When dealing with safety-relevant considerations, this issues of handling, packaging, transport, and storage of the parts during and after the finishing process are often neglected. This is a mistake, for these processes have a considerable potential for causing damage. Cases in which no further non-destructive testing is done to the parts are especially critical.
Handling of the parts during the finishing process can cause mechanical damages to highly stressed part zones (Ill. 18-1). An especially significant role in this is played by intra-facility transport using forklifts. Forklift drivers must be required to adopt certain behavioral measures that minimize the risk of damage to the parts (Ill. 18-8).
Transport in the context of shipping new parts also presents a risk for mechanical damages (Ill. 18-10).
An entirely different type of dangerous damage is corrosion cracking (Ill. 18-4). This risk can necessitate the wearing of gloves. Mechanical damages due to carelessness (Fig. "Straighrening bent parts") are also possible during the finishing process and shipping of the new part (Ill. 18-11).
Damages to parts during temporary storage (preparation) in a production process play an important role (Ill. 18-2). This primarily concerns effects from fouling (Chapter 22.214.171.124). These problems can be sufficiently safely avoided with the aid of suitable containers (Ill. 18-5). However, there are also completely different damaging influences such as exceeding the prescribed maximum allowable time spans. In this case, the material properties may chane unacceptably (embrittlement, Ills. 126.96.36.199-8 and 188.8.131.52-10 ).
Shipping also requires suitable containers or packaging to prevent damage to new parts (Ills. 18-5 and 18-9).
If damage has occurred, modern inspection methods can provide a good chance of understanding causes and making possible targeted solutions and risk assessments (Ill. 18-11).
Illustration 18-1 (Ref. 18-1): The handling of parts can result in grooves and scratches that unacceptably influence the operating behavior and/or life span of cyclically stressed parts.
Of course, not every scratch leads to a dangerous situation. If the scratch is located in a highly stressed area of the part (LCF) with an unfavorable orientation, however, it can decisively shorten the life span of the part. For example, scratches in the hub areas of disks and/or in bolt bores are dangerous. These areas seem to present themselves for provisional picking up/hanging for short transporting of the part or for submerging it into a bath (top left diagram). This can create especially dangerous axial grooves in bores. There is also a danger of axial grooves when mounting parts on equipment (Fig. "Fastening of parts during finishing"). Plane surfaces around bores are threatened by grooves of various orientations. These are created when the part is shifted during storage and transport, when force tends to be applied to these surfaces. If hard particles (e.g. blasting media, corundum) are present on the opposite surface, they will scratch the part. Especially tricky particles are those that have become stuck in a softer material (e.g. wood) and cannot be removed through sweeping, suction, or blowing.
The limited possibility of a corrosion-resistant coating in the bores of steel disks can promote corrosion scars if it is improperly stored.
If grooves have been created or noticed during handling, a specialist should be consulted to estimate their effects (Fig. "Minimizing scrap rates throuch reworking"), even if an apparently experienced colleague considers the damages to be insignificant. Only specialists, in this case represented by the responsible quality assurance department, can ensure correct assessments and provide the basis for taking appropriate steps. If a specified reworking procedure is deemed necessary and possible, it will require coordinated action in any case (Fig. "Minimizing scrap rates throuch reworking").
In order to reduce the sensitivity of parts to small (allowable) cracks, part surfaces are often preventively shot-peened. This defuses the effect of the scratches (Fig. "Effects of shot peening"). Of course, this does not mean that scratches on these parts can be safely ignored.
A few recommendations for preventing damages to parts during handling:
Illustration 18-2: The storage of parts (intermediate storage) between sequential finishing processes is a demanding task. This can be seen in the following examples.
The risk of fouling (top diagram) is increased by the following influences:
Hydrogen embrittlement during excessively long intermediate storage times (middle diagram, Ill. 184.108.40.206-14): High strength steels are especially prone to absorbing hydrogen in processing baths (Fig. "Hydrogen embrittlement by coatings" and Volume 1, Chapter 5.4.4). In order to prevent this danger, the parts must be heated to at least 180°C for several hours, beginning within two hours after hydrogen absorption (Fig. "Hydrogen absorption during etching process"). If this time window is missed, there is a risk of irreversible embrittlement.
