Why is Stainless Steel Magnetic?

People often use the magnet to absorb stainless steel to verify its pros and cons. If the magnet does not absorb stainless steel, it is non-magnetic, which is considered to be good, and the price is genuine. If it is adsorbed, it is magnetic and is considered to be a counterfeit. In fact, this is an extremely one-sided, unrealistic method of identification.

There are many types of stainless steel, which can be divided into several categories according to the organizational structure at room temperature:

Austenitic type: such as SUS304, SUS321, SUS316, SUS310, etc.;
Martensite or ferrite type: such as SUS430, SUS420, SUS410, etc.;
The austenite type is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic.

Most of the stainless steels used for decorative tube sheets are austenitic 304 materials, which are generally non-magnetic or weakly magnetic. However, due to chemical composition fluctuations caused by smelting, or different processing conditions, magnetic properties may also occur. It cannot be considered that these steels are counterfeit or unqualified. What is the reason for this?

It is mentioned above that austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Improper segregation or heat treatment during smelting results in a small amount of martensite or ferrite structure in austenitic 304 stainless steel. Thus, 304 stainless steel will have weak magnetic properties. In addition, 304 stainless steel is cold-worked, and the microstructure is also transformed into martensite. The greater the cold work deformation, the more martensite transformation and the greater the magnetic properties of the steel. Like a batch of steel strips, it produces Φ76 tubes with no obvious magnetic induction and produces Φ9.5 tubes. Due to the large deformation of the bending, the magnetic induction is obvious. The rectangular tube produced is larger than the round tube, especially the angled part where the deformation is more intense and the magnetic is more obvious.
In order to completely eliminate the magnetic properties of 304 steel caused by the above reasons, the austenite structure can be restored by high temperature solution treatment, thereby eliminating magnetic properties. In particular, the magnetic properties of 304 stainless steel caused by the above reasons are not at the same level as the magnetic properties of other materials such as 430 and carbon steel. That is to say, the magnetic properties of 304 steel always show weak magnetic properties.

This tells us that if the stainless steel is weakly magnetic or not magnetic at all, it should be judged as 304 or 316 material; if it is magnetically similar to the carbon steel, it is judged to be not 304 material.