The effect of heat on Mu-Metal

[Panrock]

I am thinking of resorting to heat to burn off old Hammerite from a CRT screening funnel.

Does anyone know if heat will adversely affect the properties of Mu-Metal? Obviously heat can cause magnets to lose their magnetism. However Mu-Metal is not a magnet - but it is nevertheless (as I understand it) highly permeable.

Steve

[kalee20]

It depends how hot you get it.

Heat treatment is generally beneficial - bending and forming mumetal clobbers its permeability. So it has to be annealed, generally in a hydrogen atmosphere (why hydrogen, I don't know. Air would tend to oxidise the stuff, but nitrogen would be inert and less hazardous. Still, factories who use hydrogen will have no difficulty filling their party balloons at Christmas).

Lower temperatures won't have such an annealing effect, but certainly should cause no harm. Of course, raising temperature above the Curie point will cause the material to lose its magnetic properties, but this is entirely reversible.

[Panrock]

Thank you for this.

Steve

[threeseven]

Mechanical stress or shock is the thing to avoid with Mu-Metal.

[Panrock]

I presume this would cause magnetisation?

When I had finished scraping and chipping off the Hammerite (with minimal help provided by Diall paint stripper) I degaussed the mu-metal pieces. They now seem to be functioning normally.

Steve

[threeseven]

No, that isn't how it works. Mu-Metal has a high permeability to magnetic flux, in other words it sort of conducts magnetic flux or field. Mechanical shock disrupts the grain structure of the material so causing area's of low permeability which allows the flux or field to pass through it.

[Anthony Thomas]

This is just a thought form my corner but to clean the "mumetal" screen how about dry ice blasting if you have such a company that offers this service.

No heat involved and I have heard that transport companies use this method to remove graffiti from painted surfaces so it should be gentle. Layers of paint can be removed one by one which is great for uncoveing historical artifacts.

Cannot offer any more info than this, a friend of mine here in Ålesund used to remove chewing gum and graffiti from marble and suchlike so I know it is / was possible.

Tony

[kalee20]

I wouldn't! It may be gentle but still not gentle enough.

Actually, thinking about it, what is wrong with Hammerite thinners (other than the expense? Or even immersing in unleaded petrol, which attacks Hammerite and makes it lift.

Mumetal is magnetically 'soft - it has a very high permeability (it is very easily magnetised by an external field) but also has, ideally, low remanance (removal of external magnetising field leaves little permanent magnetic field remaining) and low coercivity (external reverse magnetising force needed to reduce the material's small permanent magnetism to zero is also small). These factors make it valuable in high-inductance low-loss transformers, and also valuable as a magnetic screening material, as threeseven observes above.

[Bazz4CQJ]

Re dry ice blasting, another alternative is soda blasting (as in baking soda). There's lots of You Tube hits showing various DIY arrangements for doing this and I've seen aluminium parts restored very well by this method. Even if mu-metal properties are affected by mechnical impact, it's only the outer-most metal grains which would be affected by grit blasting.

B