What is the difference between RF and Magnetic Shielding?
Radio Frequency shielding is required to stop high frequency fields (> 100 kHz) and copper, aluminium, metallised plastics are normally used because they are conductive and have very little permeability. Magnetic shielding is typically found in the 30 – 300 Hz AC range.
What is the difference between DC and AC?
DC is direct current that flows in one direction only such as the fields emitted from the Earth or produced by magnets and some motors. AC is alternating current that reverses its direction over a short period and these fields are generated by typical 50-60 Hz electric power equipment. Magnetic shielding is effective for both of these types.
What is magnetic permeability?
It is a materials ability to absorb magnetic flux. It is a ratio of flux density to field strength. The higher the permeability the better the magnetic shield attenuation performance.
Do cryogenic temperatures affect the performance of Mumetal ® ?
Mumetal ® is affected by cryogenic temperatures. The saturation induction remains the same but the permeability decreases. At cryogenic temperatures we need to use a special cryogenic Mumetal ®
Can you use magnetic shielding materials at high vacuum?
Mumetal is similar to stainless steel so out gassing is minimal.
Why is a final heat treatment required for Mumetal ® , supra 50 and pure iron?
After plastic deformation a high temperature heat treatment is required to rearrange the crystal structure as well as allowing the grains to grow. Without this final heat treatment the magnetic properties and the shielding attenuation will be much reduced.
Can you weld Mumetal ® ?
Yes without a problem but it must be fully heat treated after welding
Nick Murby has worked in the specialist nickel iron alloy magnetic shielding industry for over twenty years, and has vast experience in manufacturing techniques, design and special project management within both industrial and research sectors.
Can I bend MUmetal? If I do would the permeability be lost? If so, could I get that back after bending it?
Dear Kim,
Mumetal is normally supplied in the”mechanically annealed” condition to allow for forming operations such as bending, and in this state it will mechanically react in a similar fashion to 316 grade stainless steel. This is quite different from a magnetic anneal which is a much higher temperature process (1100+degrees C instead of 900 for a mechanical anneal.)
Any mechanical deformation of mu metal after this final magnetic anneal will significantly reduce its properties, the exact amount is difficult to define as it depends on several factors:- the amount of deformation, the thickness of the mumetal, and the actual shape and direction of the field for which the shield is required, it is not unusual to find properties have been reduced to 30% of their original value by forming operations after the magnetic annealing process. In most shielding applications the rule is for optimum shielding and the magnetic anneal should be the last production process.
Regarding your second point , yes – as explained above, shield properties are fully recoverable by a further specialised magnetic annealing process (refer to my article on mumetal specifications for further information on this heat treatment process)
Hello, how do I anneal a mu-metal??
Dear Sir,
In order to achieve the optimum magnetic properties for Mu metal it requires a special heat treatment in either a low vacuum or preferably, a hydrogen furnace. For a full final heat treatment mu metal should be heat treated to 1150 deg C with a four soak. The information above is for a magnetic and not mechanical anneal. If you wish any further details, please do not hesitate to contact me.
Dear Nick Murby,
Does the magnetic shielding material (mumetal) still attract extra external magnetic field (flux) after its state of saturation?
Dear Sir,
After mumetal reaches its saturation point it will not attract additional external magnetic magnetic fields. A material such a Supra 50/Pure Iron has a higher saturation flux density and should be used.
Dear Alex
The saturation induction of mu metal is 0.76 Tesla
Kind Regards
Hi,
I’m trying to make an eddy current coil using mumetal as a core sensor in the coil. However I’ve come across some difficulties and haven’t succeeded yet. I was wondering if someone can assist me.
Are there different heat treatment procedures for mumetal depending on different applications?? The mumetal material that I used was also heat treated but I’m not sure if the treatment procedure was appropriate or not.
After doing heat treament under microscope I could see the crystal grains on the surface. However after a few months I found that the grains have started to disappear. Is this normal for grains to disappear as time passes by??
Besides mumetal, I have also tried supermalloy, permalloy..etc different kinds of NiFe alloy but I still did not suceed. The eddy current coil doesn’t work well. So I’m still wondering if it is the material problem or the heat treatment problem??
Any suggestions or comments will be greatly appreciated….!!!!
Thanks a lot!
Yes there are different heat treatments for Mumetal and different grades of Mumetal which has various trade names inc Supermalloy and Permalloy A. Take care with this particular trade name as it also is used for the 50%NiFe and the 36%NiFe grades. True mumetal should correspond to ASTM A753 and alloy 4 under this standard and is the only grade we use and has a nominal composition of 79% NI with 4% Mo and balance Fe. There is also a grade containing up to 5% Cu in replacement for most of the Mo but we do not use this grade since generally its properties are regarded as inferior.
Regarding heat treatment it must be heat treated as a last process normally this is in an atmosphere of pure dry hydrogen at a temperature of 1100-1170 degrees centigrade for 2-4 hours followed by cooling to below 250 degrees at a cooling rate around 170 degrees per hour
Grain size after this heat treatment should be large normally ASTM 9 or larger and can often be seen without any optical aid if the HT atmosphere is very good with a dewpoint better than -40 degrees centigrade. The disappearance of these grains is normally due to mild surface oxidation, mumetal is very oxidation resistant in air but the “super clean ” surface after heat treatment will oxidise mildly over time in normal room conditions.
I hope this helps , please feel free to see our website for more Heat Treatment and material details and do not hesitate to contact us if you require further help or information.