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.

Bill Swears wrote:
Hi, I'm a fantasy and science fiction writer, and was recommended to
this group for a technical question.

Well, maybe 'technical' is stretching things.

I've written a bit where snaps and shackles were made of a steel. They
tested well in normal temperatures, but shattered under load at about
-40. I used manganese as a placeholder, but would like to use a correct
alloying material to have my 'expert' character tell the journeyman to
"try using a little less (or a little more) X" where X is my manganese
placeholder.

Can anybody help me? Have I provided enough information?

Bill