Hydrogen embrittlement during welding in an atmosphere with water vapor (bottom diagram): If parts are stored in a cold location and then brought into a warm work room before welding, then condensation water may form (Fig. "Weld quality by cover gas"). If the parts are not sufficiently dried before welding, the water will dissociate and the resulting hydrogen will be greedily absorbed by the melt. After days and weeks, brittle cracking may occur (Ill. 18-3).
Illustration 18-3: Delayed cracking may occur a long time after critical finishing processes. By this time, the parts are in storage or have been delivered (Ills. 220.127.116.11-10, 17.3.1-12, and 18-2). Today, laboratory tests (SEM, Fig. "Scanning electron microscopy (SEM)") can sufficiently safely determine whether a mechanism involves delayed cracking, usually hydrogen embrittlement. It tends to occur on parts made from especially hard/strong steels. It is not necessarily limited to complex welded parts (page 18.104.22.168-2). Insufficiently disembrittled bolts (Fig. "Hydrogen embrittlement danger of bolts"), nuts, pipe connections (Volume 1, Ill. 22.214.171.124-1), threaded inserts, and springs (Fig. "Hydrogen absorption during etching process") can be affected.
If materials are in a sensitive structural state, air moisture or marine atmospheres can cause stress corrosion cracking (Volume 1, Ill. 3-5). One example is
One example is union nuts made from high-strength aluminum alloys in which the heat treatment evidently deviated from specifications (Volume 1, Ill. 126.96.36.199-1).
Illustration 18-4: Common salt causes stress cracking corrosion in titanium alloys above 450°C (Fig. "Stress corrosion cracking by process baths and hand sweat"). Experience has shown that even a fingerprint (hand sweat) is sufficient to cause cracking under powerful tensile residual stresses. The required temperatures are reached during welding, evidently including friction welding (Fig. "Flaws of friction welding"), and heat treatments.
As a preventative measure against this type of damage, suitable gloves (e.g. cotton gloves) should be worn when handling titanium parts.
Corrosion-sensitive steel parts can also be damaged in connection with hand sweat. In this case, however, the damage is not cracking, but watery corrosion in connection with condensation water. Threatened parts include roller bearings, etc.
At least due to the external appearance of corrosive attack (rust), but also because the bond strength of coatings (e.g. lacquers) can be compromised, handling parts with bare hands is not recommended. Especially sensitive surfaces include freshly etched or blasted steel parts and unprotected light metals (Al and Mg alloys).
Illustration 18-5: A suitable container for transport and intermediate storage in the finishing process is a prerequisite for avoiding damaging external influences (Ill. 18-9). Requirements for a container for parts include:
The lid should safely keep out contaminating foreign materials from the finishing process (Ill. 18-2) and from other sources (Ill.188.8.131.52-20).
The part should be fastened in the container. This prevents the part from shifting (danger of grooves or scratches, Ill. 18-1) and/or striking the walls if the container is struck or angled.
The container must be sufficiently tough with regard to the considerable accelerations (impacts) during transport. Meeting these requirements can necessitate considerable work in the case of heavy parts.
The container must be stackable and safely prevent sliding or tilting during lifting or transport.
Additional characteristics include good washability in reusable containers. Use of a transparent lid can provide visibility of the part.
Illustration 18-6: The storage of auxiliary materials for the finishing process can influence the occurrence of damages in various ways. This can be seen in the following examples.
In containers with metal powders (top left diagram) for solders, thermal spray coatings, powder metallurgic parts, or filled elastomer and synthetic resin coatings, vibrations can cause them to demix (Volume 2, Ill. 7.1.3-15; Ills. 15.1-17 and 184.108.40.206.2-6). Vibrations of the hall floor or transport are sufficient for this to occur. Demixing can locally change the powder properties in the container so much that the final product does not meet specifications. This includes low strength in PM parts, unsuitable melting ranges in solders, or poor abradable properties in thermally sprayed coatings.
Especially with hardenable organic media, such as the components of synthetic resins, adhesives, and lacquers systems (bottom left diagram), the maximum allowable storage time must be strictly observed. This is related to conditions such as maintaining a suitable temperature range (e.g. deep freezing of reactive resins). Temperature ranges are critical with prepregs (pre-impregnated layers of fiber-reinforced plastics), which react very sensitively to short-term temperature increases above the typical sub-zero storage temperatures. In these cases, the strength of the material itself and/or the bond strength of adhesives and coatings may be affected.
