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Welcome to the OMEGA RESEARCH UPDATE. This is the first in a series of quarterly newsletters to be published covering a multitude of subjects of interest to you - the Metal Finisher. If you find the topics interesting let us know. Wright us with your questions, comments, or topics you need help with.

This issue deals with baking GOOFS in the processing of actual parts or hardware. The spring issue will deal with GOOFS in the processing of test samples, coupons, or first articles. We anticipate the summer issue to deal with SPECS, or specifications, how they are changing, what impact they have on you, and how OMEGA RESEARCH can help. The fall issue will deal with the full spectrum of tests that the metal Finishing industry needs to perform on an ongoing basis. Farther out in the future we will detail the workings of Hydrogen Embrittlement tests and how they impact your operations. Also, an issue to discuss the "Metallurgy" of plating. CORROSION, probably the biggest reason the metal finishing industry exists, will also be covered in the future.

OMEGA RESEARCH has written about hydrogen embrittlement many times, in books, trade publications, research papers, information guides etc., and we hope we have put in perspective what it is all about. However, from time to time we still get basic questions from people new to the industry.

Every industry has its ....Oh No!.. problems. The computer people worry about bugs and virus. The automobile industry still has nightmares over exploding gas tanks. The beef industry is worrying now about mad cow disease, and so on.... Hydrogen Embrittlement is the worst problem our industry has. Look at some other potential problems. If the adhesion of plating is bad, you can catch it fast and it is pretty apparent if it peels off. If the surface finish or roughness is bad you can see it quickly. If the hardness is off, you can run a quick, cheap test to check it, and re-plate it if required. If the thickness is bad, you can catch it fast and re plate or rework. All of these problems are "...self evident.." as Lincoln once said, and they usually don't harm the expensive parts you are plating. Now lets look at hydrogen embrittlement. Hydrogen embrittlement is not visible 99.99% of the time. You can't see it, taste it, touch it or feel it. Worst of all it is damage that's inside the parts. We may never know it is there when the parts are shipped back. The customer may never know it is there when he installs the part in the aircraft. Our worst nightmare is that a part will fail and cause an aircraft accident. This is why we worry about, and try to control, hydrogen embrittlement.

So let's begin. Sit back, pour yourself a cup of coffee and read about:

BAKING GOOFS!

"WHAT DO I DO NOW?"

As a metal finisher, if you have never uttered those words, you are lucky. Mistakes or goof-ups happen. We would hope that our industry and society as a whole matures, mistakes will diminish. For those times when the best human intentions fall short, or when just the "moon and planets align" you need to take command of the situation quickly, and chart a course of action to recover. Let's talk first about electrolytic or conventional plating goofs, for these are the ones most prone to hydrogen damage or embrittlement. The potential for baking or embrittlement relief goofs is always there. You may have been faced with a situation where you come in on a Tuesday morning, and are told that some parts that were plated Monday did not make it into the bake oven. Uh-Oh! Now 12 or so hours have gone by compared with the 4 hour maximum delay period. (Note: some contractors or specifications require even shorter delay times. Consult with your purchase order to make sure of your customers requirements.) The time for crying, yelling, and yes cursing has passed. YOU MUST ACT QUICKLY NOW. This scenario should always be considered a ticking time bomb problem. Get the parts into the bake oven on a priority basis fast. This in itself will not necessarily fix the problem, but it is a positive action to take when time is precious and the clock is ticking against you.

Now, you have the luxury of sitting down and thinking about secondary steps that can be taken. There are a number of steps available to try and minimize the damage that may have happened. Some of these steps may not be applicable to the type of part your are processing, the customer they are for, or the ultimate user - the prime. (Note: reference is made to an aircraft prime; however, non-aircraft customers may also have specific processing requirements.) Probably the best bake schedule is to run the parts for a minimum of 24 hours at a temperature of usually 375 deg F. (Note: consult your process specification, purchase order, or actual blueprint requirements for actual baking temperatures as they vary, some being at 275 deg F.) Baking times are almost always shown as minimums, so running out to 24 hours will not harm the parts. Consult your applicable process specifications for exceptions.

A simple way of thinking about hydrogen damage is that of a military commando unit. The individual commandos are the atomic hydrogen atoms. After the commandos hit the beach, they want to join or form up into more powerful squads or teams. In like fashion, the atomic hydrogen atoms are driven to form up into molecular hydrogen. Molecular hydrogen is far more damaging to the steel. However, it takes time for these "commando" hydrogen atoms to form up. This is why there are requirements in all plating specifications calling out maximum delay periods into the bake oven. The sooner you can drive out these "commando" hydrogen atoms, the less chance they will form up into more powerful and passably devastating "teams". (Figure A depicts some effects of hydrogen damage.)

