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Old 01-13-2005, 11:20 AM   #1 (permalink)
 
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Steel is testing my sanity!

Out of the 5 books I have about welding, not one of them ever mentions a distinction between "cold rolled" and "hot rolled" steel. Is there anyone who can explain the differences between them, in terms of the manufacturing process and subsequent strength? I have heard that cold rolled is stronger, but I cannot locate it anywhere! Even my local steel mill does not supply this stuff. If cold rolled is so great, why is it that nobody stocks it? Also, would it even be considered acceptable to fabricate leaf-spring hangers out of .25" hot rolled steel plate? Is hot rolled really that weak? And one final question. Isn't your typical angle-iron, square-tube, and rectangular tube steel all hot rolled??? I see people using all this stuff for vital suspension brackets, and it does not seem to raise concern.....
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Old 01-13-2005, 11:26 AM   #2 (permalink)
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Some of you guy's need to put the books down.


I think cold is a c-hair stronger because of compressed molcules and blobidy blaw blaw.

most steel yards stock both hot and cold.

both are fine for your vital parts

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Old 01-13-2005, 11:34 AM   #3 (permalink)
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go here. I'm not gonna type up the 2 pages that my ryerson catalog has.

http://www.ryersontull.com/stocklist...tLevel&ID=DATA
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Old 01-13-2005, 12:37 PM   #4 (permalink)
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Cold rolled: put it in a vise and hit it with a hammer

Hot rolled: heat it with flame and bend it with pliers

This is the most basic example. There is some chemistry in the actual process but as mentioned above it is detailed. The steel plants do this on a much greater scale.
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Old 01-13-2005, 12:53 PM   #5 (permalink)
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basically cold rolled keeps a nice crystal latice where as the heating of hot rolled distorts that lattice and makes it slightley weaker.

Either way... if your fabing spring hanger out 3/16 steel (either type) you arnt gonna have ANY issue.
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Old 01-13-2005, 01:16 PM   #6 (permalink)
 
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Quote:
Originally Posted by az-k5
Cold rolled: put it in a vise and hit it with a hammer
Hot rolled: heat it with flame and bend it with pliers
That's an interesting way of looking at it. I've always been under the impression that if you want to bend metal without inducing fatigue, it should be heated. Maybe this is not the case...
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Old 01-13-2005, 01:44 PM   #7 (permalink)
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isn't one harder to drill , like hard as hell ?
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Old 01-13-2005, 03:04 PM   #8 (permalink)
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Have you looked through the Tech Section of this board? I think Billa Vista did a huge writeup on this type of thing. FYI.

Materials

From that link;
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"In the process, the physical characteristics of the metal are improved by breaking up the "as cast" crystal structure and "refinement" of the grain size...The yield strengths of almost all metals decrease notably with increasing temperature, so that a given amount of deformation can be achieved at much lower stress levels if the material is hot worked rather than cold worked. ..Almost all of the energy expended in the hot working of metals is dissipated as heat, leaving the metal's crystal structure largely unaffected and the metal ductile. On the other hand, while most of the energy expended in the cold working of a metal is also dissipated in the form of heat, some part of this energy remains within the crystal structure of the metal itself as "strain energy" in the form of various distortions of and dislocations in the crystal space lattice. Therefore, cold working, by decreasing the grain size and increasing the number of dislocations in the crystal lattice of the metal being worked, can increase the strength and hardness of the finished product—sometimes to a notable extent—at some cost in ductility...As you would expect, the different mill forming processes result in steel mill products of varying qualities and properties:"
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Old 01-13-2005, 03:11 PM   #9 (permalink)
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isn't one harder to drill , like hard as hell ?

No
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Old 01-13-2005, 03:27 PM   #10 (permalink)
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Cold rolled also has alot more stress in it than hot rolled and you can always tell the difference between the two. Hot rolled has a layer of black crust on it almost like paint.
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Old 01-13-2005, 03:31 PM   #11 (permalink)
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Cold rolled usually doesn't come thicker than 10 gauge (~.137"). Round stock can be fairly large in cold rolled (6" dia). Cold rolling work hardens the material, so it is stronger and stiffer, with a smooth surface. It is typically used where surface finish is important, like body panels.

Hot rolled has black oxide surface scale that needs to be cleaned off for a good finish. It can be also be purchased as pickled and oiled, which means it has been acid dipped to remove the scale, and oiled to prevent rust.
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Old 01-13-2005, 03:47 PM   #12 (permalink)
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Cold rolled machines awesome and almost enjoys being welded. If I have the choice between one or the other, especially for brackets, working with cold is just.. better. I can't describe it fully but it's just more fun to deal with. It's easier to dykem and sharpie on, it doesn't have the same mill scale that HR and P and O steel has, and you can see the HAZ much clearer on it - overall, it is just more satisfying to deal with. Laying a big, smooth fat bead on cold rolled with 75/25 MIG mix is just gravy, man. I could do that all day long.

