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

 

[EdStainless]

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

 

[GlennByrne]

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.

 

[SJones]

"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.

 

[bac47]

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.

 

[EdStainless]

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

 

[bac47]

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?

 

[WKTaylor]

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

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o We believe to be true what we prefer to be true. [Unknown]
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[bac47]

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