Thread Galling 3

[seaguy]

Our current application is a 17-4PH Condition 900, 150 KSI min. Y.S. passivated bolt (socket head cap), with 5/8-11UNC screw threads that threads directly into a 316L base weldment on an ocean going vessel. This bolt is holding a roller assembly on a Helicopter Hangar Door, so it is exposed to a salt-laden atmosphere.

Over time the sailors try to remove the bolt, and it is shearing off clean right under the head (90 degree shear plane). I suspect the bolt threads have 'cold-welded' themselves to the 316 tapped holes. It looks like anti-seize was applied to the threads, but to no availe.

I'm thinking of possibly chrome-plating the threads, to improve hardness and surface finish, while retaining corrosion resistance.

Is this the right approach, or should I further investigate corrosion versus thread galling as the mode of failure?

 

[Metalguy]

If you don't need such high strength for the bolts, you can try Nitronics 60 or equiv. It has high resistance to galling, and better corr. resistance than 17-4.

BTW, using 17-4 in the H900 cond. is just begging for hydrogen embrittlement. I thought the Navy banned its use for aircraft applications (at least for stuff that flies) way back in the late 1950's.

 

[SJones]

Check to see whether the "anti-seize" contained molybdenum disulphide. It has been proven that this compound reacts with moisture to produce H2S. Your bolts could then have suffered a 'sour service' related failure.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[EdStainless]

I like some of the thoughts above.
I think that it is likely that either hydrogen embrittlement or crevice corrosion is what kills the bolts. The high torque required to remove them just finishes it off.
You can probably find bolts in K-500 Monel that will fit also. This would be a good option.
Bolts that are 'plastic' coated would also be a good option (do both!). Such coatings are available that contain lubricants (teflon).
If you want to stay with an anti-size contact Dupont about Krytox. It is a synthetic oil/grease. The grease is available with many additives, including some non-sulfer based anti-galling agents.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[unclesyd]

As posted by MetalGuy using 17/4 PH in the H900 condition is a no-no in my book. I was advised many years ago by ARMCO never to use the 17/4 in the H 900 condition maybe the the H925 condition or better yet the H1125 condition around saltwater.

One thing that I've done is use only 70% or less thread engagement for set in fasteners in saltwater.

One type lubricant that I've had good luck in saltwater with is FEL-PRO(copper)and Thermoplate 220 (boron nitride).

Depending on the quantity Monel bolts are in short supply. Nitronic 60 is good choice but again rather slow in coming.

For any type of marine bolting and a lot of good information.

http://www.marfas.com/specialmaterials.shtml

 

[seaguy]

Uncle Syd,

Thanks for the reply.

One question, you state:

'One thing that I've done is use only 70% or less thread engagement for set in fasteners in saltwater.'

I am not sure how to interpret this. Are you saying only use 70% of Y.S. for torquing values, or allow only 70% thread engagment of blind tapped hole?

I can understand and relate to the first interpretation, but the second interpretation I do not understand the outcome of doing that...

 

[unclesyd]

I was referring to thread engagement. I actually drill the tapped holes oversize in your case .5432" for 70% but would use 35/64 drill .5469". 100% = .5068. If the drilling was not in a rigid setup I might drop back a size and use the probable hole size of the smaller drill bit. On some preexisting threads I have taken the tips off the bolt threads.

I have found this to be quite helpful in dealing with SS fasteners. This can take out some of the root/tip interference normally found in tapping threads in SS or other alloys.

One other thing that you might consider in your situation is to use threaded insert or HeliCoils in the 316 SS. We used a lot of 316 SS components and nearly all ended up with threaded inserts or HeliCoils especially if the bolted connection was disassembled often.

 

[unclesyd]

Forgot to add that the hole is easier to initially tap.

Also you can now get Nitronic 60 HeliCoils/inserts in your size. I nearly all manufacturers have them now.

One supplier

http://helicalwire.com/product.htm

A supplier plus a good article on Nitronic 60 inserts.

http://www.emhart.com/products/helicoil/techtopic/898-1.htm