The magnetism of stainless steel

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People often assume that magnets attract stainless steel in order to determine its quality and authenticity; if a magnet does not attract it, it is considered to be of good quality and genuine； If the inhaler has magnetism, it is considered to be a fake. In fact, this is an extremely one-sided and unrealistic wrong method of judgment. There are many types of stainless steel. At room temperature, they can be classified into several categories based on their microstructural structure: 1. Austenitic type: such as 201, 202, 301, 304, 316, etc ； 　　2. Martensitic or ferritic types: such as 430, 420, 410, etc ； Austenitic grades are non-magnetic or weakly magnetic, while martensitic or ferritic grades are magnetic. 　　The stainless steel commonly used for decorative tube sheets is mostly of the austenitic 304 type; it is generally non-magnetic or only weakly magnetic. However, variations in chemical composition due to manufacturing processes or differences in processing conditions can result in magnetism, but this does not mean the material is counterfeit or defective. What is the reason for this? 　　As mentioned above, austenite is non-magnetic or weakly magnetic, whereas martensite or ferrite is magnetic. Due to compositional segregation during manufacturing or improper heat treatment, small amounts of martensite or ferrite structure can form in austenitic 304 stainless steel. In this way, 304 stainless steel will have a slight magnetism. 　　Furthermore, when 304 stainless steel is cold-worked, its microstructure transforms into martensite; the greater the degree of cold working deformation, the more martensite is formed, and the greater the magnetism of the steel becomes. For steel strips of the same batch, when used to produce Φ76 pipes, there is no significant magnetic response; whereas when used to produce Φ9.5 pipes, such a response does occur. Due to the greater bending deformation, the magnetic susceptibility becomes more noticeable. When manufacturing rectangular tubes, the degree of deformation is higher than that of circular tubes; especially at the corners, where the deformation is more severe and the magnetism is more apparent. 　　To completely eliminate the magnetism of 304 steel caused by the aforementioned factors, it is possible to restore a stable austenitic structure through high-temperature solution treatment, thereby eliminating the magnetism. 　　It should be specifically noted that the magnetism of 304 stainless steel resulting from the reasons mentioned above is not on the same level as that of other types of stainless steel, such as 430 or carbon steel; in other words, 304 steel always exhibits weak magnetism. This tells us that if the stainless steel strip has weak magnetism or no magnetism at all, it should be classified as 304 or 316 material ； If it exhibits strong magnetism similar to that of carbon steel, it should be determined not to be 304 material.