Screw Material to Prevent Galling on 304 SST Threads? 1

[hydronicengr]

Currently, we have regular (304 SST) Heli-Coil inserts on the substrate we are fastening to. Most of these substrates are installed in the field already so changing out the inserts is not an option. Some of these substrates also have Nitronic 60 inserts.

Due to cleanliness purposes, we cannot use our readily available 300 series stainless screws with an anti-seize compound.

I've been looking at using Nitronic 60 screws, but for our size (3/8-16 UNRC-3A between 1 and 2 inches in length), they are quite expensive at $13-15 per screw -- even in large quantities.

What other screws materials (plated or not) are readily available, cheap, compatible with BOTH 304 SST and Nitronic 60? I'm using socket head cap screws.

Thank you!

 

[MikeHalloran]

Nylon.

... you didn't specify a strength or stiffness requirement.



Mike Halloran
Pembroke Pines, FL, USA

 

[hydronicengr]

Apologies.

Screw must have similar mechanical specifications as 300 series stainless (304 and/or 316).

 

[MikeHalloran]

You mean like soft, weak, work hardens, galls easily? Those properties? ;-)

I think there are some coppermetals that might work, except for the cheap part.

I was actually not kidding about the plastic option; the stuff can be surprisingly strong. ... but plastic is not metal.

How about you reverse engineer your application (or, long shot, find the original calcs), so you can evaluate possible alternatives against the actual functional requirements?



Mike Halloran
Pembroke Pines, FL, USA

 

[CoryPad]

What are the quantities? There is no reason to pay $15 for a Nitronic 60 part that fits in the palm of your hand. What are the limitations on surface treatment? If anti-seize paste is not an option, what about dry-film lubricant or PTFE-coatings like Everlube or Xylan?

 

[hydronicengr]

Quantity is 100.

 

[Tmoose]

If you are unable to find an alternate or coated fastener removing the 304 inserts to install Nitronic or other gall resistant inserts is not all that hard.
Helicoil sells an "extracting" tool. page 31 here -

http://www.helicoil.in/pdf/HeliCoil Catalogue.pdf

 

[moltenmetal]

CoryPad and Mike have put an envelope around the solutions in my opinion.

Fluoropolymer-coated plain-Jane 18-8 fasteners, installed carefully with a torque wrench, would be pretty cheap. Replace after each removal.

You could try some expensive anti-galling material solution (i.e. the Nitronic), which still is unlikely to work reliably if installed with no lubricant whatsoever and/or if over-torqued. That assumes that the obvious solution from a materials perspective, i.e. going with a bronze instead of a stainless steel, is not an option for some reason other than, "it isn't silver-coloured".

 

[berkshire]

Could you use Nickel bronze, AKA German silver, or would the zinc content be a problem?
However they are not cheap.
B.E.

You are judged not by what you know, but by what you can do.

 

[EdStainless]

I would suggest Si Bronze bolts in Nit60 inserts.
Change out all of the ones that are 304.
Si Bronze is regularly used for bolts for electrical connections and should be available.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[hydronicengr]

Coreypad: What are the disadvantages of using PTFE coated screws? I was unable to find some solid data on the wear rate of these things. Some of these screws may have to be removed/installed frequently. I know it depends on the coating thickness as well as curing temp and other factors, but I would like to know a ballpark value in terms of number of uses.

Thanks

 

[CoryPad]

Yes, it depends on several factors. If the coated screws engage with parts with rough surfaces (head and thread contacts), you may abrade the coating and only get 1 or 2 good uses. If the system is well-designed, manufactured, assembled and maintained, then you could expect around 10 uses without problems.

 

[tbuelna]

hydronicengr-

Use fasteners made from A286 cres. They will be harder than the 300 series cres inserts, and they should also pose no galling issues with nitronic 60 materials. While the A286 screws will be a fair bit stronger than the 300 series cres screws, I don't see why this would present a problem.

 

[Guest102023]

I've seen the aftermath of PTFE coated fasteners and to me that's a non-starter.

I think you should start by laying out your strength requirements and corrosion requirements. Check out handbooks such as Smithell's for properties between pairs of contacting metals. Could martensitic SS be an option?


"If you don't have time to do the job right the first time, when are you going to find time to repair it?"