Ever see a batman episode where Mr. Freeze freezes a large steel door and just kicks it through? in movies and TV I seems to recall that freezing things makes them weaker.
But wouldnt freezing things make them stronger? Since the molecules are more closely packed? Or is this true and TV has it wrong?
At incredibly low temperatures, most things become brittle. This is why you can break a frozen steak, but not a raw one, you have to tear the raw one. I am a cook, though, and can tell you it is much easier to cleave a raw steak in two than a frozen one.
Not sure how this works on already solids, but I believe, the "Comic book theory" on how this works goes like this:
Freezing causes things to crystalize, making them more likly to shatter. Mr. Freeze is an expert on how things freeze and knows where and how to hit this structure to force this crystalized structure to fall apart.
Most TV shows that deal with the fantastic (i.e. Comic Book Heroes, superspies, etc.) use this same sort of spudeoscience to explain thier inner logic and workings.
Actually some steels don’t have to be all that cold to become brittle. I had a need to specify some steel for use on aircraft one time so I checked with our metalurgical mavens. They warned me to make sure I specified steel that stayed ductile at low temps and helped me make the right choices.
Temperatures as high as 35[sup]o[/sup] F can cause some steels to become brittle. In fact, I seem to remember that there is a possibility that the rivets on the Titanic were of such steel, since metalurgy was sort of primitive at the time, and thus popped, causing the hull to open up large leaks and that there was no long gash as had been thought.
Steel has a microstructure which you can see here. Below the NDT (Nil Ductility Temperature) steel is brittle and above it is ductile.
To not confuse you with more details, think of steel as a mixture of clay and sand. As you cool it down, the sand percentage increases and hence it becomes more brittle.
That’s an awesome microscopy link andy_fl. Thanks 
1st of all i hope you were watching some really old Batman. I didn’t think there was anybody who watched “Batman and Robin” with Arnold Schwarzenegger. I think they are usually showing freezing things with a liquid nitrogen or something. Like in the movie Terminator 2. I think when something is that cold it is brittle but as the warms up a bit it begins to get stronger. I recall seeing a segment on a “20/20” type show that a company was using nitrogen to “deep freeze” metal objects and when they thaw out they become stronger. Obviously i am no scientist but it had something to do with the way molecules line up (parallel I think) after freezing. But then again i could be wrong.
Isn’t “brittleness” somewhat relative? Sure, frozen-to-fifty-Kelvins steel might be weaker than steel at room temperature, but brittle enough to shatter with a kick? I think not.
Thawing doesn’t apply to steel as you use it because it’s still frozen. It’s not as if the steel was saturated with water which melted.
Cryogenic treatment is quite popular in firearms. Typically barrels and other parts are gradually cooled to the temp of liquid nitrogen then warmed up again.
Nametag, some steels get brittle quickly. Some bicycle and steering wheel locks are easily shattered by chilling them with a can of freon and stiking them with a hammer.
I dont think it is recrystallisation as that would be too slow at these temperatures. Essentially (and this is true in plastics too) any perfectly rigid solid is also very brittle as it cannot prevents cracks spreading. It is cracks which destroy any metal or plastic. The colder the material, the less the atoms or molecules can move, so any energy cannot be dissipated, and so spreads in the forms of cracks
Okay, think about this. Hang a t-shirt out on a line and try to punch a hole in it. Doesn’t work right? its because the tshirt acts to absorb your energy and distriute it.
Same thing with the metal door. If it were at room temperature, it would simply dent. If you kept on hitting it, you would eventually “tear” a hole in the door. Not the best way to do it. Now, if you froze it, it becomes brittle and yo can “snap” rather than tear the door.
Nice overview of why plastics (polymers) get more brittle at cold temps