials or material states: This includes the peening of brittle coatings that tend to spall. If single-crystal materials are shot peened or intensively abrasively blasted, the plastic deformation can initiate recrystallization at sufficiently high annealing temperatures (Fig. "Single-crystal casting-flaws").· Part overpeening: This results in crack-like overlapping, called peened surface extrusion folds (PSEF, Ills. 18.104.22.168-14 and 22.214.171.124-15), which are a possible cause of significant decreases in dynamic fatigue strength.
- Burrs at edges and corners of the part due to poor blasting conditions at sharp edges (Fig. "Operation damaging effects of burrs"): These include a unique occurrence that is descriptively named elephant tail. Burring occurs most often on parts that must be peened in areas where surfaces run together at an angle. Experience has shown that these especially dangerous damages occur at the edges of the annulus at the transition of the blade slots of rotor disks (middle frame).
- Overlapping and cracking due to angled blasting (Ills. 126.96.36.199-14 and 188.8.131.52-15).
- An unfavorable surface structure on the part before blasting can be the cause of overlapping (bending of tall roughness peaks) and unpeened notch zones between roughness peaks.
- Uneven peening pattern due to poor tuning of the movements of the part and blasting nozzle (Fig. "Optimizing shot peening process"),· Insufficient coverage (Fig. "Optimizing shot peening process") provides insufficient protection from stress corrosion cracking (Volume 1, Ill. 184.108.40.206-4; Fig. "Negative effects of residual stresses from finishing"),
- Warping of parts, especially blade profiles and thin-walled housings, during the peening process or due to subsequent relaxation. Deformation of filigreed part zones such as the edges of compressor blades (rollover, top right diagram, Fig. "Damages by cleaning blasting")
- Tin-can-lid effect: snapping of flat parts, such as thin-walled compressor disks (Fig. "Negative effects of residual stresses from finishing").
- Tensile stress increases in cross-sections below the peening zone (Fig. "Influences at fatigue strength by shot peening") can have especially negative effects in case of pure dynamic tensile stresses (small stress gradient). This increases the danger of fatigue cracks occurring beneath the surface.
* Foreign material (such as lead) can smear onto shot and then be transferred to the peening surface, where it can cause cracking (LME) during subsequent finishing processes (Fig. "Damages by metallic surface fouling from finishingfinishing") or during operation (Fig. "Damaging by foreign materials on parts").Smeared foreign materials can also cause corrosion due to element formation, either directly or through alloyed structural components. One example is Fe wear products from the shot onto Cr Ni steels.
Another example is titanium alloys that form corrosion sensitive alloys with Fe wear products in the area around weld seams (Ill. 220.127.116.11-19). This type of structural change can also promote hot cracks.
· Fouling through oil or condensation water in the blasting air flow: The water can cause corrosion during storage before the next finishing step. Oil residue can hinder subsequent processes (coating, etc.).
- Unallowably pronounced roughness on areas in the air flow, resulting from fractured shot, for example.
- Damage to threading and similar profiled and filigreed part zones, resulting from unsuitable masking methods.
- Shot residue from fractured shot particles (loading effect, Fig. "Loading effect by blasting processes") can cause damage in the oil circulation system (e.g. bearings, valves).
- Cracks and flaws are peened shut (top diagram). These flaws are difficult to detect during subsequent penetrant testing.
- Dust explosions in blasting chambers (without wet suction) due to metal wear products (Mg, Ti),· Unallowably large amount of material removal from fitting surfaces (Fig. "Material removal by the shot peening process").
One problem that is related to all blasting processes is the blocking-up of hollow spaces, i.e. shot remaining in the base material. This can lead to overheating in hot parts (also see Fig. "Abrasive blasting problems"). At high temperatures, reactive shot materials can damage the base material or coatings, or promote high-temperature corrosion. If shot material enters the fuel or oil circulation systems, there is a danger of extensive operating damages.
Figure "Material removal by the shot peening process": Even shot peening, especially with sharp-edged steel grit (right diagram), causes noticeable material removal. In contrast, the material removal during glass bead peening is in the m range and should be acceptable even in the case of tight tolerances.
It is possible that a pronounced increase in roughness on the peened surface will appear to result in growth. In this case, a certain amount of settling must be expected under contact forces during assembly or operation.
