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16.2.2.7 Cracking during the Finishing Process

This chapter deals with cracking in parts that can be causally attributed to the finishing process. What is a crack? Is it a signal in a non-destructive testing method such as penetrant testing? Is every elongated material separation a crack? How can “elongated” be defined in this context? Are additional characteristics necessary in order to define something as a crack?
Although a crack can be understood as a material separation that is due to tearing of the material, experience has shown this strict definition to be impractical. It is unavoidable that flaws such as cold welding, kissing bonds, and (dissolving) corrosive attack will be labeled as cracks as soon as they have an elongated shape, i.e. as soon as a crack is indicated. For this reason, this chapter understands cracks to be elongated separations, regardless of their cause or forming mechanism (Ill. 16.2.2.7-1.1).

Cracking presents a potential threat to many finishing processes. The creation of a crack often requires combined action of several finishing steps, and can be caused by very different damage mechanisms. Because of this, cracking is dealt with in detail in corresponding chapters of this book, which are referenced in the illustrations in this chapter. The central theme of this chapter is general considerations and effects related to cracking, and is intended to assist in drawing conclusions regarding causal influences.

There are two main influences that finishing processes have on cracking, and they can act in combination: material strength losses and the buildup of dangerous tensile stresses (e.g. thermal stresses) and tensile residual stresses.

If cracks are found during finishing, the prerequisite for targeted and sufficiently effective remedies is determination of the damage mechanism (Ill. 17-11). This means that the damage-causing and damage-promoting influences must also be understood, which makes it possible to identify crack-causing finishing steps and estimate risks. Important conclusions can be drawn on the basis of typical, structure-specific crack characteristics (Ill. 16.2.2.7-3), the crack pattern on the surface (Ill. 16.2.2.7-4), and the analysis of exposed crack surfaces (Ill. 16.2.2.7-5).

Statistics regarding flight accidents show how dangerous cracks are for part safety. Damaged or cracked parts that can be traced back to machining (Ill. 16.2.2.7-6) evidently play an important role. Cracking and its influence on part behavior is dealt with using the example of grinding cracks (Ill. 16.2.2.7-6). In addition, stress corrosion cracking and hydrogen embrittlement are treated as typical damage types that require multiple influences, such as tensile stresses, notches, and acting media (Ills. 16.2.2.7-7, 16.2.2.7-8, and 16.2.2.7-9).

Special attention must be given to the prevention of finishing-related cracking. Motivators for this include costs, prestige, and especially safety concerns. If cracks have occurred, a safe and targeted approach with effective solutions must be ensured (Ill. 16.2.2.7-11)

Illustration 16.2.2.7-1.1: At first, it seems simple to answer the question “what is a crack?” In practice, however, the answer is not clear. Usually, every elongated flaw is labeled as a crack. This is especially true in the case of penetrant testing. These “crack indications” can indicate very different flaws, such as fractures due to material overstress, or cut pores and cavities that were created without cracking.

Fractures due to cracking (top left diagram) can be traced back to overstress. Forced cracks (also creep crack, SCC, hot crack) usually occur under static loads. Dynamic fatigue cracks are the result of a fatigue process with cyclical crack growth. In all cases, the crack edges have fracture characteristics.*
Bonding flaws, folds, and overlapping (top center diagram) are not the result of cracking. This includes cold welding (15.2-2) and oxide coatings (15.2-10) on cast parts, bonding flaws in connections (soldering, welding, Ills. 16.2.1.1-8 and 16.2.1.3-38) and coatings, forging folds and lamination (Ill. 15.2-24), as well as flaws due to excessive blasting (Ill. 16.2.1.6-15).

Chemical dissolving (top right diagram): These crack-like flaws are created through increased local dissolving, usually at grain boundaries (Ills. 16.2.1.7-5 and 16.2.1.7-10). This is referred to as intergranular corrosion (IGC). These flaws usually create networks and can be the result of either corrosion or etching.

Experience has shown that specialists must deal with another question in the case of small cracks. At what point can a crack be viewed as technically relevant? One answer to this is “a crack size (or corresponding flaw size) that can be sufficiently reliably detected using serially-implementable non-destructive testing methods. This interpretation of the concept is especially important for the design of highly-stressed parts. The term “technical crack” was defined specifically for this case (middle diagram). This is a surface flaw with semi-elliptical borders and a length of 0.8 mm and depth of 0.4 mm (middle diagram).
Cracks occur when the mechanical stress levels exceed the fracture strength. Although this seems trivial, these conditions can occur in many different ways during the finishing process, and are not always obvious (bottom diagram).

A loss of strength occurs during many processes with increased temperatures. A typical example is grinding with insufficient cooling and/or overly aggressive parameters. In steels, the loss of strength is marked by a significant loss of hardness (annealing effect). If conditions for stress corrosion cracking (Ill. 16.2.1.7-8) are present, the strength will be reduced significantly. A similar effect occurs under the influence of metal melts, and is called liquid metal embrittlement (see Chapter 16.2.2.3). The more brittle a material is, the greater the cracking effect of small flaws. Hydrogen embrittlement can be mentioned in this context (Ill. 16.2.1.7-14).

