I did search, but the answers I got were WELL over my head.

I'm just wondering what the difference is between the two?

For instance, if you had a 2x4x.250 wall peice of rectangle tube in both flavors....Is one stronger/better than the other? Is it comparing apples to oranges, or are the differences minimal, and nothing I'm going to notice?

Typically cold rolled will more closely adhear to its published dimensions. Cold rolled is somewhat stronger because it becomes work hardened in the process of forming it.

As stated, cold rolled will be marginally better on tolerance. It's also a helluva lot cleaner than hot rolled and won't be covered in mill scale. It's way easer to machine and weld. And it costs about 50 to 100 percent more depending on size.

I'm having a hard time finding local places, that'll sell me small amounts of what I need. Nor can I find anyone willing to sell me scrap. Maybe it's me not looking hard enough, but everyone I talk to, only sells to large accounts, or they have a 3-500 dollar minimum.

Then I find places like http://metalsdepot.com/, and others. They have exactly what I need, and there's no minimum order, but it's all hot rolled.

So, I'm wondering if I hold out for cold rolled stuff, or if I'll be fine, buying the hot rolled stuff online?

I dont waste my money on cold rolled unless it is the only option in the size that I need, or if buying solid steel to use as a shaft of inner bearing surface for light loads...

Later,
Jason

I dont waste my money on cold rolled unless it is the only option in the size that I need, or if buying solid steel to use as a shaft of inner bearing surface for light loads...

Later,
Jason



Ok, that makes sense.

Thanks for the info:beer

There's really no such thing as cold-rolled tubing. It's rolled as a sheet, then bent & welded into a tube.

Work hardening doesn't affect STRENGTH - it changes the elasticity (stress/strain), but the material's strength stays the same.

When I need scraps, I hit the local fabrication/machine shops & recycling yards.

I only buy cold rolled just because cleaning the hot rolled is a bitch.

There's really no such thing as cold-rolled tubing. It's rolled as a sheet, then bent & welded into a tube.

Work hardening doesn't affect STRENGTH - it changes the elasticity (stress/strain), but the material's strength stays the same.

When I need scraps, I hit the local fabrication/machine shops & recycling yards.

You are right, the sheet is either cold rolled or hot rolled, and then made into tubing. However, if it does not effect the strength, then why dose CREW cold rolled electric weld tubing have a tensile strength about 10k psi higher than HREW tubing? In fact, CREW is nearly the same tensile strength as an equivalent sized piece of DOM tubing. I realize the guy posting the question is inquireing about structural steel, not round tubing, but I would think that it would apply to rectangular tube as well as round.

It's certainly conceivable that the mfr. is using higher-strength steel for CREW & DOM than for HR. But you also have to remember that hot-rolled shapes don't get the same tempering that rolled sheets get, so it's most likely just the TEMPER of the base sheet stock that's giving the CREW more strength than the HR shapes.

Again: work-hardening is NOT 'work-strengthening'. It only makes the steel LESS pliable.

http://www.offroadfabnet.com/forums/showthread.php?t=340

OnlineMetals.com is another metal distributor online... they have no minimum order and have pretty much anything you would ever need... the shipping is a little expensive though

OnlineMetals.com is another metal distributor online... they have no minimum order and have pretty much anything you would ever need... the shipping is a little expensive though


Yeah, I've looked at them too.
One of them is alot cheaper than the other...I think it's onlinemetals.

Again: work-hardening is NOT 'work-strengthening'. It only makes the steel LESS pliable.

Incorrect,

Per Joseph Shigley

"Cold rolling and cold drawing have the same effect upon mechanical properties. The cold-working process does not change the grain size but merely distorts it. Cold working results in a large increase in yield strength, and increase in ultimate strength and hardness and a decrease in ductility."

Well, before this gets turned into a debate, that's gonna fly well over my head, I just wanna thank everyone for their help:beer

I'm having a hard time finding local places, that'll sell me small amounts of what I need. Nor can I find anyone willing to sell me scrap. Maybe it's me not looking hard enough, but everyone I talk to, only sells to large accounts, or they have a 3-500 dollar minimum.

Then I find places like http://metalsdepot.com/, and others. They have exactly what I need, and there's no minimum order, but it's all hot rolled.

