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]

The short I-am-not-a-metallurgist answer: if it doesn't hurt the paint, it probably won't touch the steel.

I don't know too much about automotive sheet, but I've been looking into this question with low-carbon structural steel plate, and the meaningful temperatures should be roughly the same.

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.

There isn't too much effect in the next few hundred degrees either; at 800F that same graph shows at best a 20% reduction. (Which is interesting because I know plenty of fabricators who like to heat things to about 500F thinking it helps things along.) It's after that point that the yield point really starts dropping.

If you're looking to keep the paint intact, I don't think you should bother with the heat. If you do start dealing with higher temperatures, though, stay under 1200F (for low-carbon steel) or you risk embrittling the steel.

Hg

 

[mendadent]

thank you for your reply, would you or anyone say the same applies to aluminum, which is increasingly being used in automotive panels?

 

[EdStainless]

Many auto body steels are 'bake hardening'. They actually increase in strength when the paint is being cured. Re-heating to low temperatures should cause no problems, but it won't make forming any easier.
Be careful with Al since it will get much softer, much faster. It would be easy to over do it.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[CoryPad]

Is the temperature 250 [°]C or [°]F? At 250 [°]C, you have a huge problem. At 250 [°]F (120 [°]C), you have a medium problem. What is the time at temperature? Try to keep the temperature lower, say below 100 [°]C, and keep the time low, say less than 30 minutes.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[HgTX]

Cory--which problem, and are you talking aluminum or steel? Or paint?

Are you talking about the "blue brittle" range of steel (about 400-700F)?

Hg

 

[CoryPad]

I didn't read the original post closely enough. My information was regarding aluminium alloys used for automobiles. Steel is unaffected by 250 [°]C. Paint would be damaged at 250 [°]C, so this must be 250 [°]F.

There is no advantage to heating steel panels for this - the elastic and plastic properties do not change enough to help with the dent removal (when keeping the temperature below the degradation limit of the paint).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mendadent]

thanks again, i am talking fahrenheit. in our field we are increasingly taking on larger size dents, some the size of ; say, a football, and creases where someone or something grazes the panel. our largest obstacle seems to be following the energy from impact where the metal buckles or pinches up slightly. once we find these areas of pressure we knock them down and can usually end up with a sussessful repair. the heat subject has come up as a possible way to make the pushing easier.(and is necessary on some to soften the paint for the initial pushes to keep from cracking) In my research i came up with the conclusion offered here, however; my source was a very old brittanica encyclopedia. well that being said as metallurgist's i knew you folks would know the answer.
as far as time, it was suggested to keep a constant heat on the area of the dent keeping it on during the lenght of repair.

another question is can anyone please guide me to a source explaining how metal reacts under various forces, and how the forming of these auto panels react to these forces of damage as the energy dissapates?

and a final question:i've read that auto panels often have to be stamped twice because of spring back, why does it have this spring back? and why does it have memory?

thanks again,
mendadent

 

[HgTX]

Get thee to a basic "strength of materials" textbook for principles of stress & strain. You may or may not have the math for it (there's calculus in there) but the principles and the graphs may make sense.

I'd like to try answering that last question, non-metallurgist to non-metallurgist, mostly to see if I can. The metallurgists can clobber me if I've misunderstood it all this time.

As you know, you can bend a piece of metal so far and it'll just spring back to where you started. If you bend it beyond a certain point, it'll move a lot faster, and will stay bent. But that initial amount that would have just sprung back, it's still going to spring back no matter what. So your permanent change is what you moved it to minus that springback piece.

Here's my really simplistic understanding of why there's so far you can move it without permanent change and then after that it does change (here's where the metallurgists may clobber me), and why the springback is still there even after you've set in a permanent bend:

There's a certain amount of "springiness" to the atomic structure of the metal. The atoms can move with respect to each other by a certain amount and still maintain their bonds, like bungee cords. If they're pushed too far, though, those bonds break (the hooks let go) and then reform between new sets of atoms that are now next to each other (the hooks catch the next hole down). That shift is permanent, but the same amount of springiness remains, and the bungee will unstretch by the same amount when you let go.

How far off am I?

Hg

 

[mendadent]

i've read something similar , and here's a quote from a metal dictionary,that may apply:

Modulus of Elasticity When a material is subjected to an external load
it becomes distorted or strained. With metals,
provided the loading is not too great, they return
to their original dimensions when the load is
removed, i.e. they are elastic. Within the limits
of elasticity, the ratio of the linear stress to the
linear strain is termed the modulus of elasticity
or more commonly known as Young's Modulus.

 

[NickE]

I probably should wait for someone more knowledgable to reply Hg; however I think you hit the nail on the head.