While the parts are in the bake oven, quickly sketch out a recovery plan - a potential course of action that can salvage or rework the parts. Always put yourself in the shoes of your customer, primarily the Material Review Engineer (M.R.Engineer). This individual has the thankless job, as he/she is the one that has to sit down, look at the problem and decide the disposition for the parts. I have seen M. R. Engineers in the aircraft industry with a ready made rubber stamp inscribed SCRAP. This is an easy and painless route to take if you are an M. R. Engineer. You, the plater, need to do the right things, and provide the data needed by the M. R. engineer to issue a proper disposition on the parts. However, we do not want to imply that all Embrittlement relief or baking goof-ups are capable of being corrected, or reworked. the hard reality of life in the aircraft industry is that many times the discrepant parts will be disposition ed as scrap due to flight safety concerns.

Now that the parts are in the bake oven, you are regaining some control over the situation as continuing hydrogen embrittlement damage stops. We say continuing as meaning that on-going progressive damage stops when the parts get to temperature in the bake oven. There is still the potential that previous, possibly irreversible damage, has already occurred. Anyway, let's talk about additional steps that can be taken to help out. Depending on the type of part, its strength or hardness level, the alloy it is made from, prior plating steps and specification or contractor requirements, the potential to go back, strip off all coatings, bake again, exists. What this does is provide a bare surface or interface to allow more effective bake-out of hydrogen. Many protective coatings such as cad, silver, zinc, etc. can act as a barrier to effective bake out of hydrogen depending on plating brightness or other factors. HOWEVER, stripping of coatings can cause harm itself, potentially inducing worse hydrogen embrittlement. Additionally, it may effect other prior coatings, installed parts, etc. and therefore the economics may preclude doing a strip back and bake. Potential tertiary or third level steps could involve bake outs at elevated temperatures, test on actual parts for Embrittlement , or tests on newly plated coupons to mimic the goof scenario. All of the above have potential use for the M. R. engineer in the disposition of the parts. The disposition of the ports should always be made by the customer. Therefore, after you have initiated a damage control path and developed a recovery plan, you must notify your customer, the sooner the better. Put together a proposed recovery plan with as much background information as possible on what happened to the parts. In the past, potential recovery steps or port dispositions made by aircraft primes, and known to OMEGA RESEARCH are:

1) SCRAP OUT the parts. This is the easiest route for M.R. engineers

2) USE AS IS without further operations or baking. This is rarely seen.

3) BAKE the parts for an additional time at normal baking temperature

4) BAKE the parts for additional time at an elevated temperature, i.e. 425-450 deg F. (this must only be directed by the M.R. engineer.)

5) STRIP & BAKE for an additional time either the standard or elevated temperatures.

6) TEST the actual parts. This usually involves cutting up a part and making a notched test bar, or may involve testing a complete intact part with elaborate fixtures or machines. This is rarely done due to time and cost issues. However, I have seen some hundred thousand dollar tests preformed on helicopter rotor head parts to salvage components and program schedules. Testing of actual parts for hydrogen is rarely done because hydrogen contents as low as 5 ppm have caused embrittlement failures in the past. The accuracy of hydrogen chemical content tests down in the region is poor with a scatter range of plus or minus 5 ppm possible.

7) TEST standard notched tensile bars that have been plated, baked etc. exactly per the recipe or schedule that the goofed-up parts experienced. This is done more often these days as it mimics any embrittleling conditions that might have occurred. The strength level of the notch bar used could be normal 260-280 KSI, or a lower strength bar closer to that of the actual parts may be utilized. This is a judgment call for the M.R. engineer. Many times combinations of the above steps may be utilized, such as BAKE - TEST, or STRIP - BAKE - TEST etc.

A good M.R. engineer will do the following when receiving notification of a problem: he will pull the engineering drawing or blueprint to look at the part, its application in the big scheme of things, its alloy type, hardness, strength, failure mode, and criticality of use. As an example, a goof up in the plating of an ash tray may not cause much heartburn to the M.R. engineer, as compared to a main wing attach fitting or helicopter rotor head. They will go back to the problem history of the part, look at its safety factors, and the economics of the exposure they would feel if a goofed up part was reworked or used as-is. A good M.R. engineer will earn his/her keep in a hurry , for the decisions they make can affect not only the economics and deliveries of an aircraft program, but also flight safety. (I remember numerous M.R. engineers i my professional career as ones with nervously blinking eyes, and nicotine stains on their fingers from chain smoking!) Of course there is always good news out there when goofed up parts are of high safety factors, or are less prone to Embrittlement. Many times, the effects of embrittlement are secondary and other effects such as hardness, corrosion resistance or electronic properties are paramount. In these cases, corrective action may be simple, straightforward and cheap.

