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