Figure "Problems by angular shot peening": For maximum blasting intensity, a blasting angle of 90 o, i.e. perpendicular impact of the peening particles, is optimal. This creates the greatest impulse. However, these conditions are not always attainable, especially when the geometric properties of the part prevent proper positioning of the blasting nozzle. The smaller the angle at which the peening media strikes the surface, the greater the undesired material removal. In addition, overlappings on a micro-scale must be expected (left diagram). These can act like cracks and endanger the safety of the part (also see Ills. 18.104.22.168-14 and 22.214.171.124-15).
The intensity decreases with the sine of the angle of impact. Virtually perpendicular impact of the peening media can be achieved in poorly accessible part surfaces in many cases through a suitable arrangement of deflectors made from hard metal (Ref. 126.96.36.199-1). The peening particles are directed onto the peening surface by a deflector surface that is positioned at an angle relative to the nozzle axis (right diagram). A typical example of this is the use of a conical bar made from tungsten carbide (WC) in order to peen bore walls. If the deflector surface is already integrated in the blasting nozzle, it creates an angled nozzle with a similar effect.
Figure "Overkill effect of shot peening": This example shows a case in which the fracture of a front compressor rotor blade led to a crash. The fracture originated in a dynamic fatigue crack in the contact surface of the blade root (bottom right diagram), which was evidently promoted by an insufficient shot peening process. A metallographic cross-section (detail) revealed signs of peened surface extrusion folds (PSEF, Ref. 188.8.131.52-6). These flaws are a sign of poor peening conditions and can compromise the dynamic fatigue strength (Fig. "Peened surface extrusion folds").
Figure "Peened surface extrusion folds" (Ref. 184.108.40.206-6): A few minutes after takeoff, the depicted engine damage occurred with uncontained fragments (top diagram).
Subsequent investigation of the affected engine revealed that, in addition to extensive penetration damage to the turbine housings, the low-pressure shaft was broken and the housings had separated axially by a few centimeters. The first stage of the high pressure turbine rotor (HPT1, middle diagram) had suffered a haircut. A large annulus segment of the disk, including about 14 blade slots, was missing (bottom left diagram). The temperature-related discolorations showed that this was evidently not consequential damage. Two additional blade slots in the remaining annulus segment had radial cracks in their bases. One had already grown roughly 25 mm into the space between the reinforcements around the bores.
The oxidation of the fracture surface revealed that the fracture-causing crack was also located at the edge of a disk slot.
The surface at the fracture origin had the typical appearance of shot peening. There were flat, overlapping-like flaws about 25 mm deep around the entire edge (also see Fig. "Overkill effect of shot peening"). These are called peened surface extrusion folds (PSEF). These flaws indicate a very high peening intensity and a peening angle that is too low. Experience has shown that the highest dynamic fatigue strengths are achieved in most materials with moderate peening intensities that do not cause any overlappings. The PSEF were the point of origin of an intergranular LCF crack typical of the disk material (forged Ni-based alloy IN 718), and this crack was interrupted by zones with signs of forced fractures. PSEF were also found in the origin points of the other cracks. The unexpectedly serious influence of PSEF can also be seen with flat gradients of the tangential stress (Fig. "Beneficial limits of compressive residual stresses"). High peening intensity, i.e. a pronounced compressive stress zone around the edge, should induce corresponding tensile residual stresses beneath the surface. These overlay with the tensile stresses from operation. If this increases the mean stress considerably, it will result in a loss of LCF strength. Earlier disk damages that were traced back to other causes showed that the disk zone in which the cracking occurred was under especially high operating stresses. This would also explain the catastrophic effects of these relatively small flaws.
Remedies for problems and damages related to shot peening
Ensuring replicable, specification-conforming shot peening procedures requires strict observation of the optimized process parameters (process stability) as well as the use of prescribed equipment and auxiliary materials (e.g. masking covers). This is especially important because the results of shot peening can not be satisfactorily evaluated on the part using non-destructive methods. This applies at least to all surfaces that influence LCF life, as well as zones under high HCF loads. Therefore, the quality of the peening must be verifiably ensured with the aid of monitoring and documentation of the peening process. These requirements become considerably stricter if the dynamic fatigue strength achieved by peening is a part of the part design, and not merely an additional safety measure (Volume 3, Ill. 14-24). The prerequisite for an optimally effective peening treatment is suitable constructive design of the parts.
In the following, several fundamental guidelines are given with no claim to completeness.
- Shot peening should be used as a reinforcing measure, not as part of the strength design.
