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Re: Steel in very cold temperatures

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>I know that the low temperatures reduce the ductility of the steel.  Is it
>therefore necessary to reject the use of such steel in lower temperatures
>under all circumstances?
I would. And I would also take particular steps in specifying weld 
procedures that will pass impact tests at low temperatures because weld 
problems seriously aggravate loss of ductility. You shouldn't have much 
trouble getting steel with improved toughness. Look for steel which can 
be supplied to meet specified Charpy V-notch tests. Typically these will 
have a higher manganese/carbon ratio and be normalized or produced to 
fine grain practices. Good welding practies are extremely important.

>I would think that if the stresses are low, then
>the brittleness wouldn't have an effect.  If this is correct, what would be
>an appropriate stress threshold?
You're right. The rule of thumb in pressure vessel work is 2-5 ksi. That 
level will arrest a moving crack provided there's enough material 
stressed at that level. And compressive stress fields prevent crack 
propagation, of course.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw