Annealing mu-metal in hydrogen to retain max permeability?

Overall, using hydrogen in the annealing process helps to ensure the purity and superior magnetic properties of mumetal. Other methods, such as using a vacuum furnace or surface coating with ceramic, may also work to reduce impurities, but using hydrogen is the most widely used and effective method for annealing mumetal. In summary, hydrogen is used in the annealing process of mumetal to provide a protective atmosphere and prevent oxidation, ensuring the purity and superior magnetic properties of the metal.
  • #1
metalplastic
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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?
 
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Hydrogen is used in the annealing process of mumetal because it is an inert and non-reactive gas that provides a protective atmosphere for the metal. It helps to prevent oxidation, which can cause impurities and weaken the metal’s magnetic properties. Hydrogen also helps to reduce the thermal conductivity of the metal, preventing uneven heating during the annealing process.
 
  • #3
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 ?
 
  • #4
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.
 
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  • #5
Milas said:
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.
 
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  • #6
metalplastic said:
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.
 
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1. What is annealing mu-metal in hydrogen?

Annealing mu-metal in hydrogen is a process used to increase the permeability of mu-metal, a type of soft magnetic alloy. This process involves heating the mu-metal in a hydrogen atmosphere to a specific temperature for a certain amount of time to achieve the desired permeability.

2. Why is hydrogen used in the annealing process?

Hydrogen is used in the annealing process because it helps to remove impurities and improve the magnetic properties of the mu-metal. It also prevents oxidation during the heating process, which can affect the permeability of the metal.

3. What is the maximum permeability that can be achieved through annealing mu-metal in hydrogen?

The maximum permeability that can be achieved through annealing mu-metal in hydrogen varies depending on the specific composition of the mu-metal and the annealing process used. However, typically, permeability values in the range of 100,000 to 500,000 can be achieved.

4. How does the annealing process affect the magnetic properties of mu-metal?

The annealing process can greatly improve the magnetic properties of mu-metal, specifically its permeability, which is a measure of how easily it can be magnetized. By removing impurities and aligning the crystal structure, the annealing process allows for a higher permeability and better magnetic shielding capabilities.

5. Are there any potential drawbacks to annealing mu-metal in hydrogen?

One potential drawback of annealing mu-metal in hydrogen is the risk of hydrogen embrittlement, which can weaken the metal and make it more prone to cracking. It is important to carefully control the temperature, time, and hydrogen concentration during the annealing process to avoid this issue. Additionally, the cost and time involved in the annealing process may also be considered drawbacks.

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