I have used old bed frame angle for a few projects and have also noticed that it seems more brittle than plain 1020 steel.

Thanks for the info on high carbon content and heat treatment.
Wouldn't have thought of heat treating the bed frame but it makes sense to not have it break during "use".

My projects using bed frame angle have not had cracking welds but this may be due to me using stainless wire when I tig carbon steel.
I have a lot of stainless and nickel alloy wire but little carbon steel tig wire.