The positions and angles of containers must be strictly observed if they are marked (right diagram).
A special danger is confusion of materials. For this reason, all possibilities for this must be avoided. Even similarities between containers can promote mistakes. This must be considered when refilling media into smaller containers.
Damaged markers such as torn tags or smeared labels (top right diagram) on containers (also see Volume 1, Ill. 5.5-4), or deformed stamps or worn color markings on raw parts, can make dangerous confusion more likely. Even if a case of mistaken identity is discovered early in the finishing process, there is a danger that a large number of parts are affected, resulting in high costs and considerable time delays.
If the procurement of auxiliary materials such as hand washing pastes or protective creams (left middle diagram) is a task for the storage department, it must be ensured that the products used are explicitly approved. Otherwise, it is possible that, for example, silicon-based products could compromise penetrant testing (Fig. "Non specified materials in the finishing process") or reduce the bond strength of lacquers or coatings.
Illustration 18-7 (Ref. 18-1): Typical handling and transport damages are shown using the example of a cooled high-pressure turbine blade.
Blocking of cooling air ducts and bores (“1”): Dust and small particles can enter into unsealed openings and worsen cooling during later operation. This will disproportionately shorten part life (Volume 3, Ills. 12.5-4 and 12.6.2-5). If the particles have an aggressive effect during operation, they can cause hot gas corrosion and dangerously damage the part (Fig. "Operation behaviour of parts by surface fouling").
Spalling of brittle coatings (“2” and “3”): This concerns thermal spray coatings such as thermal barriers and labyrinth armoring. If these are nicked, they can break off or out. This will limit their function. Locally ineffective thermal barriers lead to increases in part temperature and a reduction of part life (see “1”). Spalled armor on labyrinth tips worsens the rubbing process, thereby promoting cracking and/or material buildup with overheating. This increases the risk of catastrophic failure in case of rubbing (Volume 2, Ill. 7.2.2.-4).
Damaging the rubbing surfaces of moving seals (“4”) such as thermally sprayed coatings and honeycomb seals. Seal surfaces made from sprayed coatings can break out, and the filigreed structure of honeycomb seals makes them especially sensitive to deformations caused by the application of
violent force. If this occurs and creates a leakage flow of hot gas to the mechanically highly-stressed blade root, the blade will fail prematurely.
Damage to brittle diffusion coatings (“5”): These coatings include Al diffusion coatings for oxidation protection (Fig. "Powder pack Al diffusion coatings"). Coatings around edges are especially affected by spalling and/or cracking under striking forces due to their exposed location. This will locally worsen the oxidation protection, shortening part life. In addition, sharp notches and cracks can unallowably reduce the dynamic fatigue strength. Thermal fatigue and high-frequency vibrations can lead to crack growth and part failure.
Damage to tip armor (“7”): In order to minimize clearance gaps, hard particles are soldered onto the tips of rotor blades in compressors and turbines (Volume 2, Ill. 7.1.4-14). Even if the armor does not spall, there is a danger that the brittle ceramic hard particles will lose their cutting effect. If this happens, the clearance-gap optimizing rubbing will be compromised from the very beginning of operation. The blade tips will be damaged more than expected (also see Fig. "Surface reworking influencing operation behavior i"). If this is followed by increased oxidation with clearance gap increases (Volume 2, Ill. 7.1.4-14), it can affect the efficiency of the entire engine (deterioration; typical values in Volume 2, Ill. 7.1.4-2).
Deformation of contact and fitting surfaces (“8”): If centering surfaces are deformed, it will cause problems during assembly. Cracks and grooves in the direction of the joining forces (also see Fig. "Fastening of parts during finishing") can noticeably reduce the LCF life. If higher joining forces prevent contact with provided positioning surfaces, there is a risk of settling during operation. This can result in loosening of the threaded connections and/or imbalances with extensive consequential damages.
Deformations on contact surfaces used for force transmission (e.g. on blade roots) can cause dangerous stress peaks that lead to fatigue cracking and failure.
