Cold rolled vs hot rolled

Hot rolled has the gray mill scale on it, cold rolled should have a much better surface finish. Most angle iron, beams, and all rebar will be hot rolled product. Plate can be either but the thicker it is (.250" and up) the more likely it is hot rolled. Thinner tube (under 1/4") is mostly made from cold rolled sheet, slit, curled up, and welded, thick wall is likely hot rolled.
Cold rolled product needs to be around 1018 (.18% carbon) give or take some to get a decent product. Hot rolled can be anything from 1008 to .40 and even higher.
The lower carbon welds better, the less carbon the less issue you have with the HAZ (Heat Affected Zone). Higher carbon will need preheats and post weld heat.
Yield strength will depend on the carbon content but cold rolled will have higher yield due to the cold working process.
For home projects you will usually wind up with hot rolled plate, cold rolled sheet (

Here's an article I wrote on the subject some time ago. It was aimed more at blacksmiths, so if there's something in there that doesn't make sense, feel free to ask. --------------------------
Hot Rolled vs. Cold Rolled, What?s all the Fuss About? ---------------------------------------------------------------- Here?s an email that I received today. It asks a pretty common question: ?> Hello Pete, need some help understanding the difference between hot for one Theoretically, the only difference between hot rolled and cold rolled steels is that hot rolled steel is rolled to its final dimensions while hot enough to scale (over 1700 degrees F) while cold rolled steel is rolled to its final dimensions well below 1000 degrees F. So----- If you are making ½? square hot rolled steel, you have to estimate what the final size will be after the product cools, whereas you can finish the cold rolled steel to much closer tolerances right in the sizing rollers and that is what you get. There are some other things to consider, too: -The finished tolerances on hot rolled steels are looser than on cold rolled. Not only the plus or minus tolerance from nominal size, but the "square-ness" of the product. And, I can tell you from personal experience that there?s a lot of out-of-tolerance A36 (hot rolled steel) out there. So, if you need a specific size and you are going to go to a ?surplus? place, bring your ruler, square and micrometer to make sure you get what you need. - I have been told that, in order to get the cold rolled steel to come out with a nice finish, they might use "cleaner" ingots from which to roll the product. This means that you?d get fewer slag or carbon inclusions with cold rolled steels. -Note that I haven't talked about the chemistry of the steel at all. You can cold roll or hot roll 1045 and you can perform either process on C1018. But since we often talk about using "mild" steels, the two steels that we end up having around most often are C1018----which is quite often sold in cold rolled form and A36 which is always hot rolled. -One other difference that may be of interest to the blacksmith is that if you buy "1018?cold rolled steel", you can be pretty sure that it has 0.18% carbon content and few other impurities. But the spec for A36 can let the carbon content go as high as 0.29% and it can contain many more impurities. More carbon makes it harder to forge. -You generally have to pay about twice as much money for cold rolled steel as for hot rolled steel, for reasons which are probably obvious from the above. So far, you are probably feeling that, in dealing with mild steels, cold rolled steel is clearly the better stuff to have if you can afford it. Well, yes, usually, but---- since the hot rolled steel IS rolled while hot, it has a chance to normalize after the last rollers, so it is pretty much stress free when you get it. But machinists who usually buy cold rolled steel, often have the stuff twist and warp on them as they machine the first side or two. This is because the cold rolled steel actually work-hardens in the rolling process. For blacksmiths, this isn't much of a problem, since we are usually going to heat it up and reform it anyway. Finally, in my experience, the more popular (to the steel yard) sizes of mild steel usually come in both cold and hot rolled. I buy hot rolled whenever I can for blacksmithing. Except if I'm going to put a LOT of work into a piece. Then I buy cold rolled steel to minimize the possibility of having a crack appear in the shaft of my fancy flesh fork after about an hour of forging and an hour of filing and chasing. But in some sizes, for instance 1/4" square, the steel yards in our area only carry it in cold rolled, at twice the price of hot rolled---- so if I want any of that for S-hooks and for nails, etc., I'm stuck with the higher priced stuff--- unless I want to order a ton or two to get it in hot rolled form!
I hope this is more of an answer than just added confusion, Pete Stanaitis --------------
Later addition: There is a third choice; P&O,or "Pickled and Oiled". This material is basically A36 that has been acid dipped to remove scale and then oiled to prevent rusting. ----------------------------------
Ivan Vegvary wrote:

What he said, and said it well. Also chuck up a piece of 12L14 ledloy in a lathe, then a piece of hot rolled, machining both, and you will get an education on tool bits. People say you can't weld ledloy very well, but I do it all the time, to a cold rolled plate. A shouldered 11/16" stud protrudes flush through a hole in the plate and is welded around. Not the best thing in the world, but that is what is speced. I found if one gets the heat just so, and the travel just right, it will will outgas as it welds to eliminate those gas bubble inclusions. This is an item that is later plated with nickel chrome, so the weld has to be decent.
RJ
> Hot rolled has the gray mill scale on it, cold rolled should have a much > better surface finish. Most angle iron, beams, and all rebar will be hot > rolled product. Plate can be either but the thicker it is (.250" and up) > the more likely it is hot rolled. Thinner tube (under 1/4") is mostly made > from cold rolled sheet, slit, curled up, and welded, thick wall is likely > hot rolled. > > Cold rolled product needs to be around 1018 (.18% carbon) give or take > some to get a decent product. Hot rolled can be anything from 1008 to .40 > and even higher. > > The lower carbon welds better, the less carbon the less issue you have > with the HAZ (Heat Affected Zone). Higher carbon will need preheats and > post weld heat. > > Yield strength will depend on the carbon content but cold rolled will have > higher yield due to the cold working process. > > For home projects you will usually wind up with hot rolled plate, cold > rolled sheet (

Blush!
Also chuck up a piece of 12L14 ledloy in a I didn't bring up machining, the OP didn't sound like he was that far into it. But chucking up a random chunk of "steel" and trying to machine it will really make you long for the leaded, free machining steel.

If its got a brown/black scale on it..its likely to be hot rolled. This works fine for welding Stuff. And its cheaper than cold rolled with tends to be bright and silvery colored.
Gunner
"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner

Thank you Roy and Pete for the very well written education. For others reading this thread almost all bar steel in Europe is cold rolled and if turning is desired, this material must be annealed before machining or severe warping will result. The stuff being stocked in the steel warehouses are there to satisfy the local demand and their processes. Since machining is much more costly than stamping and forging, alloys that lend themselves to those processes are what is available on the shelf Steve