I noticed a friend's appliances are not magnetic, and mine are. Both are stainless steel. Are there two types of stainless steel or something? Thanks
Some stainless steel are magnetic and some aren't. The real high grade stainless steels aren't magnetic.Stainless is just regular steel with varying degrees of nickel and chromium alloys mixed in the batch. The more nickel and chromium the less likely to be magnetic. A lot of stainless exhaust pipes and barbecue grills claim to be stainless, and yet they still rust and tend to be magnetic. They must use the bare minimum of nickel and chromium in the melted batch just to claim the title of stainless steel. Nickel and chromium are quite expensive and they are what gives the steel the anti-corrosion and extreme hardness characteristics along with loss of magnetism of the steel. If you have stainless steel that is magnetic , it is a good sign it is stainless steel that barely meets the criteria of true stainless steel.
There are many types of stainless steel. Some are magnetic and some are non-magnetic. The magnetic properties of stainless steel are very dependent on the elements added into the alloy, and specifically the addition of nickel can change the structure from magnetic to non-magnetic. Poor heat treatment or high heat input welding of normal or high carbon austenitic stainless steels will cause sensitization, ie formation of chromium carbides. The formation of carbides not only reduces the corrosion resistance of the stainless steel but also tends to form martensite around the carbide. This martensite is magnetic and the more severe the sensitisation, the stronger are the magnetic properties. When nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. Martensitic stainless steels are magnetic. *Wrought, austenitic stainless steels, such as 304 and 316, are generally regarded as non-magnetic in the annealed condition, ie they are not attracted significantly by a magnet. However, if they are cold worked they will be attracted to a permanent magnet. The change occurs because the cold work deformation induces a transformation of the microstructure from austenite to martensite. The effect is less marked in alloys with high concentrations of austenite stabilisers such as nickel, nitrogen and carbon. Once the martensite is formed, it may also become magnetised. *In contrast to the austenitic alloys, ferritic stainless steels such as 409 or 3Cr12/5Cr12 and martensitic stainless steels such as 420, are strongly attracted to a magnet even in the annealed state. The duplex and super-duplex stainless steels will also be strongly attracted because they contain about 50% ferrite in their microstructure. *
Is Stainless Steel Magnetic
If the non magnetic one is has a brighter finish it may be that it is a ferritc stainless steel which depends only on high chromium content to keep it stainless,the duller one is likely to be the austenitic type which contains nickel as well as chromium and is usually non or only faintly magnetic and with a dull finish.Chromium rich stainless steels(ferritic and martensitic grades) are designed to be used for cutlery and strength application are always strongly magnetic (quite often permanent magnets).Stainless steels containing at least 18%chromium and 8+% of nickel are designed to be corrosion resistant and weldable,this type of steel(austenitic) is not ferromagnetic in the annealed state but the lower grades do become magnetic after cold work(hammering bending etc.The difference between magnetic grade or not rests in the crystal structure.In the austenitic types the structure is the same as that of gold and copper which is cubic close pack and,in steel,is a non magnetic form;but in low grades cold working can cause some breakdown of the austenite to the magnetic room temperature form of iron known as ferrite.Ferrite has the ordinary body centred body centred cubic form of iron which is magnetic.All of the stainless steels depend upon Chromium to form an anti-corrosion barrier at their surface;but this is only reliable in oxidising conditions(like the open air)They nearly all discolour and even rust if trapped in damp conditions where oxygen potential is low(as under wet plastic or underground ).
