What the heck is the difference?

Chris

 

one you roll cold .. and the other you roll hot

 

You must be a rocket scientist!!

Chris

 

after the initial hot roll which shapes (forms) the steel, it can further be rolled into a more accurate thickness. it is called cold rolled because by the time the second pressing occurs the metal has cooled closer to room temperature.

 

So is cold rolled considered higher grade then? Is it tougher at resisting dents and scratches?

Chris

 

Actually they are both ways of forming steel. Cold rolling is done at room temp and shapes the metal to what ever shape you need, but there is a limit to the amount of change in shape you can do. If you want to make a smaller piece than you can do in one cold working process you need to anneal the metal by heating it up to release the stresses built up in it. And then you can further cold work it. Hot working is done by deforming the metal at a temperature above the recrystallization temp, the metal continuously recrystallizes as it is formed. So hot working can take a very large piece and make it small by running it through a series of operations without having to anneal it or anything, making it an easier process. Sorry for the babble, but basically cold-worked steel is stronger, but more brittle due to stresses that are left in the steel after shaping. Cold worked steel is not good for welding applications since the heat will just make the metal lose it's strength (near the weld) that was gained through cold-working. Hot worked metal is weaker but more ductile and better suited for welding applications. I could be wrong just kinda a quick explanation off the top of my head, but I think this is correct.

 

arsrmk (that is an interesting moniker, what the heck does that mean/imply/stand for?)

you are correct as far as i have learned in welding class, but w/one minor discrepency: it is my understanding that before a cold process, a hot one must occur. perhaps the hot process could be considered the annealment before the shaping? also, i believe that the reshaping in the cold process has more to do w/thickness than w/a "shape."

 

There doesn't have to be a hot working process before a cold worked one, however in many cases there might be because hot working allows a bigger, quicker change in the size of the piece. But cold working does give a more precise size piece than hot working does. Annealing doesn't change the shape, just a heat treatment to eliminate some or all of the effects(stress wise) of cold working. Cold working doesn't have to be a change in just thickness, if you take a wire and bend it, you have cold worked it. So many times it is used to change the thickness, but can be a process used to achieve alot of different shapes and things. I could get more in depth but that would just start to sound meaningless technical babbling.

 

do not want to hijack ravencr's thread (though in actuality you ARE answering his/her question), but i do not find what you are saying babble. in fact, i am learning a bit here and am very interested in what you have to say.

 

Try a google search as there are numerous websites that explain steel and all about it in detail.

 

yes, but learning is more effective w/interaction

 

So if welding, I should use Hot rolled, right?

Chris

 

you can use either . Hot rolled steel looks nasty from the mill slag , cold rolled is much cleaner and pickled and oiled is a nice finish from what I understand .

 

Two major categories are to be considered as far as the steel surface is concerned; namely cold-rolled steel and hot-rolled steel. All steel is originally hot-rolled, while some of it goes through the cold rolling process later. Basically, hot-rolled means that the steel is heated in the furnace to a cherry-red temperature and then passed through a series of rolling mills to reduce it to the desired size. When cold-rolled steel is wanted, the hot-rolled steel is reduced only part of the way in gage and then allowed to cool and then finally passed through a series of reducing mills without further heat being applied to the metal. As metal is worked from one reducing mill to the next, the grain structure is rearranged in such a manner that the steel becomes harder and more brittle. To offset this embrittlement the steel must be periodically "softened' or annealed between cold working operations. Annealing is accomplished by passing the metal through a furnace again, heating it to a cherry-red temperature. These extremely high heats actually burn the surface of the metal thereby producing scaly deposits that makes it necessary to pass the metal through an acid pickling tank after annealing and for a second time just before the final cold-working. The second pickling operation is a 'must' if an even and brighter surface is to be expected. The burning or oxidation can be largely prevented, however, by passing the metal through what is known as an atmospherically controlled furnace. In this type of furnace all oxygen has been removed, thereby eliminating any possible scale production. Without oxygen no oxidation can take place, therefore the metal emerges in much the same condition surface wise as when it entered the furnace.

There is a definite difference in the amount of reactivity to chemicals between hot-rolled and cold-rolled surfaces. Cold-rolled, steel has a much finer grained, smoother and less porous surface. This is so because cold rolling tends to 'fold in' and close up the pores. The less porous the surface, the less surface area is presented to chemical attack by acids, alkalis, and phosphating compounds. By the same token, soft hot-rolled steel is more porous and therefore more reactive to chemicals than is hard, tempered steel. With soft steel the metal is cold-worked. and then annealed just before the final pass which is a light 'skin pass' through the rolls which reduces thickness no more than a couple of thousands of an inch. This amount of final cold reduction is not enough to close up the pores as much as hard tempering, in which the steel is reduced by several cold rolling after annealing, giving it a shinier surface.

For your application Chris, I'd get whatever is cheapest at the steel mill or you can find cheap/free as scrap, if buying you want hot rolled for welding, especially what you are doing. You're going to pretty it up to beat it up anyway! I use hot-rolled as that is what we have the most of lying around!