# What stainless steels are good for induction heating?

I'd like to know which stainless steels are good for heating on an induction stove. Wikipedia notes that the surface resistance resistivity is a good figure of merit. However, that article only lists the surface resistance resistivity for two types of stainless steel -- the better being 432. However, I cannot find 432 from my favorite supplier, although they do have 430.

Will 430 have a similar surface resistance? Is the a resource where I can find the surface resistance of other stainless steels? I have searched around but only found results for the two stainless steels that wikipedia lists.

EDIT: Looking at the IEEE article that underlies the wikipedia article, I think that the wikipedia article should use "surface resistivity" instead of "surface resistance". I was using the terms in the wikipedia article. Sorry for any confusion. I will edit the wikipedia article if appropriate.

• You said "I'm under the impression that surface resistivity is a function of the skin depth and resistivity" -> You are (dangerously) mixing terms. ResistIVITY is a property of the material and expresses the resistance of a unit cube of material but does not tell you actual resistance that results. ResistANCE is the resistance to current flow of the actual material used in the shape/dimensions used. The pot base offers Ohmic resistance to induced current as a result of material resistance for a slab the shape of the base and a depth controlled by the skin depth at the frequency used. Jun 14 '15 at 7:19
• Maybe the issue is that I haven't defined the term. The IEEE article that underlies the wikipedia article defines surface resistivity as proportional to $\sqrt{resistivity \cdot permeability \cdot frequency}$ (i.e. resistivity divided by skin depth). It seems to include all the ingredients you suggest. Is this a good figure of merit? dx.doi.org/10.1109/TIA.1973.349892 Jun 14 '15 at 16:55

... that article only lists the surface resistance for 430 & 432 stainless steel -- the better being 432. However, I cannot find 432 from my favorite supplier, although they do have 430. Will 430 have a similar surface resistance?

Yes.
430 and 432 are listed as having almost identical resistivities - see below.

Surface resistivity by itself is not an adequate figure of merit.
Inductive heating is a function of AC skin depth and resistivity. At lower frequencies skin depth increases and higher resistivity materials are required to produce the desired load. As frequency rises skin depth decreases and total material resistance increases due to less depth x area volume and lower resistivity materials are better.

The recommendations to use stainless steels is related to the typical frequencies of current induction cookers. This is mainly limited by the economics of currently available high power electronic drivers. As driver costs fall with time frequencies will be able to be increased, reducing skin depth and allowing lower resistivity metals to be used.

430 and 432 are listed as having almost identical resistivities - see below.

Care should be taken to distinguish between resistIVITY and resistANCE.

ResistIVITY is a property of the material and expresses the resistance between opposite faces of a unit cube of material but does not tell you actual resistance that results when dimensions are other than a unit cube.
Resistivities are usually expressed either in microOhm.inch or microOhm.centimetre. I'll call these mOin and mOcm respectively.

• The reason for the units of Ohms x length is that, starting with a unit cube resistance increases with increasing thickness and decreases with increasing face area. So R for some other shape = Resistivity x t/A Ohms.
To have units of Ohms, resistivity must cancel the t/A, so units of resistivity = Ohms x A/t = Ohms x length.

ResistANCE is the electrical resistance to current flow in Ohms of the actual heating material used in the shape/dimensions used. The pot base offers Ohmic resistance to induced current as a result of material resistance for a slab the shape of the base and a depth controlled by the skin depth at the frequency used.

Resistivities are usually expressed either in microOhm.inch or microOhm.centimetre. I'll call these mOin and mOcm respectively.

1 mOin = 2.54 mOcm

Links to many Stainless steel data sheets here

430 - 24 micro-Ohm.Inch (mOi) From here

430 ultraform (added Titanium) - also 24 mOin (so suspect)
From here

432 - 24.5 mOin
304 - 29 mOin

304 - 69 & 72 mOcm (about 28 mOin)
347 - 72 mOcm (~= 28 mOin)
316 - 75 mOcm (~= 30 mOin)
All from here
(suspiciously) Identical figures here

Skin effect & skin-depth:

Skin effect - Wikipedia

Skin deth formula & notes

Skin effect depth calculator - does not have an entry for stainless steel - but see simple associated formula. Skin depth scales with 1/(sqrt(relative permeability))

Depth ~= Sqrt(Resistivity/(Pi x Freq x absolute permitivity)) see above reference

Textbook chapter 7 page pdf.

• I'm under the impression that surface resistivity is a function of the skin depth and resistivity, basically in the way you describe. Is this not so? Jun 14 '15 at 5:20