does steel "weaken" at higher temperatures?

A friend of mine and I were having a discussion about whether fire weakens structural steel in buildings, and I must admit that neither of us has the
technical background to answer this.
Does this question have an easy answer?
--
Bob Fleischer
Groton, MA 01450
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Bob Fleischer wrote:

Let us start simple.
Any matal or alloy heated to near the melting point (liquidus for some alloys) is mechanically weaker than when far below the melting point.
Temperature is effectively a measure of thermal vibrational energy of the atoms or molecules that make up a substance. So, at high temperatures, the atoms/molecules have more energy which they may attempt to use to wiggle past each other or to wiggle past or around whatever was keeping them in place.
If you have a propane torch and a safe place that won't catch on fire, you can clamp pieces of coathanger in a vice and do some heating and bending. You will feel the metal bend easily when red hot. It will bend much more easily than when at room temperature.
You can also play around with heating and bending any other small metal specimens you may have. You'll feel the same effect.
The reduction in flow (and fracture) strengths at high temperatures is used in many metals forming operations such as hot forging either in a die or as the old fashioned blacksmith did with hammer and anvil and the furnace.

and Methods, (incidentally, an excellent book by a great architectural educator, which I've enjoyed both as a student and teacher). The section on fireproofing steel includes the classic graph (p371, f9.64 in my old edition) of yield strength versus temperature for ASTM 36 standard U.S. structural steel.

There are papers which show the generalized trend of decrease of strength with dimensionless temperature (test temperature/Melting Temperature in absolute temperature scales).
For several metals(alloys) of defense interest, you can get the elevated temperature data in MIL HDBK 5 (Military Handbook 5) and with some digging around, you can download a free copy in PDF form, or you can actually pay for it if you like. MIL-HBK5.PDF is the file name on my hard drive and it is over 40 MB in size.
Jim
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I haven't actually done that to feel the effect on 304 stainless steel rod. I will sometime. :)
At this point tho, I do know that with a 3/32" thick 304 stainless steel rod, I can hang a "low alloy, high carbon steel" pocket knife blade or spring from it while heat treating, and it doesn't sag like a coat hanger would! :)
I've even been using the same piece over and over. :)
So there are differences in material, when and how much it softens, but the general rule still applies? ;)
Alvin in AZ
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You may think about 9/11. It's the prove tha at 1/3 of the melting temperature the metal structure waekens and if loaded collapse. The mane of the phenomenon is "creep".
andrea
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YES

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There are two questions 1 is steel weaker when it is hot: answer yes it starts to soften from about 300C. See the following for some basic data. http://www.corusconstruction.com/legacy/fire/images/fireres_section6.pdf
http://www.corusconstruction.com/legacy/fire/images/fireres_section7.pdf
2 is steel weakened by having been heated in a fire: answer it can be, but often structural steel will still have acceptable strength after a fire.
http://www.corusconstruction.com/legacy/fire/images/fireres_section16.pdf
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