Steel stud corrosion stress problem? 2

[jimhokie]

I have an application where I want to use a carbon steel (SAE J429) grade 8 stud to hold a piece of equipment to its foundation. It is a 1.125" stud that will be torqued to 650-700 ft-lbs. I'm hearing there is some concern about some kind of stress corrosion cracking or hydrogen embrittlement in highly stressed carbon steel fasteners in a possibly wetted environment. Can anyone explain what the issue may be, or point me to somewhere I can learn about this potential problem?

Thanks in advance for any assistance offered.

Jim

 

[CoryPad]

The issue is that atomic hydrogen diffuses into the iron lattice and reduces the strength of the steel, eventually causing fracture.

You can learn a huge amount regarding this issue. First, perform a search on this site using the term "hydrogen embrittlement". Next, you can perform an Internet search (e.g. Google). Other places to look include standards development organizations (ASTM International, SAE International, ISO), materials suppliers (Carpenter, Allegheny Technologies, etc.), and coatings suppliers (Doerken, Magni, etc.).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

Use a protective coating for the stud (other than electroplating, which can induce hydrogen embrittlement). I would suggest a phosphate coating with an organic film topcoat, such as E-coat, although there are many other such coatings. Also, I would not rely on torque to tighten the nuts, as preload can vary considerably. Use the turn of of the screw type method or use an angular indicating wrench.

 

[arto]

How about Dacromet/Geomet?

http://www.metal-coatings.com/

 

[jimhokie]

Thanks to all for the information and coating suggestions. I am contractually limited in my coating options--I can use one coat zinc molybdate primer or epoxy primer, followed by a finish coat of enamel. I'm guessing the key (in addition to avoiding electroplating) is simply to isolate exposed surfaces of the material from atmosphere to prevent hydrogen ingress? Would I be on firm ground to tell my customer that the allowed paint coating will eliminate any concern for hydrogen embrittlement of this fastener (for its planned 50 year life)?

 

[arto]

Check out the NSTM on fasteners:

http://www.fas.org/man/dod-101/sys/ship/nstm/ch075.pdf

Lotsa good stuff on your problem.

"Grade 8 bolts are sensitive to hydrogen, which causes hydrogen embrittlement. You have to be careful, therefore,
about what corrosion protection coating you use in what environment: in certain environments some
coatings will release atomic hydrogen into the fastener."

 

[swall]

Your stated paint options might last 50 weeks in a wet environment, not 50 years. Can you go to a galvanized grade 5 stud instead?

 

[jimhokie]

It shouldn't really be a "wet environment" per se, just the possibility of getting wet on rare occasions due to a leak or spray from adjacent equipment, and it will likely be repainted many times over its life. We had to go to grade 8 strictly for the strength.

 

[Screwman]

The UTS of your fastener is 114,000 lbs. I ran a quick calculation using .2 as your nut factor and come up with a clamp load of about 35,000 lbs. Why are you using such a high strength part if you are not loading it up to it's full capability? I would expect a clamp load of about 85,000 lbs. for that size and grade of bolt.

With that being said, at the low clamp load levels that you will be seeing at that torque level you should not have problems with either SCC or embrittlement cracking. The suseptability of fasteners to those two failures tends to be proportional to the hardness of the parts (UTS) and the applied stress (clamp load). Low lovels of one or the other greatly reduce the potential for failures.

Dick

 

[jimhokie]

While your numbers vary from mine, you make a good point. (I have a proof load of 91,600 lbs per SAE J429 Gr. 8, and a calculated preload of 54,900 lbs. at 700 ft-lbs torque on lubricated threads, equating to about 60% of proof load. Granted, there is uncertainty with translating torque to preload.) Is there some established percentage of proof load, or a rule of thumb below which hydrogen embrittlement concern goes away?

Also, per the NSTM referenced above by arto, hydrogen embrittlement is only a concern for fasteners with a tensile strength above 150 ksi. SAE J429, Gr. 8 tensile strength is right at 150 ksi, so it sounds like I may be borderline as is, and if being preloaded only to 60% of proof makes it even less susceptible, maybe I have good argument for my stud material?

 

[TVP]

jimhokie,

The minimum tensile strength requirement for Grade 8 bolts according to SAE J429 is 150 ksi, so all of them will be susceptible to hydrogen embrittlement. Even with the coatings that you mentioned, this will not be a conservative design for 50 years of service. Are you sure that the coatings for the rest of the structure also apply to the bolts, or is it possible to use high performance organic coatings such as Magni, Doerken, or Metal Coatings?