Annealing mu-metal in hydrogen to retain max permeability?

[metalplastic]

Mumetal is a high magnetic permeability alloy. It is known any kind of mechanical deformation changes its microstructure leading to loss of superior magnetic properties. Annealing of machined parts fabricated from mumetal is standard.

I keep reading in literature, perhaps everyone getting that from the same source, that it is ideal to anneal mu-metal at 1120degC in hydrogen and then have controlled cooling.

My question is why hydrogen specifically? What is the scientific reasoning behind this?

I am assuming they just want zero oxygen or contaminant environment? Cant this be done just as well in a vacuum furnace or perhaps surface coat the mumetal part in a ceramic coating before heat treatment in moderate oxygen environments? Reduce impurities? Perhaps that could have been done with other steps instead instead?

 

[Milas]

Hi

We are trying to hydrogen anneal a mu-metal alloy ( 80% Nickel and 5 % molybdenum) with a view of increasing its permeability.

How can we calculate the holding and cooling temperatures, for how long and what the cooling rate should be. Additionally, how do we clean or degrease the parts prior to annealing and how big a part does the <-40 dew point play in all this ?

 

[f95toli]

I would be surprised if there is a straightforward way to calculate those parameters. Your best bet is to find a paper or similar which describes the process.
I have annealed cryoperm (very similar to mumetal) on a couple of occasions. Unfortunately I no longer have the parameters I used (the last time I did it was about 15 years ago), but I do remember finding the parameters somewhere. Places like CERN and other large faculties do tend to publish technical reports and similar which often contain this type of information.
As far as I remember, I used an old silicon furnace which could provide a hydrogen/argon(?) mix.

 

[berkeman]

Hi

We are trying to hydrogen anneal a mu-metal alloy ( 80% Nickel and 5 % molybdenum) with a view of increasing its permeability.

How can we calculate the holding and cooling temperatures, for how long and what the cooling rate should be. Additionally, how do we clean or degrease the parts prior to annealing and how big a part does the <-40 dew point play in all this ?

I've had good luck working with Magnetic Shield Corporation here in the US:

https://www.magnetic-shield.com/science-research-education/

I have used them for standard and custom mu-metal shields, and for consulting on shield design. Have a look at the above page and look through their design resources. I haven't seen any explicit information on the annealing process, but I may have missed it. You could also send them an e-mail to ask some of your questions.

 

[Astronuc]

My question is why hydrogen specifically? What is the scientific reasoning behind this?

Hydrogen provides a reducing environment - which ideally precludes oxidation of the metal being annealed in the environment. Mo and W metals are often sintered/annealed in hydrogen, since they don't form hydrides. Stainless steels are 'bright annealed' in a hydrogen environment. Alloys of Ti, Zr and Hf are melted/annealed in a vacuum, since they do form deleterious hydrides.

See the following discussion - Heat treatment of MuMetal - Vacuum or Hydrogen?
http://mumetal.co.uk/?p=111

Importantly - "Vacuum is merely a description of the process and in practice vacuum heat treatment equipment may vary considerably in the actual vacuum achieved." How strong (or low) a vacuum? Some folks may use a Ti getter to absorb residual oxygen, or perhaps use an argon flush, which is more expensive, than simply using a 'dry' hydrogen environment. An Ar/H₂ environment would also work, as long as there is no residual oxygen/moisture.

Hydrogen also has a high thermal conductivity as far as gases go. It helps to transport heat into the product, especially where complex shapes are involved.