All austenitic grades have very low magnetic permeabilities and hence show almost no response to a magnet when in the annealed condition; the situation is, however, far less clear when these steels have been cold worked by wire drawing, rolling or even centreless grinding, shot blasting or heavy polishing. After substantial cold working Grade 304 may exhibit quite strong response to a magnet, whereas Grades 310 and 316 will in most instances still be almost totally non-responsive.
The change in magnetic response is due to atomic lattice straining and formation of martensite. In general, the higher the nickel to chromium ratio the more stable is the austenitic structure and the less magnetic response that will be induced by cold work. Magnetic response can therefore be used as a method for sorting grades of stainless steel, but considerable caution needs to be exercised.
Any austenitic (300 series) stainless steel which has developed magnetic response due to cold work can be returned to a non-magnetic condition by stress relieving. In general this can be readily achieved by briefly heating to approximately 700 - 800°C (this can be conveniently carried out by careful use of an oxy-acetylene torch). Note, however, unless the steel is a stabilised grade it could become sensitised to carbide precipitation. Full solution treatment at 1000 - 1150°C will remove all magnetic response without danger of reduced corrosion resistance due to carbides.
If magnetic permeability is a factor of design or is incorporated into a specification, this should be clearly indicated when purchasing the stainless steel from a supplier.
Many cold drawn and/or polished bars have a noticeable amount of magnetism as a result of the previous cold work. This is particularly the case with grades 304 and 303, and much less so for the higher nickel grades such as 310 and 316. Even within the chemical limitations of a single standard analysis range there can be a pronounced variation in the rate of inducement of magnetic response from cold work.
Magnetically Soft Stainless Steels
In some applications there is a requirement for a steel to be "magnetically soft". This is often required for solenoid shafts, where it is necessary for the plunger to respond efficiently to the magnetic field from the surrounding coil when the current is switched on, but when the current is switched off the magnetic field induced in the steel must quickly collapse, allowing the plunger to return to its original position. Steels which behave in this way are said to be magnetically soft. For corrosion resisting applications there are ferritic stainless steels which are magnetically soft, usually variants of a grade "18/2" (18% chromium and 2% molybdenum) but with very tightly controlled additions of silicon and often with sulphur added to make them free machining. Special mill processing guarantees the magnetic properties of the steels.