Smeared foreign materials (“9”) are especially dangerous if they react with and damage the base material or coatings at operating temperatures (Fig. "Damaging by foreign materials on parts"). Examples include embrittlement or damage to the natural protective oxide coating, resulting in increased oxidation.
Illustration 18-8: Forklifts, which are a frequently-used transport method in the finishing process and shipping of the new parts, have a considerable potential for causing damage. For this reason, it is important to sufficientyl sensitize and motivate their operators. They should be capable of correctly evaluating any accidents and to take appropriate measures. A prerequisite for this is that the relevant properties of the parts and materials being transported are known to the operator. This includes sensitivity to transport-related overstress in filigreed structures, strict dimensional tolerances, and brittle behavior. An additional characteristic is safety-relevance (e.g. in rotor parts) with an idea of the possible consequences of damages. Finally, the value of parts is also important. It should be roughly understood (e.g. relative to the cost of a mid-size passenger car).
Containers that cannot be safely stacked should be recognized early. Precarious stacking must not be allowed.
In many cases, the external appearance will indicate possible damages caused by arranging or lifting of containers. If this is not the case, then the incident must be reported to the responsible departments. This action is certainly not always a matter of course to the person who caused the damage. It requires a suitable corporate culture (Fig. "Negative motivation"), as it may be related to personal consequences.
Vibrations of the type resulting from uneven transport paths (cross-ridges, gravel) can lead to scratching and wear on parts that come into contact with one another in unsuitable packaging. In assembled parts with roller bearings, such as pre-assembled engine modules, there is a risk of damage to the bearing races (brinelling, Ill. 18-10) if the storage/transport containers are not sufficiently damped. The result is serious damages during operation.
Another consideration is damage to the forklift itself. This primarily concerns fracturing of the fork, causing containers to fall. For this reason, the forks must be inspected for cracks at regular intervals (using X-rays or penetrant testing). Special attention must be given to the inner corner of the transition to the sled for vertical movement (middle right diagram).
An example of this was the fracture of a fork arm which caused a provisionally-packaged fighter aircraft engine to fall from a height of about 1 meter. Understandably, this incident resulted in extensive and extremely expensive inspections and (dis)assembly work, even if no actual damage occurred.
Illustration 18-9: Storing and transporting finished parts and part groups can necessitate elaborate packages and containers (top diagram) in order to prevent damages through the various potential external influences (bottom frame). This means that, at least for the adaption to specific parts, packaging becomes a specialized task that requires an understanding of the part characteristics and safety demands.
Mechanical packaging characteristics: This includes primarily the influence of external forces during transport, stacking, and handling (Ill. 18-8). The demands on the packaging increase along with the sensitivity and weight of the parts. Not only the outer shell is important, but also the bedding for the part inside, using materials such as foam cutouts, foam packing pellets, or paper sructures. In addition to work/costs, environmental considerations (waste) must also be taken into account.
Corrosion is usually the result of packaging that does not seal properly in the atmosphere in question, thereby allowing air moisture to enter. If conservation is not sufficient, condensation water can act in combination with marine atmospheres or aggressive industrial atmospheres to cause corrosion damage. Races in roller bearings are especially sensitive to this type of situation. These problems should be avoidable through the use of sufficiently sealing packages, suitable conserving of the parts, and desiccants (silica gel) in the packages.
If containers are not sealed properly, large temperature differences, such as between day (sunshine) and night (e.g. in desert regions) and/or during air transport, can cause air exchanges with the surrounding atmosphere, thereby intensifying the inflow of moisture and/or fine dust particles. If these conditions are present over longer periods (e.g. months), elastomers (seals, coatings) and synthetic resins (adhesives) can age (embrittlement, strength losses) and/or lose their bond strength. It is also possible that, over extended storage times, media such as unsuitable conserving oil may be unacceptably altered.
A similar effect can also be seen in combination with air pressure fluctuations. These can be expected, for example, when changing altitudes, such as during transport over high passes or in an aircraft. It is also possible that components of a liquid media could evaporate more intensively in this situation.
Illustration 18-10 (Ref. 18-1): Transport damages are a special aggravation for the sender, receiver, and the transporter. If they are not recognized, they can unacceptably influence the operating behavior of a part. In this case, there is a potential safety risk in addition to time delays and increased costs.
