The magnetic properties of stainless steel are very dependent on the elements added to the alloy. A basic stainless steel has a "ferrite" structure and is magnetic by the addition of chromium, which can be hardened by the addition of carbon, making it "martensitic", however, the most common stainless steel is "austenitic" "-they have a higher chromium content and also have nickel added. Nickel is what changes the physical structure of steel and makes it theoretically non-magnetic.

304 stainless steel contains chromium (minimum 18%) and nickel (minimum 8%). This is an austenitic steel that responds only slightly to magnetic fields. It also contains 18-20% chromium and 8-10.5% nickel, as well as small amounts of other elements.

316 stainless steel is a molybdenum alloy steel that also responds insignificantly to magnetic fields, which means it can be used in applications that require a non-magnetic metal. It also contains other elements in varying concentrations.

Since both 316 stainless steel and 304 stainless steel are austenitic, when they cool, the iron remains in the form of blackite (y-iron), which is a non-magnetic iron phase. The differences in solid iron correspond to different crystal structures, and in alloys of other steels, this high-temperature phase of iron transforms into a magnetic phase when the metal cools. The presence of nickel in stainless steel alloys stabilizes the austenite against this phase transformation when the alloy cools to room temperature. This corresponds to a larger magnetic susceptibility than we would expect for other nonmagnetic materials, but still much lower than what might be considered magnetic.

However, that doesn't mean you should measure with such low sensitivity on any 304 or 316 stainless steel material you come across, any process that can change the crystal structure of stainless steel can transform austenite into ferromagnetic martensitic body or cable body form, these processes include cold working and welding. Further complicating matters, austenite may also spontaneously transform to martensite at low temperatures, and the magnetic properties of these alloys depend on the alloy composition. Significant differences in magnetic properties can be observed for a given alloy within the allowed variation of Ni and Cr.

Both 304 and 316 stainless steel have paramagnetic properties. As a result of these properties, small particles (eg spheres of approximately 0.1-3mm diameter) can be attracted to strong magnetic separators located in the product stream. Depending on their weight, especially their weight ratio to the magnetic attraction force, these small particles will remain on the magnet during the production process.

These can be removed during magnet cleaning operations. From our experience, 304SS small particles are easier to keep in the flow than 316 stainless steel particles because it is slightly more magnetic.