Whats the difference and how do their characteristics differ durring fabrication? What are some applications for each?

 

Short story:
Use cold-rolled for welding.

Longer story:
Hot-rolled means just that, the metal is red-hot when shaped. As it cools it gets all sorts of scale on/in it. It's a bitch to weld (ok, for me at least,) always spitting and popping like it has a lot of crap in it, which it does. And that's *after* I ground it until shiny...

OTOH, if you're just drilling holes in it and bolting stuff together then it's just as good as cold-rolled.

 

Typically hot rolled will be more ductile because it formed before the grains were fully formed, cold rolled is work hardened which gives it a smaller grain structure and makes the alittle harder. Most metal that you go out and buy is hot rolled because it is less expensive

 

Hot rolled looks like **** when you make stuff with it unless you mill or grind the scale off of it.

 

from a machinist perspertive, i havent used hot rolled that much, its pretty junky and if i remember it leaves at bad surface finish, steel likes to tear when its cut unlike stainless, but cold rolled you can get a way better finish on it with the right speeds and feeds