So, I'm wondering if I hold out for cold rolled stuff, or if I'll be fine, buying the hot rolled stuff online?

Do you have a Steelco by you? They have always sold to me in the smallest amounts, you don'thave to pay any online shipping, and they usually give me scraps free or almost free (I'm not talking a bunch of steel here, if I see a piece I want out in the shop by the cutter, they usually hand it to me).

As far as cold/hot rolled, unless I am getting some DOM tubing, everything I have ever fabricated for the truck was I guess hot rolled, though it seems nice and smooth. that site you link to shows all the A500 and what not structural rectangular tubing as hot rolled.

Again: work-hardening is NOT 'work-strengthening'. It only makes the steel LESS pliable.
Incorrect,

Per Joseph Shigley

"Cold rolling and cold drawing have the same effect upon mechanical properties. The cold-working process does not change the grain size but merely distorts it. Cold working results in a large increase in yield strength, and increase in ultimate strength and hardness and a decrease in ductility."

uh oh :toothless :popc1:

uh oh :toothless :popc1:

Studied that 2 weeks ago, it was fresh in my brain...:thumbup

Well, before this gets turned into a debate, that's gonna fly well over my head, I just wanna thank everyone for their help:beer

:lolup

Cold rolling affects the yield strength a lot, and the tensile strengh little (the effect is there, but it's not as large).

Like Mike and others pointed out, the main advantage is that the material is a lot cleaner. All of the mill scale flakes off when you cold roll the material.

Steve -- I doubt cold rolled/cold formed benefits too much from the temper, since most common cold rolled materials (at least normal sheet and tube, spring steel and the like is also cold rolled and that's a very different story) are low carbon and don't have a very strong response to heat treatment at all. There may be an effect, though. I just don't know how much.

I always use cold rolled sheet versus hot, since you just get a better quality surface. As for shapes, it depends on what I'm doing. I do a bunch of stuff out of rectangular stock, which means A500B, which is hot rolled. Round tube would depend on the application, but I'd probably just go for the hot rolled unless the appearance was critical. If strength with minimal weight were the critical issue, I'd plan on geometric changes more than material changes for extra strength, since the effect is usually greater.

...large increase in yield strength...A change in yield strength IMO doesn't constitute a change in the material's strength, per se. But what does Mr. Shigley consider "large"? Is he comparing this gain to the strength of a more expensive alloy & categorizing the increase as "large" based on the cost of buying better steel versus work-hardening cheap alloy? Or is he saying it's truely significant on its own? Like 15% or more? Have you gotten to break anything in labs yet?
...and increase in ultimate strength...But not a significant increase, unless I'm remembering my materials labs results wrong. IIRC, it was usually LESS than 105% of the annealed ultimate strength for most carbon steels.

Hope you can see this, my scanner sux...

Annealed AISI 1018 Steel has a Yield Strength of 32,000 psi and an ultimate strength of 49,500 psi.

Given 15% cold work, final yield strengths are Yield = 57,100 psi and Ultimate = 58,200 psi.

This is an increase in yield and ultimate strength of 178% and 118% (respectively) over standard annealed strengths.

As far as yield strength constitutitng a change in the material strength...I design all of my bolted connections and breakaway grooves (brass and 304SS) to 2/3 of the material yield strength.

IMO, a material's yield strength is the actual strength of that material, since inelastic deformation occurs after the yield point (the part is useless after that).

...Have you gotten to break anything in labs yet?...

Considering he sat for the PE last Friday, I somehow suspect he got to break plenty of things in the materials labs at least four years ago. :thumbup

...As far as yield strength constitutitng a change in the material strength...I design all of my bolted connections and breakaway grooves (brass and 304SS) to 2/3 of the material yield strength.

IMO, a material's yield strength is the actual strength of that material, since inelastic deformation occurs after the yield point (the part is useless after that).

First, for bolted connections -- are you referring to bolted connections where the bolt is loaded in tension, or normal structural bolted connections where the bolt is in single (or occasionally double) shear?

