High strength nut/bolt for marine application 4

[bac47]

I have been researching materials for use in a marine fastener application, but could use some input from some of you with experience. We have a part with a socket hex drive and 3/8-16 UNC thread and a mating square captive nut. The parts will be completely submerged in seawater for most of their life. The nut in particular is captive in a closed pocket where seawater will likely collect and become stagnant. I believe this will create a significant risk for crevice corrosion. High strength is required for these components in the range of 80 ksi yield or better.

We have used monel 400 or 500 for similar applications but there have been reports from our production staff of the hex drive stripping out easily during assembly. I am thinking this phenomenon is limited to the 400 alloy since it is not as hard, but I don't know because our drawing allows fabrication from either.

We have also used Inconel 718 for similar applications and I am leaning toward using it for at least one component here. I am concerned about galling between the nut and bolt. These parts may be disassembled on a monthly basis and I don't want them to lock up. Is there another comparably strong and corrosion resistant material that would work well with 718 for the mating part? We have used Nitronic 60 and Ni-Al-Br to prevent galling in the past but I don't think either is suitable for this application.

Also of note, we need a way to assemble these components so that they will resist loosening from vibration. Threadlocker seems like a good solution so I don't think lubricating the threads is an option to prevent galling.

Thanks

 

[TVP]

You may want to consider a high-strength copper-nickel alloy such as Marinel 220 or Marinel 230. These grades were developed for immunity to corrosion in seawater, and have relatively high strength (yield strength > 700 MPa, similar to ASTM 193 B7 steel fasteners. They are also highly resistant to galling, and do not require a lubricant for assembly/disassembly. Thread adhesive would therefore be a possibility. Here are some links with additional information:

http://www.tachart.com/material/marinel-220/

https://www.copper.org/applications/marine/cuni/alloys/high_strength_alum.html

http://www.azom.com/article.aspx?ArticleID=1732#_The_Development_of

https://www.nickelinstitute.org/~/m...130902-14054 ALLOYS FOR MARINE FASTENERS.ashx

 

[EdStainless]

Don't use 718, 625, or Nit 60.
As a first step I would require K500, with no options. You do require that these for formed (either cold or warm, but never hot), and then threaded and aged? The sequence is critical.
Marinel is a good option.
A "C" alloy (276, 22, 622, 686, 59, ...) would also give you the desired corrosion resistance.
When it comes to galling using dissimilar metals usually helps, so mixing Marinel and K500 should be a good combination.

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P.E. Metallurgy, Plymouth Tube

 

[bac47]

Ed - Don't disagree, but why not Inconel 718 or 625? Also, why not hot finished K500? Couldn't the material be machined and heat treated/aged afterwards regardless of the condition of the raw stock?

I should mention this is a very low volume application. This may impact material selection and mfg processes mentioned. I am going to check with a couple of our material suppliers, but is Marinel readily available or is a mill run required?

Thanks very much for the suggestions, this is very helpful information.

 

[EdStainless]

I have seen both 718 and 625 suffer crevice corrosion in seawater service. Granted both were in cases where there was plenty of oxygen and biological activity.
I have also seen K500 bolts that were made wrong, they had variable grain size with coarse structure at the shank-head transition. These bolts failed at low loads even after proper HT. There are people out there making K500 bolts regularly, and doing it right.

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P.E. Metallurgy, Plymouth Tube

 

[bac47]

I noticed Langley doesn't list Marinel as an available alloy on their site anymore. It doesn't look like these high strength cupro-nickels are covered by an ASTM spec either. I had checked on Langley's Hiduron alloys previously and it looks like they are difficult to procure in the US and expensive. This tells me that most of the guys specifying materials for offshore/marine applications are using something different. Is there something else that combines high strength with marine corrosion resistance and galling resistance that is readily available?

 

[EdStainless]

Does Materion (formerly Brush Wellman) make fasteners in Toughmet?
I don't know.

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P.E. Metallurgy, Plymouth Tube

 

[bac47]

Waiting to hear back from a couple materials suppliers on availability of toughmet.
I have been reading as much as possible and did see mention of corrosion of Cu based alloys in seawater near land due to contaminants from waste.

I think I may be over-emphasizing the galling requirement. Offshore/Oil&Gas/Marine bolting IAW A193 and A194 recommend 316 bolts with 316 nuts which would seem to represent a horrible combination for galling. Also, in literature regarding offshore and marine bolting, galling comes up surprisingly few times.

