heating automotive sheet metal 5

[mendadent]

in need of help:
i am a paintless dent repair technician, with that being said, can anyone tell me whether heating a panel at low temperatures, say 250-300 degrees will change the composition of the panels? i already know it won't harm the paint, but is there any benefit to mee in heating the metal, ie..limbering,or softening the panel?
thankyou for any replies.
ronnie

 

[HgTX]

Sounds kind of like what I see in the girder shops when they go chasing "buckles" out of the larger panels of built-up girders. They heat it one way to get the bulge out here, and it pops up somewhere else, and they chase it around for a while till what they have is within tolerance. Some people have a better feel than others about where exactly to apply the heat to minimize the creation of a new problem someplace else.

Note that I'm talking about heat straightening, using thermal stresses to induce deformation. Different mechanism than what you use.

Either way, an exact science it ain't, not on any practical level. There's something called finite-element analysis that uses a computer model of the exact shape and deformation of the part in consideration, but it takes so much time to build the model that it's not financially practical (not to mention that the software costs $$$$).

Regarding a forum that's more practically oriented than this one but still with good technical clue...dunno. I know of one for welding, but not metalworking.

Hg

 

[mendadent]

hgtx said
"Sounds kind of like what I see in the girder shops when they go chasing "buckles" out of the larger panels of built-up girders."
that is exactly what it's like. except as you all know in automobile design, most panels have four corners, yet may have several creases,or rounded,or inverted body lines, so we don't have to chase it all over the car. but with the above being said, we do have to find it and every curve, crease,or body line,and even internal glued brace inside the panel, will have an effect on the "buckling" you described above.

 

[mendadent]

hgtx said
The "softening" is the lowering of the yield strength of the steel. It'll take less force to get the steel moving. The higher the temperature, the lower the yield point, but there is very little effect in the first few hundred degrees. I have in front of me a graph (source unknown) of yield point vs. temperature for various structural steels and at 400F they're showing at best a 10% reduction in yield strength


if i give you a fax number can i get a copy of that graph hgtx? or is it online?
thanks in advance ronnie

 

[metman]

mendadent,
I have been trying to mispronounce your handle all wrong as mendad-ent these past few days while following this thread . It finally came to me (duuhh) to pronounce it mend-a-dent!

You commented above "is this what we dent technicians do if we have over worked(pushed) a panel? young's modulas, and is it also the term work hardened? for there are times when we work a dent wrong from the beginning, we stretch instead of shrink and create bigger mess for ourselves, and when it occurs we have spongy but yet lumpy metal, and at best we can only lightly sand at this point and rebuff."

Not to rebuff Cory's suggestion to educate yourself on the basics rather maybe the followimng will encourage you to persue it: When you have overworked the dent, you have pushed the metal too far past Youngs Modulus (the straight line/elastic portion of the stress strain curve -- yes as someone mentioned, the stress strain curve will indeed help you to understand this.) Once you have pushed it into the plastic region, the metal moves/stretches permanently. No, Youngs Modulus is not work hardening rather work hardening occurs when you stretch the metal beyond the elastic limit so that the next time you work it (next hammer blow), you have to stress it above the increased (work hardened) level that you just induced.

 

[HgTX]

Does anyone have an online link to a graph showing effect of temperature on yield strength? I don't think what I have would survive a scan.

Hg

 

[mendadent]

it amazes me what you gentlemen know.
you see it is easy to pull out tools and start "working a dent" but it is fascinating (whether necessary or not) to understand the intricacies of why what i do works or does not work.
thanks metman
and hgtx, thank you too! i would like to see the graph.

let me propose another question regarding heat: while i now know from all your experiences that heat doesn't change working characteristics unless it is heated very highly; is there merit to say if a quarter size door ding is heated and kept at a constant temperature of 200 degrees, that the 10% it does change,(mentioned by hgtx' graph) ,could possibly be enough to make the heated metal want to escape outward to the cooler metal if being pushed upon?

 

[BillPSU]

mendadent
The strength of the material is not affected by heat however the material will expand causing additional stress to be applied because the heated area will expand and the cold area around the dent will not. It may give the material the additional stress to pop back into the original condition with a little persuasion. The only caveat is that if the material has seen plastic deformation (strain beyond yield point) that material will never return to the orginal shape. This area of plastic deformation may be small enough where it is not visible but I have no experience.