Second ... well, the part isn't always useless after that. Are you designing to 2/3 yield stress in the extreme fibers, or 2/3 ultimate strength (as in entire section yielding, not ultimate tensile strength)? Depends strongly on the part and the loading as far as whether it's useless. A solid rectangular section is only at 2/3 of its plastic moment capacity when the extreme fibers start to yield (Mp/My = 1.5), and even the thinner among the W sections still usually have around an 8-10% reserve strength (Mp/My ~ 1.1). :toothless

There's a big difference when you try to cut cold rolled with a hack saw compared to hot rolled. :toothless

First, for bolted connections -- are you referring to bolted connections where the bolt is loaded in tension, or normal structural bolted connections where the bolt is in single (or occasionally double) shear?

Both, tension against pressure (think flanges) and shear for moments about the axis of the flange (distributed over the number of bolts in the connection - flange acting as a coupling), plus a buttload of other failure modes (any possible impact forces - our containers hold flammables and corrosives). My parts have to break at specified points to keep the valves intact.

Second ... well, the part isn't always useless after that. Are you designing to 2/3 yield stress in the extreme fibers, or 2/3 ultimate strength (as in entire section yielding, not ultimate tensile strength)? Depends strongly on the part and the loading as far as whether it's useless. A solid rectangular section is only at 2/3 of its plastic moment capacity when the extreme fibers start to yield (Mp/My = 1.5), and even the thinner among the W sections still usually have around an 8-10% reserve strength (Mp/My ~ 1.1). :toothless

I design to 2/3 the yield strength of the material (adds even more of a factor of safety). My breakaway grooves are thin-walled tubes usually, I model them as though the strain is equal across the cross section. True though, the part is not useless at yield. :thumbup

Given 15% cold work, ... This is an increase in yield and ultimate strength of 178% and 118% (respectively) over standard annealed strengths.Obviously, it's been WAAAAY too long since I crunched any numbers on this. :brownbag

Combined stresses! Sounds dangerous. :rockon :toothless

As an amusing aside, if you ever want to get a feel for someone's materials knowledge, ask 'em this:

You have a 1" cube of A36 steel with a tested yield point of exactly 36ksi. If you submerge this cube in water to 83,100 ft, where water pressure exceeds 36ksi, what happens?

My advisor always likes to throw that one out at the beginning of his inelastic structures class, just to see where he's starting with the students. :popc1:

I've never heard that, or considered it, but I'm gonna guess it deforms by becoming slightly concave on each face, & more dense due not only to the pressure, but also the temperature. If it were a sphere, I'd expect it to ONLY become more dense.

Of course, the problem is that we'll never actually know since that's more than double the deepest known point on this planet. Then, it also begs the question: what happens to WATER at that pressure??? ;)

You could duplicate the conditions in a specially constructed pressure vessel if you really wanted to. :wacko :toothless

This question applies equally to a 1" sphere of the same material.

The stress is equal to the outside pressure, which is the same on all sides (hydrostatic) ... does it yield? Why or why not? :shrug ;)

I say nothing happens

I think it would become a billet, slightly smaller & denser than the original specimen. I think it would technically "yield" at a microscopic level, but not fail since it can't actually crush in on itself at that pressure. On the sun, maybe - but not in water.

I still think it would become slighly concave on each face, but I'm not sure why...

You have a 1" cube of A36 steel with a tested yield point of exactly 36ksi. If you submerge this cube in water to 83,100 ft, where water pressure exceeds 36ksi, what happens?


It will depend on how much the 36ksi is exceeded. For example 0.1ksi where as at 1ksi things may start to happen.

To answer Blue's question, let's say this 36ksi yield steel is submerged to 40ksi. Heck, let's make it 50ksi.

One answer above is basically correct -- but which, and why?

I still say nothing happens because that 50ksi is compressing it from all angles. It can't compress one part without it bulging out somewhere else, and since all the weight is evenly applied all around, and pushing on it from all directions, it wouldn't change shape in any way.

Even if it was an aluminum beer can that was opened and brought under water to 100ksi I don't think anything would happen to it. You could even have it full of air and let the water come in as the pressure rises and I don't think it would deform the can in any way. Because its going to be the same pressure inside and out and from all sides, even though theres a tiny air pocket inside, nothing should happen.

I don't know, but I'm guessing thats it

I still say nothing happens because that 50ksi is compressing it from all angles. It can't compress one part without it bulging out somewhere else, and since all the weight is evenly applied all around, and pushing on it from all directions, it wouldn't change shape in any way.

