Coated CS bolting for cryogenic service 1

[XL83NL]

I have spec which reads that for a cryogenic application (design T approx. -325 F), A320 L7 bolts + A194 gr 7 nuts, both PTFE coated, will be used.
From B31.3 (applicable design code), I believe L7 may not be used at that temperature. Environment is inside a building, highly corrosive fluids.

From an application point of view, would anyone suspect a problem?
I have no background in cryo's, but something tells me L7 at -325 F may become too brittle. However I dont know what the PTFE will 'do to it'.

 

[TVP]

The PTFE is really a two-step coating: the first layer is zinc phosphate, then there is an organic coating deposited (typically an epoxy with some PTFE particles distributed within it). This coating does not have an effect on the temperature capability of the fasteners.

ASTM A320 requires impact testing of L7 at -150 F, with a minimum energy of 20 ft lbs. Cryogenic applications at -325 almost always use an austenitic stainless steel such as grade B8 from ASTM A320.

 

[XL83NL]

What I already suspected.
Put short, it means L7 wont do the job, and they'll probaly become too brittle and break at some notable event.

 

[EdStainless]

Do people use anything other than austenitic stainless or Ni alloy bolts at these temps?

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[XL83NL]

@ Ed: I've no/little experience in low temp/cryogenics, but I'd suspect the same thing ...

 

[Duwe6]

Stainless and aluminum are the materials of choice for cryogenic service. Stay with B8 or B8M bolting [304 & 316, respectively], and adding a teflon coating is a very good idea. It prevents galling.

 

[TechThreads]

These fasteners are actually pretty easy to come by... the Xylan line of coatings (fluoropolymer based) are what is typically supplied when people ask for "PTFE coated". In addition to what Duwe6 said regarding galling, the corrosive effects will be severely diminished by the protective coating on the fasteners. Xylan 1424 is the most common, and has the best chem resistance for your environment, but it won't do well for the cryo application. Xylan 1014 is more common for cryo, and still has overall strong environmental resistance.

 

[XL83NL]

Are coated bolts+nuts in general (say for standard applications, -200 upto 550 deg C) always a good option?
For prevention on galling, corrosion, decrease bolt stress, etc.?

 

[EdStainless]

With SS and even some Ni alloys it is common to use coatings to minimize galling.
Just be aware that many of the coatings also become brittle at cryo temps.
I have seen bolts made of 625 used for service that required the combination of high strength, corrosion resistance, and wide temp range (625 will handle -400F to 1400F).

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