Mendadent- The quote you give is the description of modulus, this is the reason for the Springyness. When you begin to permanently deform the metal you have passed out of the linear or elastic portion of the stress strain curve. Now adding force (stress) produces strain (deformation) relatively easily.

 

[mendadent]

quote fron nick e
"Mendadent- The quote you give is the description of modulus, this is the reason for the Springyness. When you begin to permanently deform the metal you have passed out of the linear or elastic portion of the stress strain curve. Now adding force (stress) produces strain (deformation) relatively easily"

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.

 

[CoryPad]

mendadent,

You are asking so many basic questions that we cannot educate you sufficiently. You need to read the book/take the class Metallurgy for the Non-Metallurgist available from ASM International. Go here to learn more:

http://www.asminternational.org/Tem...=Ecommerce/ProductDisplay.cfm&ProductID=11068

http://www.asminternational.org/Tem...=Ecommerce/ProductDisplay.cfm&ProductID=11069

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NickE]

Ummmmm... not quite.

first a question:
have you looked at a stress-strain curve yet?

If not then please do before we go much further.

Ok the Young's Modulus, or elastic modulus, describes the dimesional changes due to application of force in the material. This is only relevant while the material will return to the same shape and form when it is released. This is the straight line that starts at 0,0 on the stress-strain curve. This is also know as the elastic region. Teh slope (rise/run) of this line is the modulus. Once a certain amount of stress (force) has been applied the material begins to take on permanent deformation. This is the part of the curve that is generally not straight. This is known as the plastic region. To make a permanent dent in steel you have to exceed the yeild strength. To remove a permanent dent you also need to exceede the yeild strength.

Over working the material will result in the steel being really strong, but brittle and easily torn.

Nick
I love materials science!

 

[HgTX]

Stress-strain curve 101:

http://www.shodor.org/~jingersoll/weave/tutorial/node4.html

Strain is stretch, loosely speaking.

Definitely look at the Metallurgy for the Non-Metallurgist book Cory mentioned. There's an online course on the material covered in the book for "only" $700 if you want to go that far, and a take-home course for $375. Or you could just read the book.

http://www.asminternational.org/Tem...te=ConferenceDescription.cfm&ConferenceID=297

http://www.asminternational.org/Tem...=Ecommerce/ProductDisplay.cfm&ProductID=11070

Hg

 

[mendadent]

hmm, i got the correct answer, perhaps i have potential to be a metallurgist.
thank you all, this is information i was looking for in original post.
not being a mettalurgist myself, are there none of you that would find my questions from on the job training as challenging, as well as entertaining?
for example: if a door hits another car and puts a crease through a body line, do any of you know where the energy is likely to end up? or what step i would take to start the repair?
thanks to you all in advance, i feel if i can just bounce some questions off engineer's "in the know" i can get a broader understanding as to why what i do in paintless dcent repair works.

 

[HgTX]

Dent repair is definitely your area, not ours. "Where the energy winds up" isn't a question that really makes sense. The damage is where you see it; don't overthink it too much. Dent repair is probably like welding in a way--sure, there's theory to be applied but when it comes down to actually doing a good job, there's an art to it, and there's no substitute for actual experience. Not a lot of experienced dent repair folks here, and we can't do it for you.

With your level of inquisitiveness, though, you might check out some engineering technology classes in your area to see if maybe you indeed have missed your calling.

Hg

 

[mendadent]

hgtx, i love that post, yes i'm inquisitive and certainly don't want to aggravate you folks,lol
but metal is fluid,right? energy does dissapate from point of impact. it causes buckes at times farther from point of impact.
other than this site, do any of you know where or who i can converse with and ask questions?

 

[NickE]

Metals are crystalline solids.

(Ok well most are, there are some really neat and very new metals that can be cast in an amorphous shape. These have a structure like glass. The max size is really small, and they are really expensive.)

 

[mendadent]

okay, after reading some on crystalline soilds, and taking hgtx comments into consideration, i can see that me knowing composition of metal can hardly help at all(other than me just plain wanting to know the information) and i conclude other than heating it to a point that will certainly melt my paint, that unless i'm working with alumimum,the point is moot. i observe that i should be more concerned with the design phase,ie..why this panel's shaped this way or that way to lessen forces of an accident on the occupants. if any of you have seen a slow motion film of an accident you will see the rippling effect of force on the metal. my intent is to find a way to do bigger and larger panels(without painting) and to understand the effects of force on the metal, because as technicians i have to find and unlock this pressure to do what i do. example: if you're sitting at your desk and cross your leg and notice the ripples in your pants leg(automotive body line similation) and press your finger down in the middle of one of them(pants leg ripple) now notice the indention,and watch the force outward on the pants leg. well this is what we in the dent repair business have to locate(the outward force) in order to bring up the original dent.