Even if it was an aluminum beer can that was opened and brought under water to 100ksi I don't think anything would happen to it. You could even have it full of air and let the water come in as the pressure rises and I don't think it would deform the can in any way. Because its going to be the same pressure inside and out and from all sides, even though theres a tiny air pocket inside, nothing should happen.

I don't know, but I'm guessing thats it

Do you realize that submarines have a maximum depth before they crush?

Do you realize that submarines have a maximum depth before they crush?

Yes. Do you know how a submarine is different than a solid block of steel or an opened beer can when its under water?

the open can of beer is a whole different situation. It would still collapse. The water would be forced out.

But where's the rum?

the open can of beer is a whole different situation. It would still collapse. The water would be forced out.

How would the water be forced out, crushing the can, when theres equal pressure holding it in through the open top?

Blue -- Mike is exactly right for a number of reasons.

First, on the solid cube or sphere -- like he pointed out, the pressure is the same in all directions in the material. This is important because metals never yield in tension or compression, only in shear. You may have seen "pure tension" and "pure compression" tests that resulted in yielding, but tests referred to that way are misnamed. They are really "uniaxial tension" and "uniaxial compression" tests, where tensile stress exists in only one direction. That means on a plane 45* from the direction of tension, you have shear, and that's what produces the yielding you see.

In the example case I gave, since there is no shear, the material will just shrink elastically, proportional to the bulk modulus of the material. This is related to the "elastic modulus," Young's modulus, usually referred to E - the measurement of the stiffness of the material under uniaxial stress, which is easy to look up for pretty much any material man commonly uses. However, it also involves Poisson's ratio, which is a ratio that relates uniaxial elastic compression in one direction with the zero-stress elastic expansion that will occur in the other two directions (sideways) under a unidirectional load.

Now, as for the beer can analogy, the critical thing is the fact that it's open. You can sink an open beer can to whatever depth you want, and it'll never deform significantly. The pressure is the same on the inside and outside, so there's no meaningful force on the can. A submarine is different because it is full of air. You have compression on the hull plates in only two directions, not three, which means lots of shear. However, odds are good it will buckle under the compression before it yields. If you opened the hatch on the sub and let the air out, you could sink it to the bottom of the Marianas Trench and the hull would be fine. The poor crew would be dead and every container in the sub that held air would be crushed, but the hull wouldn't know the difference.

As another aside -- notice that while I said metal doesn't yield under pure (three axis) tension, I didn't say it wouldn't fail. Metals can fracture without yielding under those conditions. You basically get elastic-elastic-elastic-snap. However, fracture doesn't occur under compression.

That's why it's an interesting question to ask ... to most people, the answer really isn't intuitive, but once you know the answer, a lot of other things make sense. :thumbup

I understand the logic of your explanation, but I still think there would be deformation on each face of the cube, relating to Poisson's ratio. But I can't think of a practical way to perform an experiment to check since taking a measurment that precise within a fluid at 40Ksi would be VERY expensive.

I guess you could use a cube with a much lower modulus of elasticity, but equally homogenous as steel, and perform the experiment at a lower pressure. You'd have to make sure the cube was large enough for the results to be valid, though. I can't think of a homgenous elastic material that weak & available in large solid forms. Maybe you could melt a bunch of PVC into a solid block. :shrug

I understand the logic of your explanation, but I still think there would be deformation on each face of the cube, relating to Poisson's ratio. But I can't think of a practical way to perform an experiment to check since taking a measurment that precise within a fluid at 40Ksi would be VERY expensive.

I guess you could use a cube with a much lower modulus of elasticity, but equally homogenous as steel, and perform the experiment at a lower pressure. You'd have to make sure the cube was large enough for the results to be valid, though. I can't think of a homgenous elastic material that weak & available in large solid forms. Maybe you could melt a bunch of PVC into a solid block. :shrug

Why do you think the face would squeeze in and not the edge or the corners?

Way to Go Mike2...you were right on with your thoughts...you are fart smeller...i mean smart feller...:thumbup

Like I said: I'm not sure, but I have this feeling it relates to Poisson's ratio. I'm not saying it WILL - I'm just saying I suspect it might. :wacko