Increase in stress levels: The local stresses are also strongly affected by finishing processes. Thermal stresses occur during many different processes, including welding, annealing, machining (boring, grinding, thermal spraying). Residual stresses (Ill. 16.2.2.4-5.0) can be expected after processes such as electroplating, welding, heat treatment, thermal spraying, and machining. The notch effect of stiffness jumps and flaws, especially in brittle base materials and coatings (Ill. 16.2.2.7-1.2), can dangerously increase the local stresses. If tension forces occur during finishing, they will especially
threaten thin-walled parts with brittle coatings (Ills. 16.2.2.5-11 and 16.2.2.7-1.2). There is also a danger of cracking during straightening processes that have loads in the plastic range, i.e. above the flow limit (Ills. 16.2.2.5-12 to 16.2.2.5-14).

Illustration 16.2.2.7-1.2 (Ref. 16.2.2.7-10): Al-diffusion coatings are widely and successfully used for oxidation protection on hot parts. A special application is on turbine rotor blades and guide vanes (diagram). A problem is that these coatings behave brittly from room temperature to about 400°C. The diagrams illustrate this in the low fracture strain, depending on the operating temperatures. The Al content is not evenly distributed in the coating. It is distributed in a process-specific pattern (detail) that also depends on the coating composition (Ill. 16.2.1.8.1-2). The brittleness, and therefore the sensitivity to mechanical damages (Ill. 18-5), clearly increase with the aluminum content. Unfortunately, high oxidation resistance also requires a high Al content. This can vary according to specifications or due to deviations in the coating process. This means that the parts may have different levels of sensitivity. In this case, it cannot be assumed that proven handling methods (e.g. packaging, storage) will provide certain protection from damage. If unexpected coating cracks occur, it is recommended that the Al content of the coating be tested.

In contrast, coating thickness, which is also dependent on the process parameters, will not have a great influence on brittleness, as long as the thickness is within the usual range (bottom diagram). If cracking occurs, however, the crack depth will increase along with the coating depth, which exacerbates its damaging influence.

Illustration 16.2.2.7-2: The formation of cracks in coatings, and their effects on the dynamic fatigue strength of the parts, i.e. the notch effect on the base material, are strongly dependent on the coating properties. This can easily be recognized in extreme cases:
Elastomers (e.g. rubber) have a very low modulus of elasticity, i.e. high resilience. Cracks in this type of coating will not have a significant fracture-mechanical effect on the base material. Therefore, dynamic fatigue strength losses cannot be expected. This is also true of porous spray coatings (right diagram).

The bond strength of the coating to the base material has a strong influence. The greater the bond strength, the greater the forces that can be transferred and cause crack initiation and growth in the base material. Diffusion coatings usually have a greater bond strength with high hardness and minimal toughness. This behavior can be seen in Al diffusion coatings, which are a popular oxidation protection for Ni alloys. In steels, this role is filled by nitriding and case-hardening. If these are present, they will raise the dynamic fatigue strength considerably. However, if cracking occurs, the pronounced notch effect of brittle breakouts will result in a significant loss of dynamic fatigue strength. This effect is very important in the handling and clamping of coated parts such as turbine blades during finishing (Ill. 18-5).

Micro-cracking occurs in ceramic spray coatings (thermal barriers, Ill. 16.2.1.8.2-3), where it is desirable in the form of so-called segmentation cracks. Crack fields are also typical in galvanically applied Cr coatings (middle diagram) and desirable in certain forms. The cracks minimize the usual dynamic fatigue strength losses (Ill. 16.2.1.8.3-3) in two ways, by breaking down tensile residual stresses and/or increasing the resilience of the coating.
If a coating delaminates in the crack area, it will not have an influence on dynamic fatigue strength.

In contrast, a coating with a high modulus of elasticity and high strength will reduce the dynamic fatigue strength of the part considerably if individual cracks form.
The thicker the coating, the greater the crack depth that fracture-mechanically acts upon the base material. The coating will lower the dynamic fatigue strength accordingly (left diagram, Ill. 16.2.1.8.3-3).

Illustration 16.2.2.7-3: In order to specifically prevent cracks and take retroactive measures against further cracking, the causal influences must be understood. In this way, the time and location at which cracking originated can be sufficiently narrowed. This allows conclusions regarding crack-initiating finishing processes, possible temporary anomalies, and the probability of additional affected parts.

An important characteristic is the pattern of the crack in the structure. Cracks along the grain boundaries, i.e. intergranular cracks, are distinguished from those that pass through the grain, i.e. transgranular cracks. Naturally, the crack patterns can change if there is a change in conditions, such as stress levels, temperatures, and corrosion.
Transgranular cracks are primarily found in tough materials in cases of short-term overstressing. This can occur during clamping, straightening, expanding, and machining. Examples include comma cracks in forged nickel alloys (Ill. 16.2.1.1-2).

