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Why is stainless steel also magnetic?

It is often assumed that a magnet attracts stainless steel to verify its quality and authenticity. If it doesn't absorb non-magnetic material, it's considered good and real stainless steel; if the suction cup is magnetic, it's considered fake. In fact, this is an extremely one-sided, unrealistic, and wrong identification method.
There are many types of stainless steel, which can be divided into several categories according to the structure at room temperature
1. Austen type: 201, 202, 301, 304, 316, etc.;
2. Martensite or ferrite: such as 430, 420, 410, etc.;
Austenite is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic. A small amount of martensite or ferrite will be formed in austenitic 304 stainless steel due to component segregation or improper heat treatment during the smelting process. In this way, 304 stainless steel will have weaker magnetic properties.
In addition, the structure of 304 stainless steel will transform to martensite after cold working. The greater the cold working deformation, the more martensitic transformation, and the greater the magnetic properties of the steel. Like a batch of strip steel, it can produce Φ 76 tubes without obvious magnetic induction, and Φ 5 tubes. Due to the large cold bending deformation, the magnetic induction is more obvious. Since the deformation of the rectangular tube is larger than that of the circular tube, especially the corners, the deformation is stronger and the magnetic induction is more obvious.
In order to eliminate the magnetism of 304 steel caused by the above reasons, the stable austenite structure can be restored by high-temperature solution treatment, thereby eliminating the magnetism.
In particular, the magnetism of 304 stainless steel due to the above reasons is not at the same level as that of other stainless steels such as 430 and carbon steel, which means that the magnetism of 304 stainless steel always shows weaker magnetism.
This tells us that if stainless steel is weakly magnetic or not magnetic at all, it should be identified as a 304 or 316 material; if it has the same magnetic properties as carbon steel, it shows strong magnetism because it is not a 304 material.

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