Ed - While 625 and 718 suffer crevice corrosion, it seems like K500 is susceptible to hydrogen embrittlement, correct? Also, what is the mechanism that causes a material that has been hardened to corrode in seawater as opposed the same material in its annealed condition? Does this also apply to age hardened alloys like K500 and 718?

Would a K500 fastener and C276 or Inco 686 nut make sense? Could I get away with something cheaper for the nut?

Another detail, the majority of the system is composed of thick, forged 316L tubing.

 

[metengr]

I would look closely at Aquamet 22 for bolting material. It has good strength and excellent corrosion resistance in seawater.

 

[bac47]

Aquamet 22 looks like Nitronic 50 with a different name, right? I don't think Nit 50 has the corrosion resistance I need for this application. I think at least a super duplex or 6Mo is required. We have a number of examples of small parts made from Nit 50 showing substantial crevice corrosion in a system installed very near to our own.

 

[metengr]

No, they are similar but not the same alloy. I have seen this material used as shafting and keel bolting in sea water with great success. Seeing this material in a keel bolting application with no crevice corrosion speaks for itself.

 

[EdStainless]

47, You are correct on the Aq22, it is cold finished Nit50, and while it works fine for shafting I wouldn't use it for fasteners because of crevice corrosion unless it was to put together and leave in place.

The strengthening mechanism in K500 has no impact on its corrosion resistance. I have only seen hydrogen embrittlement in material that was heavily cold worked and direct aged.
718 has mediocre aqueous corrosion resistance in any condition. I did a bunch of slow strain rate testing and it is susceptible to Cl induced cracking. The K500 and 25-6Mo were not.
A 6%Mo alloy should work, but in fasteners a Ni alloy will be the same price.

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P.E. Metallurgy, Plymouth Tube

 

[bac47]

Want to keep nut and bolt materials different to lessen the chance of galling. K500 and C276 look good but they appear somewhat far apart on a galvanic chart. The boat the system is installed on has cathodic protection but I am not sure how close we are to an anode. Is galvanic corrosion likely to be an issue here?

 

[EdStainless]

My ref for galvanic potentials in gently flowing seawater show Monel at -0.20V and Ni-Mo-Cr alloys at -0.05V (both vs SCE). This is generally not enough difference to be of concern.
Good to check though.

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P.E. Metallurgy, Plymouth Tube

 

[SJones]

http://www.haynesintl.com/alloys/al...istant-Alloys/c-22hs-alloy/principle-features

http://www.nace.org/cstm/Store/Product.aspx?id=2a60691f-82ed-e111-ac69-0050569a007e

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[metengr]

Viable material to consider.

 

[EdStainless]

Viable, yes.
Actually produced, not really

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P.E. Metallurgy, Plymouth Tube

 

[bac47]

As a preliminary search I use a site that shows how many mills and distributors in North America make or carry a specific alloy. When I searched for C22 it looked like a fair amount of both, which would lead me to believe the material is readily available. It actually looked like a really good choice to me, there is an ASTM spec for the cold worked and/or age hardened stuff which makes things easier for the engineer.

I'm only basing my comment on a single site, is C22 hard to get? Keep in mind these are designed parts, we cannot use standard fasteners for this application. Qty will likely be in the 200-300 range total for each part.

Another question, among other things, I have been referring to Norsok M-001 and EEMUA 194 for some guidance on materials selection. Are these viable sources? I ask because EEMUA practically writes off Monel because of failures, and M-001 doesn't even mention it. We have been using small Monel K500 SHCS screws on "wetted" parts of our system for decades without issue. We also use quite a few very high strength 718 SHCS in probably the most critical structural connection that we deal with, again without any issue that I know of.

 

[metengr]

Norsok is very good as a reference document for materials applications.

 

[SJones]

Of the two documents, EEMUA 194 is far superior, not least because it gives the reasoning behind materials recommendations. It also tends to be less dogmatically conservative than the NORSOK standard.

Like most alloys dependent upon a heat treatment process, or processes, if it is not carried out correctly the material could be rendered extremely susceptible to hydrogen embrittlement. The good experience with the Monel screws could be down to correct vendor selection, amenable dimensions for good heat treatment,low stress regime, and so on. It's not a foregone conclusion that cracking will occur, but the likelihood side of a risk assessment does start to head upwards. Issues did arise with N07718 processing resulting in the generation of API Std 6A718, and then API Std 6CRA. The latter document may well be worth a review to get an idea of key quality control requirements.

It was a DNV report, 2002-3301, that effectively killed off K500 for the oil and gas industry, and that is promulgated in EEMUA 194.



Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.