Ni78 soft magnetics alloy annealing

[marcus4]

Hello. I need small quantity parts to be annealed and wonder can I use dental vacuum furnace? Something like this:

https://www.ebay.co.uk/itm/BIODENT-Keramikofen-Typ-SYSTOMAT-M-Vakuumpumpe-9887-/232481417446

Most of them are capable to reach 1200c but I'm not sure are those furnaces good enough for this purpose. Any comment is welcome.

 

[MagBen]

you are talking about permeally Fe-78Ni, right? then i donot think the furnace is good enough. the key to anneal to optimize magnetics is the cooling rate, temp, and atmosphere. temp 1200C is high enough, atmosphere prefers dry H2, and yet the cooling rate needs to be controlled depending on Mo content in the alloy.

 

[marcus4]

Hi MagBen, yes, Fe ni 78 or 80liek this 0.02 C, 0.50 Mn, 0.35 Si, 80.00 Ni, 4.20 Mo, Bal. Fe. As far i know dry H2 is recommended for the maximum permeability and to clean impurities, but I will be happy with the average results. This is from the manufacturer datasheet:

"For maximum softness and optimum magnetic and electrical properties, Carpenter HyMu "80" alloy should be
annealed in an oxygen-free, dry hydrogen atmosphere with a dew point below -40°F (-40°C) at 2050/2150°F
(1121/1177°C) for 2 to 4 hours. Furnace cool to 1100°F (593°C). From 1100 to 700°F (593 to 371°C), cool at a
rate between 350 to 600°F (194 to 334°C) per hour.
Oil, grease, lacquer and all other contaminants must be removed before annealing. The individual parts should be
separated by an inert insulating powder such as magnesium and aluminum oxide during hydrogen annealing.
Vacuum heat treating can be employed. Generally, there is some small sacrifice in magnetic properties compared
to heat treating in a dry hydrogen atmosphere. "
I see "Furnace cool to 1100°F (593°C)". Is that a problem? I think, (maybe I'm wrong) that it can be achieved manually. Or I miss something?

 

[dgallup]

We anneal quite a few materials in vacuum that are recommended to be annealed in dry hydrogen and they work very well in our applications. I can't tell from the link how good a vacuum that furnace will pull (I don't read German). You don't say what your application is or how critical the properties are. Unless you can pull a really hard vacuum (~1 micron Hg), you are sure to get scale.

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[marcus4]

Hi dgallup. I cant find technical details about this furnace, but I give this one as the example. There is a lot of other small (and affordable) dental furnaces on the market and i can choose the right one, but not sure which one? Also, if you know exact model please recommend.
My application is annealing Ni78 or ni80 soft magnetics material and since I need small quantities I I can't use any of local heat treatment services. All of them must be filled with 200kg and up. Unfortunately my annealing process is not so popular to share the service with some more people. That's why i look for the small lab furnace.
When you say "We anneal quite a few materials in vacuum that are recommended to be annealed in dry hydrogen", did you tried to anneal soft magnetics alloys to improve permeability? If yes, what cooling rate you suggest?

 

[dgallup]

Cooling rate is going to be material specific but for silicon iron & ferretic stainless steel we use a maximum of 110 degrees C per hour down to 535 C and no strict control below that. Below 180 C air cooling is permissible.

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[MagBen]

high Ni permeally is a different animal, cooling rate is an important parameter to control magnetics (for other alloys, including low Ni (49%) permeally, cooling rate is not much practically critical as long as not too fast to cause thermal stress).

Furnace cool, or whatever cool, to 1100F is not a problem. cool rate at 700-1100F needs to be controlled to get a right order/disorder ratio structure for a nearly Zero magneto-crystalline constant for a highest permeability.

By the way, if you can get a high vacuum, and you material is not dimensionally thin (say strip). Vacuum or H2 or Ar atmosphere does not make much difference.

 

[marcus4]

Hi guys, thanks for the tips. Since Im not sure about the amount of vacuum inside this little furnaces I presume that I have to find one locally and do the experiment. Unfortunatelly, parts I want to anneal are thin (0,2mm laminate) and I presume that I have to use Mgo powder to separate them. One detail still remains unclear to me. What does it means "Furnace cool to 1100°F (593°C)'. They dont give any rate. Does it means that rate from annealing to 593c temperature is not important?

 

[EdStainless]

If you need to cool in a vacuum you may need to backfill with Ar in order to cool faster.
Are you lams uncoated? No insulation layer?
You might just need to make sure that they are not in a tight stack, and that the edges don't line up.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[marcus4]

thanks @EdStainless. Yes, uncoated As I mentioned above, I'm not sure how fast should be the cooling from annealing temp to 593°C? Can I just switch off the heating and wait temp. to drop or I need some inert gas to speed the process? If I need to speed it up, what rate?

 

[hacksaw]

NiFe anneal,

Marcus, you don't really want a vacuum anneal, dry inert gas or H2 works fine.

Fundamentally, you are after a stress anneal at temperature well below 1200C.

Rate of cooling is secondary as long as it is not a quenching cool down.

As with all annealing, you 'll have to go through a trial and error process.

good luck,

 

[MagBen]

80 Ni is very sensitive to heat treatment. Both atmosphere and cool rate at low temperature (700-1100F which is the order/disorder transition temp) are primary.

You will still get a high permeability (>100,000) ignoring these parameters, but I hope the right alloy is used for the right people who can fully take advantage of the alloy with potential of much higher permeability up to 400,000, or even 500,000, if people treat it well.

For thin gaged material, Dry H2 is preferable since there is an additional benefit, i.e. to remove impurities. Vacuum and Ar will yield lower perm to some degree. In any case, vacuum needs to be less than 10^-3 torr and chamber be leak free. N2 is not recommended.

Ed raised a good point. At that high temperature, the bare lams should not be touched each other. Use MgO, or at least Al2O3 powder to separate lams to prevent welding.

The cool from 2100F to 1100F is not important, one can use whatever convenient way to cool.

Fine alloy deserves fine treatment for fine applications.