- Optimal peening intensity should be selected (Fig. "Common Almen intensities"). If in doubt, it should be determined using part-relevant tests. Experience has shown that in most materials, moderate peening intensity results in the greatest increase in dynamic fatigue strength.
- Special care must be given when peening edges. PSEF (Ills. 220.127.116.11-14 and 18.104.22.168-15) and elephant tails (Fig. "Shot peening undesirable effects i") must be avoided under any circumstances. They occur in case of overpeening, or due to excessively low peening angles.
- Accessibility to angled surfaces (blade slots in disks), bores (Fig. "Problems by angular shot peening"), and tight radii must be guaranteed. If necessary, the peening shot diameter must be selected accordingly.
- The work sequence should be designed in such a way that the peened surfaces are not heated to excessively high temperatures. This can occur, for example, during welding, heat treatment, or the application of coatings (Fig. "Relaxion limits application of shot peening").
- Do not peen any coated surfaces that could be damaged by effects such as spalling or even minor material removal.
- If regeneration through renewed shot peening is necessary during operation/overhauls, it must be ensured that this is possible.
- Testing of the peening shot during loading and continuous control during the peening process (e.g. roundness, proportion of fractured shot) are necessary.
- Ensure continuous separation of fractured shot.
- Test the shot for fouling through corrosion tests on a peened test plate.
- Optimize and test the peening process.
- Monitor the peening process. Affix the Almen plate in a part-relevant, meaningful position.
- Inspect the peened surface through microscopic examination (REM) of synthetic impressions.
- Series-relevant testing of the peening equipment and masking devices is necessary.
- Verify the peening effect using operation-relevant part tests and monitoring engine operation.
In this context, a special focus should be the part temperatures, as they may cause a certain breakdown of the peening effect due to relaxation.
- Smoothing of peened surfaces should be avoided, even if it is done with the aid of “suitable” abrasive processes.
- There should be no sharp border around the peened area, but rather a gradual transition (Fig. "Shot peening undesirable effects i").
- Sharp edges should not be peened. Edges should be chamfered or rounded. The peened parts should be inspected for PSEF and elephant tails.
- Sensitive areas and hollow spaces susceptible to blocking should be masked with suitable covering equipment or masking material. Avoid fouling the shot with wear products from the masking/covers. Unsuitable masking material includes lead strips, since lead residue on hot parts can cause catastrophic damages (Fig. "LME fracture of titanium screw").
- The surface conditions before peening should be optimal (e.g. no corrosion or deep machining grooves).
- Different material types (e.g. Ni alloys and steels) should not be processed in the same machine with the same load of shot. There is a danger of material transfer (corrosion).
- Shot peening should preferably be the last step in the finishing process.
- After peening, the parts should be dried, and not stored without conserving measures.
- Special care must be taken if there is a possibility that flammable/explosive dusts may be created (Al, Mg, Ti). This danger is greater during abrasive blasting. If necessary, use wet blasting processes.
Figure "Common Almen intensities" (Ref. 22.214.171.124-1): As a controlled machining process, shot peening requires specifications (drawings, parameters, operation charts) regarding the Almen intensity. This is a requirement for ensuring the desired operating behavior of the part.
Peening should at least be outlined using the following parameters:
- Clear indication of the surfaces to be peened
- Almen intensity
- Peening shot material
- Shot size
- Coverage in %
- Equipment, masking, and covers, if requiredIn the case of safety-relevant parts, the extent to which these parameters must be approved by the OEM and/or responsible authorities must be determined.
Achieving the best possible peening effect requires a peening intensity that is specifically matched to the part. This intensity must not be too low, but also not too high (Ills. 126.96.36.199-14 and 188.8.131.52-15).
Determining the optimal peening parameters is done in an iterative process between part-relevant peening tests and specimen evaluation:
- The peening tests should be evaluated by inspecting the peening specimens. This includes using metallographic cross-sections, which can be used to determine the deformation and hardness patterns. * Residual stress measurements are used to determine their progression into the material, and their maximums (Ref. 184.108.40.206-1, Ills. 220.127.116.11-2 and 18.104.22.168-5.3).
- Microscopic surface analysis (REM) of the degree of coverage (Ref. 22.214.171.124-3, Fig. "Optimizing shot peening process") and the surface structure (e.g. damages due to broken shot or overpeening).If shot peening is already considered during the new design construction of a part, then suitable geometric ratios for the peening process should be incorporated in order to make peening as effective as possible. It may be necessary to specifically design the geometry of the part or to develop suitable peening equipment.