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High strength nut/bolt for marine application
4

High strength nut/bolt for marine application

High strength nut/bolt for marine application

(OP)
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

RE: High strength nut/bolt for marine application

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/al...

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

https://www.nickelinstitute.org/~/media/Files/Tech...

RE: High strength nut/bolt for marine application

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

RE: High strength nut/bolt for marine application

(OP)
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.

RE: High strength nut/bolt for marine application

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

RE: High strength nut/bolt for marine application

(OP)
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?

RE: High strength nut/bolt for marine application

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

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P.E. Metallurgy, Plymouth Tube

RE: High strength nut/bolt for marine application

(OP)
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.

RE: High strength nut/bolt for marine application

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

RE: High strength nut/bolt for marine application

(OP)
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.

RE: High strength nut/bolt for marine application

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.

RE: High strength nut/bolt for marine application

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

RE: High strength nut/bolt for marine application

(OP)
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?

RE: High strength nut/bolt for marine application

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

RE: High strength nut/bolt for marine application

Viable material to consider.

RE: High strength nut/bolt for marine application

Viable, yes.
Actually produced, not really

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P.E. Metallurgy, Plymouth Tube

RE: High strength nut/bolt for marine application

(OP)
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.

RE: High strength nut/bolt for marine application

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

RE: High strength nut/bolt for marine application

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.

RE: High strength nut/bolt for marine application

In crude applications people shouldn't use Monel because of Mercury issues.
But in simple seawater service (provided that it isn't heavily cold worked and direct aged) it works well.

C22 is a good option.
I used to make high strength shafts and we saw issues with 718, found that heavily cold worked 625 did better even though it wasn't perfect.
We had K500 fasteners (Ferry cap head style) custom made in quantities of a few hundred at a time.

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P.E. Metallurgy, Plymouth Tube

RE: High strength nut/bolt for marine application

Check out RW Ross of NiDi paper 617 NACE 2000 and KO Fischer of DNV paper 3017 NACE 2003. These are useful reviews of high strength alloys for marine fastener applications.

RE: High strength nut/bolt for marine application

Quote (Roger Francis from Oil & Gas Corrosion and Material Selection group on LinkedIn)


"When I worked for Weir Materials we supplied a lot of lightly cold worked superduplex stainless steel fasteners (UNS S32760) (strength equivalent to B7 steel) with solution anneraled nuts. These have given good resistance to crevice corrosion in seawater up to ~35°C without galling because of the hardness difference beteen lightly cold worked and SA material. If these fasteners are to be CP'd subsea, they will pick up hydrogen, but failures will not occur unless the fasteners are stressed close to the 0.2% proof stress (>95%), which is rare for fasteners."

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.

RE: High strength nut/bolt for marine application

(OP)
I had been taking a second look at the super duplex alloys for this application. The reason being that 718 is strong but doesn't have the corrosion resistance of most of the other alloys in question, 625 is close, but not quite strong enough in the annealed condition (looks like it can be PH to 90 ksi yield or so though?), K500 is strong and corrosion resistant, but I am worried about proper aging, C-22HS is very strong and corrosion resistant but is a different UNS number than regular C-22 and looks like it may be hard to get and probably difficult to machine due to hardness. The high strength cupro-nickels are also very difficult to get. That leaves super duplex, probably 2507.

We are considering using a coating to reduce the likelihood of galling as well. Maybe an electroless nickel or TDC?

Glenn - thanks for the two papers. I had already reviewed the Nidi paper, but I am trying to get my hands on the other one.

RE: High strength nut/bolt for marine application

Any of these will be tough to source, but you have the right ideas.
Just two things to keep in mind.
1. The nut and bold should have different hardness (either same alloy in two different conditions or two different alloys) to minimize galling risk. The nuts are usually the softer part.
2. The bolts should have rolled threads (this is part of why the bolts are usually stronger).

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P.E. Metallurgy, Plymouth Tube

RE: High strength nut/bolt for marine application

(OP)
I think we are going to go with S32760 duplex for the fastener and nut as suggested. We have a few small inserts in an FRP structure that will interface with the head of the "bolt". I am thinking of specifying K500 as an option for these plates because FEA shows we need high strength for these parts as well. I want to make sure the material is properly aged after it is machined. Is there specification available that that will increase the chances of avoiding improper aging? Looks like some API specs exist for 718 mentioned earlier, is there something similar for K500 or does something like QQ-N-286 suffice?

RE: High strength nut/bolt for marine application

bac47... are You familiar with...???

SAE J1781 Materials for Fluid Systems of Marine Vehicles
SAE J2270 Ship Systems - Fasteners - Test, Inspection and Installation Requirements
SAE J2280 Ship Systems and Equipment - Fasteners - Selection and Identification Requirements
SAE J2295 Fastener Part Standard - Cap Screws, Hex Bolts, and Hex Nuts (Inch Dimensioned) [for ships]
SAE J2295M Fastener Part Standard - Cap Screws, Hex Bolts, and Hex Nuts (Metric) [for ships]

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: High strength nut/bolt for marine application

(OP)
I'm not, but I will certainly give them a look. Thank you.

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