Some transport damages can be prevented by the sender. Engine parts, especially, are usually very expensive. They are high-precision, sometimes filigreed, or corrosion-sensitive. Packaging must take these properties, as well as the expected transport stresses, into account. If the transported parts are modules with roller bearings, vibrations (e.g. from an uneven road) or repeated shocks (e.g. rail joints, shunting shocks) can lead to wear damages on the roller bearing races (brinelling, also see Fig. "Dynamic fatigue cracks by ultrasonoc cleaning"). Naturally, the packaging costs must be reasonable relative to the value of the part. Shocks should be kept especially from edges, moisture should not be allowed inside, and stresses in a stack need to be considered. Inside the packaging, parts must not come into contact with another in order to prevent dangerous notches or the breaking out of brittle coatings (e.g. ceramic thermal barriers). In order to prevent wear and dynamic fatigue strength losses, the parts must not rub against one another during vibrations. Therefore, the packaging must securely fasten the parts. It has been revealed, for example, that honeycomb seals with their typical thin sheet metal ridges can be especially easily damaged at the edges. This could occur due to transport shocks on parts packed in plastic boxes.
The contact surfaces of the packaging must be so soft and flexible that they do not damage parts. Protruding metal parts such as nails are not acceptable.
If, in spite of good packing, a part arrives at the receiver`s location with damages, one may suspect that the transport was unusually rough. How can this suspicion be verified? Modern laboratory methods, especially SEMs (Fig. "Scanning electron microscopy (SEM)"), make it possible to understand the causes of transport damage at an acceptable cost level. This inspection method can reveal the geometry of the deformation, as well as permitting reconstruction of the deformation process, perhaps with the aid of a chemical analysis of wear products or smeared materials.
For example, the receiver of a shipment suspected that a protruding nail in the transport box was the cause of transport damage. The damage in this case was to the outer diameter of a compressor disk. However, laboratory tests were able to detect typical cement wall residue in the damaged area. This proved that the part had struck a wall. Apparently, either the packaging failed under extreme impact loads, or the part struck the wall after being removed from the packaging.
In another case, plastic packaging residue was found in bent honeycomb cells. This indicated that the plastic used for packaging was too hard.
The color of packaging can also be a cause of expensive damages. For example, there was a case in which the yellow aramid containment bandage of a smaller fan engine was accidentally sliced open with a knife. The personnel had mistaken this important engine component for a common yellow packing material.
Several recommendations for preventing part damage during handling:
Illustration 18-11: The prerequisite for targeted early prevention of transport and handling damages (Fig. "Straighrening bent parts") is the determination of the causes of cases that have already occurred. If the damages are safety-related and/or expensive, a problem analysis is recommended (Ill. 17-11). Experienc has shown that the most important process in determining the necessary facts is SEM inspection (Fig. "Scanning electron microscopy (SEM)"). It can be used to document even minute damages, recognize geometric anomalies, as well as trace their causes. The composition of any smeared material or impressed particles can also be analyzed. Due to the specimen size limits dictated by the specimen compartment of the SEM, it may be necessary to make impressions of the damaged area (Fig. "Non destructive microscopic inspection"). Naturally, this proscribes chemical analyses, unless particles remain stuck in the impression.
The geometric shape of the damage provides important information regarding the shape of the part that struck it. This makes possible identification of nails, bolts, other parts, or the packaging.
The chemical analysis of smeared material and particles makes it possible to draw conclusions regarding their type and origin.
In the above case of a damaged turbine disk, it was possible to confirm that the damage must have occurred due to contact with a concrete wall at the location where the parts were received.
The results of SEM inspections can also be decisive for subsequent evaluation of the reusability of the parts or possible reworking (Fig. "Minimizing scrap rates throuch reworking").
18-1 A.Rossmann, “Unser Beitrag zur Qualitätssicherung”, 1998, Turboconsult.
18-2 Metals Handbook “Volume 11, Failure Analysis and Prevention”, ASM 1986, ISBN 0-87170-007-7. pages 211, 212.
18-3 I.E.Traeger, “Aircraft Gas Turbine Engine Technology”, Second Edition, Glencoe, ISBN 0.07-065158-2, 1994, page 399.