Hydrogen embrittlement on 12.9 bolts 4

[dougbevi]

I have always been of the presumption that to eliminate hydrogen embrittlement in hardened bolts that have been zinc plated it is necessary to bake them after plating. I am being told by our QC guy that it is not recommended to plate the bolt at all as complications can still result even if they have been baked. Any opinions on this mater would be appreciated, I suspect that fastener load is crucial because of the nature of its application.

Thanks

Doug

 

[swall]

Yes, for many reasons, high strength fasteners that are electroplated can fail due to hydrogen embrittlement, even though baking and the normal precautions are in place. The safest way to avoid problems is to use an organic type of coating, like Dacromet or some of the newer products. I'm sure others will elaborate on those.

 

[dbooker630]

In ASTM F568M it states in section 4.6 that class 10.9 and 12.9 bolts, screws and studs should not be hot-dip zinc-coated. There is no similar statement in the standard for mechanically deposited zinc, but I agree with your QC person. We do not use 12.9's in safety related or corrosive applications.

 

[CoryPad]

This is a complex subject. Baking after electroplating for high-strength parts is required due to the hydrogen generated during the electroplating process. The reason ASTM F 568 prohibits hot dipped galvanizing is because of the acid pickling step prior to immersion in liquid. The pickling introduces hydrogen, and the thick zinc coating can trap the hydrogen in the part. Since mechanical zinc does not use the heavy acid pickling, and since it is a thin coating, it is not prohibited.

However, there is an argument that says avoid anodic coatings on high-strength steel altogether to prevent hydrogen generation during exposure to a corrosive environment. This is common in the structural fastener market, e.g. ASTM A 490 prohibits metallic coatings on fasteners.

Some good information on this subject is available in Specification for Structural Joints Using ASTM A325 or A490 Bolts from the Research Council on Structural Connections (free download):

http://boltcouncil.org/files/2004RCSCSpecification.pdf

Recently, work was done to qualify Dacromet for ASTM A 490 fasteners. Dacromet is one of several non-electrolytic zinc multilayer coatings that provide high corrosion resistance with improved resistance to hydrogen embrittlement. A review of this work is in the latest edition of Fastener Technology International, available for free here:

http://www.fastenertech.com

 

[MotorVib]

Grade 5 & 8 bolts are common to be plated. Greade 5 is about 10.9 and grede 8 is about 12.9. The Key that that I have found is to have a good vender. If you do have hydrogen embrittlement you will know in a few days.

The sad part about this is the only good way that I have found to not have problems with hydrogen embrittlement is just a good vendor.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[swall]

Not to be picky, but an SAE grade 5 fastener corresponds to a metric grade 8.8/9/8 and an SAE grade 8 corresonds to a metric grade 10.9.

 

[dbooker630]

The only concern with Dacromet is the hexavalent chrome content. There is a dacromet free of Cr+6 called Geomet that works.

 

[MotorVib]

swall your right I miss read my table.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[Flesh]

To reinforce what others have stated, avoid electroplating high strength fastener Grades 8 and 9 (10.9 and 12.9), especially in safety/critical applications. If coating is necessary, I always prefer one of the organic types i.e. Dacromet, Geomet, etc. Even if you can find a reputable supplier for electroplating, things can foul up. Additionally, your purchasing department is most likely looking to do things cheaper all the time. So when they switch suppliers, your left with questionable product and a large headache. I see this too often (in fact 3 weekks ago). Had to quarantine one lot at several plants after fastener heads started popping off within 48 hours. Generated a nice pile of expensive re-work. All because an Asian source was selling fasteners a bit cheaper.

 

[Tmoose]

"Fastener heads started popping off within 48 hours."

A decade or 2 ago I worked in a shop that did specialist engine machine work.
A set of Chrysler 426 Hemi rods were rebuilt with new Direct Connection (Chrysler factory high performance division) bolts. The finish machined rods were set on the bench in the balance section. The next day several bolt heads had popped off. Not slight intended against Chrysler then or now, but I believe there is surely some science to making bolts.

 

[CoryPad]

Tmoose,

There is considerable science to making bolts, however much of it has been discovered in the last 2 decades, which was not able to help you and Chrysler in the past.

 

[ornerynorsk]

Have never, repeat, never, had a problem with zinc plating high strength fasteners as long as there was no shortcuts with the post-plate bake. We do from 1/4 to 1/2 million fasteners per year in various grades.

 

[swall]

Sometimes the OEM's aftermarket division gets the MRB stuff.