Hopefully someone with more detailed knowledge may chip in but what you
are interested in is the ductile-brittle transistion temperature for the
material. For common structural steels it is not much below freezing
IIRC from college days. Above this temperature failures exhibits ductile
behaviour and below it brittle behaviour but with a transition between
to two dependant on the material. IIRC this property can be an issue in
artic conditions. So I could speculate that freon on a carbon steel lock
may cool it sufficiently to make it brittle, but for alloy steel it may
not. IIRC stainless steel maintains its ductility to lower temperatures
than carbon steel.

Kamus of Kadizhar wrote:

I've got a purely hypothetical question:

ISTR that at one time, freezing locks with Freon was a popular way to
break them. The theory being that you freeze the lock mechanism or
shackle until it becomes brittle, then use a hardened hammer to smash it.

Now I'm being told that's an urban myth.

I come here seeking expertise on frozen metal - is it practical / possible
to freeze steel to where it becomes brittle using a can of Freon? The
technique was to use Freon under pressure, then "spray" it on the lock.
The Freon cools as it expands, freezing the metal.

--Kamus