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telectrix

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something that has been bothering me for some time now. stainless steel is steel with approx. 7% -10% of chromium added to make it rust resistant, this is due to the chromium producing a molecularly bonded chromium oxide coat. al this i understand. what i can't get my head round is that it is still 90% ish ferrous, so why is it not magnetic?
 
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M

MarkieSparkie

Austenitic stainless steel (300 series) is non-magnetic due to the high nickel content. In a typical non-magnetic stainless steel (18/10) there is 18% chromium and 10% Nickel in the mix. Austenitic stainless steel accounts for about 70% of World production. Ferritic, Martensitic, Duplex and Precipitation Hardening the other major stainless steel types are magnetic.

It is commonly stated that “stainless steel is non-magnetic”. This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a relative magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1. Typical values for standard austenitic stainless steels can be in the order of 1.05 – 1.1.
It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.
In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases, it is often necessary to agree a maximum magnetic permeability between customer and supplier. It can be as low as 1.004.

Stainless steel is usually divided into 5 types:

  1. Ferritic – These steels are based on Chromium with small amounts of Carbon usually less than 0.10%. These steels have a similar microstructure to carbon and low alloy steels. They are usually limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required they offer a wide range of applications. They cannot be hardened by heat treatment. High Chromium steels with additions of Molybdenum can be used in quite aggressive conditions such as sea water. Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels. They are magnetic.
  2. Austenitic - These steels are the most common. Their microstructure is derived from the addition of Nickel, Manganese and Nitrogen. It is the same structure as occurs in ordinary steels at much higher temperatures. This structure gives these steels their characteristic combination of weldability and formability. Corrosion resistance can be enhanced by adding Chromium, Molybdenum and Nitrogen. They cannot be hardened by heat treatment but have the useful property of being able to be work hardened to high strength levels whilst retaining a useful level of ductility and toughness. Standard austenitic steels are vulnerable to stress corrosion cracking. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. They are nominally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel.
  3. Martensitic - These steels are similar to ferritic steels in being based on Chromium but have higher Carbon levels up as high as 1%. This allows them to be hardened and tempered much like carbon and low-alloy steels. They are used where high strength and moderate corrosion resistance is required. They are more common in long products than in sheet and plate form. They have generally low weldability and formability. They are magnetic.
  4. Duplex - These steels have a microstructure which is approximately 50% ferritic and 50% austenitic. This gives them a higher strength than either ferritic or austenitic steels. They are resistant to stress corrosion cracking. So called “lean duplex” steels are formulated to have comparable corrosion resistance to standard austenitic steels but with enhanced strength and resistance to stress corrosion cracking. “Superduplex” steels have enhanced strength and resistance to all forms of corrosion compared to standard austenitic steels. They are weldable but need care in selection of welding consumables and heat input. They have moderate formability. They are magnetic but not so much as the ferritic, martensitic and PH grades due to the 50% austenitic phase.
  5. [FONT=&amp]Precipitation hardening (PH) - These steels can develop very high strength by adding elements such as Copper, Niobium and Aluminium to the steel. With a suitable “aging” heat treatment, very fine particles form in the matrix of the steel which imparts strength. These steels can be machined to quite intricate shapes requiring good tolerances before the final aging treatment as there is minimal distortion from the final treatment. This is in contrast to conventional hardening and tempering in martensitic steels where distortion is more of a problem. Corrosion resistance is comparable to standard austenitic steels like 1.4301 (304). [/FONT]
 
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snowhead

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As all that Googled rubbish above probably says, Some lower grade Stainless is magnetic.
 

telectrix

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  • Thread Starter Thread Starter
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so, following on from that excellent post, could non-magnetic stainless steel be used to manufacture enclosures where cable entries would generally give rise to eddy currents, in order to eliminate the problem?
 

Richard Burns

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Eddy currents are generated by a changing magnetic field (generated from the current flow through the cable) interacting with a conductor, it does not need to be a magnetic conductor. The standard experiment is having a high magnetic field between two plates and dropping a sheet of copper between the plates, the sheet's rate of fall is considerably reduced as it passes through the magnetic field.
 
M

MarkieSparkie

The short answer to your question is no.
The only way to completely eliminate eddy currents flowing in such circumstance would be to manufacture the enclosure from an electrically non-conductive material eg. plastic.
Eddy currents can be reduced in iron and steels by making them less conductive by adding Silicon to metal mix during manufacture.

Other methods of reducing eddy currents include:
Thinner material.
Laminating thin sections together.
Slotting the material so the current cannot circulate, but it may weaken the structure.
Reducing the magnetic field by cancellation by combining the feed (L) and return (N) conductors (and any protective conductor) of a circuit into the same cable and/or twisting the line conductors together in pairs, triples or quads as appropriate.
Slow the relative speed of change of the magnetic field by reducing its frequency, this is not usually an option where the supply frequency is fixed.

Unfortunately this issue is clouded in BS7671:2008+A1:2011 521.5 by it focusing on ferromagnetic materials, wrongly giving the impression that the use of non-ferromagnetic materials instead eliminates or reduces the problem of eddy currents. This is compounded by the fact that a number of well known electrical installation text books misleadingly recommend passing single core cables through non-ferrous plate (usually aluminium) and that any glands and connectors are made from similar non-ferrous materials in order to reduce eddy currents.
 
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so according to what your saying Mark, then AWA single core cables also give rise to eddy currents in the AWA?
im intereseted to see how this thread develops.

Its nice to have a proper technical discussion too!
 

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