Mumetal Grain Size and Magnetic Properties Correlation

I have a sample of transformer laminate (mumetal) with unknown magnetic properties. My goal is to achieve the same magnetic properties with raw material that has the same chemical composition and thickness, but I can’t measure the permeability results. Is it possible to do that by comparing the grain size/shape and repeat the heat treatment until they become the same?

If the target’s sample permeability is unknown, do you have approach to achieve same magnetics qualities with raw material? I’m aware that repeating the heat treatment and trimming it’s parameters will be the solution, but I would like to do it under the lab environment (lab tube furnace with protective atmosphere). Diameter of the tube is large enough for few parts but to prove final results (measuring inductivity of the choke) I need 80 parts. That leads me to large furnace I don’t have…

Thickness is 0,35mm but properties of material are posted really rough on manufacturer site. I can achieve wavelength-Dispersive X-Ray Flurescence Spectrometry if it’s needed but here is chemistry from their site: Ni76.6 Cu4.5 Mo3.3 Fe14.7 other% Mn, Si

Target permeability has to be approx. u5=11000 but I can’t measure it. I know it’s low permeability for this kind of material but that’s the subject of my post here.

Is it clever to start with lower temperature, say, 1000, than if the permeability is lower than expected to repeat treatment with higher temperature? Of course, with H2 protection and clean furnace.


Grain size is indeed one of factors which determine magnetics, at a certian range, the bigger the grain size, the less the grain boundaries, the less the pinning sites of magnetic domain displacement, and so the higher the permeability. However, as for Mumetal, the ratio of short-ranged disordered/ordered takes a big role to determine soft magnetics. A same grain size with a different disorder/order structure can create a big difference in permeability.

I can recommend heat treatment parameters to fully recover the magnetics:

  • Try 1100-1200C/4h + 150C/h cool to 700C, then 180C/h cool. better use dry H2 atmosphere (dew point < -40C).
  • I assume u5 is the permeability at H =.005 Oe at 60 Hz. 60-70,000 is expected. initial permeability at B=40Gs, 60 Hz is about 50-60,000. these are impedance permeabilities, if using inductance permeability, the values are even higher.

Note the cooling rate below 700C is critical to yield partially ordered structure. High temperature is for grain growth as well as effective removal of C, S. If the temperature is too low, you will get very small grain size and high amount of remaining C, S, both of which could bring you properties far away from desired.

[b.t.w., Even low Ni grades, say Carpenter High Perm 49, can give you 11,000 at .35mm heavy.]

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