knowing the material type and some of the process helps alot. The heat treat is probally used to bring the material back up to strength after annealing it for processing so the arm doesn't bend during normal use. Cold bending may still cause it to break and would add stress to the part. It would be more of a luck thing if it broke. If you heat it up first you will most likely anneal it so you could run the risk of it bending in use. The best way would be to anneal then bend then heat treat again.
A little data for this material is in the following link. There is a ton more if you google 8620 steel
http://www.metalsuppliersonline.com/propertypages/8620.asp
