Magnets sticking to stainless steel?

Discussion in 'Physics & Math' started by MetaKron, Jun 18, 2005.

  1. MetaKron Registered Senior Member

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    5,502
    I have been told that magnets don't stick to stainless steel. Yet, I have seen permanent magnets sticking to stainless steel. These were small neodymium magnets I purchased from All Electronics. They stick to every kind of stainless steel I could find at home and at work. Ceramic magnets don't.

    This experiment is one more thing that tells me that experiments should be tried and not just dismissed because theory says something. Also, it's just interesting. I suspect that it's because fairly pure iron exists in small crystals near enough to the surface to be attracted to the magnet.

    This also tells me that if you want to use a magnet to test whether a sample is stainless or not, either use a weaker magnet or know what you are doing. Ceramic magnets don't stick at all, as far as I can tell. A neodymium magnet will stick and hold up its own weight, but it is easily removed. I doubt there's much call to distinguish between stainless steel and nickel-copper alloys that don't stick even to neodymium magnets.
     
  2. MetaKron Registered Senior Member

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    Is that banner supposed to be there?
     
  3. funkstar ratsknuf Valued Senior Member

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    I think so. Sciforums has to live somehow...

    (It appears below the first post on every thread page.)
     
  4. MetaKron Registered Senior Member

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    I just never noticed it before.
     
  5. James R Just this guy, you know? Staff Member

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    I suspect you're right.
     
  6. Billy T Please use Sugar Cane Alcohol Fuel Staff Member

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    20,601
    I am not sure exactly what you are agreeing to James, but I think I have a slighly different take on this. I thought his basic question was why did one type of magnet show different effects from the other?

    First, note tht if the stainess steel is only weakly feromagnetic, perhaps only paramagnetic. Then placing it up against the magnet face, will not reshape the field distribution greatly, as say placing a strongly feromagnetic material (iron, nickel etc) would. Thus lets assume, as a first approximation, that the both types of magnets have the same external field pattern and it is not greatly changed in shape when in contact with the SS.

    If his magnets were of different size and with fields of different shape, there could be explanations in this fact, and I bet it likely they were, but I a looking for some reason for a difference if the two magnets differed only in the strength of the field, but not it their field geometery. The only thing I can think of under this constraint, is that one (the stronger one) may saturate small feromagnetic domains in the SS, especialy those near the face of the magnet (one surface of SS in contact with magnet). Now we must also consider that there can be an interactions between domains in the SS.

    This does not envolve any motion of the atoms in the SS, only the spin alingments of the unbalanced orbitals. (neclecting nuclear spins) Perhaps the domain- domain interaction in the presence of the stronger magnet type is significant but not in the case of the weaker magnet. That is, perhaps the randomly oriented domains in the presence of the weak magnet remain random, but in the presence of the stronger magnet, domains flip and align. Perhaps this does start with micro feromagnetic crystals near the surface and almost instanteously propigate inward.

    One could experimentally explore this concept with a high sensitivity audio amplifier and a pick up coil wound around the front face of the magnet so it is essentially in contact with the SS surface. The slow movement of the magnet to contact the SS will not produce any audio noise, but if a cascade of domains flipping is heard with one magnet and not the other, my speculatons are supported. (I probably don't have the spelling correct, but this audio noise is called "Barkhausen noise" if memory serves me correctly)
     
  7. kevinalm Registered Senior Member

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    993
    "Stainless steels" aren't steels at all. Well, not in the Fe alloy sense anyway. Most are a nickel chromium alloy, no iron. A weak magnet is attracted, just so feably you don't notice.
     
  8. MetaKron Registered Senior Member

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    5,502
    Chemistry info for 304 alloy stainless steel


    Chemistry info for 316 alloy stainless steel

    The stainless steel that I can buy online runs 68-82 percent iron. Nickel is a bit more expensive than iron even if we do use it in currency, so I would expect iron to be the metal of choice as a base for stainless steel. One thing about nickel is that it doesn't have to be alloyed with chromium to become unable to corrode. It comes that way out of the box. Nickel is what they used to plate things with before it was decided that it was too poisonous. It damages the central nervous system much the way manganese does. Stainless steels do contain around 8 to 14 percent nickel.
     
  9. Rogue Physicist Registered Senior Member

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    Yes this is quite true: there is a variety of interesting alloys, especially 'steels' that are deliberately made not to interact with magnetic fields. They are used in motors, and mechanical devices that require magnetically sensitive parts to be protected from stray magnetizing effects. A good example is your typical hard-drives, especially larger ones. You can pick some up at a surplus store and extract metal parts that have the weight and strength of steel, but cannot be magnetized and don't stick to magnets. These materials are very useful in building experimental apparatus cheaply.

    By the way, nickel is also magnetic, so the fact that you have an alloy made of nickel instead of iron is not an adequate explanation. You'll have to look deeper into the theory behind para- and dia-magnetic material alloys.

