This paper shows how two different penetrant methods used in conjunction can provide a more complete inspection coverage and minimized risk of failure. A process is described where krypton gas penetrant imaging complements traditional liquid penetrant to achieve higher detection reliability of anomalies in aircraft engine components than either method can obtain independently. G.P. Singh Associate Contributing Editor

Figure 1-3
Figures 4-6
Figure 7-9
Table 1

This year marks the 27th anniversary of the first successful use of krypton gas penetrant imaging for the detection of cracks, casting porosity, hot tears, cold shuts, dross, etc., in aircraft engine components. In the late 1960s the US Air Force first applied this method to the detection of porosity in sleeve bearings used in the nose cones of T-34 turboshaft gas turbine engines (Figure 1).

These bearings, made of Babbit metal, were failing unexpectedly. Millions of dollars were being spent on unscheduled maintenance and repairs to subsystems. The Air Force felt that cavitation erosion was the cause of these unexpected failures. However, the krypton method identified the porosity as the cause of the fatigue failures and not cavitation erosion. Subsequent corrections to the casting procedures eliminated the porosity and the failures.

This original application of the krypton evaluation technique, or simply KET as it's called today, involved the use of hand wrapped photographic film. Since then numerous improvements have been made to the process, most notably the replacement of the photographic film with a sprayable emulsion. KET is now a robust, low cost production process much like liquid penetrant inspection and is depicted in Figure 2.

 

A Closer Look at the KET Process
KET is a batch process similar in throughput, procedure, and cost to the familiar liquid penetrant method. The krypton gas used in the early experiments is still the working fluid in today's process. It consists of 95 percent inert krypton and 5 percent krypton 85.

 

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Millions of dollars were being spent on unscheduled maintenance and repairs to subsystems.

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Initially the part is cleaned and placed in a chamber where it is outgassed to remove the layer of air molecules adsorbed along the surfaces of both the cracks and the outer surface. The evacuated air in the vessel is replaced with the mixture of krypton gases in which only the krypton 85 atoms are active.

The part is left in the krypton until all of the exposed surfaces contain adsorbed krypton atoms. Following exposure, the krypton gas is pumped out of the vessel and air is readmitted. During this step, the krypton atoms along the free surfaces desorb more readily than those trapped in the cracks. This results in relatively little surface activity but high concentrations of activity at crack sites. The average amount of gas retained in a crack is substantially greater than that retained along outer surfaces. Krypton 85 atoms, because of their extremely small diameter, diffuse into very tight, oxide filled cracks or casting hot tears.

Imaging the concentrations of krypton gas trapped in these microcracks is made possible with a liquid emulsion (sometimes referred to as a dispersion). The dispersions used for the KET process are sensitive film emulsions developed by Eastman Kodak Co., Rochester, New York. They are liquid gelatin, a water soluble material, containing silver halide particles. Dispersion coatings 250-500 µm (1-2 milli-in.) thick are applied under darkroom conditions.

After the dispersion is dry, the part is allowed to sit for an appropriate time while the krypton 85 gas in the cracks exposes the dispersion. Finally, the exposed part is developed by normal photographic techniques as if it were a sheet of exposed film.

The dispersion allows the inspector to readily identify the microcrack. Indications appear black against a white background and may be read under white light conditions. KET images are in exact registry on the part where the cracks reside and will remain distinct for some time.

The KET process can be adjusted to meet individual customer requirements. Film exposure time for casting porosity, hot tears, cold shuts, dross, etc., and low cycle fatigue cracks will vary. Every effort is made to produce an optimum signal for each application whether film, dispersion, or electronic sensors are used.

A unique feature of the KET process is its built-in magnification factor. Beta particles emitted by the krypton 85 atoms radiate laterally and can penetrate 500-1,250 µm (2-5 milli-in.) of material. This allows one to see microcracks not normally visible with the naked eye. This enlargement factor varies from 1,250 to 2,500 µm (5 to 10 milli-in.) so that a 25 µm (0.1 milli-in.) crack opening would show up as a 1,250 to 2,500 µm (5 or 10 milli-in.) KET indication. Frequently, tight microcracks or hot tears with a 25 µm (0.1 milli-in.) surface opening will behave in this manner. The enhanced visibility of microcracks through KET increases the likelihood that a technician will spot potentially life limiting microcracks in a part before unnecessary investments are made in machining a part of questionable quality or risking the shipment of such a part to a customer.

