Induction heating: facts or sales talk? I need information! 1

[kingnero]

I've been invited by a sales person to check out the new setup of a welding program, using induction heating.

Induction heating is new to the firm I work for and for me personally.
I vaguely know the principles, but not much more than that.
We always use flames (oxy/acetylene) and ceramic carpets (resistance heating) for preheating.

The guy baffled me today with eddy currents and Faucoult currents and molecular movement and such, so can anyone point me to somewhere where all of this is properly explained, preferably a bit more scientific than good ol' google seems to find?

Another question: is it true that, when a part is heated using induction, it keeps the heat "in itself" for longer than if it was preheated (to the same temp.) using flames or ceramics? I find this hard to believe, however he really tried to persuade me that is't like that...

Any insights much appreciated...

 

[metengr]

Suggest you talk to the professionals regarding induction heating. We have used them at our Plants many times...

http://www.manningsusa.com/industries.html

 

[metengr]

If you're still interested in knowing the induction heating concept with a little science behind it - have fun

http://www.uie.org/webfm_send/441

 

[1gibson]

Another question: is it true that, when a part is heated using induction, it keeps the heat "in itself" for longer than if it was preheated (to the same temp.) using flames or ceramics? I find this hard to believe, however he really tried to persuade me that is't like that...


If you consider the "part temperature" to be the surface temperature, then yes. Think of it as microwaving compared to toasting. Which are you going to use to cook the inside of a baked potato?

 

[racookpe1978]

I have worked with Mannings for many years - Get hold of them for tech details.

 

[kingnero]

@ metengr: thanks, I'll see whether they have dealerships/offices in Europe. If so, I might pay them a visit, if not I'll call 'em in the USA.
thanks for the pdf, I've printed it and will have a look at it this weekend.


@ 1gibson: when I said 'temp', I ment core temp, as of course surface temp isn't representable for a workpiece.

I rephrase: when 2 identical parts are fully heated to a certain temp, using flames for one part and induction for the other,
and conditions are the same (same ambient temp, no forced cooling, ...), will the induction heated part be warmer for a longer time?

I find this hard to believe, however the sales guy tried to explain this with molecular movements and a lot of technical verbiage but I am still not impressed.

 

[btrueblood]

"when 2 identical parts are fully heated to a certain temp, using flames for one part and induction for the other,
and conditions are the same (same ambient temp, no forced cooling, ...), will the induction heated part be warmer for a longer time?"

No. If the internal temperatures are the same, the parts will cool at the same rate.

 

[racookpe1978]

Problem is, the flame-heated part will be heated from the outside - so, when (if!) the inside gets to the same temperature that the induction heater has achieved, (parts of ) the outside of the flame-heated MUST BE much hotter than the assumed inside temperature. Also, when that inside temperature is reached, then the smaller volume areas closer to the outside surface will be even hotter than the average inside temperature - a hollow casting, for example, will be unevenly heated inside. A casting or part with large area/lower volume areas near the outside surface will find those volumes cooling faster than the interior of the rest of the "average" part.

A flame-heated part will (usually) get passed through the flame, or the flame passed on over the outside surface, so areas of the outside are getting unevenly heated and cooled off at various times as the whole part gets up to temperature. An air oven, uniformly raised to some delta T over the desired final part internal temperature, will more uniformly raise temperatures. No part of the whole assembly can get higher than the air temperature - which can be regulated much smoother than an open flame that (by definition) is much, much higher than the surface.

 

[kingnero]

That is true, but we have a very good control over the core temp, even when using flames.

Imagine a railroad track. That's similar to an I-beam, only much heavier.
When we need to (repair / clad) weld the upper surface (the "flange"?), we heat the middle part (the "web"?) until the surface of the contact area (with the train wheels) has the requested temp.
That means that the heat went upwards, coming from below.
The temp we measure at the top, is (only slightly) lower (due to losses of radiation) than the core temp.
Thus, the exact opposite of regular procedures.

But, as I understand the responses I got here, the sales talk that the railroad track will keep the heat longer than using flames, isn't true at all. There is no different cooling down curve for the metal, depending on the way you heat it up.

Thanks for your thoughts on this!

 

[dinjin]

Induction heating is often used as in case hardening and it normally does produce greater depths of hardeness below the surface for thick parts. For thin parts, there probably is little difference.

 

[Mr168]

What amazes me most about your encounter with the sales rep, is the fact that he was trying to explain the laws of thermodynamics to you, more so than pitching the real reason why fabrication facilities use induction heating equipment for preheats - speed.

 

[btrueblood]

Induction heating still produces a non-uniform temperature profile, as most of the heating power is induced near the surface. Not quite as steep a profile as a flame, for sure, but similar. metengr's link shows some nice plots and equations demonstrating this.

My point was that, for the same internal temperatures (perhaps I should have said bulk-averaged temperatures , the flame-heated part won't cool faster because of some residual or latent heat produced by the induction process. I.e., an effect like superheating of water due to rotational frequency excitation of the water molecules by a microwave oven, does not occur in induction heating of metals. That I know of.

 

[weld7777]

We have 8 induction heating machines (Miller Electric), they are a great time saver, and also allows weldor to weld on part while keeping interpass temp in range.

 

[stanweld]

Induction preheating has some distinct benefits: faster attainment of preheat temperature, and the high frequency AC mitigates arc blow.