Stainless Steel 316, Magnetic?????

316SS is almost non-magnetic. It will have a relative permeability above 1, depending on the field strength and the manufacturing process. If there was a lot of work hardening, the permeability will be higher. I use a very weak magnet to sort SS; it will attract but not hold 316.

We had a similar problem recently with 304 stainless which is very close to 316. The parts are machined so I guess they could work harden somewhere in the process & had become magnetised. We tried de-gaussing the finished assembly but some residual magnetism remains. We use Mu-metal shields to protect our devices from the Earths magnetic field but that's not much help if the magnetism is inside the equipment.

Cold working of 316 or 316L stainless steel will cause it to become magnetic. Fully annealed 316 is nominally non-magnetic but the extent of the magnetic peermeability is a function of the actual composition of the particular heat of material. If you review the specifications of 316 sst you see the metals in the alloy must fit into a range of percentages of the element in the heat. If you look on the Scheffler diagram, you will see that 316 alloy can overlap other alloys. In short all 316 is not the same. The magnetic permeability of each heat can vary so the amount of "magnetism" in a particular heat can vary.

Annealing the parts will return them to nominally "non-maagnetic" condition.

These particular parts would have been vacuum stoved at 1000°C & allowed to cool naturally. They then get assembled without being worked any further but might have had a light TIG weld at one or both ends. I haven't looked up the annealing temperature for stainless but I would have thought that they would be fully annealed yet they still remain magnetised.

Hi all,

Any austenitic SS of the 302 family (304, 316, 321, 347, etc) in the normal "as delivered condition" that's solution annealed, are fully austenitic and non-ferromagnetic, so no magnet attraction is possible.

If any of this SS are welded, the melted metal which usually is not subjected to a new solution heat treatment can have a little portion of δ-ferrite which is ferromagnetic. Just take a look to the Schaeffler-De Long diagram.

Many specifications and codes ask for that small amount of δ-ferrite (usually in the range of 3 to 12%) because it decrease the cracking sensibility of the weld. One of the methods to get the percentage of δ-ferrite or the FN (ferrite number, which is a similar measurement) is through magnetic devices. In this cases, the attraction of the SS by a magnet increases as FN does.

Some other question: Austenite structure in those SS is metastable and can transform to martensite (ferromagnetic) depending on the degree of cold deformation, the temperature and the exact composition of the steel. In fact there are studies in which a parameter is used to measure the susceptibility of a particular steel to that phenomenon. It is called M_d by similitude with M_s which is the temperature to which Martensitic transformation Start. In this case the suffix d stands for "deformation".

If the case you mention is cast SS, probably the problem is in the solution heat treatment. In the case of hot formed 316 this phenomenon of transformation to martensite dosn't occur. Be secure of the real heat treatment status of the supplied SS and in case of doubt a metallographic inspection would give you the answer.

We use SS pump shaft made from 316 all the time. It has always had some magnetic property to it. Sometimes enough to hold a magnet.