Naturally, there are also intergranular forced cracks. They tend to occur as hot or thermal cracks (Ill. 15.1-8) at temperatures that soften the grain boundaries, and are typical for processes such as welding (Ill. 16.2.1.3-10), grinding, and heat treatments. If high temperatures are combined with
tensile stresses for extended periods, creep cracking can occur. These cracks usually run along the less creep-resistant grain boundaries (Volume 3, Ill. 12.5-7).
During hydrogen embrittlement as the result of hydrogen absorption during galvanic processes, diffusion treatments (case hardening), or etching, brittle cracks form along the grain boundaries (Volume 1, Ill. 5.4.4.1-2). Similar crack symptoms result in the micro-range during stress corrosion cracking, in which the corrosive media acts under sufficiently high tensile stress. These conditions can be present in etching baths. If halogens (chlorine, fluorine, Ill. 16.2.2.7-7) act on titanium alloys at increased temperatures, they will be especially sensitive to this damage type.

If wetting metal melts act on a surface under tensile stress, they can cause intergranular cracking called liquid metal embrittlement (LME; Ills. 16.2.2.3-10.1 and 16.2.2.3-11). Typical examples include metallic fouling (Ills. 16.2.2.3-10.2 and 16.2.2.3-11) such as dripping solder residue during heat treatments (Ill. 16.2.2.3-5).
A special case is crack-like grain boundary attack through corrosion or etching without tensile stresses. This is called intergranular attack (IGC, Ill. 16.2.1.2-7).
The type of crack opening can also provide important information: if cracks such as welding cracks gape open, it indicates a plastic compression process before cracking occurred. The result is high tensile stresses and cracking during cooling. On the other hand, cracks can also be pinched shut under the influence of compressive residual stresses from spring-back or thermal strain.

Illustration 16.2.2.7-4: The crack pattern relative to machining marks and part geometry can provide important information regarding the origin of cracking in the finishing process.
Crack orientation: The path of a crack relative to part characteristics such as machining direction, part shape, structural textures (e.g. grain orientation from forging, solidification direction) or weld seams is an indicator of causal influences. Even a cracking pattern that is oriented along the direction of machining is informative.

Grinding cracks, chatter marks, or comma cracks from reaming or turning processes tend to run across the machining direction that can be recognized in the machining marks.

Orientation along the machining marks is more likely in cases of thermal fatigue (Ill. 16.2.2.5-7). If the crack is oriented along the part contours, thermal stresses may be the cause. A lack of any recognizable orientation is also an important characteristic. It precludes the above possibilities with sufficient probability. The likely cause that remains is overstress. This type of force influence indicates clamping forces, straightening processes, or damages during handling.

Crack fields: Parallel cracks are an indicator of large temperature gradients or high tensile stresses perpendicular to the cracks. Typical examples are grinding cracks (Ill. 16.2.1.1-4).

Crack networks, i.e. with no orientation, rather indicate even stress conditions such as in Cr coatings. Another possibility is corrosion-influenced grain boundaries.
Cracks around weld seams
always indicate that welding was a causal influence. Naturally, other factors may have been involved. The crack orientation and the location of the crack relative to the weld are influenced by residual stresses, form notches (undercuts, excess weld metal, Ill. 16.2.1.3-2), structural characteristics, toughness (e.g. embrittlement), and strength in and around the weld seam (Ill. 16.2.1.3-12).

Cracks around surface changes such as discoloration and roughness: If discoloration of the part surface is positioned or distributed in a recognizable pattern relative to cracking, it is an important clue regarding the cause of the cracking (Ill. 16.2.2.6-13).
Discoloration can have several causes that point to different influences, and these may act in combination. Discoloration often occurs in the form of tarnishing at the high temperatures of a machining process. If tarnishing occurs during a heat treatment or welding (Ill. 16.2.1.3-18) under cover gas or in a vacuum, it is an indirect indicator of possible material changes. Titanium alloys, especially, tend to embrittlement and cracking after absorbing oxygen.

Titanium alloys are also sensitive to fouling such as residue from baths or auxiliary materials (e.g. cooling lubricants, Chapter 16.2.1.1.1). This can cause tarnishing without noticeable temperature influences. If cracking occurs in these areas, and there is residue containing halogens (usually chlorine), stress corrosion cracking is likely (Ill. 16.2.1.7-8).
Unintended brittle coatings can also be the cause of discoloration and cracking. These include creep coatings under protective coverings during the Al diffusion coating of turbine blade roots (Ill. 16.2.1.8.1-3).

Roughness changes, e.g. as a result of undesired etching, can delineate themselves as surface changes. Crack indications in these areas suggest corrosive attack (e.g. due to an etching bath, Ill. 16.2.1.7-10). Undocumented local reworking can be revealed by changes of typical machining surfaces (e.g. grinding, turning). Cracks in this area indicate that they were already present before the reworking or that there was a related problem. Material changes (Ill. 16.2.1.3-8) such as cracking in the area of repair welding can change the appearance of the surface. This effect can occur during blasting (Ill. 16.2.1.6-10), vacuum annealing, or etching, and is seen especially in complex cast parts.

