Why Does 300 Series Stainless Steel Sometimes Have a Low Level of Magnetism?

Potentially, any metal containing iron can be magnetic in certain states. 300 series, of which iron is the largest constituent element (as measured percent by weight), is no exception to this rule. However, it is far more prevalent in heavily cold worked tube (tube which has been worked to maximum tensile strength for that diameter/wall). The more the tube is worked, the more that the crystal structure of the stainless steel gets transformed from austenite into martensite. The strain induced from tube drawing yields a body centered crystal structure which is magnetic. A tube that is fully annealed (austenitic) has a face centered crystal structure which is not magnetic.

One might ask---why not just anneal the tube to eliminate any magnetism? The most important reason is that any desired mechanical characteristics like high tensile strength or hardness values will be lost once the tube is annealed into a soft state. The other reason is that if the tube is straightened (assuming it is an annealed state), this operation always happens after the final anneal because heat treating can distort the tube straightness. Straightening adds a tiny amount of cold work which can barely be registered. But even the smallest amount of work---say from a slight bend or straightening---can create a tiny amount of magnetism that could be objectionable for a very particular application. In these cases, considering an alternative alloy may be the best route.