    Non-Magnetic Steels - Definition: Austenitic steels such as the 14% manganese steels and the 303 type 18/8% chromium-nickel stainless steels.

    Stainless steels
    are high-alloy steels that have superior corrosion resistance than other steels because they contain large amounts of chromium. Stainless steels can contain anywhere from 4-30 percent chromium, however most contain around 10 percent. Stainless steels can be divided into three basic groups based on their crystalline structure: austenitic, ferritic, and martensitic. Another group of stainless steels known as precipitation-hardened steels are a combination of austenitic and martensitic steels. Below are the general compositional contents of these groups.
    Grades

    Ferritic grades:
    Ferritic stainless steels are magnetic non heat-treatable steels that contain chromium but not nickel. They have good heat and corrosion resistance, in particular sea water, and good resistance to stress-corrosion cracking. Their mechanical properties are not as strong as the austenitic grades, however they have better decorative appeal.

    Martensitic grades:
    Martensitic grades are magnetic and can be heat-treated by quenching or tempering. They contain chromium but usually contain no nickel, except for 2 grades. Martensitic steels are not as corrosive resistant as austenitic or ferritic grades, but their hardness levels are among the highest of the all the stainless steels.

    Austenitic grades: Austenitic stainless steels are non-magnetic non heat-treatable steels that are usually annealed and cold worked. Some austenitic steels tend to become slightly magnetic after cold working. Austenitic steels have excellent corrosion and heat resistance with good mechanical properties over a wide range of temperatures. There are two subclasses of austenitic stainless steels: chromium-nickel and chromium-manganese-low nickel steels. Chromium-nickel steels are the most general widely used steels and are also known as 18-8(Cr-Ni) steels. The chromium nickel ratio can be modified to improve formability; carbon content can be reduced to improve intergranular corrosion resistance. Molybdenum can be added to improve corrosion resistance; additionally the Cr-Ni content can be increased.

    It also bothered me the first time I found out about non-magnetic steels.
    Perhaps this post below will help you:


    --------------------------
    Date: Mon Feb 21 10:45:07 2000
    Posted By: Bob Novak, Sr Process Research Engineer,
    Carpenter Technology
    Area of science: Physics

    Hi xxxxxx,

    This is an interesting question, and one that metallurgist and physicists
    are still discovering answers for. It is the electrons in a localized
    region called a domain that must interact for a material to become
    magnetic. When the interaction of the electrons with a magnetic field
    results in a lowered energy state, the material can retain a magnetic
    field. Anything that changes the relative positions of the electrons in a
    crystaline structure will affect how they interact and their ability to
    become magnetized. The different elements that are added to iron change
    its crystaline structure, and thus its ability to be magnetized.
    Additionally, not all crystaline forms of iron are magnetic. The face
    centered cubic structure of iron is not magnetic. The interaction of the
    electrons in a face centered cubic iron structure does not result in a
    lowered energy state when it interacts with a magnetic field.

    This is a simplified answer to a complex materials characterization
    question. In summary, it is the interaction of the electrons in a domain,
    and not any single pair of electrons, which determines how magnetic any
    given alloy system will be.

    Bob Novak
    Specialist -Process R&D
    Carpenter Technology
     
    Last edited: Jun 19, 2005
  10. MetaKron Registered Senior Member

    Messages:
    5,502
    I take it that more precisely, the metals of the shells of hard drives won't become magnetized themselves. They are transparent to a steady magnetic field, not so transparent to an AC field, and won't acquire a permanent magnetic field under any conditions.

    Nickel is good for making nonmagnetic materials. Even rare earth magnets won't stick to the nickel used in currency, not even the plated nickel over copper. At one time I read that the most nonmagnetic metal made was an alloy of nickel over copper. That was when the most magnetic metal known was cobalt-copper-samarium. "Neodymium" magnet sounds like something out of H.G. Wells. :D
     
  11. metallurgy.ir Registered Member

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    5
    Austenitic stainless steels are non-magnetic but ...

    Austenitic stainless steels such as 304 and 301 are renowned for their non-magnetic properties which is the first tangible characteristics when these steels are compared with ferritic and martensitic stainless steels. However, austenitic stainless steels become magnetic after the sufficient cold deformation due to the formation of strain-induced martensite. Therefore, these steels cannot further be used as non-magnetic materials. For more info:

    [1] H. Mirzadeh and A. Najafizadeh, Correlation between processing parameters and strain-induced martensitic transformation in cold worked AISI 301 stainless steel, Materials Characterization 59 (2008) 1650-1654.

    [2] H. Mirzadeh and A. Najafizadeh, Modeling the reversion of martensite in the cold worked AISI 304 stainless steel by artificial neural networks, Materials & Design 30 (2009) 570-573.

    [3] H. Mirzadeh and A. Najafizadeh, ANN modeling of strain-induced martensite and its applications in metastable austenitic stainless steels, Journal of Alloys and Compounds 476 (2009) 352–355.

    :cool:
     

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