PS I am a fab dork.

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Old 01-13-2005, 04:35 PM   #13 (permalink)
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Quote:
Originally Posted by Quick & Dirty
Cold rolled usually doesn't come thicker than 10 gauge (~.137"). Round stock can be fairly large in cold rolled (6" dia). Cold rolling work hardens the material, so it is stronger and stiffer, with a smooth surface. It is typically used where surface finish is important, like body panels.

Hot rolled has black oxide surface scale that needs to be cleaned off for a good finish. It can be also be purchased as pickled and oiled, which means it has been acid dipped to remove the scale, and oiled to prevent rust.
im assuming u are talking about plate only? as a machinst i have seen plenty of cold rolled mild in the 2" thick and thicker range.

and they are both about the same to drill to clarify that other guys question.
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Old 01-13-2005, 05:40 PM   #14 (permalink)
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Go read the ASTM standards if you like the books soo much.
There is a chemical composition difference between hot and cold, along with tensil strength ect. All depending upon which grade you are dealing with.
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Old 01-13-2005, 06:03 PM   #15 (permalink)
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i don't know if you have read carrol smith's book "engineer to win" yet. it has a lot of imformation in it. from the manufacturing of the steel to the finish product. here is a figure from his book (sory for the quality of the scan):
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Old 01-13-2005, 06:07 PM   #16 (permalink)
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i just finished making three gas tanks for my friends' land cruisers out of 16 ga hot rolled. when i built my tank i used 16 ga cold rolled and it bent and welded great. the hot was harder to work and the scale flaked off in the brake. welding was no where as good as the cold. the only reason i used hot on the new tanks was i could not rember what i used before. i will not make that mistake again.
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Old 01-13-2005, 07:15 PM   #17 (permalink)
 
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Lots of good information here. One thing I noticed, deals with the 4-digit identification code. I just looked at the steel I've seen using, and cannot find this code anywhere! Imagine that, I'm building hangers and don't even know the type of steel I'm using! None of the mild-steel plates at Home Depot, or even my local steel mill have this code. The question is, when you guys want to use some special alloy steel, where do you go to get it, and how do you know what it is?
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Old 01-13-2005, 07:27 PM   #18 (permalink)
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go search "steel" on mcmaster.com and browse around a bit....
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Old 01-13-2005, 07:45 PM   #19 (permalink)
 
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I work with steel all day, and the difference is not even worth understanding if you are fabricating. All steel sheet/plate is hot rolled, then some is further worked to create a smoother appearance and a more consistand grain. The problem is that it dosn't matter, because if you want 11 gauge and thinner it will more than likely be cold rolled and thicker stuff is typically just kept in a hot roll condition. Obviously bar stock is available in many shapes as cold rolled. It really is that simple and dosn't mean poop for brackets or anything like that. I laser cut the stuff and definately prefer the HRS pickled and oiled because the surface is better than plain HRS.
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Old 01-13-2005, 07:47 PM   #20 (permalink)
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Quote:
Cold rolling work hardens the material, so it is stronger and stiffer, with a smooth surface. It is typically used where surface finish is important, like body panels.
Not saying your wroong but from a machinist standpoint everytime you surface cold rolled the center gets hotter and cools leaving a dip inthe center of the cold rolled.....it really sucks to make precision parts out of.
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Old 01-13-2005, 09:29 PM   #21 (permalink)
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Quote:
Originally Posted by HoloceneBound
Lots of good information here. One thing I noticed, deals with the 4-digit identification code. I just looked at the steel I've seen using, and cannot find this code anywhere! Imagine that, I'm building hangers and don't even know the type of steel I'm using! None of the mild-steel plates at Home Depot, or even my local steel mill have this code. The question is, when you guys want to use some special alloy steel, where do you go to get it, and how do you know what it is?
You have to request MTR's. I have rejected entire 40K truckloads loads of material that did not have the proper MTR's. The home depot stuff will most likely be hot rolled A-36 material. The only # you will find on your steel are the heat # if you are lucky, or maybe the grade depending on what mill rolled it.
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Old 01-14-2005, 01:37 AM   #22 (permalink)
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Cold rolled machines poorly because it is drawn-forged. This process makes the outside of the material dense, not hard. When you mill/drill cold rolled, it relieves the pressure that is stored in the center of the material. Too much milling/drilling, and the material turns into a soap dish. The black crust on hot rolled is called mill scale. It develops on the material as it is cooling. If you heat a material with a torch, you get a similar effect. Mill scale should be ground off before welding. I have a pair of 3"x4" pieces of cold rolled that i use in my 50 ton shop press.
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Old 01-14-2005, 08:14 AM   #23 (permalink)
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Not sure if this is along the same lines. If not I will remove it and start my own post..

Whats the difference between ASTM-A512 & ASTM-A513?


I have searched the board and through the tech articles but haven't found any specific differences. The search I did on the net lists them both as DOM material, but no specifics...
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