24
February 2016
Stainless Steel and Magnetism
Stainless Steel and Magnetism
Stainless steel is not a “virgin” material we can find in nature but an alloy of metals
and elements, one of which is chromium; if this element’s proportion is higher than 12% the metal gets its anti-corrosion properties. On the other hand the magnetic properties of stainless steel are created or not by the composition of the alloy. Stainless steel is classified in various qualities (series) according to the proportions of its composing materials. There are 4 basic groups for stainless steel: a) Martensitic, b) ferritic, c) ostenitic and d) duplex. What remains to be examined is what causes the magnetic or non-magnetic properties.
Chromium increases the magnetic properties of stainless steel as opposed to nickel that decreases them. In stainless steel series 300 and more the penetration of nickel transforms the composition to austenitic (non-magnetic) and especially after being cold-worked the metal shows magnetic percentages close to 0% (AISI 316 especially after being cold-worked is non-magnetic and AISI 304 has very low levels of magnetism).
- Martensitic stainless steel is magnetic, the percentages of chromium are between 12% to 17%, of carbon is 0.5% and after being heated it becomes more hard. This kind of metal is used for the production of turbine propellers, blades and cutlery.
- The ferritic stainless steel metals have many advantages like the low heat expansion and as a consequence higher tolerance to corrosion that can be provoked by high temperatures, the levels of yield are higher, they are less prone to corrosion caused by mechanical stress and have thermal conductivity that makes them perfect for exchangers and electric irons. Last, ferritic stainless steel are magnetic and that can prove to be very useful in various applications.
- Austenitic stainless steel properties differ as there are many series of qualities we can find, the most popular one is 300 and then we have 200, 400 and the super-austenitic 900. Basic setsquare for the non-magnetic property of austenitic stainless steels is the content in austenite or γ-Fe, that is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element.
- Duplex or Austenoferritic stainless steels have mixed proportions 50-50 or 40-60, with good mechanical properties and corrosion resistance. They are ferromagnetic but not like Ferritics as elements interpolate in levels that modify their properties.
Sources:
-Wikipedia
-Pencom Technical Bulletin "Magnetism and Other Properties of Stainless Steel"
-G.Christoforidis "Steels and Stainless Steel"