If cracks are deformed, pressed shut, or smeared, it can limit the time frame of the crack initiation in some cases. The same is true for cracks that were pressed shut by the impact of blasting media and/or are filled with blasting media (detail).

Cracks around coatings: In this case, due to differences in crack-influencing characteristics, a distinction should be made between applied coatings and diffusion coatings.

Applied coatings: A common development is cracks due to high tensile residual stresses in galvanically deposited coatings, such as Cr coatings. The cracks that occur during the finishing process are usually limited to the coating. Defined cracking is often desirable in order to ensure certain coating characteristics, such as dry running operation after lubrication system failure. The breakdown of dangerously high tensile residual stresses can be another goal. If cracks occur in the base material below the coating, they are usually caused by stress corrosion cracking. In this case, there are usually high tensile residual stresses in the base material surface. These are usually the result of a less-than-optimal grinding process before the coating was applied (Ill. 16.2.2.4-12). If there is coating residue in the crack (Ill. 16.2.2.7-4) or the crack pattern can be seen in the coating makeup, it is a certain indicator of the temporal order of coating and crack initiation.

Diffusion coatings such as Al, nitriding, and case-hardening have grown together with the base material. Therefore, cracking will always affect the base material (dynamic fatigue strength). If these coatings, for example Al diffusion coatings, behave brittly at low temperatures (Ill. 16.2.2.7-1.2), it may indicate the cause of cracking. A typical occurrence is plastic deformation under high clamping forces, straightening attempts, or due to handling errors. With a bit of luck, concentric crack fields can indicate the origin of the external force and the type of overstress (Ill. 16.2.2.5-11).

Illustration 16.2.2.7-5: If it is possible to break open a crack during destructive testing, a fracture surface analysis can be done. The macroscopic and microscopic inspection of crack surfaces provides important information regarding the damage mechanism, causes, finishing process, and time of origin.

Microscopic crack surface inspections in a SEM are especially useful (Ill. 17.3.2-7). They can be used to identify typical fracture surface structures that can clearly be attributed to the crack causes. These include hot cracks, forced fractures, embrittlement, dynamic fatigue, corrosion, stress corrosion cracking, and material weak points. Microanalysis in an SEM shows the buildup of coatings. This makes it possible to draw conclusions regarding the influence of coating processes, diffusion, and penetrating metal melts (LME).

Typical fracture surface characteristics can be grouped into three main categories:
Coatings and foreign particles: These are coatings that have entered into the crack. They can come from galvanic, chemical, or diffusion processes. The type of deposit can be determined with a microanalysis in an SEM. Aggressive etching baths can be identified by residue such as chlorine. The same is true for corrosion products.
If the crack was already present during penetrant testing, it may be possible to detect traces of the components (penetrant oil, developer). This would show that the crack was not discovered by the tests.

Oxide coatings indicate overheating of the crack surfaces in oxygen, which is typical of heat treatment in atmosphere.
Even particles such as blasting media fragments, residue of thermal spray coatings, or coating powder can become stuck in the mouth of an existing crack and indicate the time cracking originated.

Discoloration: These characteristics enable conclusions regarding the exact location at which cracking began. This improves the chances of determining the crack-causing influences (Ill. 16.2.1.4-15). The identification of the crack initiation point is a prerequisite for targeted micro-inspections. If the crack surfaces were heated in oxygen (air), tarnishing can be expected. This usually reveals heating. However, conclusions regarding quantitative temperatures based on tarnishing on crack surfaces must be viewed even more skeptically than similar attempts on part surfaces. The fresh metal surfaces of crack edges, the limited accessibility for oxygen in deeper cracks, changes in material composition due to diffusion, and possible deposits can have a confusing effect on tarnishing. If the cracked part was subjected to a reductive annealing treatment in a vacuum, hydrogen, or corrosive atmosphere (Cl, F), it may have removed tarnishing. This promotes misinterpretations.
Crack surfaces can also be discolored by coatings from baths, penetrating metal melts (LME), and diffusion-related changes (e.g. Al diffusion coating). A micro-analysis may be able to attribute the discolorations to these processes.

Fracture structure: Even the macroscopic analysis of crack surfaces with discolorations can permit conclusions regarding primary crack initiation zones. A typical example is a pronounced intergranular fracture zone in an otherwise ductile fracture. A concentric arrangement around a delineated zone is an additional indication. This type of fracture pattern is indicative of hydrogen embrittlement or stress corrosion cracking (Volume 1, Ill. 5.4.4.2-3). An edge at the surface (Ill. 16.2.1.7-10) can indicate intergranular corrosion.

Illustration 16.2.2.7-6: Local overheating with a danger of cracking under the influence of effects such as sparks and hot particles (Chapter 16.2.2.6) occurs primarily during machining (top diagram).

