Dumb Question - Hot rolled vs. Cold rolled??

R

ritzblitz

Joined
Dec 31, 2006
Messages
293
Hi Everyone,

I know this is kind of a stupid question but I'll ask it anyways...

What is the difference between hot rolled and cold rolled steel? Does it matter which one I choose if I'm going to be forging the steel? Is there any composition difference between the two?

Thanks alot!

Alex
 
You know they always say in an education setting that there is no such thing as a stupid question. Well, there is so.

Yours isn't stupid though.

The only thing I know is one is rolld out cold and the other is rolled out hot :) Big help huh :o
 
Haha, as far as I know, there is no difference between the two other than the finish. So I'm going to say that I answered my question, and it was stupid to make a thread.
 
Not a stupid question at all. In fact many makers have been sorely disappointed in steel because of not knowing the difference. 416 stainless for instance can be had both ways. The surface looks better in cold rolled. I use it because it cleans up a little better in the milling vice. You have to be really careful with 440-c for instance. Hot rolled bar, is uglier on the surface. But is good stuff. There is a lot of sheared 440-c out there from cold rolled plate. It can have a very strong tractor tire tread grain when trying to finish. Seems that hot rolled is just a bit more dense. In steel going to be used forging, it probably doesn't matter. You can improve on the grain in cold rolled by forging, but hot rolled is damed good just as it comes form the mill. There is a lot of talk about grain refinement. It is mostly BS. You can get a shape that you want by forging with less metal that stock removal, But the grain refinement comes from the heat treating. Not from forging. Such things as edge packing, and breaking up the molecules is pure smoke and mirrors. Sells well to the UN educated. But is nothing more than Flem-flam at it's best. Mike
 
Well here is my 2 cents,
A few weeks ago at a class on blacksmithing I asked the instructor what kind of scraps from the scrap bin from work I could bring in. He said hot roled was preffered over cold rold for forging. I didn't ask why.
BUT..... I would Try Both if you can and deciede for your self. Kind of home project,:thumbup: and as others have said " Most of all have FUN":)
 
Most high carbon tool steel is hot rolled, if my memory serves me correctly. I don't know about stainless. I don't think tool steel responds well to cold rolling, makes it weak and brittle. Although if you were just talking about forging mild steel, I'm not sure what is really preferrable. If you know how to anneal, it shouldn't matter if it's hot or cold roll, from my understanding.
 
CR mild steel is good for fixtures and such. The surfaces are flatter and edges are more perpendicular and more crisp. Dimensional accuracy is better.
You can get CR high-carbon easy enough. I have a couple hundred pounds of CR W1 in different sizes from several different vendors. Also have CR 1095 on the shelf, CR 1075 and a few other things. Sometimes you have to get it sheared from plate if you want a particular steel in CR. HR is cheaper.
 
There is no difference in density of hot or cold rolled ! If you're going to forge get the hot rolled as you're not concerned with finish.All steel starts out as hot rolled then some is cold rolled as a finish operation .This usually has no scale and is dimensionally held to tighter tolerances.
 
"How it is made", prolly the best show ever made for gearheads,
had a segment on how they make steel. Wuz most excellent indeed.

Oftentimes you see a blacksmith forging a bar (making, say,
damascus), working with something like 1"x1"x10" bar.

Now, imagine that bar is 15 feet x 15 feet x 20 feet, being forged
by a gian press, while being held in giant tongs. That's what it looks
like.

Before that happens, they smelter iron and additives in a giant
smelter, with 3 giant electrodes. Must be megaAmps of current
doing the job. As the metal melts, they add alloying elements and
frequently check the resulting "mix" for proper percentages, adjusting
as needed. As they pour, filters will catch larger debri. Of course,
to qualify for NASA, this process needs to be pretty darn clean. It might
have to be repeated few times, to get progressively cleaner, better steel.


Some metals, like brass, can be hardened by "working" it . Steel doesn't
exhibit this feature. Still, when they roll, a lots of stress ends up in the
metal . Also, when hot rolling, exterior layers loose carbon and other alloying
elements. Nasty, hard scale forms. It has to be removed - this makes steel
"decarb" free and it also brings it down to exact dimensions (precision-ground) or just slightly oversized (called just that). Another way to remove the scale is to pickle the steel in acid, but of course it is not precise at all.

