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Re: High Temp in Bolted Joints

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Title: Mensaje
Fascinating.  I've never heard rationale for such a requirement mentioned at ASTM or ASME.
I can understand where someone wouldn't want to use non-structural bolts like SAE Grade 5s or Grade 8s which are almost always zinc plated.  Most often they have a supplementary chromate-based finish that typically loses its effectiveness and can lead to other problems as temperatures get over 200 deg. F.
As A490s are not permitted to be coated, perhaps that is at the root of the requirement.
However, that wouldn't explain why an uncoated A325 would be prohibited.  This is odd because the tempering temperature of an A325 could quite easily be much higher than that for an A490.  Not only that, ASTM A325 assumes that for bolt diameters 1-1/8" and above we don't necessarily get a uniform quench ---- hence the reduced tensile strength requirement of 105ksi in the larger diameters.  Contrast that with large diameter A490s which we assume we DO get a uniform quench (mandated to be done in oil).  Hence, no drop in tensile strength in the larger diameters.  Thus, in practice the A325 is the more conservative of the two.
Depending upon a fastener manufacturer's material selection --- one would generally expect the A325 to be a more suitable candidate for high temperatures (even above 650 deg. F.,) although limited research has been done in this area.  (If interested, see Journal of Structural Engineering Vol. 119, No. 11, November 1993.) 
It would be very interesting to hear the rationale for any requirement pertaining to temperatures between 176 and 650 deg. F.
David Sharp
TurnaSure LLC
Somewhere I read the following specs. ". . .bolted joints subjected
to temperatures higher than 80º C ( 176º F ),shall be design with
A490 bolts, and with diameters not less than 1"...."
Is there any reason to prevent A325 bolts being used with a high 
temp. environement ?  ( paint protected bolts, bolts in black or galvanized bolts ? ).
Appreciate some enlightenment on this issue.