In article SGu1f.8119$oc.7024
@newsread2.news.pas.earthlink.net,
says...
Actually, it's the plain carbon steel that is brittle at low temps,
adding some molybdenum does wonders for low temp strength.

You might want to read up on the 'Charpy' test for impact resisitance.
You put a large notch in the sample piece, use a pendulum to swing a
heavy weight into the sample. At -40 (F or C) carbon steel just snaps.


Also search on "brittle to ductile transition temperature"

Ned Simmons