Intense local heat development occurs in the contact area of the tool. High temperatures (bottom center diagram) to the point of local (grain boundaries) melting of the material can occur. In this phase, the expansion of the heated volume is restricted by the surrounding material (also see Ill. 16.2.2.6-3). If this results in compressive stresses greater than the flow limit, plastic compression will occur. Very rapid cooling follows after heating. The surrounding cold material and external cooling, usually with cooling lubricants, provides intensive heat removal. During this process, the compressed volume attempts to contract, but this is prevented by the surrounding material, resulting in high tensile stresses. If the grain boundaries are still doughy, hot cracks (bottom right diagram, Ill. 15.1-8) with typical fracture characteristics (Ill. 16.2.1.3-10) can form. If residual stresses are not broken down sufficiently by the cracking, high tensile residual stresses can be expected after cooling.

This creates a possibility of cracks ocurring only at a later point in time, for example, under corrosive influences during storage (Ill. 18-3) or under dynamic loads during operation.

Illustration 16.2.2.7-7: Stress corrosion cracking (SCC) can occur in many materials, such as steels, brass, and aluminum alloys. The higher the strength of a material, the more sensitive it is to this crack-initiating damage mechanism.
In engine construction, there is a special interest in stress corrosion cracking in titanium alloys (Ill. 16.2.1.7-8). This is due to the extensive use of titanium alloys in highly-stressed, safety-relevant parts such as rotor components and pressurized housings.

Requirements for stress corrosion cracking are:

  • Corrosion-sensitive material state
  • Corrosive media, suitable for the material state
  • Sufficiently high tensile stresses

These damage-causing influences are not only present in engines, but also in the production process. Operating damages (Ills. 16.2.2.7-9 and 16.2.2.7-10) are often made possible only by tensile stresses and/or material influencing from the production process.

Titanium alloys do not require watery corrosive media (Volume 1, Ill. 5.4.2.1-8) like SCC systems in steels do. High tensile stresses will even make solid halogen connections dangerous. In the finishing process, this usually concerns chlorine compounds such as common salt. Fluorine, which is released by processes such as the overheating of fluorinated hydrocarbons (Teflon, separating agents, lubricants), is another potential corrosive media from finishing. Temperatures above 450°C, and especially above 500 °C, are necessary to initiate cracking, at least with chlorine compounds (top left diagram). These temperatures occur during heat treatments (Ill. 16.2.2.3-16), welding, and soldering. The required tensile stresses are often residual stresses in the raw parts (forging) or from the finishing process (grinding, welding, top right diagram).

The necessary chlorine can originate in many different sources in the finishing process.

  • Hand sweat from handling of the parts (Ill. 16.2.2.3-16)
  • Cooling lubricants or cutting pastes containing Cl
  • Cleaning agents or baths containing Cl

Sufficiently high temperatures break down the chlorine compounds, releasing the aggressive chlorine. This is true of deposits of fouling such as common salt (NaCl) and thin reaction coatings (Ill. 16.2.1.7-8).
In consideration of these factors, the ban on degreasing baths made from chlorinated hydrocarbons (trichloroethane = Tri, perchloroethane = Per) is a welcome development.
Despite improvements in cutting performance, from the point of view of safety it is incomprehensible why cooling lubricants containing Cl are still recommended for drilling titanium alloys today (diagram, Ref. 16.2.2.7-7).

Illustration 16.2.2.7-8 (Ref. 16.2.2.7-4, Example 16.2.2.7-1): Although this case was more likely related to an overhaul, the described problems are also highly relevant to the finishing of new parts. They concern the use of unsuitable or potentially dangerous auxiliary materials. As this case shows, influences or damages from the finishing process may only become active at a much later time under conditions typical during operation (Ill. 16.2.2.7-10).
The damage, a fracture of the low-pressure shaft made from high-strength steel, occurred during climbing flight (top diagram). Therefore, it occurred during the highest fan performance and under the highest shaft stress (see engine diagram).

This example shows the danger of operation cracks caused by unsuitable stripping and cleaning baths. In this case, the steel shaft was attacked by chlorinated residue. The corrosion notches acted as initiating points for fatigue cracks following torsional vibrations. This caused the low-pressure shaft (bottom diagram) to break into many pieces (Ill. 16.2.2.7-9).
The central vent tube (CVT) in the shaft is made from a titanium alloy. It became embrittled due to hydrogen and chlorine and broke after the shaft as consequential damage.

