Hydrogen Embrittlement ASTM A574

[TugboatEng]

Today I have inherited several vessels with obsolete drive units and no factory support. The drives have submerged.structural flanges and are notorious for breaking bolts. 30 years later, I think it's time to look into a cause. The fastener are socket head caps screws of ASTM A574 standard and some shoulder bolts of no specific standard. My understanding is that the shoulder bolts have never been found broken. It appears the shoulder bolts are 7/8" and 32 HRC. The bolts that are breaking are 1-1/8" and 37 HRC. Another manufacturer of similar equipment uses exclusively class 8.8 fasteners, I assume to reduce the risk of hydrogen embrittlement.

Here is a picture of one failed fastener. Are their any visual clues of HE? Can HE be determined if the fracture faces are corroded?

I'm thinking of specifying ASTM A193 B7. The problem is that lower strength socket head cap screws are not commercially available in imperial dimensions.

 

[3DDave]

Off the top: https://www.boltcouncil.org/files/HydrogenEmbrittlementInSteelFasteners-Brahimi.pdf

Not sure it's hydrogen. It could be, but what I have seen in hydrogen embrittlement has been like the part is made of glass; just shatters right away.

However I cannot say I have seen every possible case and usually the fracture, such as from fatigue, goes perpendicular to the axis rather than this weird cone.

The usual source of hydrogen is some part of the manufacturing process, such as pickling or electroplating or both. This report describes what the metallurgists I worked with had to say on the subject; mainly that plating or passivation should be followed by baking to cause the hydrogen to exit.

It also mentions the failure of fasteners that are subject to high temps and the problem that cadmium plate poses as a stress risk. There was some engine the supplier had used cad plating on the exhaust headers and they had a 100% fail rate. The spec for the fastener specified "NOT FOR USE ABOVE X00ºF" of some similar and suspiciously low limit. ( I forget the exact temperature. It's been 20 years.)

So, if unsure, you could simply require baking the fasteners at a few hundred ºF for several hours before installation as mentioned in the report.
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This site has pictures of various failures; the one that matches is stress corrosion fracture: https://smrp.org/News/Solutions-Arc...y-Learn-to-Recognize-Mechanical-Failure-Modes

Stress Corrosion Cracking

Bolts being under constant tension leave them susceptible to stress corrosion cracking (SCC) if they are exposed to water-based corrosion agents. The effects of any corrosive environment are made worse at warmer temperatures. Over time, bolts exposed to the wrong compounds can begin to suffer cracking and eventually fail. Each material type is susceptible to different corrosion agents. Hardened carbon steels are at risk of SCC when exposed to a variety of compounds including ammonia, caustics and sulphur-based compounds. Stainless steels are most susceptible to chloride-based cracking.

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Other failures of this type produce a cup-and-cone surface pair when the fastener is not strong enough, though those seem to have a lot more tensile section reduction. Maybe the stress corrosion cracking causes the tensile failure without allowing much reduction in section.
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Recommendation - install the bolts with polysulfone rubber sealant. Apply liberally to threads and ensure all small cracks that could let water get into the threads are filled, like under the head. It cannot corrode if it cannot get wet.

 

[MintJulep]

ASTM A574-17: 5.5 Standard Finishes—Unless otherwise specified, the screws shall be furnished with one of the following “standard surfaces as manufactured” at the option of the manufacturer: (1) bright uncoated, (2) thermal black oxide, or (3) chemical black oxide. Hydrogen embrittlement tests shall not be required for screws furnished in these conditions.

So, the folks that wrote the standard weren't worried about hydrogen embrittlement with these three finish options.

Looks like your screws have one of the two black oxide finishes.

The shape of the fracture looks most like stress corrosion cracking from the pictures at the link provided by @3DDave.

Probably not hydrogen embrittlement.

Black oxide is mostly useless at preventing corrosion, as evidenced by the all the corrosion on your screw.

These days you should be able to get A547 SHCS with Aluminum-Zinc Flake coatings (i.e. Geomet or Dacromet). Far better corrosion protection than black oxide, and the process does not cause hydrogen embrittlement.

The Unbrako catalog sort of suggests that it's possible. https://unbrako.com/images/downloads/Unbrako_US_Product_Guide.pdf

Where are these installed? What's the loading?

 

[LittleInch]

Do you have a sectional diagram of the bolt installation?

That looks more to me like crevice corrosion / fatigue of the bolt on the left caused by stress concentration at the point it enters the hole. but its not really possible to see is it he bolt that's the issue or the design?

 

[TugboatEng]

Stress corrosion cracking, ISO 898-1 calls this out as a problem for these fasteners. The zinc-aluminum flake coating sounds like a very good solution and should be commercially available. I will pursue this route.