Bill

 

[mendadent]

I believe this is what "we" in our field call" stretched metal". and surprisingly we do have a couple tricks for that. the most common is coming up from underneath on the parameter and making "high spots" and knocking them back down. but if we use sharp enough tool from the very beginning, we accomplish the sameand the untrained eye will never know the dent was there.

this may see elementary but if i were to do the opposite and heat the panel around the dent leving the dent cool, are you then suggesting the exact opposite from your quote above? since the metal expands, that it will now be expanding my dent farther inward?

you have answered my question with this statement and i thank you .
"The strength of the material is not affected by heat however the material will expand causing additional stress to be applied because the heated area will expand and the cold area around the dent will not. It may give the material the additional stress to pop back into the original condition with a little persuasion."

 

[BillPSU]

If you heat around a cold area the outer area will expand and actually cause a tension on the cool area. This type of heating may cause the same affect as heating the small area as one puts the some area under compression while the other cause the small are to be in compression.

 

[mendadent]

sounds like i may want to make some conical shaped device and heat the paramater and keep the dent cold, and as you say be in compression. i recently bought a variable temp heat gun, for a whopping $99.00 i can go from 120 degrees to 1500 now. of course my key temperature is under 240.

 

[NickE]

Hmmmmmm..... been watching this thread...

A question for mendadent... If I warm my paint to ~150-175C then is there less of a chance of cracks forming when I flex the sheet metal?

 

[mendadent]

yes, and the colder the temperature outside the more that's true. why do you want to flex your metal?

 

[NickE]

Just wondering... I leave body repair to the experts.

 

[mendadent]

nicke, i notice you are a young metallurgist, what is it that you do? or are we not to list that info?

 

[NickE]

I'd rather not tell you WHO I work for... (Although I've definately left enough clues around this place)

What I do:
Mainly Research and Development on specific types springs/hard steel products.

This could involve in any one week:
Rebuilding machines
Writing Code
Wiring sensors
Fatigue testing
Heat Treating
Mechanical Design
FEA/CFD Analysis
DOE
Failure Analysis
Sweeping the Floor
Changing Lightbulbs
Welding
Other various tasks as necessary. <-Directly from my Job Description



Nick
I love materials science!

 

[mendadent]

do you have any additional input that might help me in my quest?

 

[mendadent]

i started my search on this site, because i wanted information on heat and you all were able to provide it and again i thank you all.You all lead me to the physics of it all.I had hoped to find an explanation that revealed why we in my field could make these repairs,with the help of metallurgist's input, but it appears that what paintless dent repairmen do defies text book logic.
What i have learned is that if you're in the plastic deformation stage(stretched metal) of the stress strain curve,that i not only have to get back to flat, but beyond it to the percentage that had plastic deformation. This alone is what seperates the professionals in our craft from others that just can't do it.I mentioned the last time i made a post here on the site that we have to locate "crowns" i think you refer to them as "metal strain hardens" and beat them down in sometimes many places before we even begin to work the point of impact.
In closing i've come to find there are far too many variables in what causes a dented panel, and even more for how the metal will react after the force is applied due to bracing, body rolls,inverted panels etcetera.
it seems that what we do is magical, because we work this stretched metal back to pre- damage with most times no detection of damage.finally, i leave you with a question wouldn't a measurement of the stress strain curve be a completely different measurement in every conceivable location of an auto body panel over entire car after stamping due to the strength the metal takes on after stamping if the exact impact could be duplicated?

 

[HgTX]

Yes and no. Pretending for a moment we have idealized behavior, the curve goes up and to the right pretty straight, then curves over and is more or less level, and then goes up a little more, and then may or may not come back down before it breaks. If you unload your ideally-behaving piece of metal while still in the elastic range, it'll retrace its steps back down the straightaway. If you unload it after it's yielded, the elastic part of the unloading will give you a parallel path to the original straight elastic part of the curve, displace to the right by whatever your permanent deformation was. If you reload it at that point, it comes back and rejoins the curve like nothing ever happened (more or less). So if you unload and reload several times during plastic deformation, what you get is the usual stress-strain curve with some branches (or very thin loops) that all poke down and left parallel to the elastic portion.

http://www.shodor.org/~jingersoll/weave/tutorial/node6.html

Hg

 

[mendadent]

that was beautiful hgtx!is your response to my final question or to my understanding in 2nd paragraph:"if you're in the plastic deformation stage(stretched metal) of the stress strain curve,that i not only have to get back to flat, but beyond it to the percentage that had plastic deformation."

thanks for your resonse and the diagram.