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Old 07-04-2011, 01:33 PM
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Default Does Steel Shrink when it cools

Does Steel Shrink when it cools

Often stated is a statement such as "steel shrinks when it cools".

Something to think about today as your are enjoying a beer, eating a hot dog and wiping the mustard off your lip with an image of the American flag.

This is often a part of panel welding discussions; it is rarely mentioned in welding of chassis, press frames or any heavier steel fabrication. If "steel SIMPLY shrinks when it cools" is a fact then the heavier work must also shrink when it cools.

Steel expands when it is heated at the rate of 6 millionths of an inch per inch per degree of heat rise. That results in about 1/2 of one thousandth of an inch expansion per foot per 100 degree temperature rise. At a fusion heat (welding) of about 2500 degrees rise: a 1 inch length will expand an additional 15 thousandths of an inch. Source "General Motors Technical Institute".

Today, In Minneapolis, the predicted high is 87 degrees, the low last nite was 70, it was windy and raining during the night. Thus I will presume the steel in the steel rack at Discount Steel cooled to 70 degrees. Their steel in lengths of about 20 feet are stored outdoors in racks covered by steel roofs.

I will suppose the steel will today reach about 120 to 150 degrees in the racks. for sake of using easy numbers, let's look at a 20 foot length of 10 inch by 10 inch square steel tubing of 1/4" wall. That piece of tubing is now almost 20 thousandths of an inch longer than it was last night,and has grown a bit more than 1/2 thousandth in width and height. The thickness has enlarged slightly also.

What will happen tonight when that length of square tube cools down?

If "heated metal shrinks when it cools"; then the tubing should be shorter than 20 feet and smaller than 10 inches square tomorrow. If that piece happens to lay in the yard for a year it may become a piece of 9" square tubing 18 feet in length.

By the same logic; the roof on our cars get hot and cool every day. Why don't the roofs shrink to flat sheets stretched across the heavier framework. Of course if "steel shrinks when it cools" is true in it's simplest form then the whole car is getting smaller. However if that is true then a 1929 Caddilac should be about the size of a chevrolet by now. The Caddy has experienced the daily heating and cooling cycle 22,895 times over the past 83 years.

The whole story I present sounds a bit foolish! the Caddilacs have not turned into Chevys and our cars roofs are not flat. The steel I've had on my shelf for 10 years is still the same size.

Does Steel Shrink when it cools? Or is there more to the story?
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Old 07-04-2011, 02:18 PM
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I often approach ideas a bit differently than many do. Some may see the above post as a bit sarcastic, others may see it as foolish. Others may call it "thinking outside the box".

I consider ideas and rules of thumb as points to begin. I've never allowed myself to get boxed in. I've been shut out, but not shut in.

The above post simply questions if the statement posed is valid. The statement is valid, however the application of the principal has been taken out of context.

In order to learn, we have to think. The above was written to provoke thought. Think about "Does steel shrink when it cools"

Point to ponder (1): The phrase "steel shrinks when it cools" is missing one very important word that begins with an "R". Over the years that word has been dropped from the phrase. Why? I have some ideas but do not know for sure.

Point to ponder (2): before steel can cool and possibly shrink. I must be heated and possibly expand.

Post your thoughts; in a day or two I'll add on to the discussion.
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Old 07-04-2011, 02:30 PM
David Gardiner David Gardiner is offline
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Who ever said that steel shrinks when it cools?.

As far as I know the accepted wisdom is that if a piece of steel is heated it will shrink beyond it original state. This is fact and if you want to test this heat one side of a round or square length of tube let it cool and watch it curve. Of course large structures shrink after welding and in large construction care and planning are needed to ensure that distortion does not cause problems.

If you have ever boxed a chassis you will know that it is easy to end up with a chassis that curves.

Outside the box? probably because it shrank.

David
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Old 07-04-2011, 02:58 PM
Janne Janne is offline
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Well, i don´t know nothing about nothing. ...but...the phrase you mean, must be/ or is, "Steel shrinks when it cools RAPIDLY...

If the steel has to shrink, it has to be heated, so high, that it´s crystal structure has to change.Molten metal has no crystal´s.When molten metal frozens/cools, the atoms form in to a regular crystals.And they realign the crystals structure.Basicly they go, where there is a empty space..or something...You guys can correct me.. . Man i sound wise..

This is too hard to translate in to english..but i like the post
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Old 07-04-2011, 03:14 PM
RockHillWill RockHillWill is offline
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Nice work Janne.

I'm with ya on this one. That is my understanding as well. The expansion from mild temperature changes does not effect the crystalline structure of rolled steel, and it will return to its original size. Heating it above a certain temperature will realign the crystalline structures in a more compact arrangement as it cools.

I have been wondering about this issue as well and reading in a few books, that is what I am believing at this time, but I am certainly open to other thoughts.

I concur with Richard that it effects thicker and larger steel structure as well.
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Old 07-04-2011, 04:02 PM
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Kerry Pinkerton Kerry Pinkerton is offline
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The question as it relates to this forum is Does SHEET METAL shrink when heated. Let's not get into structural steel because it's off topic.
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Old 07-04-2011, 04:16 PM
cameron cameron is offline
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Steel expands when it's heated, shrinks when it cools. More specifically, its volume increases when heated, decreases when cooled.

It is virtually impossible to prevent this change of volume with changing temperature, the pressures involved would be beyond anything even heavy metalworking machinery can contend with.

We can constrain one or two dimensions of the metal, however- its length, or its area. If we constrain the area, the volume will still increase with heating. If the area can't increase, the thickness must increase to accomodate the extra volume. At the same time, compression stresses are created, and the compression yield stress is diminished with the increased temperature

If the compression stress exceeds the yield strength, the metal yields (Duh!!). In the case we're considering, the metal increases in thickness. Unless we can apply sufficient tension stresses as the metal cools to reverse this increase in thickness and restretch to the original thickness, we are left with smaller area and greater thickness- we say the metal has shrunk.

BUT- the volume is the same as it was originally.

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Last edited by cameron; 07-04-2011 at 04:27 PM. Reason: Change "compression strength" to "compression stress".
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Old 07-04-2011, 04:22 PM
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My impression is that it comes down to the width of the temperature range (coldest to hottest) the steel has been exposed to, and how close this range gets to the change of state between a solid and a liquid. The hotter and closer we get to that change of state, the more shrinkage we can anticipate when the steel cools.

Sounds good to me, but then again, I'm an engineer who never went to school to take engineering, so I might be missing some details I should know about.
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Old 07-04-2011, 04:27 PM
weldtoride weldtoride is offline
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Probably the best explanation I ever saw pertaining to the heating/cooling expansion/contraction topic was in a little movie that used to be distributed by Lincoln Welding Foundation: Prevention and Control of Distortion in Arc Welding, the film was an animation from the Disney studios.

In the film a piece of round bar stock is placed in a vice end-to-end and heated. As it is heated it expands. The steel wants to expand in every direction, but the vise jaws prevent it from expanding longitudinally, so all expansion is radial. As it cools, it shrinks in all directions, and therefore becomes shorter, and drops out of the vise.

I duplicated this experiment for my students. If you want to try, do not over-tighten the vise, just enough jaw pressure to hold the bar stock initially will produce the desired results.

The film makes the point that welding heat is localized heat, and that expansion and contraction both are controlled by surrounding non-heated metal, analogous to the vice.
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Old 07-04-2011, 06:02 PM
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The "R" word was RESTRICTED.

This discussion is getting a bit too far off the metalshaping topic; so let's get back to shaping some parts and sharing those experiences.

Thanks for your input.
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