Comments on the conclusions drawn in the selectively cited investigation report in Example 16.2.2.7-1:
The presence of lacquer indicates that the LPT shaft is evidently made from a high-strength steel. In addition to the fracture into many brittle pieces, the dynamic crack-initiating corrosion notches caused by the chlorinated bath residue indicate that crack initiation and/or crack growth occurred under the influence of corrosion (stress corrosion cracking, corrosion fatigue).
The fracture of the CVT, which is made from a titanium alloy, was a consequential damage of the shaft failure, but the brittle fracture pattern is unusual. Evidently, signs of hydrogen embrittlement and chlorinated deposits were found, supporting the following interpretation:
In the example, it is suggested that hydrogen embrittlement was involved. Titanium alloys in cleaning baths tend to embrittling hydrogen absorption (Ill. 16.2.1.7-9) if ferrous fouling (e.g. in chafe marks) is present. However, for embrittlement that reacts to impact stress, brittle titanium hydrides must have formed. If the hydrogen is dissolved in the lattice as it is in steels, it can be assumed that crack-initiating embrittlement will require static loads over a long period of time (Volume 1, Ill. 5.4.4.1-6). However, the micro-cracking described in the literature seems to indicate a different damage mechanism that is typical for titanium.
It is possible that, as described in Ref. 16.2.2.3-8, a thin reaction layer of the titanium with chlorine from the cleaning bath was formed (Ill. 16.2.1.7-8). Understandably, this cannot be removed through subsequent rinsing. Under the operating stresses and temperatures of more than 450°C (?), it is possible for the observed micro-cracking to occur due to stress corrosion cracking. Unlike the steel LPT shaft, this corrosion type does not require a watery electrolyte (Ill. 16.2.2.3-16).

Illustration 16.2.2.7-9 (Ref. 16.2.2.7-4, Example 16.2.2.7-1): The diagram shows a detail (Ill. 16.2.2.7-9) of the fractured LPT shaft made from high-strength steel. The dynamic fatigue cracks occurred under the influence of corrosion. The many brittle fragments (top diagram) and the branching crack pattern (bottom diagrams) point to additional influences. The fracture pattern indicates hydrogen embrittlement. High-strength steels are sensitive to hydrogen-induced stress corrosion cracking. During this process, the material absorbs hydrogen during corrosion (Volume 1, Ill. 5.4.4.1-4). This would plausibly explain the findings and the fracture of the shaft. If one assumes that the operating temperatures of the shaft would not permit a watery corrosive media, then it seems likely that standstill times with condensation water development could have played an important role in this damage case.

Example 16.2.2.7-2 (Ref. 16.12.2.7-4):

Excerpt: ”…The takeoff was normal, but between 400 and 500 feet in the initial climb the flight crew heard a loud `bang' from the No 2 engine….The No 1 engine (fan) speed indication for the No 2 engine was reducing rapidly and the corresponding exhaust gas temperature (EGT) indication was in excess of 900°C….several witnesses around the airport noted that for a brief period flames and smoke had appeared from the rear of the No 2 engine…An uneventful…single engine landing was carried out.
All engines utilize a four stage low pressure turbine (LPT) which drives a single stage fan and three stage…low pressure compressor (LPC) through the LPT shaft.(The examination of the engine) indicated that the low pressure (LP) shaft had failed since the fan, which appeared undamaged, was able to rotate freely whilst the low pressure turbine (LPT) did not rotate…The position of the damage on the No 4 (bearing) outer track…indicated that the rear section of the LP shaft had moved aft prior to the bearing failure…There had been no growth in the bore diameter of the LPT disks, and thus the LPT had no overspeed…The LPT shaft had failed over a length between , approximately, the No 3 bearing and the aft end of the HPC. In this area the shaft had fragmented into some seven segments of various sizes…
The external surface of the shaft showed clear evidence, over its full length, of an aluminum-loaded protective paint finish, …there were no obvious signs of corrosion. The inner surface, however was devoid of this protective paint throughout its sealed length, bounded by the front and rear center vent tube (CVT)/LPT shaft seals, and the shaft had been affected by surface corrosion over this same length. The corrosion was most severe in the fragmented region close to the areas of contact made by the forward CVT support ring, although there was evidence of the protective paint within the front and rear support ring areas of contact. Detailed examination showed the paint in these areas to have adhered to the surface, indicating that the corrosive attack on the inner paint surface had occurred after painting and the whole length of the shaft had probably been coated in paint. The shaft's inner surface was covered in compact patches of dark brown powdery deposit…which suggested that the whole surface had been covered prior to the failure…The shaft had failed as a result of torsional fatigue mechanisms, from multiple origins, precipitated by corrosion pitting of the shaft inner surface… A microsection taken near the primary origin revealed the presence of a multi-branched stress-corrosion cracking in addition to fatigue mechanism…the cracking mechanisms had been relatively longterm.
The protective paint…is essentially a silicon-based paint loaded with 40% aluminum…The painting process involves, after degreasing, the application of several layers…The degreasing process involves the use of chlorine based solvents (such as trichloroethylene)…An element analysis of the corrosion deposits, however, revealed the presence of chlorine in the form of aluminum (and other) chlorides. Evidence of chlorides were also found…throughout the sealed region of the LPT shaft.
The CVT had failed as a direct result of the LPT shaft failure…(the) significant fragmentation had occurred due to hydrogen embrittlement of the titanium with the outer surface showing the presence of small oxidised cracks in which chlorine enrichment was detected…it originated from a chlorine aromatic derivative from the chlorobenzene family. This potentially corrosive agent is commonly used as a paint stripper……it probably derived from an agent introduced inadvertently, or used as temporary alternate
This was the first failure of this nature to have occurred to this engine type…although cracks in at least one shaft had been detected, during a shop visit, in the outer surface (of the LPT) resulting from an area of fretting.

