Liquid nitrogen is commonly used today as part of the process to produce ultra-tough tool steels. When used properly as part of the "tempering" process, it causes the grain size of high carbon steels to become extremely small and well organized, resulting in a tool steel that can be honed to an extremely sharp razor edge and is tougher than steels hardened other ways so that it holds its edge longer.
This is a technology that has developed over the past several years. The steels treated this way are becoming fairly common now and are marketed as "cryogenic". These make especially good woodworking tools - chisels and plane blades.
If you immerse a steel in liquid nitrogen long enough for it to attain severe sub-zero temperatures, it can become quite brittle and easily shatter - while still cold. But once warmed up to room temperature there is no long term effect. The effect is only attained if the steel is treated when it is hot, as part of the tempering process, when the crystals are free to re-orient themselves within the structure.
You can learn much, much more by searching "cryogenic steel" in Google. Here is a Wikipedia article that gives a good overview:http://en.wikipedia.org/wiki/Cryogenic_hardening
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