When using hot roll for removal-method knifes, one has to be careful not to end up with a warped blade, again due to the stress of hot-rolling being hidden in the metal bar. At best, you will have to spend lots of time orrecting it.
 
Ok this was just on history channel and a while back how its made.

When steel is heated up the molecules align in to what they called a matrix. This becomes a stronger steel. In cold rolling they start with a huge billet of steel and run it throguh rollers compressing the steel. In cold roll steel youll have the same matrix as you did in the original billet (for the most part its really less change to the matrix). Cold rolling also uses many more rollers and or passes to get the stock to the final size and shape this is why it will tend to have a nicer looking finish.

With hot rolled steel the matrix will change become more dense and well aligned. In stock removal this is a good thing as it will grind and file etc smoother for us (more consitancy) than cold rolled. In the end though if your heat treating is good other than the ease of finishing both will end up the same or very close to the same in hardness grain structure etc.

Remember on raw hot rolled youll need to remove a portion of the mettal from top bottom and both esdges to get past the scale layer.

OT sort of but intresting. You know that alluminum foil you use to wrap your lunch in? Well it is a multi layer or damascus alloy. To get foil they use hot roller to thin out a very huge billet of aluminum. Once its at a given thickness it is folded in half and ran through the rollers again to increase its strength. Depending on how heavy a duty its to be used for it may be folded again and again ran through the rollers. So you heavy duty foil is folded twice giving a 4 layer damscus aluminum foil :)

Just a fun little fact i picked up watchign how its made heheh
 
Hot rolled is, as its name says, rolled hot. Has scale, is less finished. If you are gonna forge it, buy hot rolled. Costs less, and you are going to forge it anyway.
Cold rolled is dimensionally more accurate, crisp corners, things like that. No scale. But it's more expensive.
 
Rashid and Novaflare, metals do not have molecules, only atoms , crystals and grains. Scale [iron oxide] and decarb are two different things . The scale can be removed by pickling or grinding .Decarb must be removed by grinding through the decarb layer .Precision ground stock usually removes decarb.Most high carbon steel is HRA [hot rolled annealed] which should have no stresses.Cold working steel actually INCREASES volume - less dense !!!
 
Exactly.
Cold working induces stresses. There's actually a very accurate article on Kevin's site.
 
If you are forging out the metal it does not make a difference. But if you was going to utilize the stock removal system it would. The hot roll is half hardened steel. And the cold roll steel is supposed to be completely in the anneal state. This is why most knife makers preferred using cold roll steel. You can cut, shape, drill, stamp, mill, and so one with hardly any stress on your cutting tools. I have tried to put this in a nutshell for you, without writing a book. There are books out there that can go over this issue in precise detail. I hope this has been helpful and may you have a blessed day.

Barkes
When I leave this world I would like to be known by the one who made his knives from start to finish. And I sell what I make and I make what I sell.

http://my.hsonline.net/wizard
http://my.hsonline.net/wizard/TerryBarkesKnives.htm
:thumbup:
 
Duh?
Every source I have says it's THE OTHER WAY ROUND!
Hot rolled is soft and annealed.
Cold rolled is work hardened.
Now I'm in serious doubt I've undestood anything...
:confused:

edit: My impression under the file is that hot rolled iron stock is softer, but that maybe because it's mild iron...
Is perhaps the opposite way with steel? Carbon content in hot rolled steel could make it harden while it's cooled passing under the cold rolling machine?
 
Hot rolled is half hardened ??? The condition after hot rolling depends on the alloy . A 1095 HR will have a pearlitic structure , annealed 1095 will have a spheridized structure .If you want to machine it get HRA.The cold rolling of 1095 is only a slight finish rolling .The Admiral CR lists low carbon steels which are cold rolled substantially to make a spring. ....But what do I know , I'm only a metallurgist !!!!
 
mete said:
Rashid and Novaflare, metals do not have molecules, only atoms , crystals and grains. Scale [iron oxide] and decarb are two different things . The scale can be removed by pickling or grinding .Decarb must be removed by grinding through the decarb layer .Precision ground stock usually removes decarb.Most high carbon steel is HRA [hot rolled annealed] which should have no stresses.Cold working steel actually INCREASES volume - less dense !!!
Click to expand...

Pure metals like iron are not molecular, but steel is not a pure metal. The Carbide fraction of steel shares the outer electrons with the metallic bond, but the rest is a covalent/ionic bonded molecule. Thus steel is a hybrid partly metal/partly molecular lattice.
 
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