Re-baking to save Hydrogen embrittled fasteners
Any thoughts on the risks of accepting electroplated fasteners which have initially failed a hydrogen embrittlement torque test, then rebaked (and finished) then torque-tested again?Dave Daigle
On an aircraft, rocket,ordnance or auto ---- no. Risk someone else's tail, not mine. There has been some work done on re-embrittlement that would make this frightening. To a certain extent, the damage has been done. The reprocess just mitigated it enough to pass. Of course, this is a personal opinion. I do not think that it is the intent of a Mil Spec to allow this.James Watts
- Navarre, Florida
James, Thanks for your input. The fasteners in question DO NOT go on any aircraft, rockets, ordnance or automobiles. I should have made this clear. But you are correct, I don't see how rebaking would heal any microcracks or microvoids. Rebaking is not an option we accept, and the problem has not come up, but one fastener supplier did suggest rebaking as an option to me the other day.Dave Daigle
for a non critical use, stripping, annealing and then hardening including the proper draw might be a tolerable recovery. JimJames Watts
- Navarre, Florida
The problem you described is more typical than anyone would like to believe. The solution proposed is a common one and can be effective in salvaging material. There seems to be a misunderstanding in the way hydrogen damage or embrittlement works...
If a sample has developed microcracks or microvoids then the proposed fix is of course worthless, but typically fasteners do not show this type of damage until after they have been stressed. The damage that hydrogen causes requires a tensile stress, either residual or applied.
My job over the last 8 years has been performing failure analysis studies. Last year alone we did over 550 failures, many of them dealing with fasteners and hydrogen embrittlement.
The one caution I do have for anyone that does what was suggested is as follows: After stripping, the parts should be given an embrittlement relief because the pickling or stripping operation is a known hydrogen contributor. If this intermediate bake is not performed you can effectively worsen the situation because of the added amounts of hydrogen that can be introduced.
It is not a trivial decision and the follow-up testing needs to be agreed upon before hand....Hugo Menendez
failure analysis director, Senior Metallurgist
The effectiveness of a delayed baking process is dependent on the severity of the embrittlement. Mr. Menendez is correct that stress is required to create nonrecoverable damage from hydrogen. If enough hydrogen is absorbed, even minor residual stresses are enough to cause unrecoverable damage. In my experience, steels with moderate hardness can recover from delayed baking. High hardness steels, 40 HRC, probably will not. The best bet is to try a rebake and redo the testing. If the testing is OK, the parts will probably be acceptable for noncritical applications.
materials testing laboratory
I agree that delayed baking and retesting of the parts may alleviate the hydrogen related failures.
I would like to suggest the possible use of mechanical plating as an alternative to the electro-deposited coating. If this has been a one time occurrence than some sort of parameter was out of spec when these parts were coated. If this has been an intermittent or ongoing difficulty than the use of mechanical plating method would alleviate this concern.Bob Bea
I do have a question for the illustrious panel. We manufacture shock absorbers for automobile market. What tests can be performed to confirm whether a batch of piston rods have been through a hydrogen embrittlement process or not?.Srinath Nandyal
I have a question to Hugo Menendez in regards to additional hydrogen embrittlement during stripping: you said: "because the pickling or stripping operation is a known hydrogen contributor".
Most of the stripping methods used for stripping coating from high strength steels do not emit any hydrogen while the stripping takes place: these are based on oxidation reactions versus simply dissolving in acid where the metal being stripped (and the base metal too!) substitutes the hydrogen in the acid...
Here are the examples of stripping methods that I consider as not causing any additional hydrogen:
1. Stripping of Chrome plate anodically in alkaline solution.
2. Stripping of Cadmium in ammonium nitrate solution.
3. Stripping of Cadmium, Nickel, Zinc, Tin, Copper, etc. from steels in alkaline nitro-sulphonic acid with cyanide solutions (there are several proprietary and generic variations of this).
4. Stripping of silver plating from steels by anodic treatment in cyanide solutions.
5. Stripping of silver, nickel, cadmium, copper, etc. from stainless steels (in case you wonder, there are several high strength stainless steels susceptible to embrittlement - mostly PH series) by immersing in ~50% nitric acid.
In fact, I am not sure if I can name a stripping method that anyone concerned with embrittlement would use on high strength materials which will emit hydrogen while stripping.
Keeping the above in mind, why do you want to perform a bake after stripping?Max Stein
captive metal finisher - Montreal, Quebec
Most tests for hydrogen embrittlement use a sustained static tensile force. If the test samples fracture or crack, hydrogen embrittlement is indicated. If no cracks occur, the parts are free from embrittlement.
The trick is to determine the magnitude of force and the length of time for the test. For a component like a cylindrical rod, an axial tensile force would be ideal. But without access to an appropriate machine for this, I would probably devise a three-point bend test using a vice or dead-weight loads. A notch in the test sample concentrates the stress and gives a more severe test for embrittlement.
The sample should be bent to just under the material's yield strength. You can determine this by bending and releasing a sample until it maintains a permanent set - for the test use a bend that is slightly less than the smallest bend or load causing a permanent set.
A typical time period for the test would be 200 hours.
materials testing laboratory
I would like to find from Mr. Hanke, when do they use the 2 million cycle vibration fatigue test for hydrogen embrittlement and what is its purpose. Is the sample under tensile load under those circumstances?Mandar Sunthankar
- Fort Collins, Colorado
I disagree with most of you on this one! I feel that the technician or engineer in the testing laboratory may not be privy to the thoughts and plans of the design engineer, and not in a position to try to develop a test to verify that the part will hold up in service. Only the designer can look at the service conditions and decide what confidence level s/he needs.
Parts are supposed to be hydrogen relieved within a specified period, whether it be 2 hours, 4 hours, or 24 hours. To say that it is okay to relieve them days or weeks or months later, as long as they pass some test sounds like pretty dangerous business. The B-8 committee of ASTM struggled with this time-delay factor for the better part of a year, and it seemed to have ended in some acrimony (the way I read the columns in the Plating journals). It doesn't sound safe to override the standards committees and the design engineers. Fix the process that yielded defective parts; don't try to fix the defective parts.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
For a number of years now, our company has specified that all chrome plated carbon steel (usually alloy 1050 Rc 40-50) pen clips receive a "hydrogen embrittlement" bake at 500 F for 8 hrs within 24 hrs following the plating cycle. In testing of complex stamping forms (i.e. sharp transitions and small radius bends), this practice has proven to yield extended flex cycling performance compared with parts which are not baked.
Elimination of the bake process is now being considered for some of our less complex design pen clips. This proposal is because we have found that water spots become permanent stains when the clips are baked. If the parts are not baked, the water stains simply wipe off. We have worked hard to eliminate the visible water spots, but our company standards concerning cosmetic appearances is quite rigorous.
I am concerned about increasing the potential for mechanical failures (hydrogen embrittlement) just to improve cosmetic appearances and manufacturing yields. Can anyone provide any comments or recommendations concerning this issue?Tom Clem
- Lincoln, Rhode Island
If the new clips have no need for flex, you can make make them out of plain steel rather than spring steel, and not bake them.
If they must flex, they must continue to be made of spring steel, and you must continue to bake them.
I think you need to fix the first problem (the rinsing-drying), rather than talking yourselves into creating a second problem of unbaked parts. Good luck.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
I address only the staining issue mentioned by Tom Clem. You could use a commercial demineralizer which would eliminate any visible stains. This assumes you blow-dry the parts after final demineralized rinse to a reasonably dry finish. We used such a device for copper plating and the results were better than we expected. The cost was also reasonable. Hope this helps.John H. Brown
- Damascus, Virginia
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