Wednesday, 9 March 2016

Stainless Steel Magnetism

Is all stainless steel non-magnetic?  The answer is “No”. 

The scientific answer is of this nature:

“The magnetic behaviour of stainless steels varies considerably, ranging from the paramagnetic (nonmagnetic) in fully austenitic grades to hard or permanent magnetic behaviour in the hardened martensitic grades.

“All austenitic stainless steels are paramagnetic … [but the magnetic] permeability increases with cold work due to deformation-induced martensite, a ferromagnetic phase.  For certain grades such as types 302 and 304, the increase in magnetic permeability can be appreciable, resulting in these grades being weakly ferromagnetic in the heavily cold-worked condition.  The susceptibility of a particular grade to becoming ferromagnetic when heavily cold-worked depends on the stability of the austenite, which, in turn, depends on chemical composition and homogeneity.”


The layman’s explanation is of this nature:

Stainless Steels are identified by series numbers ranging from 100 to 600.  Each series is organised by the alloy and grain structure.  The ones we are interested are mainly the 300 and 400 series.  The 300 series have an austenitic grain structure and the 400 series has a martensitic structure. 
"In the martensitic structure at the atomic level all the iron atoms are acting as mini magnets aligned in the same direction.  Cumulatively they are all adding to the overall magnetisation of the material, this is known as ferromagnetism.  However the addition of nickel disrupts this regular structure giving it an austenitic nature, so inhibiting the magnetism of the stainless steel."  

This makes the 300 series in its un-worked state non-magnetic and the 400 series magnetic.

A relatively general characteristic of the 300 series is the approximate 18% chromium and 8% nickel (among other alloying elements) content.  There is an interplay between the chromium and nickel content.  Chromium allows the stainless steel to have a magnetic structure, while nickel reduces the magnetic properties of the steel.  The 300 series mostly contain enough nickel to make them non- magnetic or only weakly magnetic. 

However, cold working to form the sheets into vessels or other objects can break down the non-magnetic structure of the steel by aligning the atoms together.  So some highly worked 300 series steels can become magnetic, although their corrosion resistance does not change.  The 316 stainless steel contains higher amounts of nickel than others and exhibits almost no magnetism in its cold worked state.  But 304 with less nickel does become mildly magnetic. Another advantage of high nickel content is that assists the chromium to form a passive surface layer, so resisting corrosion. This is of assistance to kiln formers, as it reduces spalling.  But note that magnetic response is a function of the metallic structure, not the corrosion resistance formed by the chromium and nickel (as well other trace metals) content.

Based in large part on: Magnetism and Other Properties of Stainless Steel, by Gregg V. Summers, P.E. Director of Product Development, Peninsula Components Technical Bulletin 

The addition of nickel in the 300 series eases the workability and welding of stainless steel over the 400 series. You are more likely to find the 300 series in worked vessels and other kitchen equipment.  But the stainless steel knives are much more likely to be of the 400 series.  Both of these stainless steels have a high degree of corrosion resistance.