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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 donīt 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 itīs crystal structure has to change.Molten metal has no crystalīs.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.

Dave Cameron

Last edited by cameron; 07-04-2011 at 04:27 PM. Reason: Change "compression strength" to "compression stress".
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Old 07-25-2011, 03:22 PM
derb derb is offline
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Quote:
Originally Posted by Richard K View Post
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.
Metal does indeed shrink when it get cold, it also grows as it gets warmer. Steel is no exception. When the piece of steel changes temperature uniformly it results in even growth or shrinkage that will reverse itself as the temperature returns back to its starting point. In your example the growth from the day time high reverses itself as it cools to the nigh time low. The next day when it gets up to 120 deg F again it will be the same size as the day before.

When locally heated and then rapidly cooled, metal can only shrink in one plane if it grows in another. Otherwise there would have to be a reduction in material. The metal that is being heated, by a weld or flame, will grow. The unheated metal that surronds it will not. This surrounding metal resisists the hot metals attempts to grow out horizontally and forces the metal to expand vertically. As this hot metal cools it shrinks and pulls in on itself from all directions. In its cooled state it has shrunk by "6 millionths of an inch per inch per degree of heat rise". This new smaller dimension horizontally causes a shrink in the panel.

To illustrate this, heat a 2 foot steel rod in the middle of its length. Do it so that the cherry red only extends halfway through and the, to prevent the heat from evenly heating the rod all the way through and promote rapid cooling, throw a wet rag on it. What is happening here is the hot spot trys to grow but the metal on the other side resists its length ward growth. As a result the hot spot has to grow out from the rod instead of lengthwise. When the hot spot has cooled it shrinks in all directions and causes the bar to bend. If we heated the whole rod width of the rod evenly it would have grown and then shrunk back to it original orientation.


Wow, that sound way more concise in my head.
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Old 07-25-2011, 09:51 PM
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Default shrinking metal

I haven't been around much lately, returned to college in Jan. majoring in Mechanical Engineering. took a break from my studies to see whats new on the site and found this post. I'm taking my ferrous metallurgy final this week so maby I can shed some light on this debate Simply put the changes in low carbon steel (1040 in this case) are dependant on the temperature you heat it to. under 300 degrees there is little change in the granular structure of the steel and it will return to it's original state. To cause shrinkage or distortion the metal must be heated above it's lower transformation temprature (about 1200 degrees) a weld puddle will get you above that temp. The distortion or shrinkage is a result of localized heat increase. Warming the area around the weld will help(but not too hot) If cooled slowly the metal will have little physical change. If the metal is quenched, the rapid cooling will cause a smaller grain structure thus the metal "shrinks". Low carbon steel doesn't harden well this is the reason it is used for bodywork. We have a Rockwell tester in our lab. mabe I could test some weld samples to see just how much the area is effected
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Old 07-26-2011, 12:12 AM
Overkill Overkill is offline
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Default Shrinking Disc

Ted,

I've never measured the temp the shrinking disc brings the metal up to, but I believe it to be far less than 1200 degrees. So - is it correct to say that metal heated in the range from 300 to 1200 degrees, will shrink if quenched in proportion to the amount of heat input? So if it's 500 degrees, it will shrink a little, and at 1000 degrees it will shrink more?
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