2024-11-22
Stainless steel-What can be done to minimize the magnetization of stainless steel components?
Both 316 and 304 stainless steels are austenitic; when they cool, the iron remains in the form of austenite (gamma iron), a phase of iron which is nonmagnetic. The different phases of solid iron correspond to different crystal structures. In other alloys of steel, this high-temperature phase of iron transforms to a magnetic phase when the metal cools. The presence of nickel in the stainless steel alloys stabilizes austenite against this phase transition as the alloy cools to room temperature. As a result, a relative permeability of K ∼ 1.002 to 1.005 are typically reported for 304 and 316 stainless steels in their annealed state. This corresponds to a somewhat larger magnetic susceptibility than we might expect for other nonmagnetic materials, but is still well below what might be considered magnetic.This does not mean that you should expect to measure such a low susceptibility on any item of 304 or 316 stainless steel that you encounter. For example, values of K∼1.8 have been found in randomly selected 304 SS fasteners. Any process which can change the crystal structure of stainless steel can cause austenite to be converted to the ferromagnetic martensite or ferrite forms of iron. These processes include cold working and welding. It is also possible for austenite to spontaneously convert to martensite at low temperatures. To complicate matters further, the magnetic properties of these alloys depend on the alloy composition. Within the allowed ranges of variation of Ni and Cr, significant differences in magnetic properties may be observed for a given alloy.