Why is my Stainless Steel Magnetic? ...Or is it?

Summit Pump - April 25, 2019

In the case of plain/standard 316-SS, it is usually because the piece has been cold worked and will pick up some magnetic properties. Note if you were to compare the 316-SS to a 400 series stainless you would see an increase in magnitude of the magnetic attraction. It will not attract other ferritic metals, only the magnet, as a result the material itself is not necessarily magnetic, it is ferrous. The magnetic response has no effect on any other property. Austenitic 300 series steels like cold drawn 304 (and to a lesser degree 316) are slightly attracted to a magnet, but this has no effect on its corrosion resistance.   

Cold Working: Parts that are highly worked (due to machine operations), such as sleeves and shafts that have been machined, ground and polished, will lend themselves even more to this phenomena.

Corrosion: In 316-SS it is the chrome content that makes it stainless, but it is the nickel content that makes it nonmagnetic. 

In the process of cleaning the materials at the foundry and the machine shop, there could be some residual ferrite (iron) from the cleaning process which temporarily alters the surface.  The surface may become contaminated on the work site as well. The remedy is to pickle and passivate the surface once again. Stainless surfaces must be kept clean, so the surface can generate the passivation layer and remain as new. It is the chrome oxide film that stainless naturally forms that keeps it from corroding.   

Rolled or Cast: There is a difference in cast stainless steels versus wrought stainless steels that aggravates and differentiates the issue of magnetic attraction. In the case of cast stainless steel, like CF8M, the chemistry and micro structure are purposely different from rolled steel, but the physical and corrosion properties are similar. AISI 316-SS is wrought steel and is notably less or nonmagnetic altogether, as compared to CF8M.  

We want some ferrite in the CF8M cast steels to increase the strength and increase its resistance to corrosion cracking. The small amount of ferrite also reduces some forms of corrosion and helps with weldability. 

Duplex and Super Duplex Materials: CD4MCu is a duplex stainless steel with magnetic attraction, due to its ferrite content. 

If you remain concerned about the material’s magnetic properties, please discuss with engineering or your RSM. It’s possible to perform and document a witnessed PMI test at no charge.

- The Summit Pump Team

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