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Q & A: ductile-brittle transition

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Most recent answer: 04/27/2018
Q:
Is the embrittlement resulting from extreme cold to carbon steel permanent or would the lattice structure return to normal after an isolated exposure to extreme cryogenic temperatures such as those applied to piping during a line freeze procedure. A line freeze procedure consists of external application of cryogenic temperatures to a pipe to form an ice plug internally to temporarily block flow. Repairs are performed, the external application is removed and the carbon piping returns to normal service temperatures.
- Timothy McKeand (age 50)
Houston, Texas, USA
A:

This is an interesting new question for me. So far as i can tell from both th practical engineering guidelinse on various websites and from the more physical descriptions of the cause of the low-temperature brittleness, the normal ductile behavior is restored once the steel is warm again. The reason is that the brittleness comes from the inability of some defects to move around quickly enough at low temperature. If the steel isn't strained too much while cold, that causes no problem because there's no need for them to move. Thhere's one important warning, however. It's possible for some low-temperature strain to cause damage (little cracks) that's not big enough to cause obvious immediate failure, but would still permanently weaken the steel. So it's important to make sure that the repairs are not stressing the frozen region. Perhaps you could check the pipe using one of the metal fatigue testing methods used by airlines. Or you could try the whole procedure on a test pipe and then test it afterwards to see if it was weakened.

A very different sort of cold effect occurs in some other metals. Tin, for example, changes crystal structure at low temperature, causing it to gradually crumble. Warming it back up won't fix it once that happens. Fortunately, steel isn't like that.

Mike W.


(published on 04/27/2018)

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