Induction Heating

[MaddyDetroit]

Hi, I am a newbie to Induction Heating. My question is this: If I am using a conventional helical Cu Coil as Inductor with Solid Steel cylinder as Work piece, Will the heating be from the core of the steel to the surface or the other way. In other words, will the core temperature of the steel work piece be very high and gradually decreases to the steel surface or VICE VERSA..I am confused in which direction this occurs..Also, instead of Solid work piece, if i use a hollow pipe of reasonable thickness (but same diameter), will the property change?
Please help...

 

[Compositepro]

Temperature will still always be highest at the surface. The magnetic field is strongest at the surface and will diminish with depth as the work piece absorbs energy from the magnetic field. The heating is caused by eddy currents in the work piece so changes in cross-sectional area in the work piece can cause concentrated heating in some areas.

 

[IRstuff]

Since the work piece is metal, its surface effectively shields the remainder of the material from the RF field. The heating of interior regions is done strictly by conduction, much like the way induction cooktops cook food in metal cookware.

TTFN
faq731-376

 

[EdStainless]

If it is a steel that you are heating it actually a bit more complex.
When you start the field cannot penetrate very deeply, once the material reaches the curie temperature the field we reach much deeper and heating will become more uniform.
The surface will always be the hottest, but as soon as you turn off the power it starts cooling quickly.

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[MaddyDetroit]

Thanks Gentlemen...all agree that outermost surface is the hottest..So,I can safely assume that "Depth of Current penetration" into the metal is also from the surface to the core and in turn, it decreases from the surface to the core...

 

[MagBen]

The magnetic field the work piece "see" from the Cu coil, is bascually the same from surface to center. the difference is the circulating eddy current cancels in the center, while reinforce in hte surface. this is so-called skin effect. the skin
depth is proportional to square root [resitivity/(magnetic permeability*frequency)].
if the work piece is magnetic steel, permeability >>1, so the skin depth is small, at the same time, the heating is easier since there are additional heating contribution from hysteresis losses of the steel itself. when Tem >= Tc, the extra contribution from the steel disappears, but in the meanwhile the increase in resistivity with temperature makes heating easier. The induction heating is not a static process.

 

[EdStainless]

As the temp rises the electrical resistance increases. And then once you are above the curie temp the steel is no longer magnetic.

This is why modern power supplies run at variable frequencies. The freq that give the best coupling and efficiency will change as the part heats. You can't change capacitor on the fly, and even though the inductance of the coil is fixed the inductive loading from the work piece is changing.
At first you have to be careful to not overheat the outer surface. After that it become a balancing issue to use the power most efficiently.

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