Comments: See Ills. 16.2.2.7-8 and 16.2.2.7-9.

Important notes:

  • Do not use any auxiliary materials that have not been approved, especially baths for cleaning and degreasing.
  • Special care must be taken during the unavoidable use of chlorinated auxiliary materials on titanium alloys.
    * Caution is required when
    approving new auxiliary materials and changing suppliers or products.

Illustration 16.2.2.7-10: Many specialists can point to examples from their experiences in which parts that were cleared as crack-free after intensive crack testing later exhibited cracks without being influenced by any significant external loads. These cracks are usually found during temporary storage or by the customer after delivery, i.e. at a much later point in time. Usually, the first reaction is that the crack testing process failed to find the cracks. This may be the case (Ill. 17.3.1-2), but it is also entirely possible that delayed cracking (Ref. 16.2.2.7-9, Ills. 16.2.1.3-18 and 18-3) occurred. However, one must be sure not to label every crack discovered after crack testing as a result of delayed cracking.

There are various causes of delayed cracking:

  • Unfavorable heat treatments that are responsible for influences such as unfavorable structure, high tensile residual stresses, and notches.
  • Absorption of hydrogen resulting in hydrogen embrittlement. This is encouraged by tensile stresses and high material strength.
  • Stress relief cracking” in welds (page 16.2.1.3-19, Table 1) as a type of creep cracking. This also occurs during subsequent heat treatments (stress relief annealing).
  • Stress corrosion cracking under the influence of suitable media and sufficient tensile stresses.Another type of late cracking can occur in parts with brittle coatings. Al-diffusion coatings used for oxidation protection are sensitive to this if improper handling causes local plastic strain. There is a good chance that laboratory inspections can distinguish these late cracks from delayed cracking on the basis of typical damage characteristics.

Illustration 16.2.2.7-11: Cracking threatens the safety of parts and must be reliably prevented. Certain approaches can help with this (no claim to completeness):

APrevention of cracking: This early approach is recommended.

A1”: Sufficient testing of the quality assurance and finishing processes is absolutely necessary for documenting the optimized processes. This is also true for licensed finishing. It must be ensured, for example, that the behavior of the tools and equipment meets the requirements of the licenser. A specific example is electron beam welding, the results of which are highly dependent on the tool properties (Ill. 16.2.1.3-26). Under no circumstances can there be independent deviation from these specifications/work instructions.

“A2-5”: There are often unexpected problems that occur during the transferral of optimized processes out of finishing process development or from a licenser to serial production. Technicians and engineers evidently have a strong desire to not accept anything without “improving” it. Often, the reasons for serially implemented finishing processes that do not appear optimized are not understood at first glance. Experience has shown that this creates a risk of problems recurring in serial production that were already solved during development of the finishing processes. This type of “disimprovement” (Volume 1, Ill. 3-3) can already begin with a type of machine that is different from that used in testing. The change to seemingly more effective auxiliary materials such as different cutting materials or cooling lubricants is almost impossible to verify using non-destructive methods, but it can have fatal consequences. These include tensile residual stresses which unallowably shorten the cyclical life spans of parts during operation. These considerations can also be applied to quality assurance.

“A6”: The handling of parts, including fastening, clamping, storage, and packaging, must be done in line with specifications. If overstress with plastic deformation occurs, for example through accidental impact stress, it can cause cracking, especially in brittle coatings (Ill. 18-5).

“A7”: When starting a series, inspection of the first parts between the individual processing steps has proven to be an effective method of damage minimization. If applicable, crack testing should be done before a large series starts. A typical example is penetrant testing following grinding. In order to prevent the typical problems of smearing and reduced detectibility of cracks, an approved heat treatment cycle should be used before testing (Ill. 17.3.1-7).

“B” Procedures during cracking: If cracks have already occurred, then risk minimization is the primary concern with regard to safety, time, costs, and prestige (Ill. 17-11).

“B1”: Causes must be determined as a prerequisite for targeted measures. Usually, there is not a single cause, but several that all contributed to cracking. Knowledge of the causal influences increases reciprocally with understanding of the damage mechanism. Today, this can be aided by the highly sensitive and reliable testing methods of metallography (Ill. 17.3.2-6) and SEMs (Ill. 17.3.2-7).

“B2”: If the causes have been understood and evaluated as much as possible, the corresponding finishing processes can be identified. This can limit the time frame in which cracking occurred, and also limit the extent of potentially affected parts.