While KET is able to detect cracks not visible to the unaided eye, it often misses visible cracks which have large crack openings. Krypton atoms desorb rapidly from wide open cracks when the krypton treated part is returned to the atmosphere. Frequently, the activity emitted by a large crack is equivalent to that coming from a good surface, and the crack signal blends in with the noise. Subsequent KET inspection often will not show these large cracks, which are easily detected by fluorescent penetrant inspection. For this reason, it is advisable to use KET after parts have been liquid penetrant inspected. Parts failing liquid penetrant need not be KET inspected; those that areacceptable after liquid penetrant inspection should be KET inspected.

 

Comparison of Penetrant Systems
There are basically two kinds of penetrant systems: liquid penetrants and gas penetrants. The liquid fluorescent penetrant inspection method has become the industry standard since it was first discovered in 1942 by Joe and Bob Switzer (Flaherty, 1986). In many ways that original process is very similar to the process we use today. Early applications were in the maintenance and overhaul of aircraft engines. Over the intervening years aircraft engines have changed considerably and the need to find smaller cracks has increased immensely. KET answers this need. The krypton gas penetrant system was never meant to compete with or replace the current industry standard. KET effectively identifies the smaller cracks that fluorescent penetrant inspection has difficulty imaging, hence it is complementary. This paper describes how both methods used together are much more effective than either method used alone.

Figure 3 depicts the penetrant problem. Liquid molecules are much larger than krypton gas atoms. Frequently liquid molecules are only absorbed by small portions of a casting porosity as in this case, at the larger opening in the center of this field of clustered microshrinkage (arrow). When only some of the liquid penetrant is absorbed, as shown here, the inspector has no idea as to the size of the microshrinkage cluster. When he dabs this area with alcohol and the indication grows fainter, he's led to believe that it's an innocuous surface anomaly.

Liquid penetrants are generally useful above 3 mm (0.125 in.) crack length but their probability of detection falls off rapidly below that size. If the crack is compressively stressed, as is usually the case for gas turbine compressor and turbine disks, then chances for detection are reduced further. And, of course, foreign debris and oxide products are usually contained in cracks, thus making it still more difficult. Liquid penetrants offer a four-sigma level of quality over all size ranges.

To achieve a six-sigma level of quality or detection reliability, one must obviously find smaller cracks with the same level of proficiency that the larger cracks are found. Original equipment manufacturers (OEM) use multiple inspections, acid etching, thermal cycling, and even engine running of turbine blades to increase the probability of detecting life limiting casting porosity, hot tears, cold shuts, dross, etc., in new parts, but the results have been mixed. New environmental regulations on cleaners pose new challenges for liquid penetrants (Rummel, 1996).

A far simpler and less costly approach to this problem is krypton gas penetrant imaging of parts that pass the liquid penetrant inspection. If in fact liquid was not absorbed by an existing crack or porosity, hot tear, cold shut, dross, etc., then chances are excellent that a gas would be adsorbed.

Our research has shown that compressively stressed cracks or oxide filled hot tears actually getter the krypton gas. Signal to noise ratios are enhanced several times by this effect.

This oxide gettering effect of krypton could also be applied to airfoil repairs where the tail of a crack is densely packed with oxide product and not detected by liquid penetrant inspection. Weld or braze repairing an airfoil with residual oxide results in internal voids and cracks. Expected life and repair cost savings are consequently compromised.

If one includes KET in the quality assurance strategy, there are new options and possibilities. For example, acid etching and thermal cycling prior to liquid penetrant are not necessary when KET is part of the quality assurance strategy. Using KET for small crack detection reduces the need for multiple penetrant inspections and could very possibly allow the use of water soluble penetrants and environmentally friendly cleaners.

 

Process Demonstration and Validation
As one approaches the lower limit of sensitivity for any given inspection method, the probability of detection decreases rapidly. The lower limit of sensitivity for gas penetrant is much lower than that for liquid penetrants. For this reason KET has a very high probability of detection over the range of crack sizes liquid penetrant must detect at its lower limit of sensitivity. This situation is shown graphically in Figure 4.

Some case histories will now be discussed briefly to show why KET should be used as a complement to fluorescent penetrant inspection.

Case 1. A large quantity of retired B1900 turbine blades from an overhaul facility were obtained and characterized by KET according to percent microshrinkage prior to inspections by two major original equipment manufacturers (OEMs) (Table 1). Ten blades representing the best to worst cases were then selected for study.