Another potential problem that has cropped up in the past is a cycle interruption on a properly initiated bake, i.e. a power failure, or controller glitch. In this case it is always best to continue the bake operation at proper temperature and times as prescribed. DO THIS WITHOUT DELAY. After completion of the proper cycle, review the situation as noted above, contact the customer/prime and request M.R. action.

Another baking goof know to have happened are parts that are baked at too low of a temperature than required (i.e. 275 deg F. vs 375 deg F), or are baked at too short of a time. If this is caught during the bake, raise the temperature immediately and document the times at the lower temperature. For insurance purposes, go as long as possible on the bake time. Many M.R. disositions in the past have called for simply adding the correct time and temperature bake IMMEDIATELY ONTO the prior low temperature, or short time bake. A short commentary on baking is in order here. Embrittlement relief or baking is a diffusion controlled solid state physics process, a process governed by the geometrical relationship called Ficks Law. What is boils down to is that temperature goes a long, long way in the diffusion movement of hydrogen out of the parts. If you were to compare the efficiency of baking at 350 deg F. vs 400 deg F. (the 375 deg F set point, plus/minus 25 deg range), you would find that you will bake out twice as much hydrogen at 400 deg F vs 350 deg F. So you see, temperature is a powerful factor in baking. Don't try and cut corners on either temperature or time. However, NEVER try to bake at a temperature higher than called out for your specific parts.

Let's talk about the baking goof we metallurgist all fear. This is where a carburized, induction hardened or ball bearing steel part is baked at too high a temperature, i.e. 375 deg F vs 275 deg F. What happens here is that the steel itself becomes over-tempered by baking at the higher temperature. Most of the time this type of good is dispositioned as SCRAP.

In summary, remember the following:

1)Damage control. Act quickly to limit the damage by getting the parts into the bake over fast.

2)Stake out a recovery plan, with potential options. Put down all pros and cons to the steps that can be taken. Gather up as much data as possible, but keep it organized and logical. Give the M.R. engineer the "meat and potatoes" he/she needs to know to make a proper decision. Put yourself in the position of the M.R. engineer - don't force them to go out on a limb without good data or you will probably not like the 5 letter disposition that might come back - SCRAP.

For non electrolytic metal finishing, such as Electroless nickel, phosphate coatings, black oxide finishes, etc. the potential for damage diminishes, but is still present just from the acidic or caustic solution environment. Proceed as detailed above quickly.

Other metal finishing goofs involving paints, primes, dry film lubricants, fill and drain etc. can have a processing impact financially speaking, but usually do not affect the core integrity of the alloy being processed, i.e. the potential for permanent damage to the parts is less. However, always think about the downstream effects of these "minor" goofs. Several years ago some fatal aircraft accidents occurred due to minor changes or differences in the coatings being applied o propeller parts. What might be minor or inconsequential to you could result in increased coffin samples to the funeral home industry! It's the little problems - the minor day to day variances when stacked up over time - that accumulate and cause accidents. Use your head when plating and processing your parts. The next time you sit down in an airline seat, you won't feel as nervous.

OMEGA RESEARCH hopes this article will be of help to you in the future. Call us if you have questions, comments or future input. More in depth discussions on hydrogen Embrittlement can be found in the OMEGA RESEARCH publication, Hydrogen Embrittlement - a Guide for the Metal Finisher,copyright 1991 and Time and Temperature Effects on the Embrittlement Relief of High Strength Steel, W. Craig Willan, Hydrogen Embrittlement of Fasteners International Technology Conference, Denver, Colorado, May 1995.

Our Next issue will deal with routine monthly or lot testing sample GOOFS and failures. Although of lower "panic" level, the potential or production shut down looms overhead!

Figure A

FIGURE A

This is an actual photograph of a setion of aircraft aluminum tubing that has experienced hydrogen damage IT IS NOT HYDROGEN EMBRITTLEMENT; however, it does show the power of atomic hydrogen, combining to form molecular hydrogen. The formation into molecular or gaseous hydrogen creates a tremendous partial pressure, literally tearing the high strength 2024-T42 aluminum tubing apart, and forming a giant bubble or blister. This photograph is of a cross section of the tubing at 50x magnification. All of the fine black poits are individual hydrogen gas pockets. Aluminum and its alloys do not experience hydrogen embrittlement. Thisphotograph is shown simply to illustrate the power of hydrogen to penetrate into solid metal and cause damage.



 
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