In real life, most people think that stainless steel is non-magnetic, and it is unscientific to use magnets to distinguish stainless steel. People often think that magnets absorb stainless steel, and verify its quality and authenticity. If it does not attract or has no magnetism, it is considered good and worthy of the name; if it is magnetic, it is considered a counterfeit. In fact, this is an extremely one-sided, impractical and wrong way to distinguish. There are many kinds of stainless steel, which can be divided into several types according to the structure at room temperature
1. Austenitic types such as 304, 321, 316, 310, etc.;
2. Martensite or ferrite such as 430, 420, 410, etc.; austenite is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic. Most of the stainless steel used to decorate tube sheets is austenitic 304 material, which is generally non-magnetic or weakly magnetic, but it may also appear magnetic due to fluctuations in chemical composition or different processing conditions due to tempering, but this cannot be considered as Counterfeit or unqualified, what is the reason? It is mentioned above that austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Because of composition segregation or improper heat treatment during tempering, a small amount of martensite or iron in austenitic 304 stainless steel will be formed. Body arrangement. In this way, 304 stainless steel will have a weak magnetism. In addition, after 304 stainless steel is cold-worked, the structure will also transform to martensite. The greater the degree of cold-working deformation, the more martensite transformation, and the greater the magnetic properties of the steel. Like a batch of steel strips, Φ76 tubes are produced, without obvious magnetic induction, and Φ9.5 tubes are produced. The magnetic induction is more obvious because of the larger cold bending deformation. The deformation of square and rectangular pipes is larger than that of round pipes, especially at the corners, and the deformation is more intense and the magnetism is more obvious. In order to completely eliminate the magnetism of 304 steel caused by the above reasons, the stable austenite structure can be recovered by high-temperature solution treatment, so as to eliminate 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, that is to say, the magnetism of 304 steel always shows weak magnetism. This tells us that if the stainless steel has weak magnetism or no magnetism at all, it should be judged as 304 or 316 material; if it has the same magnetism as carbon steel, it shows strong magnetism, because it is not 304 material. Both 304 and 316 are austenitic stainless steels and are single-phase. Has weak magnetism.