It should be mentioned here that OEM and field reports indicated that a high percentage of these turbine blades were cracking and failing unexpectedly. In all cases microshrinkage was present at the site of stress rupture cracking. The engine manufacturer had placed low life limits on these blades after one failure occurred at 80 h.

Two large OEMs were contracted to inspect these ten blades with their production fluorescent particle inspection lines. Using their own accept-reject criteria, they were asked to pass judgment on these blades. OEMs allow between 1-2 percent microshrinkage as does most of the aerospace industry. KET rejected eight of the ten blades based on this rejection criteria. Following the OEM's fluorescent penetrant inspection, all ten blades were accepted. Subsequent metallurgy by Battelle confirmed that the eight blades which KET rejected, but the OEMs accepted, contained microshrinkage exceeding the OEM's own fluorescent penetrant inspection standards. Blade #247 is shown in Figure 5.

Three important conclusions were drawn from this study:
1. Repetitive fluorescent penetrant inspection does not ensure the detection of microshrinkage. Prior to service these blades were inspected once at the foundry and two times during manufacture by the OEM. During this study they were inspected by two different OEMs. In all, they were inspected five times by four different fluorescent particle inspection lines. Failure to detect was not operator dependent but method dependent.

2. The KET inspection of fluorescent particle inspection approved blades is a more accurate quality assurance strategy than the standard practice of repetitive fluorescent particle inspection.

3. To ensure that investment castings are hippable (i.e., can be hot isostatically pressed) they should be KET inspected prior to hipping. Such inspections would offer assurance that the microshrinkage is not surface connected and that hot tears, cold shuts, and other non-hippable casting discontinuities are not present. The industry standard today is fluorescent particle inspection prior to hipping.

Case 2. The Radian Corporation in The Conduit (1995) reports how casting porosity produced invisible leaks of a highly reactive gas from pressurized canisters (Ellis, 1995). The leaks were in the stub portion of special gage assemblies made of cast 316L stainless steel (Figure 6). Gas leaks became visible a few days after installation as the escaping gas reacted with the atmosphere and caused discolorations.

In this case 20 percent of the cast 316L gage assemblies leaked in the cast stub portion but had passed liquid penetrant inspection. Under the scanning electron microscope (SEM) no visible surface perforations were observed. Only after metallurgical analysis was the band of microshrinkage responsible for the leaks visible. This example demonstrates the effectiveness of gas as a penetrant. More importantly it points out the need to KET inspect parts at the foundry after fluorescent particle inspection to ensure that the investment casting process is optimized before parts reach the customer.

Case 3. In 1993 the US Navy's Aviation Supply Office (ASO), now called the Naval Inventory Control Point (NICP), completed a 6,000 blade study comparing fluorescent particle inspection with KET (Mahorter, 1993). However, in this case the Mar M-246 turbine blades used in this study received a two hour cyclic engine test prior to fluorescent particle inspection (i.e. type ZL22). This OEM green run requirement for hot tear detection was expensive and resulted in the prime being the only source who could comply, thus restricting acquisition to the prime. The ASO's goal was to qualify KET as an alternative to the green run plus fluorescent particle inspection requirement.

The results of this study demonstrated the superiority of gas penetrant over liquid penetrant in the identification of hot tears:

• KET identified 12 blades with hot tears, none of which were found by the foundry fluorescent particle inspection nor the green run plus fluorescent particle inspection. A typical blade is shown in Figure 7.
• The KET hot tear blades were then subjected to a high sensitivity fluorescent particle inspection (i.e. Magnaflux ZL30) in a Navy Research Laboratory. Fluorescent particle inspection could only find hot tears in four of these blades.

Navy researchers concluded that engine running turbine blades first and then doing fluorescent particle inspection was ineffective in detecting hot tears up to 2.5 mm (0.1 in.) long. KET is now used by the Navy in place of the engine green run plus fluorescent particle inspection. Savings to date are in the millions of dollars and no blade failures have been reported.

 

Applying KET to Practical Problems
During the past several years the demand for industrial gas turbines has grown rapidly worldwide. This growth has been fueled to a large extent by the OEM's use of aero technology in the design, development, and manufacture of industrial engines.

In one case, an industrial engine from an aero derivative design having a 35,000 h design life began experiencing unexpected second stage turbine blade failures. These failures occurred as low as 800 h in this unshrouded IN792 Hf blade design. Investigators identified hot tears at the trailing edge above the blade platform as the cause. The manufacturer investigated various inspection methods to find the anomalous parts but had little success. Most methods were unreliable, with some costly and impractical in a production environment.

