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  #11  
Old 07-04-2011, 06:15 PM
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jhnarial jhnarial is offline
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  #12  
Old 07-04-2011, 06:22 PM
Dyce Dyce is offline
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I've look at the refining processes for steel. Oxygen induction, electric, and coal. The longer you "cook" the more carbon the steel will loose. I beleive the crystalline structure changes when heated but I beleive more goes on. The properties change as the metal is heated time after time to it's molten state (welded). I have no proof but have come to beleive each weld is a kind of "refining". Thaking away a little carbon each time making the metal harder and less resillient and more likly to crack. I'm just speculating....

The low temps you are talking about Richard I think isn't enough to change the steel. I think the temp changes you are talking about compare to freezing water to ice, and warming it back up. Ti changes volume but not the properties. It takes alot of energy to change the properties of anything in so many things in this world......
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  #13  
Old 07-05-2011, 04:10 AM
robert robert is offline
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how would a shrinking disc work if the steel didnt shrink more then it expands after/during heating???

the word restricted i dont understand what you mean richard,its probably my english but i would like to know cause im missing the point now
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  #14  
Old 07-05-2011, 06:46 AM
ojh ojh is offline
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Quote:
Originally Posted by cameron View Post
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
I don't quite get this Dave, and i think i need to. This sounds kinda important, can you give another try at explanation - use an example if possible.
Thanks, oj
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  #15  
Old 07-05-2011, 08:26 AM
cameron cameron is offline
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oj,

Try this purely mental experiment.

Suppose you take a disc of sheet steel and attach its outside circumference rigidly to a heavy ring of steel.

Now heat the disc- its volume increases but the area can't increase, it has to expand in thickness. Actually, it will probably buckle and bulge outward, but we push or hammer it back flat.

Now, the tricky bit. While the metal is still hot, detach its rim from the disc and let it cool. Since there is now nothing constraining the metal (or restricting it as Richard K puts it), it retains its increased thickness and shrinks back to a smaller area than it had originally, and it still has its original volume.

Obviously, we can't do this every time we want to shrink using heat. But, unless some modified conditions of this experiment are present, we don't get heat shrinkage!

Heat a small area with a torch or stud welder and the thickness increases because the spot is restricted in area by the cooler metal around it. If it bulges up when torch heated, tapping it back flat will force it back into a smaller area than it wants to have.

Using a shrinking disc, pressure prevents the outward bulge which would accomodate increased area and forces the increased thickness.

What I haven't mentioned is that , in every shrinking process , the compression yield strength of the metal has to be exceeded, or the metal simply returns to its original volume, area and thickness.

I hope all this verbiage hasn't just muddied my explanation even more.

Dave Cameron
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  #16  
Old 07-05-2011, 09:15 AM
ojh ojh is offline
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Thanks Dave, i get the constrained explanation. So, if we dropped - i think it was aristotle's experiment - a piece of undisturbed steel into water and made a mark for displacement then heated/shrank the metal when we dropped it back into the tank it would displace the same amount?
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  #17  
Old 07-05-2011, 09:33 AM
cameron cameron is offline
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I think it was Archimedes, but yes.

Dave Cameron
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  #18  
Old 07-06-2011, 12:40 AM
Rich Pauza Rich Pauza is offline
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Sheet metal does expand when heated - and shrinks when it cools.

You do not see the deformation on your car roof because the curves & crown hide it.

A poor fit & weld – or bad seam placement can be easily spotted on a warm sunny day.

If you took a piece of .063 thick sheet metal & milled it square 1.000 x 1.000 - triple checked your measurement, then applied heat, it would increase in size - the more you heat, the bigger it grows.

Let it cool, and it will shrink back down to its original size at your original measurement temp.

If you take two pieces & mill them up the same way - same size, 1.000 x 1.000 - butt them up - tac em - and then weld em together - let em cool - and re measure - they will NOT be 2.000 long - they will be shorter - Why....? ( In heavy wall fabrication with major mass you will not see major shrinkage from welding – but shrinkage is there. )

When you weld thin wall tube - you can have substantial shrinkage - sometimes if not taken into consideration, you can end up with disastrous results.
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  #19  
Old 07-06-2011, 11:57 PM
JasonJamesWrenn JasonJamesWrenn is offline
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i hope this is on topic.. since I joined the site I have been wanting to try no gap with gas on sheet metal...but today I tried it with my Mig with no gap very clean and tight, I may be way off here but it seems to me the sheet metal at the weld point tends to expand and cause peeking in the sheet. is that correct thank you!
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  #20  
Old 07-07-2011, 04:21 AM
robert robert is offline
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Quote:
Originally Posted by JasonJamesWrenn View Post
i hope this is on topic.. since I joined the site I have been wanting to try no gap with gas on sheet metal...but today I tried it with my Mig with no gap very clean and tight, I may be way off here but it seems to me the sheet metal at the weld point tends to expand and cause peeking in the sheet. is that correct thank you!

that is correct cause you have no gap and adding material
welding mig u need a gap 1 mm that way there is place for the welding material, i tack every inch or more with 1 mm gap then make a few but welds in one pass and then grind then repeat the process

i find grinding before everything cooles the metal is not restricted to shrink back to the original place if you dont grind emediatly the weld holds back the process because it is much thikker then the sheet
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