“B3”: In some cases, there are questions as to why cracks were only detected at a late point in time, especially if several finishing and quality assurance steps have occurred since cracking, or if the cracks were only discovered on the finished part.
These questions must be answered in order to recognize weaknesses in the quality assurance process, usually non-destructive testing. It must be remembered that even the best non-destructive testing methods are not 100% guaranteed to find cracks (Ill. 17.3.1-2). Of course, case-specific optimization is recommended. A typical reason is an unfavorable work sequence. This is the case, for example, if penetrant testing is done right after the grinding of nickel alloys, but the heat treatment required for opening smeared cracks only occurs after several additional processing steps.

“B4”: Experience has shown that the probability of mistaken conclusions is especially high in the case of finishing damages. The reason for this is the complexity of the damage mechanisms and the wide variety of possible causal influences. In order to minimize this risk, the verification of conclusions may be decisive. For this reason, the meticulous and very work-intensive tracing of production processes, including any applicable additional intermediate tests, may be necessary. Another possibility is reproduction of the damages in technical tests. This should be done in the original finishing process, if possible.

“B5”: It seems self-evident that the finishing steps must be stopped until the causes have been satisfactorily determined and corrective measures have been implemented. However, practical pressures such as delivery deadlines and costs can make this very difficult. The minimum requirement for resuming production is that at least the first chronological cause of cracking is understood and eliminated.

“B6”: The definition of targeted solutions presupposes a sufficiently accurate understanding of the crack initiation mechanism and the causal influences involved (“B1”). After corrective measures have been implemented, their effectiveness must be verified, at least for the first subsequent batch of the series. Ideally, in addition to the testing sequence of the series, this should be done after every process step. Only after the corrective measures have proven to be effective should a return to series requirements occur.

“B7” and “B8”“: In addition to the direct prevention of cracks through corrective measures, there is the issue of whether or not cracked parts were delivered. If this cannot be sufficiently safely determined, the risks must be estimated. This includes identifying and determining the number of possibly affected parts (e.g. serial numbers), as well as determining their location, i.e. end user.

The influence on the operating behavior of potentially flawed parts is an important safety risk and basis for making decisions. In order to estimate this, it is necessary to inform and involve the following groups sufficiently early:

  • Technical departments (e.g. strength)
  • Safety boards
  • Authorities and regulators
  • Licensers and customers

Any further necessary measures (e.g. monitoring of installed parts) must be developed and agreed upon together. The above bodies must also cooperate in determining possible reworking (Chapter 17.5) of affected parts.

“B9” and “B10”: Experience has shown that the issue of possible reworking (Ill. 17.5-1) arises when there are important pressures (delivery deadlines, costs) and an accepted risk (Ill. 17-11). This must be tested by the responsible bodies (“B7”). Naturally, the harmlessness of reworking must also be verified, possibly through testing in serial conditions with corresponding optimization and documentation.

References

16.2.2.7-1 L.Engel, H.Klingele, “An Atlas of Metal Damage”, Wolf Publishing Ltd, 1981, ISBN 0 7234 0750 9.

16.2.2.7-2 P.Adam, “Fertigungsverfahren von Turboflugtriebwerken”, Birkhäuser Verlag, 1998, ISBN 3-7643-5971-4, page 104

16.2.2.7-3 ASM Handbook Volume 5, “Surface Engineering”, ISBN 0-87170-377-7, 1999, page 144.

16.2.2.7-4 Department for Transport, AAIB Bulletin No.: 4/97 Ref: EW/C96/11/1 “Severe internal damage to No 2 engine”, 1996, pages 1-5.

16.2.2.7-5 M. Field, J.F. Kahles, “Übersicht über die Oberflächenbeschaffenheit bearbeiteter Werkstücke, `Surface Integrity' ”, periodical “Fertigung”, Issue 5/72, pages 145-156. (2121)

16.2.2.7-6 E.Schreiber, “Die Eigenspannungsausbildung beim Schleifen gehärteten Stahls”, periodical “Härterei-Technische Mitteilungen”, 28 (1973) Heft 3.

16.2.2.7-6 E.Schreiber, “Härterisse und Schleifrisse - Ursachen und Auswirkungen von Eigenspannungen”, periodical “ZwF”, 71 (1976) 10, pages 460-465.

16.2.2.7-7 “Machining Titanium Alloys”, www.supraalloys.com/Machining_titanium.htm, 1.0.4.2004, pages 1-12.

16.2.2.7-8 H.Simon, “Oberflächenreaktionen an Titanwerkstoffen”, periodical “Metall Oberfläche” 5-1982, pages 211-217. (437)

16.2.2.7-9 ASM Metals Handbook Ninth Edition “Volume 11 Failure Analysis and Prevention”, ISBN 0-87170-377-7, 1986, pages 95, 98-99, 249, 359, 664-666.

16.2.2.7-10 E.E.Affeldt, L.Cerdan de la Cruz, L.Peichl, “Influence of Aluminide Coatings on the Mechanical Behaviour of Aero Engine Turbine Components”, presentation at the Turbine Forum “Advanced Coatings for High Temperatures”, at the “International Conference, Forum of Technology”, April 21st-23rd, 2004, Nice,/Port Laurent, France.

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