After learning about KET through Battelle, the gas turbine manufacturer conducted a lengthy evaluation program. Figure 8 shows one of the blades from this study. The tightness of the hot tear makes viewing even at 25x impossible and precludes entry of liquid penetrant. Krypton gas penetrant however, had no problem visualizing these invisible cracks. For this application KET was implemented through a process specification.

 

Risk Reduction With KET
Whenever a new engine design is taken through the manufacturing and development cycle, there are many unknowns or risks. This process is far from an exact science. New materials, cooling configurations, and manufacturing processes compound these problems. When mistakes become evident long after this cycle has been completed and production is in full swing, corrections can and do become costly for both the OEM and the customers. The costs associated with these risks can be avoided when more is known rather than unknown. KET can serve such a purpose when it becomes part of the manufacturing, development, and production cycle.

We have shown in this paper that there is a small risk of parts cracking or failing prematurely when KET is not used. Oldfield and Oldfield (1993) have shown a direct correlation between casting anomalies and the incidence of cracking and failure of turbine blades. The Federal Aviation Administration (Operational Systems Branch, Oklahoma City, OK) has records of many such events. They are reported under Service Difficulty Data and Accident/Incident Data. The Navy, Air Force, and Army overhaul depots report similar events in their engineering investigations of engine failures.

Figure 9 summarizes what we have discovered with KET over a fourteen year period about turbine blades and life limiting casting anomalies. This database included 15 different engines (i.e., military, commercial, and industrial). Approximately 100,000 blades were examined after acceptance by fluorescent particle inspection. Findings conclusively point to a small percentage of turbine blades (e.g. about 0.25 percent on average) that cause most of the reported problems. These are the blades that contain potentially life limiting anomalies and escape detection at the foundry. These blades represent a risk to the customer (i.e., the anomaly may or may not lead to a failure). The risk may be very low (it is never zero) or very high.

Because a turbine may contain up to 400 turbine blades, a 0.25 percent risk factor is very significant. That's one anomalous blade in 400. Figure 9 explains why turbine blade failures are not rare events but expected events. It only takes one blade failure to cause an engine failure.

Traditional approaches use service lives to measure risk - a costly and time consuming process. Using KET on foundry product, risk assessment is immediate for almost the cost of fluorescent particle inspection. Potentially life limiting anomalies are accurately characterized and casting parameters can be fine tuned to minimize risk. Design lives can be verified earlier in the development process, thereby allowing for less costly design changes before the production phase begins.

 

Acceptance of KET as the New Standard
The aero engine manufacturers agree that KET is superior to fluorescent particle inspection at the lower end of the discontinuity size spectrum, but they are reluctant to make KET a requirement for turbine blades. Foundries are also reluctant to use KET because it isn't an engine manufacturer's requirement. Imposing higher standards admittedly raises the cost of doing business. However, experience shows that KET provides its users with paybacks far in excess of the cost of using it.

The current standard will inevitably be raised by market forces. Global competition, demands for six-sigma quality, and new complexities in investment castings will drive the standard up. For now, it's necessary for customers to specify KET on their purchase orders for investment castings, hipping, repairs, and overhaul if they want to minimize their risk.

 

References

Ellis, P.F., II, "The Case of the Invisible Leaks," The Conduit, Radian Corporation, Austin, TX, 1995.

Flaherty, J.J., "Yesteryears: History of Penetrants: The First 20 Years, 1941-61," Materials Evaluation, Vol. 44, No. 12, Nov. 1986, pp 1371-1374, 1376, 1378, 1380, 1382.

Mahorter, R.G., "Evaluation of Krypton Evaluation Technique (KET™) for Hot Tear Detection in T56 Turbine Blades," Naval Air Warfare Center, Patuxent River, MD, Code: Air 4.4.3, Nov. 1993.

Oldfield, W., and F.M. Oldfield, "Service Failure of Hot Stage Turbine Blades: The Role and Mechanism of Oxidation Ratcheting," Metallurgical Transactions, Oct. 1993.

Rummel, W.D., "Cautions on the Use of Commercial Aqueous Cleaners in Fluorescent Penetrant Inspection Processes," Review of Progress in Quantitative NDE Conference, Bowdoin College, Brunswick, ME, Jul. 28-Aug. 2, 1996.
 

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Copyright © 1996 by the American Society for Nondestructive Testing, Inc. All rights reserved.
 

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